Structural, magnetic, and electronic properties of iron selenide Fe6-7Se8 nanoparticles obtained by thermal decomposition in high-temperature organic solvents I. S. Lyubutin, Chun-Rong Lin, K. O. Funtov, T. V. Dmitrieva, S. S. Starchikov, Yu-Jhan Siao, and Mei-Li Chen Citation: The Journal of Chemical Physics 141, 044704 (2014); doi: 10.1063/1.4887356 View online: http://dx.doi.org/10.1063/1.4887356 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/141/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural and multiferroic properties of Bi1−xInxFeO3 (0≤x≤0.20) nanoparticles J. Appl. Phys. 113, 044107 (2013); 10.1063/1.4788668 Temperature dependence of magnetic anisotropy constant in iron chalcogenide Fe3Se4: Excellent agreement with theories J. Appl. Phys. 112, 103905 (2012); 10.1063/1.4759352 Synthesis, structure, and magnetic behavior of nanoparticles of cubic ZnMnO3 Appl. Phys. Lett. 100, 252407 (2012); 10.1063/1.4729817 Synthesis and magnetic properties of iron sulfide nanosheets with a NiAs-like structure J. Appl. Phys. 107, 09A335 (2010); 10.1063/1.3367968 Magnetic properties of monodisperse iron oxide nanoparticles J. Appl. Phys. 99, 08N710 (2006); 10.1063/1.2172891
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THE JOURNAL OF CHEMICAL PHYSICS 141, 044704 (2014)
Structural, magnetic, and electronic properties of iron selenide Fe6-7 Se8 nanoparticles obtained by thermal decomposition in high-temperature organic solvents I. S. Lyubutin,1,a) Chun-Rong Lin,2,a) K. O. Funtov,1 T. V. Dmitrieva,1 S. S. Starchikov,1 Yu-Jhan Siao,3 and Mei-Li Chen4 1
Shubnikov Institute of Crystallography, Russian Academy of Sciences, Moscow 119333, Russia Department of Applied Physics, National Pingtung University of Education, Pingtung 90003, Taiwan 3 Department of Mechanical Engineering, Southern Taiwan University of Science and Technology, Tainan 71005, Taiwan 4 Department of Electro-optical Engineering, Southern Taiwan University of Science and Technology, Tainan 71005, Taiwan 2
(Received 8 April 2014; accepted 24 June 2014; published online 24 July 2014) Iron selenide nanoparticles with the NiAs-like crystal structure were synthesized by thermal decomposition of iron chloride and selenium powder in a high-temperature organic solvent. Depending on the time of the compound processing at 340 ◦ C, the nanocrystals with monoclinic (M)-Fe3 Se4 or hexagonal (H)-Fe7 Se8 structures as well as a mixture of these two phases can be obtained. The magnetic behavior of the monoclinic and hexagonal phases is very different. The applied-field and temperature dependences of magnetization reveal a complicated transformation between ferrimagnetic (FRM) and antiferromagnetic (AFM) structures, which can be related to the spin rotation process connected with the redistribution of cation vacancies. From XRD and Mössbauer data, the 3c type superstructure of vacancy ordering was found in the hexagonal Fe7 Se8 . Redistribution of vacancies in Fe7 Se8 from random to ordered leads to the transformation of the magnetic structure from FRM to AFM. The Mössbauer data indicate that vacancies in the monoclinic Fe3 Se4 prefer to appear near the Fe3+ ions and stimulate the magnetic transition with the rotation of the Fe3+ magnetic moments. Unusually high coercive force Hc was found in both (H) and (M) nanocrystals with the highest (“giant”) value of about 25 kOe in monoclinic Fe3 Se4 . This is explained by the strong surface magnetic anisotropy which is essentially larger than the core anisotropy. Such a large coercivity is rare for materials without rare earth or noble metal elements, and the Fe3 Se4 -based compounds can be the low-cost, nontoxic alternative materials for advanced magnets. In addition, an unusual effect of “switching” of magnetization in a field of 10 kOe was found in the Fe3 Se4 nanoparticles below 280 K, which can be important for applications. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4887356] I. INTRODUCTION
In recent years, iron selenide Fe-Se compounds have attracted a great interest because of promising application in peculiar electronic, optical, and magnetic devices. Their magnetic and conductive properties depend on the composition, and they could be ferro/ferrimagnetic metals, semiconductors, or even superconductors.1–4 According to the phase diagram,5–10 the Fe-Se compounds with the NiAs-like crystal structure occur over a certain composition range, in particular from 51 to 59 at.% selenium. Within this range the phase can exist as a hexagonal NiAs-like structure with approximate composition Fe7 Se8 (Hphase), and as a monoclinic deformation of the same (pseudohexagonal) phase, with approximate composition Fe3 Se4 (M-phase).9, 10
a) Authors to whom correspondence should be addressed. Electronic
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In the NiAs-type structure of iron chalcogenides Fe1-x X (X = S, Se) the Fe cations are octahedrally coordinated by six X anions (the FeX6 octahedra) and the coordination polyhedron of X is the trigonal prism created by six Fe (the XFe6 prisms) (Fig. 1(a)).11 The magnetic structure of Fe1-x X consists of ferromagnetic iron layers in the s plane which are antiferromagnetically coupled to each other along the c-axis.12–19 The total magnetic moment Mtot appears from the competition between magnetic moments of neighboring layers, and the material can be antiferromagnetic (AFM) or ferrimagnetic (FRM) depending on the value of moments of the corresponding layer-sublattices. The iron deficiency in Fe1-x Se is compensated by the vacancies at the cation sites (Fig. 1(b)). The layer moments depend on the number of the vacancies and on the way of vacancies distribution.20 A random distribution of vacancies gives rise to a pure AFM structure with a zero magnetic moment Mtot due to a total compensation of the moments in neighboring iron layers. An ordered distribution of vacancies results in different type of superstructures, and can reduce the symmetry of the system from hexagonal to monoclinic.
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FIG. 1. (a) The unit cell of the NiAs-type crystal structure of Fe1-x Se compound. Black balls are the Fe cations and grey balls are the Se anions. The intralayer direct Fe-Fe and 90◦ -superexchange Fe-Se-Fe bonds are shown by dashed lines with J1 and J2 , respectively and the interlayer 129◦ - and 62◦ superexchange Fe-Se-Fe bonds are shown by dashed lines with J3 and J4 , respectively. (b) The unit cell of the Fe3 Se4 crystal structure of the NiAs-type with cation vacancies. Solid and open circles represent Fe atoms and cation vacancies, respectively. Anions are not shown.
In the compositions close to Fe7 Se8 , these vacancies can be ordered at low temperatures in two ways differ in the repeated distances along the c-direction.5–8 Quenching from high temperatures produces a “3c” hexagonal superstructure (multiplication of the c-axis, doubling of the a-axis) whereas slow cooling gives a “4c” superstructure with a slight triclinic distortion6 (small letters denote the fundamental cell). In the superstructures, the vacancies are positioned in every second cation plane along the c-axis. Different type of superstructures may lead to either AFM or FRM magnetic structures.20 Magnetic measurements show that the easy axis of Fe7 Se8 lies within the basic c-plane near the Neel temperature (TN ) but changes towards the c-axis when decreasing temperature. This spin-rotation process depends on the type of superstructure. For the 3c structure the spin-rotation occurs abruptly at 130 K, while the process begins at about 220 K and shows a gradual shift with decreasing temperature in the case of the 4c structure.7, 8, 21 The neutron diffraction studies of crystallographic and magnetic structure of the Fe3 Se4 compound22, 23 revealed the monoclinic unit cell which can be related to a pseudo NiAs type structure with ordered vacancies (space group I/2m) isomorphous to Cr3 X4 (X = S, Se, Te). The Fe ions occupy two nonequivalent positions. A half of Fe3+ ions are located in the (001) planes containing vacancies (sites I) and remaining iron ions (Fe3+ and Fe2+ ) are statistically distributed in filled planes (sites II). The compound is ferrimagnetic with
TN = 320 K. The spins of Fe ions in I and II sites are antiparallel and directed along the c axis normal to the basic plane. The magnetic moments of Fe3+ and Fe2+ ions proved to be 3.25 and 0.65 μB , respectively, which is considerably less than their theoretic values.22 Traditionally, iron selenides are synthesized by the elemental reaction in evacuated tubes at elevated temperature, or by reaction of aqueous metal salt solutions with toxic and malodorous gas H2 Se. They are also obtained by the method of mechanical alloying. However, these processes were complicated and usually accompanied by impurity phases. Recently, the Fe3 Se4 and Co-doped Fe3 Se4 nanoparticles have been synthesized by the high-temperature organicsolution-phase method.24, 25 The magnetic selenide compound CuCr2 Se4 was obtained by the precipitation method in Ref. 26, and monodisperse nanoparticles CuCr2 Se4 were synthesized by the thermal decomposition in high-temperature organic solvents.27 In this work, we developed a facile method to directly produce the single hexagonal (H) and/or monoclinic (M) phases of iron selenide nanocrystals with the NiAs-like structure via a one-pot thermal decomposition of ferrous chloride and selenide powder in oleylamine. The structural, magnetic, and electronic characteristics were studied by several complimentary methods including the Mössbauer spectroscopy. II. SAMPLE PREPARATION AND METHODS OF CHARACTERIZATION
In this work a series of samples of the iron selenide FeSe nanoparticles was synthesized by the method of thermal decomposition of the source components in high-temperature solutions. The reagent composition, the reaction scheme and some details of the synthesis are given in Table I. In the synthesis of different samples, the ultimate high-temperature processing at 340 ◦ C was varied from 30 to 70 min. The sample compositions were measured by the inductively coupled plasma-mass spectrometer (ICP-MS). The crystal structure and phase purity of the samples were examined by X-ray powder diffraction (XRD, Mutiflex MF2100, Rigaku Co. Ltd.). The XRD analysis showed (Fig. 2) that the observed peak positions and intensities for the sample, which was finally processed at 340 ◦ C for 70 min, can be indexed on the hexagonal 3c crystal structure (H sample). This is a characteristic of the Fe7 Se8 composition. The sample processed at 340 ◦ C for 30 min has a monoclinic unit cell structure (M sample), which corresponds to the Fe6 Se8 composition. Two other samples of this series MH1 and MH2 processed at 340 ◦ C for 60 and 50 min, respectively, consist of
TABLE I. Some details of the synthesis of the iron selenide Fe6-7 Se8 nanoparticles. Fe/Se is the molar ratio of FeCl2 *4H2 O to selenide. Sample name H M MH1 MH2
Expected composition
Reaction scheme
Fe/Se
Crystal structure
Fe7 Se8 Fe6 Se8 Fe6-7 Se8 Fe6-7 Se8
FeCl2 *4H2 O + selenide + oleylamine→ 200 ◦ C, 60 min → 340 ◦ C, 70 min FeCl2 *4H2 O + selenide + oleylamine→ 200 ◦ C, 60 min → 340 ◦ C, 30 min FeCl2 *4H2 O + selenide + oleylamine→ 200 ◦ C, 60 min. → 340 ◦ C, 60 min. FeCl2 *4H2 O + selenide + oleylamine→ 200 ◦ C, 60 min. → 340 ◦ C, 50 min
1/0.9412 1/0.9412 1/0.9412 1/0.9412
Hexagonal Monoclinic Hexagonal +monoclinic Hexagonal +monoclinic
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MPMS, (Quantum Design) in an applied field sweeping from −70 to 70 kOe. For measurements of the zero-field-cooled (ZFC) and field-cooled (FC) temperature dependence of magnetization, the samples were first cooled at zero field from 300 to 78 K then the magnetization was measured in the fields of 100 Oe and 11 kOe by heating the sample from 78 to 300 K. After that the samples were cooled from 300 to 78 K maintaining the same field. Then the FC curves were recorded with increasing temperature. A vibrating sample magnetometer (VSM) was also used to measure the dc magnetization in the temperature range from 78 to 300 K and in the applied field up to 11 kOe. The Mössbauer spectroscopy was applied to examine the phase composition, structural, magnetic, and electronic properties of the nanoparticles. The 57 Fe-Mössbauer spectra were recorded at temperatures between 90 and 300 K in the transmission geometry with a standard spectrometer operating in the constant accelerations regime. The gammaray source 57 Co(Rh) was at room temperature and the isomer shifts were measured relative to metal α-Fe at room temperature. FIG. 2. The X-ray diffraction patterns of the Fe-Se nanoparticles. The indicated reflection indices correspond to the hexagonal Ni-As-type 3c-crystal structure of Fe7 Se8 (H sample) and to the monoclinic structure close to Fe6 Se8 (M sample). The MH1 and MH2 samples consist of a mixture of M and H phases.
a mixture of two hexagonal and monoclinic phases Fe6-7 Se8 (Fig. 2). The mass spectrometry analysis (Fe, Se) revealed that the compositions of the H and M compounds are Fe7.53 Se8.74 (or Fe0.862 Se) and Fe6.75 Se9.22 (or Fe0.732 Se), respectively. This indicates that the Fe:Se concentration is close to the expected 7:8 and 3:4 ratio for H-phase and M-phase. The morphology and microstructure of the particles were characterized by scanning electron microscopy (SEM) and the transmission electron microscopy (TEM, Tecnai G2 F20, FEG-TEM, Philips Co. Ltd.). The SEM and TEM images of the Fe-Se nanoparticles are shown in Figs. 3 and 4. The images in Fig. 4 show the plate shape nanocomposites of hexagonal form with the characteristic size of about 200-400 nm. The measurements of magnetization curves and hysteresis loops were performed at 5, 100, and 300 K using a superconducting quantum interference device SQUID-VSM,
III. RESULTS OF MAGNETIC MEASUREMENTS A. The field dependence of magnetizations
The magnetization curves of all samples measured in an applied magnetic field up to 70 kOe reveal pronounced hysteresis characteristics typical of a ferromagnetic (or ferrimagnetic) behavior (Fig. 5). With increasing temperature from 5 to 300 K, the magnetization area of the hysteresis loop is reduced significantly in all samples. At room temperature magnetization is almost saturated in the field higher 10 kOe, and it remains at the level of about 2–3 emu/g (Fig. 5). However, at low temperatures of 100 and 5 K magnetization is not saturated even in the highest field of 70 kOe. Here it is important to note that, in contrast to our nanoparticles, the bulk monoclinic sample Fe3 Se4 was saturated in weak fields.28 Probably, it can be explained by strong surface anisotropy of the nanoparticles. The hysteresis loops of both the hexagonal (H) Fe7 Se8 and monoclinic (M) Fe3 Se4 nanoparticles reveal unusually high values of coercive force Hc (Fig. 5). The Hc value is the highest in Fe3 Se4 (about 25 kOe), and it is almost constant in the temperature range from 5 to 100 K.
FIG. 3. The scanning electron microscopy images of the monoclinic Fe-Se nanoparticles.
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FIG. 4. The scanning and transmission electron microscopy images of the hexagonal Fe-Se nanoparticles.
FIG. 5. The applied-field dependences of magnetization and hysteresis loops recorded at 5, 100, and 300 K for the Fe-Se samples with hexagonal (H), monoclinic (M), and mixed structures MH1 and MH2. Insets show the zoomed view of hysteresis plots at 300 K.
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Recently, high coercivity Hc was observed in the Fe3 Se4 and Co-doped Fe3 Se4 nanoparticles synthesized by the hightemperature organic-solution-phase method.24, 25 Hard magnetic properties depend on the particle shape. The “giant” Hc values reaching 40 kOe at low temperature were found in the faceted- and cacti-shaped Fe3 Se4 nanoparticles, whereas Hc is somewhat lower in nanosheets.24 This unusually large coercivity was explained by the uniaxial magnetocrystalline anisotropy of the monoclinic Fe3 Se4 with ordered cation vacancies.25
B. The temperature dependence of magnetizations in monoclinic Fe3 Se4 nanoparticles
In the Fe3 Se4 nanoparticles a shift of the hysteresis loops was observed in the magnetization curves measured in the applied field up to 10 kOe. As shown in Fig. 6, the loops are shifted upwards along the magnetization axis without any shifts along the field axis. The loop-shift behavior is temperature dependent. With temperature decreasing from 300 K, the shift starts at about 280 K and continues down to 200 K. Simultaneously, the loop area decreases, essentially below 260 K, and it almost disappears below 200 K. As shown in Fig. 7, temperature behavior of the coercivity HC and remanent magnetization σ R for the Fe3 Se4 sample measured in the field of 11 kOe reveal a maximum just at about 280 K. Such behavior characterizes switching of magnetization in the field of 10 kOe. The switching process strongly depends on temperature, and below 200 K, magnetization is not switched at all. As follows from the behavior of the FC and ZFC magnetization curves (Fig. 7(b)), this effect is related to the appearance of exchange magnetic interaction between nanoparticles below 280 K, leading to the splitting of the FC and ZFC curves in the field of 11 kOe. Strong anisotropy of the exchange interaction does not allow the magnetization to change direction even in the field of 10 kOe. This feature of the Fe3 Se4 nanoparticles can be important for applications. The temperature dependences of magnetization measured for Fe3 Se4 in ZFC and FC modes are shown in Fig. 7. In the low field of 100 Oe, the ZFC and FC curves are split just
FIG. 6. Hysteresis loops measured in the applied field of 10 kOe at different temperatures for Fe-Se nanocrystals with monoclinic structure.
FIG. 7. Temperature dependence of magnetization for ZFC and FC measurements in the applied field of (a) 100 Oe and (b) 11 kOe for Fe-Se nanocrystals with monoclinic structure. (c) The temperature behavior of coercivity HC , remanent magnetization σ R and magnetization in the applied field of 11 kOe. Insets show the zoomed view of anomalies in magnetization at about 180 and 275 K.
below the Neel temperature (TN ) which is the characteristic feature of superparamagnetic and/or spin glass systems.29, 30 In the low applied field, the ZFC and FC curves are strongly different in the value of magnetization σ . In the ZFC regime, σ is very small of about 1 emu/g and almost independent of temperature. It implies that the magnetic structure of the material is close to AFM. In the FC regime, lowtemperature magnetization strongly increases to ∼14 emu/g showing an essential influence of the weak external field 100 Oe on the magnetic structure. It is well known that in the process of magnetization, a weak field at first shifts the boundaries between magnetic domains leading to a single domain magnetic structure. A relatively easy shift of the boundaries implies weak magnetic anisotropy in the inner (core) part of
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the nanoparticles. From these observations we may conclude that the surface magnetic anisotropy of the Fe3 Se4 nanoparticles is essentially larger than the core anisotropy. Similar effect was recently observed and discussed in the magnetite Fe3 O4 nanoparticles.31, 32 As temperature increases, the FC magnetization of Fe3 Se4 remains nearly constant between 78 and 160 K, and then it sharply drops at ∼180 K (Fig. 7(a)). Near this temperature, the ZFC magnetization also shows a weak anomaly (see inset in Fig. 7(a)). This can be related to the spin reorientation effect suggested in Ref. 12. Magnetization disappears at TN ≈ 340 K. It shows that the TN value in our Fe3 Se4 nanoparticles is higher than in the bulk material (320 K) previously reported in Refs. 22 and 23. In the high external field of 11 kOe, the ZFC and FC magnetization curves of Fe3 Se4 coincide above Tint ≈ 280 K (Fig. 7(b)), and this temperature can be related to the appearance of magnetic interactions between nanoparticles at T < Tint which enhances the magnetic anisotropy.4, 33–35 As shown in Fig. 7(c), the temperature variation of the coercivity Hc which is a measure of the magnetic anisotropy,34 reveals a maximum just at ≈ 280 K. C. The temperature dependence of magnetizations in hexagonal Fe7 Se8 nanoparticles
The magnetic behavior of monoclinic and hexagonal phases is very different. The ZFC and FC magnetization curves for the hexagonal Fe7 Se8 (H) sample measured in the low (100 Oe) and high (11 kOe) applied fields are shown in Fig. 8. In the low field, the ZFC and FC curves do not overlap at all temperatures up to 300 K (Fig. 8(a)). It indicates that the nanoparticles are not free but they are strongly coupled by magnetic interactions. Below 130 K, the ZFC curve in Fig. 8(a) shows drastic decrease of magnetization to nearly zero value indicating a transition from FRM to AFM state. As follows from the hysteresis loops, the coercivity Hc value of Fe7 Se8 also drastically changes (increases) at this temperature (Fig. 8(c)). The spin-rotation effect was found in the bulk hexagonal Fe7 Se8 just at 130 K.21 With decreasing temperature, the easy axis changes its direction from the basic c-plane towards the caxis. This abrupt magnetic transition in the narrow temperature region is a characteristic of the 3c type superstructure appearing due to the vacancy ordering,21 whereas gradual transition is a characteristic of the 4c type superstructure.36 From this observation we can conclude that the 3c type superstructure of the vacancy ordering occurs in our hexagonal Fe7 Se8 nanoparticles and this is in good agreement with our XRD data. The decrease of magnetization below 130 K was explained8, 37, 38 by the competition between the in-plain and the normal-to-plain magnetic anisotropies which leads to a canted spin structure. The different temperature dependence of these anisotropies can vary the canting angle, thus changing the total magnetic moment of the material. However, the change in value of magnetization at the FRM–AFM transition (along with changes in spin directions) also implies a redistribution of the cation vacancies in the adjoining iron
FIG. 8. Temperature dependence of magnetization for ZFC and FC measurements in the applied field of (a) 100 Oe and (b) 11 kOe for Fe-Se nanocrystals with hexagonal structure. (c) The temperature behavior of coercivity HC , remanent magnetization σ R and magnetization in the applied field of 11 kOe.
layers. Such a behavior was recently observed in the iron sulfide Fe1-x S nanoparticles with a NiAs-like structure.20, 31, 39 Thus we may suppose, that the drastic change in the low-applied-field ZFC magnetization of the Fe7 Se8 sample at 130 K is related to iron vacancy redistribution (from random to ordered) which leads to the transformation of the magnetic structure from FRM to AFM. This magnetic transition is also expressed as a maximum of the FC magnetization and sharp increase in the coercivity Hc (Fig. 8(c)). On the other hand, the similar magnetic transition was recently observed in the magnetite Fe3 O4 nanoparticles below the Verwey transition temperature TV .31 According to Goodenough,40 such decrease in magnetization may be explained by appearance of the direct covalent Fe-Fe coupling in octahedral sites of magnetite below TV . The spins of 3d electrons are compensated in the covalent bonds, thus decreasing the iron magnetic moment.
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In the high applied field of 11 kOe, the ZFC and FC magnetization of hexagonal Fe7 Se8 nanocrystals coincide in all temperature interval of 77–350 K (Fig. 8(b)). It shows that the applied field is strong enough to line up spins along the field at all temperatures. This implies that magnetic anisotropy of exchange interactions in the hexagonal Fe7 Se8 nanocrystals is not so high as in the monoclinic Fe3 Se4 . The magnetic transition observed in the low applied field at about 130 K (Fig. 8(a)) is shifted by the high applied field to lower temperature of about 118 K. IV. MÖSSBAUER SPECTROSCOPY CHARACTERIZATION OF NANOPARTICLES A. Mossbauer data for monoclinic Fe3 Se4 nanoparticles
All nanoparticles were investigated by the 57 Fe- Mössbauer spectroscopy at temperatures between 90 and 300 K. The representative spectra of four Fe-Se samples recorded at 90 and 300 K are shown in Fig. 9. The temperature evolution of the spectra between 90 and 300 K is represented in Fig. 10 for the hexagonal H and the monoclinic M samples. At 90 K, the Mössbauer spectra of the monoclinic (M) sample indicate that the main part of iron ions is in the magnetically ordered state (Fig. 11(a)). The low-intensive doublet in the spectrum center (about 7% of total iron content in the sample) has the isomer shift δ = 0.48 mm/s and quadrupole splitting = 0.60 mm/s which indicate the presence of Fe3+ ions in the paramagnetic state. Obviously, this paramagnetic
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component appears due to the particle size distribution, and it belongs to very small particles (d < 10 nm) whose spin blocking temperature is below 90 K. The magnetic spectrum of Fe3 Se4 (Fig. 11(a)) can be fit to four components corresponding to four nonequivalent iron sites with different values of magnetic hyperfine fields Hhf and isomer shifts δ. For two of these magnetic components, the hyperfine parameters are: δ = 0.49 and 0.30 mm/s and Hhf = 46.8 and 43.8 T, respectively, and these parameters are characteristic of the Fe3+ ions. The occupation of these nonequivalent Fe3+ iron sites is in the ratio 37/14. Two other magnetic components have the same isomer shift δ = 0.74 mm/s and different field values Hhf = 22.6 and 10.6 T. These parameters are characteristic of the Fe2+ ions, and the occupation of these nonequivalent Fe2+ iron sites is in the ratio 16/26. These Mössbauer data indicate that a part of magnetic iron ions is in the ferric Fe3+ state (about 50.5% of total iron content in the sample) and the other part is in the ferrous state close to Fe2+ (42.5%). With due regard for the paramagnetic component, the ratio of Fe3+ /Fe2+ for the iron ions in the sample is 57.5/42.5 (at 90 K). From this value, the condition of electro-neutrality of the Fe1-x Se molecule leads to the Se/Fe ratio of 1.29. This is very close to the ratio Se/Fe = 4/3 ≈ 1.33 valid for the Fe3 Se4 composition. Thus, the Mössbauer parameters reveal that the monoclinic Fe1-x Se phase of the obtained nanoparticles has the dominant composition Fe3 Se4 with very small excess of ferrous ions, apparently, from the hexagonal FeSe phase. This correlates very well with our XRay data.
FIG. 9. Mössbauer spectra of the hexagonal H, monoclinic M, and mixture-phase samples MH1 and MH2 of iron selenides at 90 and 300 K.
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FIG. 10. The temperature evolution of Mössbauer spectra of the hexagonal H and monoclinic M iron selenide samples between 90 and 300 K.
The ferric iron states were also found in the selenium doped chalcopyrite CuFeS1.6 Se0.4 by Mössbauer measurements with the field Hhf value of about 37 T.41 However, the value Hhf of about 32 T was found in the iron sulfide Fe3 S4 with the spinel structure.42 As follows from the X-ray and neutron diffraction data,5, 22, 23 the Fe ions in Fe3 Se4 occupy two nonequiva-
FIG. 11. Computer processing (the fit to four components by UNIVEM program) of the Mössbauer spectra of monoclinic M (a) and hexagonal H (b) samples of iron selenides recorded at 90 K.
lent sites located in the alternating planes containing vacancies (sites I) and in planes without vacancies (sites II). Our Mössbauer data indicate that Fe3+ and Fe2+ ions in Fe3 Se4 nanoparticles occupy both the sites I and II. It seems that the Fe3+ and Fe2+ ions with higher value of the magnetic fields Hhf = 46.8 and 22.6 T, respectively, should be attributed to the planes containing vacancies. For these sites all exchange interaction bonds Fe-Se-Fe with neighbors from the nearest layers (above and below the given layer) are preserved when the vacancies appear (see Fig. 1(b)) whereas for Fe ions in planes without vacancies some exchange bonds are broken Fe-Se.20 Thus, from the Mössbauer data we can conclude that about 53% of iron (37% Fe3+ and 16% Fe2+ ) are in planes with vacancies and about 40% (14% Fe3+ and 26% Fe2+ ) are in the planes without vacancies. The temperature dependences of magnetic fields Hhf in four nonequivalent iron sites are shown in (Fig. 12). It is interesting that the Hhf values in Fe3+ ions drop at 160–180 K, whereas this anomaly is not observed for the Fe2+ ions. Our magnetic measurements revealed a decrease in the FCmagnetization just in the same temperature region (Fig. 7(a)), which is related to the spin reorientation effect. It can be supposed that the spin reorientation is connected with vacancy redistribution which influences the magnetic state of Fe ions. As follows from the Mössbauer data (the Hhf (T) behavior), the ferric ions are more sensitive to the vacancies redistribution than the ferrous ions. This implies that more vacancies appear near the Fe3+ ions, which stimulates the magnetic transition with rotation of the Fe3+ magnetic moments.
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FIG. 12. Temperature dependences of magnetic hyperfine fields Hhf in nonequivalent iron sites of the hexagonal (H) and monoclinic (M) Fe-Se nanocomposites.
B. Mossbauer data for hexagonal Fe-Se nanoparticles
At 90 K, the Mössbauer spectrum of the hexagonal Fe7 Se8 (H)-sample (Fig. 11(b)) indicates that a part of iron ions is in the magnetically ordered state (about 60% of total iron content in the sample), and the other part is in the paramagnetic state (about 37%). In addition, the small-intensive (about 3%) magnetic component with the highest value of magnetic hyperfine field Hhf = 51.9 T was observed (green lines in Fig. 11(b)). Obviously, this component is typical of iron oxide (Fe2 O3 ) which appears from the iron oxidation at the particle surface. The main magnetic spectrum of Fe-Se (H) sample can be fit to the two components with the magnetic fields Hhf = 47.0 (35%) and 44.2 (24%) T. The isomer shifts δ of both magnetic components are close and equal to 0.48 and 0.45 mm/s, respectively. At 300 K the δ values reduce to about 0.37 mm/s. These δ values are typical of ferric Fe3+ ions. This implies that the hexagonal Fe-Se sample was transformed into the composition close to Fe6 Se9 . The nonmagnetic doublet component, observed in the (H) sample at 90 K, has the δ values of 0.45 mm/s, which
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is also a characteristic of the ferric Fe3+ ions in the paramagnetic state. The quadrupole splitting of the doublet is = 0.91 mm/s. The Mössbauer spectra lines are broaden in both magnetic and paramagnetic components, which can be connected with distribution of the particle sizes. It seems that the paramagnetic doublet component in the 90 K spectrum originates from very small particles whose spin blocking temperature TB is below 90 K. In our study, the Mössbauer measurements of the hexagonal (H) sample were performed in several months after the sample had been prepared. It seems that for this time the samples were degraded into the composition close to Fe6 Se9 since the observed Mössbauer parameters are typical of ferric Fe3+ ions. In case of the NiAs-type structure, such a material should have high amount of the cation vacancies. Two magnetic components observed in the Mössbauer spectra indicate that the two types of nonequivalent iron sites with different number of cation vacancies dominate in the transformed FeSe (H) sample. We noted that the magnetic fields Hhf and isomer shifts δ values in the transformed Fe-Se sample (H) are very close to the values for ferric Fe3+ ions in the monoclinic Fe3 Se4 , as it was discussed above. On the other hand, if one suggests that ferric Fe3+ ions in (H) appears because of full oxidation of the material, then α-Fe2 O3 and/or γ -Fe2 O3 type oxides can be expected. However, the field Hhf value in these oxides must be of about 51–52 T31 which is much higher than the values observed in our Fe-Se (H) sample. Moreover, the Neel temperature of iron oxides Fe2 O3 is about 900 K42 which is much higher than the value 450 K obtained for (H) in our magnetic measurements. Thus, the appearance of iron oxide phases in the transformed Fe-Se sample (H) should be excluded. Usually the Hhf fields values in chalcogenides are lower than in oxides due to higher covalence in the cation-anion bonds.43 It seems that the field values Hhf = 47.0 and 44.2 T found in (H) are consistent with the ferric iron state. The ferrous state of all cations was found in bulk Fe7 Se8 with the Hhf fields values in the range of 31–22 T.36 Under the similar conditions, the Hhf values in ferrous ions must be lower than in ferric ions due to the lower spin value of Fe2+ and the orbital contribution to the field.20, 44 With increasing temperature, the magnetic spectra components became asymmetric with broadening of inner parts of the resonance lines (Fig. 10). This is a signature of superparamagnetic behavior typical of nanoparticles. The area of the paramagnetic doublet gradually increases with temperature at the expense of the magnetic components areas. This indicates a transition of small-size particles from the magnetically ordered state to the paramagnetic state due to fast spin relaxation. From the temperature dependence of an average value of magnetic field Hhf (Fig. 12) we can conclude that the main part of the particles (about 80%) transforms to the paramagnetic state at about 290-300 K. In the superparamagnetic process, it can be considered as the blocking or Neel temperature of the particles with the largest size. Meanwhile, in the Mössbauer spectrum at 300 K about 20% particles are still in the magnetically ordered (or superparamagnetic) state with Hhf = 12 T.
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V. SUMMARY AND CONCLUSION
In this work a facile method was developed to produce the iron selenide nanoparticles with the NiAs-like crystal structure by thermal decomposition of iron chloride and selenium powder in the high-temperature organic solvent. Depending on the time of the compound processing at 340 ◦ C in oleylamine, the nanocrystals with monoclinic or hexagonal structures as well as a mixture of these two phases can be obtained for about one hour. The magnetic behavior of monoclinic Fe3 Se4 and hexagonal Fe7 Se8 phases is very different. Besides the Neel temperature at 340 and 450 K in Fe3 Se4 and Fe7 Se8 samples, respectively, the magnetic transitions were found at 180 and 130 K related to the spin rotation due to the vacancies redistribution. The hysteresis loops in both Fe7 Se8 and Fe3 Se4 nanocrystals indicate the unusually high coercive force Hc which value is highest (“giant”) in monoclinic Fe3 Se4 (of about 25 kOe). This can be explained by the surface magnetic anisotropy of the Fe3 Se4 nanoparticles which is essentially larger than the core anisotropy. Such large coercivity is rare for materials without rare earth or noble metal elements, and Fe3 Se4 -based compounds can be low-cost, nontoxic alternative materials for advanced magnets.24, 25 Below 280 K, an exchange magnetic interaction appears between Fe3 Se4 nanoparticles, which enhances the magnetic anisotropy. Strong anisotropy of this interaction does not allow the magnetization to change the direction even in the field of 10 kOe. Such properties of a “switching effect” in the Fe3 Se4 nanoparticles can be important for applications. The applied-field and temperature dependences of magnetization show a complicated transformation between FRM and AFM structures which is related to the spin rotation process connected with the redistribution of cation vacancies. The 3c type superstructure of the vacancy ordering was found in the hexagonal Fe7 Se8 . Below 130 K, the redistribution of vacancies from random to ordered leads to the spin-rotation and transformation of the magnetic structure from FRM to AFM. This magnetic transition is also manifested by a maximum of the FC magnetization and sharp increase in the coercivity Hc . In monoclinic Fe3 Se4 , the Mössbauer spectra reveal several nonequivalent positions of iron ions appearing due to the presence of cationic vacancies in the nearest environment of Fe. The fraction of Fe3+ and Fe2+ ions in the planes with vacancies and in the planes without vacancies was estimated. The Mössbauer data indicate that vacancies prefer to appear near the Fe3+ ions and stimulate the magnetic transition with the rotation of the Fe3+ magnetic moments. In general, the magnetic properties of nanoparticle samples are not only intrinsic to the particles but also depend on the conditions in which the particles are, such as the exact chemical composition, homogeneity of the crystal structure, shape, aggregation, crystallinity etc. In turn, these conditions are defined by the particle processing method. In particular, it is known that the shape and size of nanoparticles as well as the magnetic properties strongly depend on the amount and the kind of surfactants used in the synthesis. For example, in Refs. 24 and 25, the authors used two kinds of surfactants, the
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oleylamine and oleic acid, as the reaction solvent to prepare the Fe3 Se4 nanosheets and nanoplatelets. However, in our experiment we used only oleylamine to prepare Fe3 Se4 nanoparticles. That is one of the reasons why the results of different researchers may not be entirely the same. ACKNOWLEDGMENTS
Support by the Russian Scientific Foundation (Project No. 14-12-00848) is acknowledged. C.-R.L.,Y.-J.S., and M.L.C. appreciate the support of the Ministry of Science and Technology of Taiwan (Grant Nos. 100-2923-M-153-001MY3 and 102-2112-M-153-002-MY3). 1 X.
J. Wu, D. Z. Shen, Z. Z. Zhang et al., “On the nature of the carriers in ferromagnetic FeSe,” Appl. Phys. Lett. 90, 112105–112105–3 (2007). 2 X. J. Wu, Z. Z. Zhang, J. Y. Zhang et al., “Two-carrier transport and ferromagnetism in FeSe thin films,” J. Appl. Phys. 103, 113501–113501–6 (2008). 3 F.-C. Hsu, J.-Y. Luo, K.-W. Yeh et al., “Superconductivity in the PbOtype structure α-FeSe,” Proc. Natl. Acad. Sci. U.S.A. 105, 14262–14264 (2008). 4 D. Li, J. J. Jiang, W. Liu, and Z. D. Zhang, “Positive magnetoresistance in Fe3 Se4 nanowires,” J. Appl. Phys. 109, 07C705 (2011). 5 A. Okazaki and K. Hirakawa, “Structural study of iron selenides FeSe . I x ordered arrangement of defects of Fe atoms,” J. Phys. Soc. Jpn. 11, 930– 936 (1956). 6 A. Okazaki, “The variation of superstructure in iron selenide Fe Se ,” J. 7 8 Phys. Soc. Jpn. 14, 112–113 (1959). 7 A. Okazaki, “The superstructures of iron selenide Fe Se ,” J. Phys. Soc. 7 8 Jpn. 16, 1162–1170 (1961). 8 M. Kawaminami and A. Okazaki, “Neutron diffraction study of Fe Se II,” 7 8 J. Phys. Soc. Jpn. 29, 649–655 (1970). 9 P. Terzieff and K. L. Komarek, “The paramagnetic properties of iron selenides with NiAs-type structure,” Monatsh. Chem. Chem. Mon. 109, 651– 659 (1978). 10 P. Terzieff and K. L. Komarek, “The antiferromagnetic and ferrimagnetic properties of iron selenides with NiAs-type structure,” Monatsh. Chem. Chem. Mon. 109, 1037–1047 (1978). 11 A. Wold and K. Dwight, Solid State Chemistry - Synthesis, Structure, and Properties of Selected Oxides and Sulfides (Springer, Netherlands, 1993). 12 E. F. Bertaut, “Contribution à l’étude des structures lacunaires: La pyrrhotine,” Acta Crystallogr. 6, 557–561 (1953). 13 F. K. Lotgering, Philips Res. Rep. 11, 190 (1956). 14 L. A. Marusak and L. N. Mulay, “Polytypism in the cation-deficient iron sulfide, Fe9 S10 , and the magnetokinetics of the diffusion process at temperatures about the antiferro- to ferrimagnetic (λ) phase transition,” Phys. Rev. B 21, 238–244 (1980). 15 L. M. Levinson and D. Treves, “Mössbauer study of the magnetic structure of Fe7 S8 ,” J. Phys. Chem. Solids 29, 2227–2231 (1968). 16 H. N. Ok and S. W. Lee, “Mössbauer study of ferrimagnetic Fe Se ,” Phys. 7 8 Rev. B 8, 4267–4269 (1973). 17 W. Kim, I. J. Park, and C. S. Kim, “Mössbauer study of magnetic structure of cation-deficient iron sulfide Fe0.92 S,” J. Appl. Phys. 105, 07D535 (2009). 18 H. N. Ok, K. S. Baek, and C. S. Kim, “Crystallographic and Mössbauer studies of FeRh2 Se4 ,” Phys. Rev. B 26, 4436–4441 (1982). 19 H. N. Ok and C. S. Lee, “Mössbauer study of FeV Se ,” Phys. Rev. B 29, 2 4 5168–5170 (1984). 20 I. S. Lyubutin, C.-R. Lin, S.-Z. Lu et al., “High-temperature redistribution of cation vacancies and irreversible magnetic transitions in the Fe1−x S nanodisks observed by the Mössbauer spectroscopy and magnetic measurements,” J. Nanopart. Res. 13, 5507–5517 (2011). 21 A. F. Andresen and J. Leciejewicz, “A neutron diffraction study of Fe Se ,” 7 8 J. Phys. 25, 574–578 (1964). 22 B. Lambert-Andron, G. Berodias, and D. Babot, “Structures magnetiques des composes MFe2 Se4 (M = Ti, V, Cr, Fe, Co, Ni),” J. Phys. Chem. Solids 33, 87–94 (1972). 23 B. Lambert-Andron and G. Berodias, “Etude par diffraction neutronique de Fe3 Se4 ,” Solid State Commun. 7, 623–629 (1969).
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Zhang, G. Long, D. Li et al., “Fe3 Se4 nanostructures with giant coercivity synthesized by solution chemistry,” Chem. Mater. 23, 3769–3774 (2011). 25 G. Long, H. Zhang, D. Li et al., “Magnetic anisotropy and coercivity of Fe3 Se4 nanostructures,” Appl. Phys. Lett. 99, 202103 (2011). 26 P. W. D. Mitchell, “Morgan PED. Direct precipitation method for complex crystalline chalcogenides,” J. Am. Ceram. Soc. 57, 278–278 (1974). 27 C.-R. Lin, C.-L. Yeh, S.-Z. Lu et al., “Synthesis, characterization, and magnetic properties of nearly monodisperse CuCr2 Se4 nanoparticles,” Nanotechnology 21, 235603 (2010). 28 I. G. Kerimov, N. Aliev, D. Guseinov et al., Fiz. Tverd. Tela 18, 3328 (1976). 29 K. Binder and A. P. Young, “Spin glasses: Experimental facts, theoretical concepts, and open questions,” Rev. Mod. Phys. 58, 801–976 (1986). 30 K. Dörr, “Ferromagnetic manganites: Spin-polarized conduction versus competing interactions,” J. Phys. D: Appl. Phys. 39, R125 (2006). 31 I. S. Lyubutin, C. R. Lin, Y. V. Korzhetskiy et al., “Mössbauer spectroscopy and magnetic properties of hematite/magnetite nanocomposites,” J. Appl. Phys. 106, 034311 (2009). 32 Ö. Özdemir, “Coercive force of single crystals of magnetite at low temperatures,” Geophys. J. Int. 141, 351–356 (2000). 33 F. K. Lotgering, “Ferromagnetic interactions ferromagnetic sulphides, selenides and tellurides with spinel structure,” Proceedings of the International Conference on magnetism, Nottingham (Published in Association with Proceedings of the Physical Society, 1964), p. 533. 34 P. S. A. Kumar, P. A. Joy, and S. K. Date, “Origin of the cluster-glasslike magnetic properties of the ferromagnetic system,” J. Phys.: Condens. Matter 10, L487 (1998).
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Laiho, K. G. Lisunov, E. Lähderanta et al., “Coexistence of ferromagnetic and spin-glass phenomena in La1−x Cax MnO3 (0≤x≤0.4),” J. Phys.: Condens. Matter 12, 5751 (2000). 36 C. Boumford and A. H. Morrish, “Magnetic properties of the iron selenide Fe7 Se8 ,” Phys. Status Solidi A 22, 435–444 (1974). 37 K. Adachi and K. Sato, “Magnetization curves of Fe S and Fe Se under 7 8 7 8 high magnetic field,” J. Phys. Soc. Jpn. 26, 581–581 (1969). 38 T. Kamimura, K. Kamigaki, T. Hirone, and K. Sato, “On the magnetocrystalline anisotropy of iron selenide Fe7 Se8 ,” J. Phys. Soc. Jpn. 22, 1235– 1240 (1967). 39 C.-R. Lin, S.-Z. Lu, I. S. Lyubutin et al., “Synthesis and magnetic properties of iron sulfide nanosheets with a NiAs-like structure,” J. Appl. Phys. 107, 09A335-1–09A335-3 (2010). 40 J. B. Goodenough and A. L. Loeb, “Theory of ionic ordering, crystal distortion, and magnetic exchange due to covalent forces in spinels,” Phys. Rev. 98, 391–408 (1955). 41 Eun Jung Choi, Byung Chul Cho, Myung Hee Lee et al., “Mossbauer study of antiferromagnetic CuFeS1.6Se0.4,” J. Korean Phys. Soc. 27, 436 (1994). 42 I. S. Lyubutin, S. S. Starchikov, C.-R. Lin et al., “Magnetic, structural, and electronic properties of iron sulfide Fe3 S4 nanoparticles synthesized by the polyol mediated process,” J. Nanopart. Res. 15, 1397 (2013). 43 F. Gazeau, J. C. Bacri, F. Gendron et al., “Magnetic resonance of ferrite nanoparticles: Evidence of surface effects,” J. Magn. Magn. Mater. 186, 175–187 (1998). 44 J. Danon, “Fe: Metal, alloys, and inorganic compounds,” in Chemical Applications of Mössbauer Spectroscopy (Academic Press, N.Y., 1968), V. I. Goldanskii and R. H. Herber (eds.), pp. 159–267.
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THE JOURNAL OF CHEMICAL PHYSICS 141, 044704 (2014)
Structural, magnetic, and electronic properties of iron selenide Fe6-7 Se8 nanoparticles obtained by thermal decomposition in high-temperature organic solvents I. S. Lyubutin,1,a) Chun-Rong Lin,2,a) K. O. Funtov,1 T. V. Dmitrieva,1 S. S. Starchikov,1 Yu-Jhan Siao,3 and Mei-Li Chen4 1
Shubnikov Institute of Crystallography, Russian Academy of Sciences, Moscow 119333, Russia Department of Applied Physics, National Pingtung University of Education, Pingtung 90003, Taiwan 3 Department of Mechanical Engineering, Southern Taiwan University of Science and Technology, Tainan 71005, Taiwan 4 Department of Electro-optical Engineering, Southern Taiwan University of Science and Technology, Tainan 71005, Taiwan 2
(Received 8 April 2014; accepted 24 June 2014; published online 24 July 2014) Iron selenide nanoparticles with the NiAs-like crystal structure were synthesized by thermal decomposition of iron chloride and selenium powder in a high-temperature organic solvent. Depending on the time of the compound processing at 340 ◦ C, the nanocrystals with monoclinic (M)-Fe3 Se4 or hexagonal (H)-Fe7 Se8 structures as well as a mixture of these two phases can be obtained. The magnetic behavior of the monoclinic and hexagonal phases is very different. The applied-field and temperature dependences of magnetization reveal a complicated transformation between ferrimagnetic (FRM) and antiferromagnetic (AFM) structures, which can be related to the spin rotation process connected with the redistribution of cation vacancies. From XRD and Mössbauer data, the 3c type superstructure of vacancy ordering was found in the hexagonal Fe7 Se8 . Redistribution of vacancies in Fe7 Se8 from random to ordered leads to the transformation of the magnetic structure from FRM to AFM. The Mössbauer data indicate that vacancies in the monoclinic Fe3 Se4 prefer to appear near the Fe3+ ions and stimulate the magnetic transition with the rotation of the Fe3+ magnetic moments. Unusually high coercive force Hc was found in both (H) and (M) nanocrystals with the highest (“giant”) value of about 25 kOe in monoclinic Fe3 Se4 . This is explained by the strong surface magnetic anisotropy which is essentially larger than the core anisotropy. Such a large coercivity is rare for materials without rare earth or noble metal elements, and the Fe3 Se4 -based compounds can be the low-cost, nontoxic alternative materials for advanced magnets. In addition, an unusual effect of “switching” of magnetization in a field of 10 kOe was found in the Fe3 Se4 nanoparticles below 280 K, which can be important for applications. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4887356] I. INTRODUCTION
In recent years, iron selenide Fe-Se compounds have attracted a great interest because of promising application in peculiar electronic, optical, and magnetic devices. Their magnetic and conductive properties depend on the composition, and they could be ferro/ferrimagnetic metals, semiconductors, or even superconductors.1–4 According to the phase diagram,5–10 the Fe-Se compounds with the NiAs-like crystal structure occur over a certain composition range, in particular from 51 to 59 at.% selenium. Within this range the phase can exist as a hexagonal NiAs-like structure with approximate composition Fe7 Se8 (Hphase), and as a monoclinic deformation of the same (pseudohexagonal) phase, with approximate composition Fe3 Se4 (M-phase).9, 10
a) Authors to whom correspondence should be addressed. Electronic
addresses: [email protected], Tel.: +7(499) 135-6250, Fax: +7(499) 135-1011 and [email protected], Tel.: +886-8-7226141 ext. 33462, Fax: +886-8-7213760.
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In the NiAs-type structure of iron chalcogenides Fe1-x X (X = S, Se) the Fe cations are octahedrally coordinated by six X anions (the FeX6 octahedra) and the coordination polyhedron of X is the trigonal prism created by six Fe (the XFe6 prisms) (Fig. 1(a)).11 The magnetic structure of Fe1-x X consists of ferromagnetic iron layers in the s plane which are antiferromagnetically coupled to each other along the c-axis.12–19 The total magnetic moment Mtot appears from the competition between magnetic moments of neighboring layers, and the material can be antiferromagnetic (AFM) or ferrimagnetic (FRM) depending on the value of moments of the corresponding layer-sublattices. The iron deficiency in Fe1-x Se is compensated by the vacancies at the cation sites (Fig. 1(b)). The layer moments depend on the number of the vacancies and on the way of vacancies distribution.20 A random distribution of vacancies gives rise to a pure AFM structure with a zero magnetic moment Mtot due to a total compensation of the moments in neighboring iron layers. An ordered distribution of vacancies results in different type of superstructures, and can reduce the symmetry of the system from hexagonal to monoclinic.
141, 044704-1
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FIG. 1. (a) The unit cell of the NiAs-type crystal structure of Fe1-x Se compound. Black balls are the Fe cations and grey balls are the Se anions. The intralayer direct Fe-Fe and 90◦ -superexchange Fe-Se-Fe bonds are shown by dashed lines with J1 and J2 , respectively and the interlayer 129◦ - and 62◦ superexchange Fe-Se-Fe bonds are shown by dashed lines with J3 and J4 , respectively. (b) The unit cell of the Fe3 Se4 crystal structure of the NiAs-type with cation vacancies. Solid and open circles represent Fe atoms and cation vacancies, respectively. Anions are not shown.
In the compositions close to Fe7 Se8 , these vacancies can be ordered at low temperatures in two ways differ in the repeated distances along the c-direction.5–8 Quenching from high temperatures produces a “3c” hexagonal superstructure (multiplication of the c-axis, doubling of the a-axis) whereas slow cooling gives a “4c” superstructure with a slight triclinic distortion6 (small letters denote the fundamental cell). In the superstructures, the vacancies are positioned in every second cation plane along the c-axis. Different type of superstructures may lead to either AFM or FRM magnetic structures.20 Magnetic measurements show that the easy axis of Fe7 Se8 lies within the basic c-plane near the Neel temperature (TN ) but changes towards the c-axis when decreasing temperature. This spin-rotation process depends on the type of superstructure. For the 3c structure the spin-rotation occurs abruptly at 130 K, while the process begins at about 220 K and shows a gradual shift with decreasing temperature in the case of the 4c structure.7, 8, 21 The neutron diffraction studies of crystallographic and magnetic structure of the Fe3 Se4 compound22, 23 revealed the monoclinic unit cell which can be related to a pseudo NiAs type structure with ordered vacancies (space group I/2m) isomorphous to Cr3 X4 (X = S, Se, Te). The Fe ions occupy two nonequivalent positions. A half of Fe3+ ions are located in the (001) planes containing vacancies (sites I) and remaining iron ions (Fe3+ and Fe2+ ) are statistically distributed in filled planes (sites II). The compound is ferrimagnetic with
TN = 320 K. The spins of Fe ions in I and II sites are antiparallel and directed along the c axis normal to the basic plane. The magnetic moments of Fe3+ and Fe2+ ions proved to be 3.25 and 0.65 μB , respectively, which is considerably less than their theoretic values.22 Traditionally, iron selenides are synthesized by the elemental reaction in evacuated tubes at elevated temperature, or by reaction of aqueous metal salt solutions with toxic and malodorous gas H2 Se. They are also obtained by the method of mechanical alloying. However, these processes were complicated and usually accompanied by impurity phases. Recently, the Fe3 Se4 and Co-doped Fe3 Se4 nanoparticles have been synthesized by the high-temperature organicsolution-phase method.24, 25 The magnetic selenide compound CuCr2 Se4 was obtained by the precipitation method in Ref. 26, and monodisperse nanoparticles CuCr2 Se4 were synthesized by the thermal decomposition in high-temperature organic solvents.27 In this work, we developed a facile method to directly produce the single hexagonal (H) and/or monoclinic (M) phases of iron selenide nanocrystals with the NiAs-like structure via a one-pot thermal decomposition of ferrous chloride and selenide powder in oleylamine. The structural, magnetic, and electronic characteristics were studied by several complimentary methods including the Mössbauer spectroscopy. II. SAMPLE PREPARATION AND METHODS OF CHARACTERIZATION
In this work a series of samples of the iron selenide FeSe nanoparticles was synthesized by the method of thermal decomposition of the source components in high-temperature solutions. The reagent composition, the reaction scheme and some details of the synthesis are given in Table I. In the synthesis of different samples, the ultimate high-temperature processing at 340 ◦ C was varied from 30 to 70 min. The sample compositions were measured by the inductively coupled plasma-mass spectrometer (ICP-MS). The crystal structure and phase purity of the samples were examined by X-ray powder diffraction (XRD, Mutiflex MF2100, Rigaku Co. Ltd.). The XRD analysis showed (Fig. 2) that the observed peak positions and intensities for the sample, which was finally processed at 340 ◦ C for 70 min, can be indexed on the hexagonal 3c crystal structure (H sample). This is a characteristic of the Fe7 Se8 composition. The sample processed at 340 ◦ C for 30 min has a monoclinic unit cell structure (M sample), which corresponds to the Fe6 Se8 composition. Two other samples of this series MH1 and MH2 processed at 340 ◦ C for 60 and 50 min, respectively, consist of
TABLE I. Some details of the synthesis of the iron selenide Fe6-7 Se8 nanoparticles. Fe/Se is the molar ratio of FeCl2 *4H2 O to selenide. Sample name H M MH1 MH2
Expected composition
Reaction scheme
Fe/Se
Crystal structure
Fe7 Se8 Fe6 Se8 Fe6-7 Se8 Fe6-7 Se8
FeCl2 *4H2 O + selenide + oleylamine→ 200 ◦ C, 60 min → 340 ◦ C, 70 min FeCl2 *4H2 O + selenide + oleylamine→ 200 ◦ C, 60 min → 340 ◦ C, 30 min FeCl2 *4H2 O + selenide + oleylamine→ 200 ◦ C, 60 min. → 340 ◦ C, 60 min. FeCl2 *4H2 O + selenide + oleylamine→ 200 ◦ C, 60 min. → 340 ◦ C, 50 min
1/0.9412 1/0.9412 1/0.9412 1/0.9412
Hexagonal Monoclinic Hexagonal +monoclinic Hexagonal +monoclinic
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MPMS, (Quantum Design) in an applied field sweeping from −70 to 70 kOe. For measurements of the zero-field-cooled (ZFC) and field-cooled (FC) temperature dependence of magnetization, the samples were first cooled at zero field from 300 to 78 K then the magnetization was measured in the fields of 100 Oe and 11 kOe by heating the sample from 78 to 300 K. After that the samples were cooled from 300 to 78 K maintaining the same field. Then the FC curves were recorded with increasing temperature. A vibrating sample magnetometer (VSM) was also used to measure the dc magnetization in the temperature range from 78 to 300 K and in the applied field up to 11 kOe. The Mössbauer spectroscopy was applied to examine the phase composition, structural, magnetic, and electronic properties of the nanoparticles. The 57 Fe-Mössbauer spectra were recorded at temperatures between 90 and 300 K in the transmission geometry with a standard spectrometer operating in the constant accelerations regime. The gammaray source 57 Co(Rh) was at room temperature and the isomer shifts were measured relative to metal α-Fe at room temperature. FIG. 2. The X-ray diffraction patterns of the Fe-Se nanoparticles. The indicated reflection indices correspond to the hexagonal Ni-As-type 3c-crystal structure of Fe7 Se8 (H sample) and to the monoclinic structure close to Fe6 Se8 (M sample). The MH1 and MH2 samples consist of a mixture of M and H phases.
a mixture of two hexagonal and monoclinic phases Fe6-7 Se8 (Fig. 2). The mass spectrometry analysis (Fe, Se) revealed that the compositions of the H and M compounds are Fe7.53 Se8.74 (or Fe0.862 Se) and Fe6.75 Se9.22 (or Fe0.732 Se), respectively. This indicates that the Fe:Se concentration is close to the expected 7:8 and 3:4 ratio for H-phase and M-phase. The morphology and microstructure of the particles were characterized by scanning electron microscopy (SEM) and the transmission electron microscopy (TEM, Tecnai G2 F20, FEG-TEM, Philips Co. Ltd.). The SEM and TEM images of the Fe-Se nanoparticles are shown in Figs. 3 and 4. The images in Fig. 4 show the plate shape nanocomposites of hexagonal form with the characteristic size of about 200-400 nm. The measurements of magnetization curves and hysteresis loops were performed at 5, 100, and 300 K using a superconducting quantum interference device SQUID-VSM,
III. RESULTS OF MAGNETIC MEASUREMENTS A. The field dependence of magnetizations
The magnetization curves of all samples measured in an applied magnetic field up to 70 kOe reveal pronounced hysteresis characteristics typical of a ferromagnetic (or ferrimagnetic) behavior (Fig. 5). With increasing temperature from 5 to 300 K, the magnetization area of the hysteresis loop is reduced significantly in all samples. At room temperature magnetization is almost saturated in the field higher 10 kOe, and it remains at the level of about 2–3 emu/g (Fig. 5). However, at low temperatures of 100 and 5 K magnetization is not saturated even in the highest field of 70 kOe. Here it is important to note that, in contrast to our nanoparticles, the bulk monoclinic sample Fe3 Se4 was saturated in weak fields.28 Probably, it can be explained by strong surface anisotropy of the nanoparticles. The hysteresis loops of both the hexagonal (H) Fe7 Se8 and monoclinic (M) Fe3 Se4 nanoparticles reveal unusually high values of coercive force Hc (Fig. 5). The Hc value is the highest in Fe3 Se4 (about 25 kOe), and it is almost constant in the temperature range from 5 to 100 K.
FIG. 3. The scanning electron microscopy images of the monoclinic Fe-Se nanoparticles.
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FIG. 4. The scanning and transmission electron microscopy images of the hexagonal Fe-Se nanoparticles.
FIG. 5. The applied-field dependences of magnetization and hysteresis loops recorded at 5, 100, and 300 K for the Fe-Se samples with hexagonal (H), monoclinic (M), and mixed structures MH1 and MH2. Insets show the zoomed view of hysteresis plots at 300 K.
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Recently, high coercivity Hc was observed in the Fe3 Se4 and Co-doped Fe3 Se4 nanoparticles synthesized by the hightemperature organic-solution-phase method.24, 25 Hard magnetic properties depend on the particle shape. The “giant” Hc values reaching 40 kOe at low temperature were found in the faceted- and cacti-shaped Fe3 Se4 nanoparticles, whereas Hc is somewhat lower in nanosheets.24 This unusually large coercivity was explained by the uniaxial magnetocrystalline anisotropy of the monoclinic Fe3 Se4 with ordered cation vacancies.25
B. The temperature dependence of magnetizations in monoclinic Fe3 Se4 nanoparticles
In the Fe3 Se4 nanoparticles a shift of the hysteresis loops was observed in the magnetization curves measured in the applied field up to 10 kOe. As shown in Fig. 6, the loops are shifted upwards along the magnetization axis without any shifts along the field axis. The loop-shift behavior is temperature dependent. With temperature decreasing from 300 K, the shift starts at about 280 K and continues down to 200 K. Simultaneously, the loop area decreases, essentially below 260 K, and it almost disappears below 200 K. As shown in Fig. 7, temperature behavior of the coercivity HC and remanent magnetization σ R for the Fe3 Se4 sample measured in the field of 11 kOe reveal a maximum just at about 280 K. Such behavior characterizes switching of magnetization in the field of 10 kOe. The switching process strongly depends on temperature, and below 200 K, magnetization is not switched at all. As follows from the behavior of the FC and ZFC magnetization curves (Fig. 7(b)), this effect is related to the appearance of exchange magnetic interaction between nanoparticles below 280 K, leading to the splitting of the FC and ZFC curves in the field of 11 kOe. Strong anisotropy of the exchange interaction does not allow the magnetization to change direction even in the field of 10 kOe. This feature of the Fe3 Se4 nanoparticles can be important for applications. The temperature dependences of magnetization measured for Fe3 Se4 in ZFC and FC modes are shown in Fig. 7. In the low field of 100 Oe, the ZFC and FC curves are split just
FIG. 6. Hysteresis loops measured in the applied field of 10 kOe at different temperatures for Fe-Se nanocrystals with monoclinic structure.
FIG. 7. Temperature dependence of magnetization for ZFC and FC measurements in the applied field of (a) 100 Oe and (b) 11 kOe for Fe-Se nanocrystals with monoclinic structure. (c) The temperature behavior of coercivity HC , remanent magnetization σ R and magnetization in the applied field of 11 kOe. Insets show the zoomed view of anomalies in magnetization at about 180 and 275 K.
below the Neel temperature (TN ) which is the characteristic feature of superparamagnetic and/or spin glass systems.29, 30 In the low applied field, the ZFC and FC curves are strongly different in the value of magnetization σ . In the ZFC regime, σ is very small of about 1 emu/g and almost independent of temperature. It implies that the magnetic structure of the material is close to AFM. In the FC regime, lowtemperature magnetization strongly increases to ∼14 emu/g showing an essential influence of the weak external field 100 Oe on the magnetic structure. It is well known that in the process of magnetization, a weak field at first shifts the boundaries between magnetic domains leading to a single domain magnetic structure. A relatively easy shift of the boundaries implies weak magnetic anisotropy in the inner (core) part of
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the nanoparticles. From these observations we may conclude that the surface magnetic anisotropy of the Fe3 Se4 nanoparticles is essentially larger than the core anisotropy. Similar effect was recently observed and discussed in the magnetite Fe3 O4 nanoparticles.31, 32 As temperature increases, the FC magnetization of Fe3 Se4 remains nearly constant between 78 and 160 K, and then it sharply drops at ∼180 K (Fig. 7(a)). Near this temperature, the ZFC magnetization also shows a weak anomaly (see inset in Fig. 7(a)). This can be related to the spin reorientation effect suggested in Ref. 12. Magnetization disappears at TN ≈ 340 K. It shows that the TN value in our Fe3 Se4 nanoparticles is higher than in the bulk material (320 K) previously reported in Refs. 22 and 23. In the high external field of 11 kOe, the ZFC and FC magnetization curves of Fe3 Se4 coincide above Tint ≈ 280 K (Fig. 7(b)), and this temperature can be related to the appearance of magnetic interactions between nanoparticles at T < Tint which enhances the magnetic anisotropy.4, 33–35 As shown in Fig. 7(c), the temperature variation of the coercivity Hc which is a measure of the magnetic anisotropy,34 reveals a maximum just at ≈ 280 K. C. The temperature dependence of magnetizations in hexagonal Fe7 Se8 nanoparticles
The magnetic behavior of monoclinic and hexagonal phases is very different. The ZFC and FC magnetization curves for the hexagonal Fe7 Se8 (H) sample measured in the low (100 Oe) and high (11 kOe) applied fields are shown in Fig. 8. In the low field, the ZFC and FC curves do not overlap at all temperatures up to 300 K (Fig. 8(a)). It indicates that the nanoparticles are not free but they are strongly coupled by magnetic interactions. Below 130 K, the ZFC curve in Fig. 8(a) shows drastic decrease of magnetization to nearly zero value indicating a transition from FRM to AFM state. As follows from the hysteresis loops, the coercivity Hc value of Fe7 Se8 also drastically changes (increases) at this temperature (Fig. 8(c)). The spin-rotation effect was found in the bulk hexagonal Fe7 Se8 just at 130 K.21 With decreasing temperature, the easy axis changes its direction from the basic c-plane towards the caxis. This abrupt magnetic transition in the narrow temperature region is a characteristic of the 3c type superstructure appearing due to the vacancy ordering,21 whereas gradual transition is a characteristic of the 4c type superstructure.36 From this observation we can conclude that the 3c type superstructure of the vacancy ordering occurs in our hexagonal Fe7 Se8 nanoparticles and this is in good agreement with our XRD data. The decrease of magnetization below 130 K was explained8, 37, 38 by the competition between the in-plain and the normal-to-plain magnetic anisotropies which leads to a canted spin structure. The different temperature dependence of these anisotropies can vary the canting angle, thus changing the total magnetic moment of the material. However, the change in value of magnetization at the FRM–AFM transition (along with changes in spin directions) also implies a redistribution of the cation vacancies in the adjoining iron
FIG. 8. Temperature dependence of magnetization for ZFC and FC measurements in the applied field of (a) 100 Oe and (b) 11 kOe for Fe-Se nanocrystals with hexagonal structure. (c) The temperature behavior of coercivity HC , remanent magnetization σ R and magnetization in the applied field of 11 kOe.
layers. Such a behavior was recently observed in the iron sulfide Fe1-x S nanoparticles with a NiAs-like structure.20, 31, 39 Thus we may suppose, that the drastic change in the low-applied-field ZFC magnetization of the Fe7 Se8 sample at 130 K is related to iron vacancy redistribution (from random to ordered) which leads to the transformation of the magnetic structure from FRM to AFM. This magnetic transition is also expressed as a maximum of the FC magnetization and sharp increase in the coercivity Hc (Fig. 8(c)). On the other hand, the similar magnetic transition was recently observed in the magnetite Fe3 O4 nanoparticles below the Verwey transition temperature TV .31 According to Goodenough,40 such decrease in magnetization may be explained by appearance of the direct covalent Fe-Fe coupling in octahedral sites of magnetite below TV . The spins of 3d electrons are compensated in the covalent bonds, thus decreasing the iron magnetic moment.
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In the high applied field of 11 kOe, the ZFC and FC magnetization of hexagonal Fe7 Se8 nanocrystals coincide in all temperature interval of 77–350 K (Fig. 8(b)). It shows that the applied field is strong enough to line up spins along the field at all temperatures. This implies that magnetic anisotropy of exchange interactions in the hexagonal Fe7 Se8 nanocrystals is not so high as in the monoclinic Fe3 Se4 . The magnetic transition observed in the low applied field at about 130 K (Fig. 8(a)) is shifted by the high applied field to lower temperature of about 118 K. IV. MÖSSBAUER SPECTROSCOPY CHARACTERIZATION OF NANOPARTICLES A. Mossbauer data for monoclinic Fe3 Se4 nanoparticles
All nanoparticles were investigated by the 57 Fe- Mössbauer spectroscopy at temperatures between 90 and 300 K. The representative spectra of four Fe-Se samples recorded at 90 and 300 K are shown in Fig. 9. The temperature evolution of the spectra between 90 and 300 K is represented in Fig. 10 for the hexagonal H and the monoclinic M samples. At 90 K, the Mössbauer spectra of the monoclinic (M) sample indicate that the main part of iron ions is in the magnetically ordered state (Fig. 11(a)). The low-intensive doublet in the spectrum center (about 7% of total iron content in the sample) has the isomer shift δ = 0.48 mm/s and quadrupole splitting = 0.60 mm/s which indicate the presence of Fe3+ ions in the paramagnetic state. Obviously, this paramagnetic
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component appears due to the particle size distribution, and it belongs to very small particles (d < 10 nm) whose spin blocking temperature is below 90 K. The magnetic spectrum of Fe3 Se4 (Fig. 11(a)) can be fit to four components corresponding to four nonequivalent iron sites with different values of magnetic hyperfine fields Hhf and isomer shifts δ. For two of these magnetic components, the hyperfine parameters are: δ = 0.49 and 0.30 mm/s and Hhf = 46.8 and 43.8 T, respectively, and these parameters are characteristic of the Fe3+ ions. The occupation of these nonequivalent Fe3+ iron sites is in the ratio 37/14. Two other magnetic components have the same isomer shift δ = 0.74 mm/s and different field values Hhf = 22.6 and 10.6 T. These parameters are characteristic of the Fe2+ ions, and the occupation of these nonequivalent Fe2+ iron sites is in the ratio 16/26. These Mössbauer data indicate that a part of magnetic iron ions is in the ferric Fe3+ state (about 50.5% of total iron content in the sample) and the other part is in the ferrous state close to Fe2+ (42.5%). With due regard for the paramagnetic component, the ratio of Fe3+ /Fe2+ for the iron ions in the sample is 57.5/42.5 (at 90 K). From this value, the condition of electro-neutrality of the Fe1-x Se molecule leads to the Se/Fe ratio of 1.29. This is very close to the ratio Se/Fe = 4/3 ≈ 1.33 valid for the Fe3 Se4 composition. Thus, the Mössbauer parameters reveal that the monoclinic Fe1-x Se phase of the obtained nanoparticles has the dominant composition Fe3 Se4 with very small excess of ferrous ions, apparently, from the hexagonal FeSe phase. This correlates very well with our XRay data.
FIG. 9. Mössbauer spectra of the hexagonal H, monoclinic M, and mixture-phase samples MH1 and MH2 of iron selenides at 90 and 300 K.
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FIG. 10. The temperature evolution of Mössbauer spectra of the hexagonal H and monoclinic M iron selenide samples between 90 and 300 K.
The ferric iron states were also found in the selenium doped chalcopyrite CuFeS1.6 Se0.4 by Mössbauer measurements with the field Hhf value of about 37 T.41 However, the value Hhf of about 32 T was found in the iron sulfide Fe3 S4 with the spinel structure.42 As follows from the X-ray and neutron diffraction data,5, 22, 23 the Fe ions in Fe3 Se4 occupy two nonequiva-
FIG. 11. Computer processing (the fit to four components by UNIVEM program) of the Mössbauer spectra of monoclinic M (a) and hexagonal H (b) samples of iron selenides recorded at 90 K.
lent sites located in the alternating planes containing vacancies (sites I) and in planes without vacancies (sites II). Our Mössbauer data indicate that Fe3+ and Fe2+ ions in Fe3 Se4 nanoparticles occupy both the sites I and II. It seems that the Fe3+ and Fe2+ ions with higher value of the magnetic fields Hhf = 46.8 and 22.6 T, respectively, should be attributed to the planes containing vacancies. For these sites all exchange interaction bonds Fe-Se-Fe with neighbors from the nearest layers (above and below the given layer) are preserved when the vacancies appear (see Fig. 1(b)) whereas for Fe ions in planes without vacancies some exchange bonds are broken Fe-Se.20 Thus, from the Mössbauer data we can conclude that about 53% of iron (37% Fe3+ and 16% Fe2+ ) are in planes with vacancies and about 40% (14% Fe3+ and 26% Fe2+ ) are in the planes without vacancies. The temperature dependences of magnetic fields Hhf in four nonequivalent iron sites are shown in (Fig. 12). It is interesting that the Hhf values in Fe3+ ions drop at 160–180 K, whereas this anomaly is not observed for the Fe2+ ions. Our magnetic measurements revealed a decrease in the FCmagnetization just in the same temperature region (Fig. 7(a)), which is related to the spin reorientation effect. It can be supposed that the spin reorientation is connected with vacancy redistribution which influences the magnetic state of Fe ions. As follows from the Mössbauer data (the Hhf (T) behavior), the ferric ions are more sensitive to the vacancies redistribution than the ferrous ions. This implies that more vacancies appear near the Fe3+ ions, which stimulates the magnetic transition with rotation of the Fe3+ magnetic moments.
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FIG. 12. Temperature dependences of magnetic hyperfine fields Hhf in nonequivalent iron sites of the hexagonal (H) and monoclinic (M) Fe-Se nanocomposites.
B. Mossbauer data for hexagonal Fe-Se nanoparticles
At 90 K, the Mössbauer spectrum of the hexagonal Fe7 Se8 (H)-sample (Fig. 11(b)) indicates that a part of iron ions is in the magnetically ordered state (about 60% of total iron content in the sample), and the other part is in the paramagnetic state (about 37%). In addition, the small-intensive (about 3%) magnetic component with the highest value of magnetic hyperfine field Hhf = 51.9 T was observed (green lines in Fig. 11(b)). Obviously, this component is typical of iron oxide (Fe2 O3 ) which appears from the iron oxidation at the particle surface. The main magnetic spectrum of Fe-Se (H) sample can be fit to the two components with the magnetic fields Hhf = 47.0 (35%) and 44.2 (24%) T. The isomer shifts δ of both magnetic components are close and equal to 0.48 and 0.45 mm/s, respectively. At 300 K the δ values reduce to about 0.37 mm/s. These δ values are typical of ferric Fe3+ ions. This implies that the hexagonal Fe-Se sample was transformed into the composition close to Fe6 Se9 . The nonmagnetic doublet component, observed in the (H) sample at 90 K, has the δ values of 0.45 mm/s, which
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is also a characteristic of the ferric Fe3+ ions in the paramagnetic state. The quadrupole splitting of the doublet is = 0.91 mm/s. The Mössbauer spectra lines are broaden in both magnetic and paramagnetic components, which can be connected with distribution of the particle sizes. It seems that the paramagnetic doublet component in the 90 K spectrum originates from very small particles whose spin blocking temperature TB is below 90 K. In our study, the Mössbauer measurements of the hexagonal (H) sample were performed in several months after the sample had been prepared. It seems that for this time the samples were degraded into the composition close to Fe6 Se9 since the observed Mössbauer parameters are typical of ferric Fe3+ ions. In case of the NiAs-type structure, such a material should have high amount of the cation vacancies. Two magnetic components observed in the Mössbauer spectra indicate that the two types of nonequivalent iron sites with different number of cation vacancies dominate in the transformed FeSe (H) sample. We noted that the magnetic fields Hhf and isomer shifts δ values in the transformed Fe-Se sample (H) are very close to the values for ferric Fe3+ ions in the monoclinic Fe3 Se4 , as it was discussed above. On the other hand, if one suggests that ferric Fe3+ ions in (H) appears because of full oxidation of the material, then α-Fe2 O3 and/or γ -Fe2 O3 type oxides can be expected. However, the field Hhf value in these oxides must be of about 51–52 T31 which is much higher than the values observed in our Fe-Se (H) sample. Moreover, the Neel temperature of iron oxides Fe2 O3 is about 900 K42 which is much higher than the value 450 K obtained for (H) in our magnetic measurements. Thus, the appearance of iron oxide phases in the transformed Fe-Se sample (H) should be excluded. Usually the Hhf fields values in chalcogenides are lower than in oxides due to higher covalence in the cation-anion bonds.43 It seems that the field values Hhf = 47.0 and 44.2 T found in (H) are consistent with the ferric iron state. The ferrous state of all cations was found in bulk Fe7 Se8 with the Hhf fields values in the range of 31–22 T.36 Under the similar conditions, the Hhf values in ferrous ions must be lower than in ferric ions due to the lower spin value of Fe2+ and the orbital contribution to the field.20, 44 With increasing temperature, the magnetic spectra components became asymmetric with broadening of inner parts of the resonance lines (Fig. 10). This is a signature of superparamagnetic behavior typical of nanoparticles. The area of the paramagnetic doublet gradually increases with temperature at the expense of the magnetic components areas. This indicates a transition of small-size particles from the magnetically ordered state to the paramagnetic state due to fast spin relaxation. From the temperature dependence of an average value of magnetic field Hhf (Fig. 12) we can conclude that the main part of the particles (about 80%) transforms to the paramagnetic state at about 290-300 K. In the superparamagnetic process, it can be considered as the blocking or Neel temperature of the particles with the largest size. Meanwhile, in the Mössbauer spectrum at 300 K about 20% particles are still in the magnetically ordered (or superparamagnetic) state with Hhf = 12 T.
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V. SUMMARY AND CONCLUSION
In this work a facile method was developed to produce the iron selenide nanoparticles with the NiAs-like crystal structure by thermal decomposition of iron chloride and selenium powder in the high-temperature organic solvent. Depending on the time of the compound processing at 340 ◦ C in oleylamine, the nanocrystals with monoclinic or hexagonal structures as well as a mixture of these two phases can be obtained for about one hour. The magnetic behavior of monoclinic Fe3 Se4 and hexagonal Fe7 Se8 phases is very different. Besides the Neel temperature at 340 and 450 K in Fe3 Se4 and Fe7 Se8 samples, respectively, the magnetic transitions were found at 180 and 130 K related to the spin rotation due to the vacancies redistribution. The hysteresis loops in both Fe7 Se8 and Fe3 Se4 nanocrystals indicate the unusually high coercive force Hc which value is highest (“giant”) in monoclinic Fe3 Se4 (of about 25 kOe). This can be explained by the surface magnetic anisotropy of the Fe3 Se4 nanoparticles which is essentially larger than the core anisotropy. Such large coercivity is rare for materials without rare earth or noble metal elements, and Fe3 Se4 -based compounds can be low-cost, nontoxic alternative materials for advanced magnets.24, 25 Below 280 K, an exchange magnetic interaction appears between Fe3 Se4 nanoparticles, which enhances the magnetic anisotropy. Strong anisotropy of this interaction does not allow the magnetization to change the direction even in the field of 10 kOe. Such properties of a “switching effect” in the Fe3 Se4 nanoparticles can be important for applications. The applied-field and temperature dependences of magnetization show a complicated transformation between FRM and AFM structures which is related to the spin rotation process connected with the redistribution of cation vacancies. The 3c type superstructure of the vacancy ordering was found in the hexagonal Fe7 Se8 . Below 130 K, the redistribution of vacancies from random to ordered leads to the spin-rotation and transformation of the magnetic structure from FRM to AFM. This magnetic transition is also manifested by a maximum of the FC magnetization and sharp increase in the coercivity Hc . In monoclinic Fe3 Se4 , the Mössbauer spectra reveal several nonequivalent positions of iron ions appearing due to the presence of cationic vacancies in the nearest environment of Fe. The fraction of Fe3+ and Fe2+ ions in the planes with vacancies and in the planes without vacancies was estimated. The Mössbauer data indicate that vacancies prefer to appear near the Fe3+ ions and stimulate the magnetic transition with the rotation of the Fe3+ magnetic moments. In general, the magnetic properties of nanoparticle samples are not only intrinsic to the particles but also depend on the conditions in which the particles are, such as the exact chemical composition, homogeneity of the crystal structure, shape, aggregation, crystallinity etc. In turn, these conditions are defined by the particle processing method. In particular, it is known that the shape and size of nanoparticles as well as the magnetic properties strongly depend on the amount and the kind of surfactants used in the synthesis. For example, in Refs. 24 and 25, the authors used two kinds of surfactants, the
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oleylamine and oleic acid, as the reaction solvent to prepare the Fe3 Se4 nanosheets and nanoplatelets. However, in our experiment we used only oleylamine to prepare Fe3 Se4 nanoparticles. That is one of the reasons why the results of different researchers may not be entirely the same. ACKNOWLEDGMENTS
Support by the Russian Scientific Foundation (Project No. 14-12-00848) is acknowledged. C.-R.L.,Y.-J.S., and M.L.C. appreciate the support of the Ministry of Science and Technology of Taiwan (Grant Nos. 100-2923-M-153-001MY3 and 102-2112-M-153-002-MY3). 1 X.
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