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High-throughput search for new permanent magnet materials
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys.: Condens. Matter 26 064208 (http://iopscience.iop.org/0953-8984/26/6/064208) View the table of contents for this issue, or go to the journal homepage for more
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Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 064208 (13pp)
doi:10.1088/0953-8984/26/6/064208
High-throughput search for new permanent magnet materials D Goll, R Loeffler, J Herbst, R Karimi and G Schneider Materials Research Institute, Aalen University, Beethovenstraße 1, D-73430 Aalen, Germany E-mail: [email protected] and [email protected] Received 23 July 2013, revised 5 September 2013 Published 27 January 2014
Abstract
The currently highest-performance Fe–Nd–B magnets show limited cost-effectiveness and lifetime due to their rare-earth (RE) content. The demand for novel hard magnetic phases with more widely available RE metals, reduced RE content or, even better, completely free of RE metals is therefore tremendous. The chances are that such materials still exist given the large number of as yet unexplored alloy systems. To discover such phases, an elaborate concept is necessary which can restrict and prioritize the search field while making use of efficient synthesis and analysis methods. It is shown that an efficient synthesis of new phases using heterogeneous non-equilibrium diffusion couples and reaction sintering is possible. Quantitative microstructure analysis of the domain pattern of the hard magnetic phases can be used to estimate the intrinsic magnetic parameters (saturation polarization from the domain contrast, anisotropy constant from the domain width, Curie temperature from the temperature dependence of the domain contrast). The probability of detecting TM-rich phases for a given system is high, therefore the approach enables one to scan through even higher component systems with one single sample. The visualization of newly occurring hard magnetic phases via their typical domain structure and the correlation existing between domain structure and intrinsic magnetic properties allows an evaluation of the industrial relevance of these novel phases. (Some figures may appear in colour only in the online journal)
the RE metals Nd, Pr and Sm. The highest (B H )max values are currently given by the ternary intermetallics Fe14 (Nd, Pr)2 B (K 1 = 4.3 MJ m−3 , Js = 1.61 T, TC = 312 ◦ C, (B H )max = 450 kJ m−3 ) (Rodewald 2007) at room temperature and by the quinary intermetallics (Co, Cu, Fe, Zr)17 Sm2 at elevated temperatures ((B H )max = 100 kJ m−3 at 400 ◦ C) (Chen et al 1998, Hadjipanayis et al 2000, Goll et al 2004). FePt and CoPt based permanent magnets ((B H )max = 200 kJ m−3 ) (Buschow 1997) exhibit high uniaxial magnetocrystalline anisotropies (6.6 MJ m−3 and 4.9 MJ m−3 , respectively) from an ordered face-centered tetragonal phase (L10 phase) which develops at elevated temperatures from a disordered face-centered cubic phase. Bulk Fe–Nd–B magnets are currently the most promising candidates for a wide range of efficient energy conversion applications (Rodewald 2007, Kramer et al 2012, Lewis and Jim´enez-Villacorta 2013), while thin film L10 -FePt/CoPt magnets and regular bit patterns thereof are promising for next-generation ultrahigh density magnetic recording, realiz-
1. Introduction
High-performance permanent magnets are important materials for efficient energy conversion in high-power electric motor and generator applications. To give the maximum benefits in terms of sustainable resource efficiency and renewable energy, the permanent magnets should be based on materials with exceptional intrinsic magnetic parameters (saturation polarization Js > 1.2 T, anisotropy constant K 1 > 106 J m−3 , Curie temperature TC > 250 ◦ C) as well as optimized microstructures to guarantee long lifetime and cost-efficiency. Large Js values are required to give large remanences Jr , while large K 1 values can yield large coercivities Hc , both ensuring high maximum energy products (B H )max . The most famous high-performance permanent magnets at present are based on rare-earth (RE)–transition metal (TM) intermetallics (Fe–Nd– B, Co–Sm) and FePt/CoPt. In RE–TM intermetallics the high saturation polarization and Curie temperature of the TM metals Fe and Co are combined with the high anisotropy constant of 0953-8984/14/064208+13$33.00
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far, it is not surprising that Fe–Nd–B has remained the highestperformance permanent magnet material for 30 years. To scan through tens of thousands of alloy systems in a reasonable time frame looking for novel hard magnetic compounds is not possible by conventional methods (as-cast samples, magnetometry) and requires an alternative concept. High-throughput materials science is an approach to the accelerated discovery, study, and optimization of both novel and known materials. During experimentation different parameters or base materials are systematically varied in parallel and empirically analyzed to identify the optimum values in a shorter period. Although individual aspects of rapid materials development have been known for decades (e.g. Anderson and Moser 1958, Kennedy et al 1965), the first integrated materials-development workflow was introduced in 1970 by Hanak (1970). The workflow included complete compositional mapping of a multicomponent system in one experiment (library), a fast method to screen for properties (characterization and testing) and computer data processing (computational tools). In 1995, applications of high-throughput experimentation in materials science were reinitialized by Xiang et al (1995). Since then, the method has been put into practice to discover and optimize a wide spectrum of materials (Maier et al 2007, Potyrailo et al 2011). In comparison with the original idea of Hanak, advanced workflow now incorporates further important aspects, such as the design/concept of experiments, materials modeling by theory and scale up. High-throughput experimentation is already an indispensable tool in catalysis, genetic and pharmacological research to quickly identify active compounds, genes or antibodies that can provide the starting points for drug design and a better understanding of particular biochemical processes (Potyrailo et al 2011). The Materials Genome project is aimed at materials design with high-throughput computation to predict material properties without experimental synthesis and record the material properties in extensive databases (Jain et al 2011). For accelerated materials discovery, Materials Genome combines advanced scientific computing techniques with innovative design tools. This includes the computation of phase diagrams to find thermodynamically stable phases and study decomposition pathways, the computation of chemical reaction kinetics and the ab initio computation of the influence of additives on intrinsic material parameters using data-mined substitution algorithms. In all cases the reliability and validity of the high-throughput methods have to be well established by comparing traditional and combinatorial routes using known materials. The targetoriented empirical search for novel intermetallic compounds requires a suitable approach to efficiently identify material systems that form (many) intermetallic compounds (so-called ‘former’ systems). The prediction of ‘former’ and ‘nonformer’ material systems is typically based on (semi)empirically developed complex algorithms, taking several out of more than 60 specific elemental property parameters into consideration (Villars et al 2001, 2008, Nianyi et al 1999a,c,b). Of these parameters, Villars et al (2001) found the Mendeleev number to be a suitable elemental property. In contrast, discovering novel hard magnetic materials using high-throughput methods is a more recent development.
Figure 1. Illustration of the performance–cost landscape of common permanent magnets and the wide gap between hard ferrites and FeNdB magnets.
ing storage densities of 1 Tb in−2 and beyond (Goll and Bublat 2013, Weller et al 2013). The worldwide sales volume of Fe–Nd–B magnets in 2005 was 4 billion US$, and by 2020 sales of Fe–Nd–B magnets are expected to be over 17 billion US$ (Constantinides 2012). For economic reasons half of this market is still dominated by hard ferrites of the type (Ba/Sr)Fe12 O19 (K 1 = 0.3 MJ m−3 , Js = 0.46 T, TC = 450 ◦ C, (B H )max = 23 kJ m−3 ) (Kojima 1982). Hard ferrites are considered to be ceramic hard magnets with a low crystal symmetry and therefore a high uniaxial magnetocrystalline anisotropy. They are free of RE, however, their strength is up to 20 times lower than for Fe–Nd–B due to their lower Fe content and ferrimagnetic behavior. Besides the economic aspects, the sustainable use of Fe–Nd–B magnets is further hindered by the fact that their temperature and corrosion stability are limited, which may lead to accelerated ageing of the magnets. Up to now, the ultimate magnetic material to fill the gap between Fe–Nd–B magnets and hard ferrites on the performance–cost landscape does not exist (figure 1). We see an enormous need for action in this field. As a consequence, already known hard magnetic materials such as MnBi and MnAl(-C), to which less attention has been paid in the past, are currently undergoing a revival (Coey 2012, Kramer et al 2012, Lewis and Jim´enez-Villacorta 2013). The key issues are to increase the operating temperature and magnetization by finding suitable alloying elements with which to stabilize the phases without impairing the magnetic properties. Also well-known ideas for realizing new ultrastrong supermagnets by making soft magnets such as Fe–35Co or Fe16 N2 , with their large saturation polarization, become hard magnetic are attracting renewed attention (Kramer et al 2012, Lewis and Jim´enez-Villacorta 2013). It is expected that their large saturation polarizations of Js (Fe–35Co) = 2.45 T and Js (Fe16 N2 ) = 2.7 T in combination with the introduction of strong planar pinning centers or expansion of the lattice structure is the road to success. Yet another alternative is the discovery of completely new high-performance, cost-effective and long-lasting permanent magnet materials, either with significantly reduced rare-earth content or completely free of rare-earths. This, however, is rather complex—like looking for a needle in a haystack considering the large number of possible alloy systems. So 2
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analysis methods to identify quickly and reliably potential hard magnetic compounds in the sample and estimate their intrinsic magnetic material parameters Js , K 1 and TC . In section 6 an overview is given of the further procedures necessary to realize a permanent magnet based on interesting magnetic compounds before scaling up to mass production can be started.
Recently, both high-throughput bulk approaches (Luedtke et al 2000) and thin film approaches (Takeuchi et al 2003) have become of interest for scanning through even higher component systems in the search for new hard magnetic compounds in a reasonable period of time. The bulk approach is based on heterogeneous non-equilibrium states that form concentration gradients between different elements due to thermodynamically guided diffusion processes. As the magnetic properties of the material change along the gradients, one sample may be sufficient to cover a complete phase diagram. The thin film approach makes use of thin film libraries produced by defocused co-sputter deposition or molecular beam epitaxy synthesis from elemental targets on a (micro-patterned) wafer. Here also the wafer (or grid pattern on it) with varying nominal composition (discrete or gradient) covers a complete phase diagram. For directional crystal growth, suitable substrates are required with a lattice spacing similar to that of the hard magnetic compounds, whereas the bulk approach is independent of substrates. Furthermore, in some cases it may be possible that novel magnetic phases formed on the nanometer scale are stabilized by the substrate and do not develop in the bulk. In any case, both approaches require high-throughput analysis methods to identify and characterize promising novel hard magnetic compounds and evaluate their potential for industrial relevance. The bulk approach with phase formation on the µm scale so far relies on the typical stripe domain patterns of hard magnetic materials to directly visualize hard magnetic compounds and estimate their properties. Thin film libraries work below the µm scale of domain formation and therefore resort to changes in the crystal structure and composition throughout the wafer which can be automated by different electron diffraction (Tsui et al 2007) and x-ray diffraction techniques (Maier et al 2007, Brunken et al 2011, Gao et al 2013). Li et al (2007), in magneto-optical Kerr effect (MOKE) measurements, have described a finite Kerr rotation angle in the case of magnetic materials. For closer investigations of interesting wafer areas the wafer has to be cut into smaller pieces to perform transmission electron microscopy and magnetometry measurements (Gao et al 2013). The focus of this paper is on the high-throughput search for novel high-performance durable and commercially attractive permanent magnet materials with a significantly reduced RE content (or better still RE-free). A systematic search is a multi-level process and requires a systematic approach. In section 2 suitable cornerstones for the search are defined to limit the number of element combinations (systems). These cornerstones are: the selection of elements which are essential for good ferromagnetic behavior, raw material costs and resource aspects as well as health and environmental risks. In section 3 a prioritization process is introduced to narrow the number of systems to be investigated. Section 4 describes the development of high-throughput synthesis for realizing heterogeneous non-equilibrium microstructures to allow the scanning of the prioritized systems. The relevant part for interesting hard magnetic phases of the phase diagram can be investigated with one sample to generate a sufficient volume (∼several tens of µm3 ) of the phases. Section 5 deals with the development of suitable high-throughput microscopy
2. Search strategy and definition of the cornerstones
The efficient search for novel and commercially attractive hard magnetic compounds requires a meaningful definition of suitable element combinations (systems). In the actual work, the selection criteria reflect the prospects of elements concerning magnetic properties, raw material cost, resource availability and toxicity. The four transition metals Fe, Co, Ni and Mn were chosen as base elements since they have promising magnetic properties, e.g. large magnetic moments and high Curie temperatures. Novel compounds with a high TM content and ferromagnetic alignment of the TM magnetic moments may thus achieve high saturation polarizations. Of the four base elements, Fe is the most attractive element since it has the largest magnetic moment and is inexpensive and readily available. Stabilization of favorable structures with ferromagnetic alignment of the TM moments can be achieved by adding preferably non-magnetic additional elements. Additives may also improve the magnetic properties of known hard magnetic compounds. This can be achieved by either incorporation of an additional additive atom species in the existing structure or by replacing an already incorporated additive atom species by a more suitable one. In either case, the incorporated additive atom may influence the TM–TM distance of the compound’s crystal structure in such way that the exchange interaction, Js and Tc are increased. From the potential natural available elements, 41 elements were considered as possible additive candidates. Not included in the additive group are elements belonging to the lanthanides, actinides and noble gases or those that are toxic/radioactive and very expensive. Despite their high price, Pt and Pd were included, since both elements are known to form hard magnetic compounds with favorable properties. A further group of elements that may be relevant for the search includes the rare-earth elements, since their unique electronic structure is favorable for the formation of compounds with high crystal anisotropies. For the conducted search, seven light RE-elements were considered as attractive candidates, as they are less expensive than their heavy cousins. In particular, La and Ce are also available in larger quantities. In summary, the following numbers of elements are considered in the search: four elements as base metals, and 41 elements as additives for RE-free systems—with additionally seven elements as rare-earths for RE-containing systems. Based on the selected elements and defined frame conditions, the number of possible element combinations was calculated for binary, ternary, quaternary and quinary systems using enumerative combinatorics (k-combination). This was done separately for RE-containing and RE-free combinations. The frame conditions specify the number of ‘slots’ that may 3
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Figure 3. Prioritization concept to evaluate the potential of a ternary
system to form TM-rich intermediate phases based on the number of intermediate phases of its TM-containing binary marginal systems (‘phase index’).
Figure 2. Number of possible systems (RE-free: TM, TM–X,
TM–X1–X2, TM1–TM2–X, TM–X1–X2–X3, . . . ; RE-containing: TM, TM–RE, TM–RE–X, TM–RE–X1–X2, . . . ) based on the combination of four TM elements, 41 additive elements (X) and seven RE-elements by enumerative combinatorics (k-combination). Including unary, binary, ternary, quaternary and quinary systems, in total approximately one million unique quinary systems are possible.
species with suitable elemental properties and, therefore, may have a greater potential to form ternary intermediate phases. The number of formed compounds of the boundary systems may thus be an attractive and readily obtainable parameter to estimate the potential of the corresponding higher order system. This general concept was further adapted to the specific task of evaluating the potential of ternary TM-based systems to form TM-rich compounds, preferably with a TM content >70 at.%. The formation of such intermediate phases takes place in the TM-rich corner of the respective composition triangle and will thus be governed to a great extent by the thermodynamics of the TM atom species. Due to its rather low influence, the TM-free binary marginal system may therefore be neglected in this case. Thus, the potential of a TM–X1–X2 system may be described by the computation of a synthetic parameter ‘phase index’ based on the number of intermediate phases that each of the two binary boundary systems TM–X1 and TM–X2 forms (figure 3). Validation of the approach showed that the higher the calculated phase index of a system is, the greater is its potential to form intermediate compounds. The phase index approach was then deployed to establish a prioritized list of TM-based systems that were to be synthesized by efficient methods (see section 4). The prioritization is a threefold process, taking the potential of the system to form intermediate phases and its raw material cost into account (figure 4). In the first step the phase index PI of more than 2800 ternary TM-systems was computed and those systems identified that are not yet characterized in literature. This step yields attractive new systems from the materials science point of view. In the second step, the raw material costs of the systems were computed for a fixed composition (80 at.% Fe, 10 at.% X1, 10 at.% X2 or 80 at.% Fe, 10 at.% RE, 10 at.% X) by taking into account the current kg-cost of the involved RE and X elements (in wt%), which was then reciprocally normalized against the raw material cost of Fe14 Nd2 B (82.3 at.% Fe, 11.8 at.% Nd, 5.9 at.% B), yielding the systems raw material cost index RMCI. This
be occupied by elements from any group, e.g. a ternary system has for example three slots whereas a quinary system has five. In the case of RE-free systems, at least one slot was always to be occupied by a TM base element and an additive element, respectively. In the case of RE-containing systems, at least one slot was always to be occupied by a TM base element, an additive element and a RE-element, respectively. For binary RE-containing systems, additive elements were not considered to occupy a slot. The number of possible element combinations increases rapidly with an increasing number of available slots in the respective systems (figure 2). While there are approximately 4700 unique ternary systems, this number increases to 76 000 quaternary systems and finally reaches approximately one million unique quinary systems. It is evident that this number of possible systems cannot be investigated within a reasonable time frame and thus a further prioritization process must be applied to identify the most promising systems. 3. Prioritization of prospective systems
The prioritization method is based on the concept that the potential of a ternary or higher component system to form intermediate phases is reflected in the number of compounds that each of its marginal systems forms. On the physical level, the formation and structural stability of intermediate phases within a given system is influenced/governed by the interplay of numerous elemental property parameters (e.g. atomic size, volume and weight, melting/boiling temperatures and enthalpies, electronegativities, ionization energies, chemical potentials, number of valence electrons, and many more) of the involved atom species (Villars 1995). A ternary system, whose binary marginal systems form many intermediate phases, already comprises atom 4
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Figure 5. Approximately linear relationship between the calculated
Figure 4. Systematic computation and mapping of the ternary phase index PI. The database is obtained by adding the numbers of the intermediate phases existing for the two binary boundary systems. To rank the systems accordingly, an economic index EI, composed of a phase index PI and raw material cost index RMCI, is established. The table presents four arbitrary systems from the priority list. For the phase index given only intermediate phases with Fe content >50 at.% have been considered for the two binary boundary systems.
ternary ‘phase index’ and a system’s potential to form at least one ternary intermediate phase. The number of recorded systems with a ‘phase index’ >13 is too low to allow reliable statistical evaluation.
4. Efficient synthesis by heterogeneous non-equilibrium states
High-throughput synthesis to scan systematically higher component systems for interesting hard magnetic phases can be realized by heterogeneous non-equilibrium states (Luedtke et al 2000). Instead of a time-consuming exploration of the corresponding phase diagrams, the method is based on the formation of concentration gradients between different elements due to thermodynamically guided diffusion processes along the tie-lines in phase diagrams in diffusion couples. As only transition-metal-rich intermediate phases are interesting candidates, one diffusion couple is sufficient to cover one binary, ternary or quaternary phase diagram with a sufficient probability. Depending on the dimension of the diffusion couples, two different methods may be differentiated.
step yields attractive systems from an economic point of view. The compound Fe14 Nd2 B was chosen as a base-line since it represents today’s most powerful and widespread high-performance hard magnetic compound, against which any new compound must compete (in terms of performance and material cost). In the third and final step, an economic index EI for each system was calculated on the basis of its phase index and raw material cost index. In figure 4 it is shown by means of four arbitrarily chosen systems that the EI index is greatest for a large PI index and a large RMCI index and that its value may vary over several orders of magnitude. This final step allows a first identification of promising new systems with respect to their economic and materials science attractiveness. The prioritization process presented is a rather easily deployable approach to efficiently extract promising systems (n < 1000) out of several tens of thousands of possible systems. The ‘phase index’ concept and the accuracy of different computation algorithms were tested and validated against a data set comprising more than 570 ternary Fe–X1–X2 systems. For each system of the data set, the number of intermediate phases (ternary and binary) was recorded and a phase index computed. Analysis of the resulting data shows that there is a correlation between the computed phase index of a system and its tendency to form intermediate phases (figure 5). Analysis of a limited ternary data set further confirmed that the influence of the Fe-free binary marginal system on the above-mentioned correlation is indeed rather low.
4.1. Reaction crucible (diffusion couples on a large scale)
The reaction crucible method is based on TM-crucibles that are charged with a powder mixture of additional elements required to investigate a specified system. For diffusion couples in the crucible a liquid has to be formed due to melting point reduction by the additional elements. Intermediate phases will form between the liquid and the crucible element due to the phase equilibria if they exist in a composition range between the melt composition and the crucible material (figure 6). During cooling additional intermediate phases can be formed according to thermodynamic equilibria (Luedtke 2001, Gross 2004). The efficiency of the method is demonstrated for known hard magnetic binary and ternary systems such as Co–Sm and Fe–Nd–B. In the case of binary Co–Sm, depending on the temperature, all intermetallic phases expected according to the phase diagram (figure 7) are detected in the microsection (figure 8). For a temperature of 1200 ◦ C the phases Co5 Sm 5
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Figure 6. Reaction crucible with a clearly visible diffusion zone and a schematic illustration of the principle of reaction crucible synthesis.
and Co17 Sm2 occur. If the annealing temperature is lowered to 1100 ◦ C, in addition to Co5 Sm and Co17 Sm2 , Co7 Sm2 and Co3 Sm are formed between the Co crucible and the liquid. In the case of ternary Fe–Nd–B, all ternary intermediate phases in the Fe-rich corner of the phase diagram (figure 9) are formed (figure 10). Depending on the B content in the sample the system follows different solidification paths. However, all paths pass the hard magnet phase Fe14 Nd2 B (φ) in the Fe-rich corner. According to the corresponding microstructures, for B-rich samples the path of phase formation goes along the η-phase (Fe4 Nd1.1 B4 ) and the φ-phase, for B-poor samples the path goes directly via the φ-phase, and for B-free samples the phases Fe17 Nd2 and Fe17 Nd5 are formed.
Figure 7. Illustration of the expected phases in binary Co–Sm
crucible for two different temperatures using the binary phase diagram of Okamoto (2011).
between 6 and 125 µm, significantly influencing the diffusion processes for a given sintering temperature and time—and, consequently, the ratio of elementary regions and broad diffusion regions. With a small powder particle size the specimen is homogenized much faster compared to a larger particle size and an equilibrium state is achieved. For larger powder particle sizes Fe remains elemental due to the larger diffusion distance. The remaining Fe is surrounded by intermetallic phases of different composition, i.e. Fe17 Nd2 (close-by) and hard magnetic Fe14 Nd2 B. So the Fe content decreases with increasing distance from the remaining elementary Fe. In the search for new phases a non-equilibrium state using larger particle sizes is advantageous as more phases may appear compared to the equilibrium state.
4.2. Reaction sintering (diffusion couples on a small scale)
The reaction sintering method is based on a powder mixture of all the elements involved in a specified system (Gross 2004). The powder mixture is compacted, leading to a good connection between the different powder particles. The resulting green compact is sintered under an inert gas atmosphere at elevated temperatures (∼1000 ◦ C). Sintering enables each particle of an element (e.g. Fe or Co) in contact with a particle of another material to form a small diffusion couple, as long as the system is in a metastable state. It can be shown in known systems that different intermediate phases are generated at the contact interface (solid–liquid, solid–solid) according to the corresponding phase diagram. Depending on whether a liquid phase is formed or not during sintering, reaction sintering is called liquid phase sintering or solid phase sintering.
In many element combinations during sintering at 1000–1200 ◦ C no liquid phase is formed. In this case, all diffusion processes proceed via solid state diffusion, which is significantly slower than liquid phase sintering. In figure 12, the efficiency of the method is demonstrated for the binary system Fe–Ti. No liquid phase occurs during sintering and the ferromagnetic Fe2 Ti phase is formed. Sintering at 1150 ◦ C for 6 h leads to the formation of grains of the hard magnetic Fe2 Ti phase. By applying a longer sintering time the diffusion zone with the hard magnetic phase becomes larger. The efficient synthesis approach has currently been applied successfully to novel systems with RE-elements of greater availability and/or smaller contained amounts, such as Fe–(Nd, Ce, La, Y)–(B, X), and to novel systems completely 4.2.2. Solid phase sintering.
A prominent example for liquid phase sintering is the ternary system Fe–Nd–B. Here, melting point reduction due to Nd (TS = 1024 ◦ C) provides a largearea reaction zone and guarantees fast diffusion processes. In figure 11 it is shown that the Fe powder particle size varies 4.2.1. Liquid phase sintering.
6
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Figure 10. Diffusion zones in the ternary Fe–(NdB) crucible for
higher and lower B content.
Figure 8. Diffusion zones realized at 1100 ◦ C and 1200 ◦ C,
respectively, composed of various hard magnetic phases for binary Co–Sm. All expected intermetallic phases according to the phase diagram are detected.
Figure 11. Liquid phase reaction sintering at 1150 ◦ C for the ternary
system Fe–Nd–B from the constituent elements with iron powder of particle size (a) 6 µm and (b) 100 µm (Bodenberger and Hubert 1977). Analysis of fully developed closure-type domains is done according to model 5. The mean domain width of closure-type domains can be obtained by automatic image analysis making use of 9
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Figure 19. Closure-type domains of Fe14 Nd2 B as a function of
temperature. 5.2.3. Curie temperature TC from the temperature dependence.
Using a hot stage on the microscope, the domain pattern can be observed as a function of temperature. In figure 19, Kerr microscopy images of closure-type domains in as-cast Fe–Nd– B are shown in the temperature range between 50 and 316 ◦ C. With increasing temperature the domain contrast continuously decreases, finally disappearing when the Curie temperature TC is reached. According to the measurements, TC amounts to 313 ± 3 ◦ C, which agrees rather well with the literature value of TC = 312 ◦ C. By plotting the domain contrast K 256 (T ) versus the saturation polarization Js (T ) of a Fe14 Nd2 B single crystal (Hock 1988) for the specified temperatures a linear behavior is observed (figure 20). The correlation can be used to determine the temperature dependence of the saturation polarization Js (T ) according to
Figure 17. Schematic figure of domain width determination on
closure-type domains obtained by the line-cut method.
Js (T ) = (K 256 (T )Js (RT))/K 256 (RT)
(6)
with the domain contrast and saturation polarization at room temperature given by K 256 (RT) = 74 and Js (RT) = 1.61 T, respectively. The Js (T ) values calculated according to (6) agree well with the Js (T ) values of a Fe14 Nd2 B single crystal known from the literature (figure 20, inset). It is possible to determine the domain contrast up to 5–10 ◦ C below the Curie temperature TC . To extrapolate Js (T ) up to TC the relation Js ∼ (TC − T )0.5 can be used (Morrish 2001). The temperature dependence of the anisotropy constant K 1 (T ) can be estimated from the width of the stripe domains according to equation (4), or using the relation K 1 ∼ Js3 (Callen and Callen 1966).
Figure 18. Visualization of closure-type domains and stripe domains
with the measured domain width and corresponding calculated anisotropy constant.
the stereological line-cut method in the horizontal and vertical directions (figure 17). The mean domain width of the stripe domains can be obtained by measurements of the domain width at different locations in the grains and averaging (figure 18). The validation of the various models was done on several grains of as-cast Fe–Nd–B, and for closure-type domains and stripe domains yields anisotropy constants of K 1 = 4.6 ± 0.3 MJ m−3 and K 1 = 4.7 ± 0.9 MJ m−3 , respectively. The calculated values are in good accordance with the literature value of K 1 = 4.3 MJ m−3 . For the exchange constant the literature value of A = 7.7 pJ m−1 has been inserted.
6. Further procedures to realize permanent magnets
A multi-level approach on the high-throughput search for novel RE-containing and RE-free permanent magnet materials so far introduced is composed of four parts (figure 21). (1) Definition of cornerstones, (2) priority setting, (3) high-throughput synthesis and (4) high-throughput analysis. However, it takes more than an interesting high-performance and commercially attractive hard magnetic phase to make an applicable permanent magnet. 10
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favorable intrinsic magnetic material parameters of the hard magnetic compound are combined with a suitable microstructure composed of ensembles of small hard magnetic grains necessary for the existence of a proper magnetic hardening mechanism (nucleation or pinning mechanism) and therefore for large values of µ0 HC , Jr and (B H )max . In the case of microcrystalline sintered magnets it is beneficial when the hard magnetic phase forms a phase equilibrium with a non-magnetic phase being liquid under the sintering conditions (such as Nd-rich phases in the case of Fe–Nd–B when the composition is chosen with over-stoichiometric Nd) in order to receive hard magnetic grains magnetically isolated by a non-magnetic grain boundary phase and consequent nucleation hardening. Alternatively, the phase diagram should contain a second magnetic phase forming a precipitation structure during processing in a self-ordered manner (such as (CoCu)5 Sm in the case of (Co, Cu, Fe, Zr)17 Sm2 ) at which efficient domain wall pinning and consequent pinning hardening may occur. In the case of nanocrystalline melt-spun magnets, a pure hard magnetic phase or even a phase equilibrium with a soft magnetic phase may exist, resulting in exchange-coupled stoichiometric or composite nanostructures (nucleation mechanism). In any case, the bulk permanent magnet on laboratory scale provides the first estimates of bulk magnetic properties and valuable details for further development of the material system by improving the structure through modified processing parameters and/or additives. After finishing the laboratory scale process the process can be scaled up to mass production.
Figure 20. Illustration of the linear dependence between the
measured domain contrast K 256 and literature value of the saturation polarization from 25 to 290 ◦ C. Inset: literature and determined values of the saturation polarization with increasing temperature.
7. Conclusions
The developed high-throughput approach based on the synthesis of heterogeneous non-equilibrium states combined with analysis by microscopic methods is a promising concept to search for new high-performance and commercially attractive permanent magnet materials with drastically reduced RE content (or better still free of RE). The approach enables one to scan through even higher component systems with one single sample. The method is based on the formation of concentration gradients between different elements due to thermodynamically guided diffusion processes along the tie-lines in phase diagrams in diffusion couples. Depending on the dimension of the diffusion couples, the reaction crucible and reaction sintering methods can be used. Promising hard magnetic phases that might occur can be visualized via their typical domain structure. From the domain structure the corresponding intrinsic magnetic properties can be directly estimated. The difference in brightness (domain contrast K 256 ) can be used as a measure of the saturation polarization Js . The domain width can be taken as a measure of the anisotropy constant K 1 . By observing the domain structure as a function of temperature the Curie temperature can be determined from the temperature dependence of the saturation polarization Js . To use the high-throughput approach most efficiently, it is strongly recommended to define relevant cornerstones to sensibly limit the number of element combinations and prioritize the systems using a procedure based on the fact that the potential of a ternary or high component system to form intermetallic
Figure 21. Illustration of the multi-level approach in the
high-throughput search for new permanent magnet materials from phase discovery to mass production of permanent magnets.
The procedure from a prospective hard magnetic phase to an applicable permanent magnet is complex and, consequently, time-consuming. Compounds judged to have a sufficiently high potential for commercial applications are carefully synthesized in high purity and larger volumes (several cm3 ) for further comprehensive analysis of the intrinsic magnetic material properties and crystal structure. For the synthesis of such as-cast samples, conventional arc melting or induction melting from the constituent elements is usually used. In the next step, the installation of an optimal processing route is required to process the selected hard magnetic compounds by sintering (i.e. powdering as-cast samples and pressing the powder under an applied magnetic field for texturing, followed by sintering at an appropriate temperature), rapid quenching (i.e. quenching the melt of as-cast samples on a rotating Cu wheel) combined with polymer bonding (i.e. powdering the quenched material and bonded with polymer) or other advanced techniques into bulk permanent magnets on a laboratory scale. After processing, the 11
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compounds is reflected in the number of compounds that each of its marginal systems forms. The approach has currently been applied successfully to potential novel systems with RE-elements with better availability and/or smaller contained amounts, such as Fe–(Nd, Ce, La, Y)–(B, X), and to novel systems completely free of RE-elements, such as Fe–(Co, Mn, Ni)–X with X = Al, C, Ge, N, P, Sn . . . . This will be the topic of a forthcoming publication.
Gross F 2004 Search for new permanent magnetic phases by the reaction crucible analysis and development of new high through put methods PhD Thesis University of Birmingham Hadjipanayis G C, Tang W, Zhang Y, Chui S T, Liu J F, Chen C H and Kronmueller H 2000 High temperature 2:17 magnets: relationship of magnetic properties to microstructure and processing IEEE Trans. Magn. 36 3382–7 Hanak J J 1970 The multiple-sample concept in materials research: synthesis, compositional analysis and testing of entire multicomponent systems J. Mater. Sci. 5 964–71 Hock S 1988 Zuechtung und magnetische Eigenschaften von (Fe, Al)14 (Nd, Dy)2 B—Einkristallen PhD Thesis University of Stuttgart Hubert A and Schaefer R 1998 Magnetic Domains: The Analysis of Magnetic Microstructure (Berlin: Springer) Jain A, Hautier G, Moore C, Ong S P, Fischer C, Mueller T, Persson K and Ceder G 2011 A high-throughput infrastructure for density functional theory calculations Comput. Mater. Sci. 50 2295–310 Kaczer J 1964 On the domain structure of uniaxial ferromagnets Sov. Phys.—JETP 19 1204–8 Kennedy K, Stefansky T, Davy G, Zackay V F and Parker E R 1965 Rapid method for determining ternary-alloy phase diagrams J. Appl. Phys. 36 3808–10 Kittel C 1946 Theory of the structure of ferromagnetic domains in films and small particles Phys. Rev. 70 965–71 Kojima H 1982 Fundamental properties of hexagonal ferrites with magnetoplumbite structure Ferromagnetic Materials vol 3, ed E P Wohlfarth (Amsterdam: North-Holland) pp 305–92 Kramer M J, Mc Callum R W, Anderson I A and Constantinides S 2012 Prospects for non-rare earth permanent magnets for traction motors and generators J. Oper. Manage. 64 752–63 Lewis L H and Jim´enez-Villacorta F 2013 Perspectives on permanent magnetic materials for energy conversion and power generation Metall. Mater. Trans. A 44A 2–20 Li X F, Bao J, Zhang J, Chen G and Gao C 2007 An imaging system for high-throughput magneto-optical Kerr effect characterization of combinatorial materials libraries Meas. Sci. Technol. 18 2039–42 Loeffler R, Goll D, Guth G, Bernthaler B, Pusch V and Schneider G 2012 Lichtmikroskopische analyse der intrinsischen Eigenschaften hartmagnetischer Phasen aus der Dom¨anenstruktur Prakt. Metallogr. Sonderband 44 185–90 Luedtke A 2001 Reaction crucible analysis and magnetic domain structures PhD Thesis University of Birmingham Luedtke A, Stahl B, Harris I R and Schneider G 2000 Relationships in the Fe–Nd/Sm/Pr–B phase diagrams determined by iron crucible reaction analysis (ICRA) Proc. 16th Int. Workshop on RE Magnets and their Applications ed H Kaneko, M Homma and M Okada The Japan Institute of metals, pp 291–6 Maier W F, Stoewe K and Sieg S 2007 Combinatorial and high-throughput materials science Angew. Chem. 46 6016–67 Morrish A H 2001 The Physical Principles of Magnetism (New York: Wiley–IEEE Press) Nianyi C, Ruiliang C, Wencong L, Chonghe L and Villars P 1999a Regularities of formation of ternary intermetallic compounds Part 4. Ternary intermetallic compounds between two nontransition elements and one transition element J. Alloys Compounds 292 129–33 Nianyi C, Wencong L, Pei Q, Ruiliang C and Villars P 1999b Regularities of formation of ternary intermetallic compounds Part 2. Ternary compounds between transition elements J. Alloys Compounds 289 126–30
Acknowledgments
The authors thank C Wegierski and R Stein for the fabrication of the as-cast samples needed for the calibration measurements of the saturation polarization from the domain contrast. Special thanks go to Carl Zeiss Microscopy GmbH, G¨ottingen, Germany for providing special microscopy equipment. We gratefully acknowledge fruitful discussions with Fraunhofer Institute for Mechanics of Materials IWM Freiburg, Max Planck Institute for Intelligent Systems Stuttgart, Robert Bosch GmbH and Magnetfabrik Bonn GmbH. The work was funded by the German Federal Ministry of Education and Research. References Anderson F W and Moser J H 1958 Automatic computer program for reduction of routine emission spectrographic data Anal. Chem. 30 879–81 Bodenberger R and Hubert A 1977 Zur Bestimmung der Blochwandenergie von einachsigen ferromagneten Phys. Status Solidi 44 K7–11 Brunken H, Grochla D, Savan A, Kieschnick M, Meijer J D and Ludwig A 2011 Combinatorial investigation of Fe–B thin film nanocomposites Sci. Technol. Adv. Mater. 12 054208 Buschow K H J 1997 Magnetism and processing of permanent magnetic materials Handbook of Magnetic Materials vol 10, ed K H J Buschow (Amsterdam: North-Holland) pp 463–593 Buschow K H J, van Engen P G and Jongebreur R 1983 Magneto-optical properties of metallic ferromagnetic materials J. Magn. Magn. Mater. 38 1–22 Callen H B and Callen E 1966 The present status of the temperature dependence of magnetocrystalline anisotropy, and the l(l + 1)/2 power law J. Phys. Chem. Solids 27 1271–85 Chen C H, Walmer M S, Walmer M H, Liu S, Kuhl E and Simon G 1998 Sm2 (Co, Fe, Cu, Zr)17 magnets for use at temperature ≥ 400 ◦ C J. Appl. Phys. 83 6706–9 Coey J M D 2012 Permanent magnets: plugging the gap Scr. Mater. 67 524–9 Constantinides S 2012 The demand for rare earth materials in permanent magnets 51st Annual Conf. of Metallurgists (Niagara Falls) Gao T R, Wu Y Q, Fackler S, Kierzewski I, Zhang Y, Mehta A, Kramer M J and Takeuchi I 2013 Combinatorial exploration of rare-earth-free permanent magnets: magnetic and microstructural properties of Fe–Co–W thin films Appl. Phys. Lett. 102 022419 Goll D and Bublat T 2013 Large-area hard magnetic L10 -FePt and composite L10 -FePt based nanopatterns Phys. Status Solidi a 210 1261–71 Goll D, Kronmueller H and Stadelmaier H H 2004 Micromagnetism and the microstructure of high-temperature permanent magnets J. Appl. Phys. 96 6534–45 12
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Nianyi C, Wencong L, Ruiliang C, Pei Q and Villars P 1999c Regularities of formation of ternary intermetallic compounds Part 1. Ternary intermetallic compounds between nontransition elements J. Alloys Compounds 289 120–5 Okamoto H 2011 Co–Sm (cobalt–samarium) J. Phase Equilib. 32 165–6 Potyrailo R, Rajan K, Stoewe K, Takeuchi I, Chisholm B and Lam H 2011 Combinatorial and high-throughput screening of materials libraries: review of state of the art ACS Comb. Sci. 13 579–633 Rodewald W 2007 Rare-earth transition-metal magnets Handbook of Magnetism and Advanced Magnetic Materials ed H Kronmueller and S Parkin (New York: Wiley) pp 1969–2004 Schneider G, Henig E T, Petzow G and Stadelmaier H H 1986 Phase relations in the system Fe–Nd–B Z. Metallkd. 77 755–60 Szymczak R 1973 Observation of internal domain structure of barium ferrite in infrared Acta Phys. Pol. A 43 571–8 Takeuchi I et al 2003 Identification of novel compositions of ferromagnetic shape-memory alloys using composition spreads Nature Mater. 2 181–4 Tsui F, Collins B A, He L, Mellnik A, Zhong Y, Vogt S and Chu Y S 2007 Combinatorial synthesis and characterization of a
ternary epitaxial film of Co and Mn doped Ge(001) Appl. Surf. Sci. 254 709–13 van Engelen P P J and Buschow K H J 1990 Note on the magneto-optical properties of some Fe-rich ternary rare-earth compounds and UFe10 Si2 J. Magn. Magn. Mater. 84 47–51 Villars P 1995 Intermetallic Compounds: Principles and Practice ed J H Westbrook and R L Fleischer (Chichester: Wiley) pp 227–75 Villars P, Daams J, Shikata Y, Rajan K and Iwata S 2008 A new approach to describe elemental-property parameters Chem. Met. Alloys 1 1–23 Villars P et al 2001 Binary, ternary and quaternary compound former/nonformer prediction via Mendeleev number J. Alloys Compounds 317/318 26–38 Weller D, Mosendz O, Parker G, Pisana S and Santos T S 2013 L10 FePtX–Y media for heat-assisted magnetic recording Phys. Status Solidi a 210 1245–60 Xiang X D, Sun X, Briceno G, Lou Y, Wang K A, Chang H, Wallace-Freedman W G, Chen S W and Schultz P G 1995 A combinatorial approach to materials discovery Science 268 1738–40
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High-throughput search for new permanent magnet materials
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Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 064208 (13pp)
doi:10.1088/0953-8984/26/6/064208
High-throughput search for new permanent magnet materials D Goll, R Loeffler, J Herbst, R Karimi and G Schneider Materials Research Institute, Aalen University, Beethovenstraße 1, D-73430 Aalen, Germany E-mail: [email protected] and [email protected] Received 23 July 2013, revised 5 September 2013 Published 27 January 2014
Abstract
The currently highest-performance Fe–Nd–B magnets show limited cost-effectiveness and lifetime due to their rare-earth (RE) content. The demand for novel hard magnetic phases with more widely available RE metals, reduced RE content or, even better, completely free of RE metals is therefore tremendous. The chances are that such materials still exist given the large number of as yet unexplored alloy systems. To discover such phases, an elaborate concept is necessary which can restrict and prioritize the search field while making use of efficient synthesis and analysis methods. It is shown that an efficient synthesis of new phases using heterogeneous non-equilibrium diffusion couples and reaction sintering is possible. Quantitative microstructure analysis of the domain pattern of the hard magnetic phases can be used to estimate the intrinsic magnetic parameters (saturation polarization from the domain contrast, anisotropy constant from the domain width, Curie temperature from the temperature dependence of the domain contrast). The probability of detecting TM-rich phases for a given system is high, therefore the approach enables one to scan through even higher component systems with one single sample. The visualization of newly occurring hard magnetic phases via their typical domain structure and the correlation existing between domain structure and intrinsic magnetic properties allows an evaluation of the industrial relevance of these novel phases. (Some figures may appear in colour only in the online journal)
the RE metals Nd, Pr and Sm. The highest (B H )max values are currently given by the ternary intermetallics Fe14 (Nd, Pr)2 B (K 1 = 4.3 MJ m−3 , Js = 1.61 T, TC = 312 ◦ C, (B H )max = 450 kJ m−3 ) (Rodewald 2007) at room temperature and by the quinary intermetallics (Co, Cu, Fe, Zr)17 Sm2 at elevated temperatures ((B H )max = 100 kJ m−3 at 400 ◦ C) (Chen et al 1998, Hadjipanayis et al 2000, Goll et al 2004). FePt and CoPt based permanent magnets ((B H )max = 200 kJ m−3 ) (Buschow 1997) exhibit high uniaxial magnetocrystalline anisotropies (6.6 MJ m−3 and 4.9 MJ m−3 , respectively) from an ordered face-centered tetragonal phase (L10 phase) which develops at elevated temperatures from a disordered face-centered cubic phase. Bulk Fe–Nd–B magnets are currently the most promising candidates for a wide range of efficient energy conversion applications (Rodewald 2007, Kramer et al 2012, Lewis and Jim´enez-Villacorta 2013), while thin film L10 -FePt/CoPt magnets and regular bit patterns thereof are promising for next-generation ultrahigh density magnetic recording, realiz-
1. Introduction
High-performance permanent magnets are important materials for efficient energy conversion in high-power electric motor and generator applications. To give the maximum benefits in terms of sustainable resource efficiency and renewable energy, the permanent magnets should be based on materials with exceptional intrinsic magnetic parameters (saturation polarization Js > 1.2 T, anisotropy constant K 1 > 106 J m−3 , Curie temperature TC > 250 ◦ C) as well as optimized microstructures to guarantee long lifetime and cost-efficiency. Large Js values are required to give large remanences Jr , while large K 1 values can yield large coercivities Hc , both ensuring high maximum energy products (B H )max . The most famous high-performance permanent magnets at present are based on rare-earth (RE)–transition metal (TM) intermetallics (Fe–Nd– B, Co–Sm) and FePt/CoPt. In RE–TM intermetallics the high saturation polarization and Curie temperature of the TM metals Fe and Co are combined with the high anisotropy constant of 0953-8984/14/064208+13$33.00
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far, it is not surprising that Fe–Nd–B has remained the highestperformance permanent magnet material for 30 years. To scan through tens of thousands of alloy systems in a reasonable time frame looking for novel hard magnetic compounds is not possible by conventional methods (as-cast samples, magnetometry) and requires an alternative concept. High-throughput materials science is an approach to the accelerated discovery, study, and optimization of both novel and known materials. During experimentation different parameters or base materials are systematically varied in parallel and empirically analyzed to identify the optimum values in a shorter period. Although individual aspects of rapid materials development have been known for decades (e.g. Anderson and Moser 1958, Kennedy et al 1965), the first integrated materials-development workflow was introduced in 1970 by Hanak (1970). The workflow included complete compositional mapping of a multicomponent system in one experiment (library), a fast method to screen for properties (characterization and testing) and computer data processing (computational tools). In 1995, applications of high-throughput experimentation in materials science were reinitialized by Xiang et al (1995). Since then, the method has been put into practice to discover and optimize a wide spectrum of materials (Maier et al 2007, Potyrailo et al 2011). In comparison with the original idea of Hanak, advanced workflow now incorporates further important aspects, such as the design/concept of experiments, materials modeling by theory and scale up. High-throughput experimentation is already an indispensable tool in catalysis, genetic and pharmacological research to quickly identify active compounds, genes or antibodies that can provide the starting points for drug design and a better understanding of particular biochemical processes (Potyrailo et al 2011). The Materials Genome project is aimed at materials design with high-throughput computation to predict material properties without experimental synthesis and record the material properties in extensive databases (Jain et al 2011). For accelerated materials discovery, Materials Genome combines advanced scientific computing techniques with innovative design tools. This includes the computation of phase diagrams to find thermodynamically stable phases and study decomposition pathways, the computation of chemical reaction kinetics and the ab initio computation of the influence of additives on intrinsic material parameters using data-mined substitution algorithms. In all cases the reliability and validity of the high-throughput methods have to be well established by comparing traditional and combinatorial routes using known materials. The targetoriented empirical search for novel intermetallic compounds requires a suitable approach to efficiently identify material systems that form (many) intermetallic compounds (so-called ‘former’ systems). The prediction of ‘former’ and ‘nonformer’ material systems is typically based on (semi)empirically developed complex algorithms, taking several out of more than 60 specific elemental property parameters into consideration (Villars et al 2001, 2008, Nianyi et al 1999a,c,b). Of these parameters, Villars et al (2001) found the Mendeleev number to be a suitable elemental property. In contrast, discovering novel hard magnetic materials using high-throughput methods is a more recent development.
Figure 1. Illustration of the performance–cost landscape of common permanent magnets and the wide gap between hard ferrites and FeNdB magnets.
ing storage densities of 1 Tb in−2 and beyond (Goll and Bublat 2013, Weller et al 2013). The worldwide sales volume of Fe–Nd–B magnets in 2005 was 4 billion US$, and by 2020 sales of Fe–Nd–B magnets are expected to be over 17 billion US$ (Constantinides 2012). For economic reasons half of this market is still dominated by hard ferrites of the type (Ba/Sr)Fe12 O19 (K 1 = 0.3 MJ m−3 , Js = 0.46 T, TC = 450 ◦ C, (B H )max = 23 kJ m−3 ) (Kojima 1982). Hard ferrites are considered to be ceramic hard magnets with a low crystal symmetry and therefore a high uniaxial magnetocrystalline anisotropy. They are free of RE, however, their strength is up to 20 times lower than for Fe–Nd–B due to their lower Fe content and ferrimagnetic behavior. Besides the economic aspects, the sustainable use of Fe–Nd–B magnets is further hindered by the fact that their temperature and corrosion stability are limited, which may lead to accelerated ageing of the magnets. Up to now, the ultimate magnetic material to fill the gap between Fe–Nd–B magnets and hard ferrites on the performance–cost landscape does not exist (figure 1). We see an enormous need for action in this field. As a consequence, already known hard magnetic materials such as MnBi and MnAl(-C), to which less attention has been paid in the past, are currently undergoing a revival (Coey 2012, Kramer et al 2012, Lewis and Jim´enez-Villacorta 2013). The key issues are to increase the operating temperature and magnetization by finding suitable alloying elements with which to stabilize the phases without impairing the magnetic properties. Also well-known ideas for realizing new ultrastrong supermagnets by making soft magnets such as Fe–35Co or Fe16 N2 , with their large saturation polarization, become hard magnetic are attracting renewed attention (Kramer et al 2012, Lewis and Jim´enez-Villacorta 2013). It is expected that their large saturation polarizations of Js (Fe–35Co) = 2.45 T and Js (Fe16 N2 ) = 2.7 T in combination with the introduction of strong planar pinning centers or expansion of the lattice structure is the road to success. Yet another alternative is the discovery of completely new high-performance, cost-effective and long-lasting permanent magnet materials, either with significantly reduced rare-earth content or completely free of rare-earths. This, however, is rather complex—like looking for a needle in a haystack considering the large number of possible alloy systems. So 2
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analysis methods to identify quickly and reliably potential hard magnetic compounds in the sample and estimate their intrinsic magnetic material parameters Js , K 1 and TC . In section 6 an overview is given of the further procedures necessary to realize a permanent magnet based on interesting magnetic compounds before scaling up to mass production can be started.
Recently, both high-throughput bulk approaches (Luedtke et al 2000) and thin film approaches (Takeuchi et al 2003) have become of interest for scanning through even higher component systems in the search for new hard magnetic compounds in a reasonable period of time. The bulk approach is based on heterogeneous non-equilibrium states that form concentration gradients between different elements due to thermodynamically guided diffusion processes. As the magnetic properties of the material change along the gradients, one sample may be sufficient to cover a complete phase diagram. The thin film approach makes use of thin film libraries produced by defocused co-sputter deposition or molecular beam epitaxy synthesis from elemental targets on a (micro-patterned) wafer. Here also the wafer (or grid pattern on it) with varying nominal composition (discrete or gradient) covers a complete phase diagram. For directional crystal growth, suitable substrates are required with a lattice spacing similar to that of the hard magnetic compounds, whereas the bulk approach is independent of substrates. Furthermore, in some cases it may be possible that novel magnetic phases formed on the nanometer scale are stabilized by the substrate and do not develop in the bulk. In any case, both approaches require high-throughput analysis methods to identify and characterize promising novel hard magnetic compounds and evaluate their potential for industrial relevance. The bulk approach with phase formation on the µm scale so far relies on the typical stripe domain patterns of hard magnetic materials to directly visualize hard magnetic compounds and estimate their properties. Thin film libraries work below the µm scale of domain formation and therefore resort to changes in the crystal structure and composition throughout the wafer which can be automated by different electron diffraction (Tsui et al 2007) and x-ray diffraction techniques (Maier et al 2007, Brunken et al 2011, Gao et al 2013). Li et al (2007), in magneto-optical Kerr effect (MOKE) measurements, have described a finite Kerr rotation angle in the case of magnetic materials. For closer investigations of interesting wafer areas the wafer has to be cut into smaller pieces to perform transmission electron microscopy and magnetometry measurements (Gao et al 2013). The focus of this paper is on the high-throughput search for novel high-performance durable and commercially attractive permanent magnet materials with a significantly reduced RE content (or better still RE-free). A systematic search is a multi-level process and requires a systematic approach. In section 2 suitable cornerstones for the search are defined to limit the number of element combinations (systems). These cornerstones are: the selection of elements which are essential for good ferromagnetic behavior, raw material costs and resource aspects as well as health and environmental risks. In section 3 a prioritization process is introduced to narrow the number of systems to be investigated. Section 4 describes the development of high-throughput synthesis for realizing heterogeneous non-equilibrium microstructures to allow the scanning of the prioritized systems. The relevant part for interesting hard magnetic phases of the phase diagram can be investigated with one sample to generate a sufficient volume (∼several tens of µm3 ) of the phases. Section 5 deals with the development of suitable high-throughput microscopy
2. Search strategy and definition of the cornerstones
The efficient search for novel and commercially attractive hard magnetic compounds requires a meaningful definition of suitable element combinations (systems). In the actual work, the selection criteria reflect the prospects of elements concerning magnetic properties, raw material cost, resource availability and toxicity. The four transition metals Fe, Co, Ni and Mn were chosen as base elements since they have promising magnetic properties, e.g. large magnetic moments and high Curie temperatures. Novel compounds with a high TM content and ferromagnetic alignment of the TM magnetic moments may thus achieve high saturation polarizations. Of the four base elements, Fe is the most attractive element since it has the largest magnetic moment and is inexpensive and readily available. Stabilization of favorable structures with ferromagnetic alignment of the TM moments can be achieved by adding preferably non-magnetic additional elements. Additives may also improve the magnetic properties of known hard magnetic compounds. This can be achieved by either incorporation of an additional additive atom species in the existing structure or by replacing an already incorporated additive atom species by a more suitable one. In either case, the incorporated additive atom may influence the TM–TM distance of the compound’s crystal structure in such way that the exchange interaction, Js and Tc are increased. From the potential natural available elements, 41 elements were considered as possible additive candidates. Not included in the additive group are elements belonging to the lanthanides, actinides and noble gases or those that are toxic/radioactive and very expensive. Despite their high price, Pt and Pd were included, since both elements are known to form hard magnetic compounds with favorable properties. A further group of elements that may be relevant for the search includes the rare-earth elements, since their unique electronic structure is favorable for the formation of compounds with high crystal anisotropies. For the conducted search, seven light RE-elements were considered as attractive candidates, as they are less expensive than their heavy cousins. In particular, La and Ce are also available in larger quantities. In summary, the following numbers of elements are considered in the search: four elements as base metals, and 41 elements as additives for RE-free systems—with additionally seven elements as rare-earths for RE-containing systems. Based on the selected elements and defined frame conditions, the number of possible element combinations was calculated for binary, ternary, quaternary and quinary systems using enumerative combinatorics (k-combination). This was done separately for RE-containing and RE-free combinations. The frame conditions specify the number of ‘slots’ that may 3
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Figure 3. Prioritization concept to evaluate the potential of a ternary
system to form TM-rich intermediate phases based on the number of intermediate phases of its TM-containing binary marginal systems (‘phase index’).
Figure 2. Number of possible systems (RE-free: TM, TM–X,
TM–X1–X2, TM1–TM2–X, TM–X1–X2–X3, . . . ; RE-containing: TM, TM–RE, TM–RE–X, TM–RE–X1–X2, . . . ) based on the combination of four TM elements, 41 additive elements (X) and seven RE-elements by enumerative combinatorics (k-combination). Including unary, binary, ternary, quaternary and quinary systems, in total approximately one million unique quinary systems are possible.
species with suitable elemental properties and, therefore, may have a greater potential to form ternary intermediate phases. The number of formed compounds of the boundary systems may thus be an attractive and readily obtainable parameter to estimate the potential of the corresponding higher order system. This general concept was further adapted to the specific task of evaluating the potential of ternary TM-based systems to form TM-rich compounds, preferably with a TM content >70 at.%. The formation of such intermediate phases takes place in the TM-rich corner of the respective composition triangle and will thus be governed to a great extent by the thermodynamics of the TM atom species. Due to its rather low influence, the TM-free binary marginal system may therefore be neglected in this case. Thus, the potential of a TM–X1–X2 system may be described by the computation of a synthetic parameter ‘phase index’ based on the number of intermediate phases that each of the two binary boundary systems TM–X1 and TM–X2 forms (figure 3). Validation of the approach showed that the higher the calculated phase index of a system is, the greater is its potential to form intermediate compounds. The phase index approach was then deployed to establish a prioritized list of TM-based systems that were to be synthesized by efficient methods (see section 4). The prioritization is a threefold process, taking the potential of the system to form intermediate phases and its raw material cost into account (figure 4). In the first step the phase index PI of more than 2800 ternary TM-systems was computed and those systems identified that are not yet characterized in literature. This step yields attractive new systems from the materials science point of view. In the second step, the raw material costs of the systems were computed for a fixed composition (80 at.% Fe, 10 at.% X1, 10 at.% X2 or 80 at.% Fe, 10 at.% RE, 10 at.% X) by taking into account the current kg-cost of the involved RE and X elements (in wt%), which was then reciprocally normalized against the raw material cost of Fe14 Nd2 B (82.3 at.% Fe, 11.8 at.% Nd, 5.9 at.% B), yielding the systems raw material cost index RMCI. This
be occupied by elements from any group, e.g. a ternary system has for example three slots whereas a quinary system has five. In the case of RE-free systems, at least one slot was always to be occupied by a TM base element and an additive element, respectively. In the case of RE-containing systems, at least one slot was always to be occupied by a TM base element, an additive element and a RE-element, respectively. For binary RE-containing systems, additive elements were not considered to occupy a slot. The number of possible element combinations increases rapidly with an increasing number of available slots in the respective systems (figure 2). While there are approximately 4700 unique ternary systems, this number increases to 76 000 quaternary systems and finally reaches approximately one million unique quinary systems. It is evident that this number of possible systems cannot be investigated within a reasonable time frame and thus a further prioritization process must be applied to identify the most promising systems. 3. Prioritization of prospective systems
The prioritization method is based on the concept that the potential of a ternary or higher component system to form intermediate phases is reflected in the number of compounds that each of its marginal systems forms. On the physical level, the formation and structural stability of intermediate phases within a given system is influenced/governed by the interplay of numerous elemental property parameters (e.g. atomic size, volume and weight, melting/boiling temperatures and enthalpies, electronegativities, ionization energies, chemical potentials, number of valence electrons, and many more) of the involved atom species (Villars 1995). A ternary system, whose binary marginal systems form many intermediate phases, already comprises atom 4
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Figure 5. Approximately linear relationship between the calculated
Figure 4. Systematic computation and mapping of the ternary phase index PI. The database is obtained by adding the numbers of the intermediate phases existing for the two binary boundary systems. To rank the systems accordingly, an economic index EI, composed of a phase index PI and raw material cost index RMCI, is established. The table presents four arbitrary systems from the priority list. For the phase index given only intermediate phases with Fe content >50 at.% have been considered for the two binary boundary systems.
ternary ‘phase index’ and a system’s potential to form at least one ternary intermediate phase. The number of recorded systems with a ‘phase index’ >13 is too low to allow reliable statistical evaluation.
4. Efficient synthesis by heterogeneous non-equilibrium states
High-throughput synthesis to scan systematically higher component systems for interesting hard magnetic phases can be realized by heterogeneous non-equilibrium states (Luedtke et al 2000). Instead of a time-consuming exploration of the corresponding phase diagrams, the method is based on the formation of concentration gradients between different elements due to thermodynamically guided diffusion processes along the tie-lines in phase diagrams in diffusion couples. As only transition-metal-rich intermediate phases are interesting candidates, one diffusion couple is sufficient to cover one binary, ternary or quaternary phase diagram with a sufficient probability. Depending on the dimension of the diffusion couples, two different methods may be differentiated.
step yields attractive systems from an economic point of view. The compound Fe14 Nd2 B was chosen as a base-line since it represents today’s most powerful and widespread high-performance hard magnetic compound, against which any new compound must compete (in terms of performance and material cost). In the third and final step, an economic index EI for each system was calculated on the basis of its phase index and raw material cost index. In figure 4 it is shown by means of four arbitrarily chosen systems that the EI index is greatest for a large PI index and a large RMCI index and that its value may vary over several orders of magnitude. This final step allows a first identification of promising new systems with respect to their economic and materials science attractiveness. The prioritization process presented is a rather easily deployable approach to efficiently extract promising systems (n < 1000) out of several tens of thousands of possible systems. The ‘phase index’ concept and the accuracy of different computation algorithms were tested and validated against a data set comprising more than 570 ternary Fe–X1–X2 systems. For each system of the data set, the number of intermediate phases (ternary and binary) was recorded and a phase index computed. Analysis of the resulting data shows that there is a correlation between the computed phase index of a system and its tendency to form intermediate phases (figure 5). Analysis of a limited ternary data set further confirmed that the influence of the Fe-free binary marginal system on the above-mentioned correlation is indeed rather low.
4.1. Reaction crucible (diffusion couples on a large scale)
The reaction crucible method is based on TM-crucibles that are charged with a powder mixture of additional elements required to investigate a specified system. For diffusion couples in the crucible a liquid has to be formed due to melting point reduction by the additional elements. Intermediate phases will form between the liquid and the crucible element due to the phase equilibria if they exist in a composition range between the melt composition and the crucible material (figure 6). During cooling additional intermediate phases can be formed according to thermodynamic equilibria (Luedtke 2001, Gross 2004). The efficiency of the method is demonstrated for known hard magnetic binary and ternary systems such as Co–Sm and Fe–Nd–B. In the case of binary Co–Sm, depending on the temperature, all intermetallic phases expected according to the phase diagram (figure 7) are detected in the microsection (figure 8). For a temperature of 1200 ◦ C the phases Co5 Sm 5
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Figure 6. Reaction crucible with a clearly visible diffusion zone and a schematic illustration of the principle of reaction crucible synthesis.
and Co17 Sm2 occur. If the annealing temperature is lowered to 1100 ◦ C, in addition to Co5 Sm and Co17 Sm2 , Co7 Sm2 and Co3 Sm are formed between the Co crucible and the liquid. In the case of ternary Fe–Nd–B, all ternary intermediate phases in the Fe-rich corner of the phase diagram (figure 9) are formed (figure 10). Depending on the B content in the sample the system follows different solidification paths. However, all paths pass the hard magnet phase Fe14 Nd2 B (φ) in the Fe-rich corner. According to the corresponding microstructures, for B-rich samples the path of phase formation goes along the η-phase (Fe4 Nd1.1 B4 ) and the φ-phase, for B-poor samples the path goes directly via the φ-phase, and for B-free samples the phases Fe17 Nd2 and Fe17 Nd5 are formed.
Figure 7. Illustration of the expected phases in binary Co–Sm
crucible for two different temperatures using the binary phase diagram of Okamoto (2011).
between 6 and 125 µm, significantly influencing the diffusion processes for a given sintering temperature and time—and, consequently, the ratio of elementary regions and broad diffusion regions. With a small powder particle size the specimen is homogenized much faster compared to a larger particle size and an equilibrium state is achieved. For larger powder particle sizes Fe remains elemental due to the larger diffusion distance. The remaining Fe is surrounded by intermetallic phases of different composition, i.e. Fe17 Nd2 (close-by) and hard magnetic Fe14 Nd2 B. So the Fe content decreases with increasing distance from the remaining elementary Fe. In the search for new phases a non-equilibrium state using larger particle sizes is advantageous as more phases may appear compared to the equilibrium state.
4.2. Reaction sintering (diffusion couples on a small scale)
The reaction sintering method is based on a powder mixture of all the elements involved in a specified system (Gross 2004). The powder mixture is compacted, leading to a good connection between the different powder particles. The resulting green compact is sintered under an inert gas atmosphere at elevated temperatures (∼1000 ◦ C). Sintering enables each particle of an element (e.g. Fe or Co) in contact with a particle of another material to form a small diffusion couple, as long as the system is in a metastable state. It can be shown in known systems that different intermediate phases are generated at the contact interface (solid–liquid, solid–solid) according to the corresponding phase diagram. Depending on whether a liquid phase is formed or not during sintering, reaction sintering is called liquid phase sintering or solid phase sintering.
In many element combinations during sintering at 1000–1200 ◦ C no liquid phase is formed. In this case, all diffusion processes proceed via solid state diffusion, which is significantly slower than liquid phase sintering. In figure 12, the efficiency of the method is demonstrated for the binary system Fe–Ti. No liquid phase occurs during sintering and the ferromagnetic Fe2 Ti phase is formed. Sintering at 1150 ◦ C for 6 h leads to the formation of grains of the hard magnetic Fe2 Ti phase. By applying a longer sintering time the diffusion zone with the hard magnetic phase becomes larger. The efficient synthesis approach has currently been applied successfully to novel systems with RE-elements of greater availability and/or smaller contained amounts, such as Fe–(Nd, Ce, La, Y)–(B, X), and to novel systems completely 4.2.2. Solid phase sintering.
A prominent example for liquid phase sintering is the ternary system Fe–Nd–B. Here, melting point reduction due to Nd (TS = 1024 ◦ C) provides a largearea reaction zone and guarantees fast diffusion processes. In figure 11 it is shown that the Fe powder particle size varies 4.2.1. Liquid phase sintering.
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Figure 10. Diffusion zones in the ternary Fe–(NdB) crucible for
higher and lower B content.
Figure 8. Diffusion zones realized at 1100 ◦ C and 1200 ◦ C,
respectively, composed of various hard magnetic phases for binary Co–Sm. All expected intermetallic phases according to the phase diagram are detected.
Figure 11. Liquid phase reaction sintering at 1150 ◦ C for the ternary
system Fe–Nd–B from the constituent elements with iron powder of particle size (a) 6 µm and (b) 100 µm (Bodenberger and Hubert 1977). Analysis of fully developed closure-type domains is done according to model 5. The mean domain width of closure-type domains can be obtained by automatic image analysis making use of 9
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Figure 19. Closure-type domains of Fe14 Nd2 B as a function of
temperature. 5.2.3. Curie temperature TC from the temperature dependence.
Using a hot stage on the microscope, the domain pattern can be observed as a function of temperature. In figure 19, Kerr microscopy images of closure-type domains in as-cast Fe–Nd– B are shown in the temperature range between 50 and 316 ◦ C. With increasing temperature the domain contrast continuously decreases, finally disappearing when the Curie temperature TC is reached. According to the measurements, TC amounts to 313 ± 3 ◦ C, which agrees rather well with the literature value of TC = 312 ◦ C. By plotting the domain contrast K 256 (T ) versus the saturation polarization Js (T ) of a Fe14 Nd2 B single crystal (Hock 1988) for the specified temperatures a linear behavior is observed (figure 20). The correlation can be used to determine the temperature dependence of the saturation polarization Js (T ) according to
Figure 17. Schematic figure of domain width determination on
closure-type domains obtained by the line-cut method.
Js (T ) = (K 256 (T )Js (RT))/K 256 (RT)
(6)
with the domain contrast and saturation polarization at room temperature given by K 256 (RT) = 74 and Js (RT) = 1.61 T, respectively. The Js (T ) values calculated according to (6) agree well with the Js (T ) values of a Fe14 Nd2 B single crystal known from the literature (figure 20, inset). It is possible to determine the domain contrast up to 5–10 ◦ C below the Curie temperature TC . To extrapolate Js (T ) up to TC the relation Js ∼ (TC − T )0.5 can be used (Morrish 2001). The temperature dependence of the anisotropy constant K 1 (T ) can be estimated from the width of the stripe domains according to equation (4), or using the relation K 1 ∼ Js3 (Callen and Callen 1966).
Figure 18. Visualization of closure-type domains and stripe domains
with the measured domain width and corresponding calculated anisotropy constant.
the stereological line-cut method in the horizontal and vertical directions (figure 17). The mean domain width of the stripe domains can be obtained by measurements of the domain width at different locations in the grains and averaging (figure 18). The validation of the various models was done on several grains of as-cast Fe–Nd–B, and for closure-type domains and stripe domains yields anisotropy constants of K 1 = 4.6 ± 0.3 MJ m−3 and K 1 = 4.7 ± 0.9 MJ m−3 , respectively. The calculated values are in good accordance with the literature value of K 1 = 4.3 MJ m−3 . For the exchange constant the literature value of A = 7.7 pJ m−1 has been inserted.
6. Further procedures to realize permanent magnets
A multi-level approach on the high-throughput search for novel RE-containing and RE-free permanent magnet materials so far introduced is composed of four parts (figure 21). (1) Definition of cornerstones, (2) priority setting, (3) high-throughput synthesis and (4) high-throughput analysis. However, it takes more than an interesting high-performance and commercially attractive hard magnetic phase to make an applicable permanent magnet. 10
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favorable intrinsic magnetic material parameters of the hard magnetic compound are combined with a suitable microstructure composed of ensembles of small hard magnetic grains necessary for the existence of a proper magnetic hardening mechanism (nucleation or pinning mechanism) and therefore for large values of µ0 HC , Jr and (B H )max . In the case of microcrystalline sintered magnets it is beneficial when the hard magnetic phase forms a phase equilibrium with a non-magnetic phase being liquid under the sintering conditions (such as Nd-rich phases in the case of Fe–Nd–B when the composition is chosen with over-stoichiometric Nd) in order to receive hard magnetic grains magnetically isolated by a non-magnetic grain boundary phase and consequent nucleation hardening. Alternatively, the phase diagram should contain a second magnetic phase forming a precipitation structure during processing in a self-ordered manner (such as (CoCu)5 Sm in the case of (Co, Cu, Fe, Zr)17 Sm2 ) at which efficient domain wall pinning and consequent pinning hardening may occur. In the case of nanocrystalline melt-spun magnets, a pure hard magnetic phase or even a phase equilibrium with a soft magnetic phase may exist, resulting in exchange-coupled stoichiometric or composite nanostructures (nucleation mechanism). In any case, the bulk permanent magnet on laboratory scale provides the first estimates of bulk magnetic properties and valuable details for further development of the material system by improving the structure through modified processing parameters and/or additives. After finishing the laboratory scale process the process can be scaled up to mass production.
Figure 20. Illustration of the linear dependence between the
measured domain contrast K 256 and literature value of the saturation polarization from 25 to 290 ◦ C. Inset: literature and determined values of the saturation polarization with increasing temperature.
7. Conclusions
The developed high-throughput approach based on the synthesis of heterogeneous non-equilibrium states combined with analysis by microscopic methods is a promising concept to search for new high-performance and commercially attractive permanent magnet materials with drastically reduced RE content (or better still free of RE). The approach enables one to scan through even higher component systems with one single sample. The method is based on the formation of concentration gradients between different elements due to thermodynamically guided diffusion processes along the tie-lines in phase diagrams in diffusion couples. Depending on the dimension of the diffusion couples, the reaction crucible and reaction sintering methods can be used. Promising hard magnetic phases that might occur can be visualized via their typical domain structure. From the domain structure the corresponding intrinsic magnetic properties can be directly estimated. The difference in brightness (domain contrast K 256 ) can be used as a measure of the saturation polarization Js . The domain width can be taken as a measure of the anisotropy constant K 1 . By observing the domain structure as a function of temperature the Curie temperature can be determined from the temperature dependence of the saturation polarization Js . To use the high-throughput approach most efficiently, it is strongly recommended to define relevant cornerstones to sensibly limit the number of element combinations and prioritize the systems using a procedure based on the fact that the potential of a ternary or high component system to form intermetallic
Figure 21. Illustration of the multi-level approach in the
high-throughput search for new permanent magnet materials from phase discovery to mass production of permanent magnets.
The procedure from a prospective hard magnetic phase to an applicable permanent magnet is complex and, consequently, time-consuming. Compounds judged to have a sufficiently high potential for commercial applications are carefully synthesized in high purity and larger volumes (several cm3 ) for further comprehensive analysis of the intrinsic magnetic material properties and crystal structure. For the synthesis of such as-cast samples, conventional arc melting or induction melting from the constituent elements is usually used. In the next step, the installation of an optimal processing route is required to process the selected hard magnetic compounds by sintering (i.e. powdering as-cast samples and pressing the powder under an applied magnetic field for texturing, followed by sintering at an appropriate temperature), rapid quenching (i.e. quenching the melt of as-cast samples on a rotating Cu wheel) combined with polymer bonding (i.e. powdering the quenched material and bonded with polymer) or other advanced techniques into bulk permanent magnets on a laboratory scale. After processing, the 11
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compounds is reflected in the number of compounds that each of its marginal systems forms. The approach has currently been applied successfully to potential novel systems with RE-elements with better availability and/or smaller contained amounts, such as Fe–(Nd, Ce, La, Y)–(B, X), and to novel systems completely free of RE-elements, such as Fe–(Co, Mn, Ni)–X with X = Al, C, Ge, N, P, Sn . . . . This will be the topic of a forthcoming publication.
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Acknowledgments
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