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Subho Dasgupta,* Bijoy Das, Michael Knapp, Richard. A. Brand, Helmut Ehrenberg, Robert Kruk, and Horst Hahn Magnetoelectric coupling refers to the change of magnetic state (i.e., control of magnetism) in a material by the application of an electric field. So far, however, magnetoelectric effects in artificially structured composites have only been limited to extremely small material volumes, adjacent to the surfaces or interfaces.[1–8] To go beyond the interfacial effects, we propose an extension of the definition of magnetoelectric phenomena to include intercalation-driven electrochemical approaches which can also be pertinent to bulk materials. The concept is demonstrated for ferromagnetic iron oxide by a judicious control over the reversible lithium chemistry. The insertion of lithium ions results in a valence change and partial redistribution of the Fe3+ cations in the spinel structure, thus yielding a large (up to 30%) and fully reversible change in magnetization at room temperature. Considering the availability of a large number of intercalation-friendly magnetic materials, ample opportunities for increasing the effect size towards a complete on-and-off magnetic switching can be foreseen, thereby paving ways for applications involving micro-magnetic actuation. The control of magnetic properties in composite nanostructures by means of an electric field, known as magnetoelectric coupling, is presently among the most thriving research areas.[1–8] While magnetoelectric effects have widely been reported at low temperatures, recent results, put in the application context,
Dr. S. Dasgupta, Dr. B. Das, Dr. R. A. Brand, Dr. R. Kruk, Prof. H. Hahn Institute for Nanotechnology Karlsruhe Institute of Technology (KIT) D-76344, Eggenstein-Leopoldshafen, Germany E-mail: [email protected] Dr. B. Das, Dr. M. Knapp, Prof. H. Ehrenberg, Prof. H. Hahn Helmholtz Institute Ulm for Electrochemical Energy Storage (HIU) Albert-Einstein Allee 11, D-89081, Ulm, Germany Dr. M. Knapp, Prof. H. Ehrenberg Institute for Applied Materials (IAM) Karlsruhe Institute of Technology (KIT) D-76344, Eggenstein Leopoldshafen, Germany Dr. R. A. Brand Department of Physics Universität Duisburg-Essen Duisburg Campus, Duisburg, Germany Prof. H. Hahn KIT-TUD Joint Research Laboratory Nanomaterials Technische Universität Darmstadt (TUD) Institute of Materials Science Jovanka-Bontschits-Str. 32 D-64287, Darmstadt, Germany
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Intercalation-Driven Reversible Control of Magnetism in Bulk Ferromagnets
show that the control over magnetic properties (e.g., magnetocrystalline anisotropy, KU; Curie temperature, TC) is also possible at room temperature.[4–8] Therefore, the burst of research activities in this area is not only curiosity-driven but also motivated by the potential for a large number of technological applications, for example, spin-based, remarkably energy-efficient electronics. However, beyond the prospective super-fast spintronics, there may be other application areas, such as on-chip micro-magnetic actuation or micro-mechanics, where the sheer magnitude of the magnetic response becomes a decisive factor. The very nature of the magnetoelectric coupling mechanisms, either elastic or purely electronic, limits the change in magnetization to only few monolayers adjacent to the surfaces or interfaces. Therefore an extension of the controllable magnetic volume beyond the surface atoms poses a significant scientific challenge that may require a completely new approach. Searching for ways to maximize the magnetic response of a material to an electrical stimulus, one can obtain radical variation of magnetism in a bulk material by an electrically-driven ion insertion process. The apparent difficulty arises from the fact that such changes in chemical states are usually either not at all or only partially reversible. However, encouragement can be sought from advanced electrochemical systems, where reversible ion intercalation process (e.g., reversible lithium chemistry) has been demonstrated for thousands of charge-discharge cycles.[9–11] Therefore, in order to display the feasibility of a chemistry-controlled, yet fully reversible, change in magnetic response of a material, even in the bulk form, we have thoroughly investigated the lithium intercalation and de-intercalation processes in iron oxide spinel. The large and reproducible switching between distinctly different magnetic states have been achieved which can be related to the change in the valence state of the iron ions. We show that by a judicious selection of the discharge potential, one can simultaneously insert a surprisingly large amount of Li (up to 1 mole per formula unit) into the iron ferrite lattice while maintaining the spinel crystal structure intact; the associated highly reversible variation in the magnetization comes as a direct consequence of this nondestructive, electrochemical process. The importance of this critical control over the electrochemical process becomes clearer by referring to the pertinent literatures. Without a careful control over the degree of lithiation, typically irreversible transformation to non-magnetic rock-salt type crystal structure has been reported, where upon de-lithiation the original spinel structure could not be recovered.[12,13] The selected ferromagnetic material for this study was the archetypical defective spinel iron oxide, γ-Fe2O3, i.e., maghemite. To facilitate Li ion migration, maghemite was prepared
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in the form of nanoparticles (davg ∼ 21 nm), following a wet chemical precipitation technique.[14] Maghemite crystallizes in a simple cubic (inverse spinel) structure. In this inverse spinel, Fe ions occupy some of the interstitial sites between the oxygen ions which assume a close-packed, fcc-type, cubic structure. In maghemite, Fe is found in two different nearest-neighbors surroundings: on the A-site (Wyckoff 8a-site) neighbored by an oxygen tetrahedron and on the B-site (Wyckoff 16d-site) in the center of an oxygen octahedron. The chemical formula for maghemite is Fe2O3, however, the crystallographic non-primitive unit cell is represented as (Fe3+)8[Fe3+5/6 + 䊐1/6]16O32. Thus, eight Fe ions occupy the Wyckoff 8a-site; while the remaining nominal 40/3 Fe ions occupy the Wyckoff 16d-site, leaving 8/3 vacancies also on the Wyckoff 16d-site. A consideration of the structural and electronic aspects of maghemite brings attention to the following points: (1) an ideal maghemite contains only trivalent Fe3+ ions; (2) the Fe3+ ions on the B-site are antiferromagnetically coupled to those on the A-site; and (3) the net ferromagnetic moment results from the higher number of Fe3+
ions on the B-site. Evidently, the other interstitial sites in the fcc oxygen lattice of γ-Fe2O3 are left unoccupied and potentially available for Li insertion. The magnetic electrodes (cathode) were prepared by thoroughly mixing γ-Fe2O3 nanopowders (davg = 21 nm) with nanometer-size graphite (davg ∼ 10 nm) particles and polyvinylidene fluoride (PVDF) as binder; 1 M LiPF6 dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) in 1:1 volume ratio (Merck), was used as the electrolyte, while pure Li metal served as the anode. It had been previously observed that lithium insertion into the spinel lattice renders the lithiated spinel oxide unstable in air;[12] hence, the electrochemical cells were assembled inside an Ar-filled glove box (O2, H2O < 0.5 ppm) where they were carefully sealed for the subsequent structural (X-ray diffraction, XRD), spectroscopic (Mössbauer) and in-situ magnetic measurements. At the beginning, the most subtle and critical experimental step was to combine cyclic voltammetry (CV) (Figure 1a) with XRD (Figure 1b) to determine the suitable discharge potential
Figure 1. (a) Cyclovoltammogram of nanocrystalline maghemite vs. Li+/Li counter electrode, a pronounced lithium intercalation (around 1.6 V) and the spinel structure destruction peak (around 1.0 V) are marked with green and red circles, respectively, the blue dotted line shows the discharge potential limit (1.1 V vs. Li+/Li) used for the reversible chemistry-controlled magnetization switching. (b) Changes in the characteristic (311) Bragg reflection of maghemite, as obtained from high resolution XRD, while operating within the safe potential range (green zone in the CV) that avoids structural collapse. (c) Schematic presentation of a section of the as-prepared maghemite unit cell, i.e., the magnetic electrode at position I in the CV, a vacant octahedral (16d) site can be seen at the right hand side. (d) Illustration of the same section of crystal lattice after lithiation down to the discharge potential of 1.1 V, i.e., position II in the CV. While, the crystal structure remains intact, new Li atoms can be found to occupy tetrahedral 8a-site, octahedral 16d-site and previously vacant octahedral 16c-site, respectively. (e) A complete conversion of the spinel lattice to α-LiFeO2 type rock-salt structure upon lithiation down to discharge potential of 0.9 V, i.e., position III in the CV.
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ingly, upon pulsing the electrode potential directly between the lithiated and delithiated states, we have obtained very similar magnetic states at the two terminal potentials (Figure S4), as has been observed during the in-situ continuous sweep measurements (Figure 2a). From the combination of the structural, electronic and magnetic response to the nondestructive lithium insertion, a consistent physical picture of the whole process can be gleaned. First of all, every Li ion intercalated into the γ-Fe2O3 lattice leads to the reduction of one Fe3+ ion to the Fe2+ state (Figure 2c). The overall electrochemical reactions occurring at the respective electrodes during the intercalation/de-intercalation processes are as follows: at cathode : γ − Fe 2O3 + xLi + + xe − ↔ Li x Fe 2O3 ; at anode : Li ↔ (1 − x )Li + xLi + + xe −
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limit that would allow maximum lithium insertion but avoid any structural damage to the magnetic electrode. The CV measurement identifies the electrochemical processes occurring at the different electrode potentials. At the open circuit potential (OCV) of ∼3.5 V, the crystal structure of the magnetic electrode is analogous to as-prepared maghemite, as shown in Figure 1c. Upon discharging (lithiation), first a pronounced lithium intercalation peak appears at around 1.6 V against Li+/Li (marked with green circle), followed by the first peak indicative of structural destruction, as verified later by XRD, around 1 V (marked with red circle). The large current peak, at the end, at around 0.6 V denotes a complete collapse of the oxide lattice, resulting in the formation of metallic iron particles. Powder XRD performed on the sample, for which the discharge process was interrupted just before (at 1.1 V vs. Li+/Li) the first sign of a severe structural change at 1 V, shows a well-preserved spinel structure (Figure 1d, Figure S1c). In contrast, the XRD pattern recorded on samples that are lithiated beyond this critical potential (down to 0.9 V vs. Li+/Li) confirms that a nearly complete conversion to α-LiFeO2-type, rock-salt crystal structure has taken place with a shift of all the previously tetrahedrally coordinated (8a-site) Fe atoms to the 16c-site (Figure 1e, Figure S1d). Figure 1B compares the position of the characteristic Bragg reflection (311) of the spinel oxide (at around 2θ = 16.15°) for the as-prepared γ-Fe2O3, and at different stages of lithiation (discharge) and delithiation (charge) processes. Interestingly, the position of the characteristic diffraction peak remains nearly constant for the as-prepared sample, the sample discharged down to 1.7 V, and for the sample charged back to 3.5 V, respectively, while only a nominal shift in the peak position is noted for the sample discharged to 1.1 V. The details of all the structural parameters are summarized in Table S1. Therefore, during the in-situ magnetic measurements, in order to retain the spinel structure, the discharge (Li insertion) and charge (Li extraction) processes were always reversed at the cut-off potential of 1.1 V and 3.5 V, respectively (marked as the green zone, in Figure 1a). Consequently, a highly reversible variation in magnetic response accompanies the electrochemically-driven intercalation process (Figure 2a). Similarly, reversibility in terms of intercalation/de-intercalation redox couple (redox peaks) has also been observed when CV measurements are conducted within this defined potential window (1.1 – 3.5 V vs. Li/Li+) (Figure S2). In contrast, discharging down to 0.9 V has resulted in a complete transition to a rocksalt type crystal structure (Figure S1d), thereby producing a nearly paramagnetic state and in reference to both the initial magnetic state and the spinel structure, they cannot be recovered upon charging (delithiation) (Figure S3). The lithiation dependent magnetization variation was measured with a potentiometric constant current (115 mA/g, unless otherwise specified) discharge and charge processes for many consecutive Li insertion and extraction cycles (as shown in Figure 2a), the magnetic response was simultaneously recorded. Noticeably, the discharge-charge cycles and the observed magnetization response were found fully reversible and can always be correlated to the electrode potential (Figure 2a,b). In fact, after completion of each charging cycle at an upper cut-off potential of 3.5 V, the measured magnetization matches exactly with the magnetic state of the as-prepared (delithiated) sample. Interest-
(1)
(when, x mole of Li ions are considered to be intercalated or de-intercalated). However, for an overall understanding, one also needs to correlate, at every critical lithiation state, the total magnetization with the charge states of the Fe ions (here, it also means Fe spin states) and their distribution over the unit cell. To simplify the situation, we consider the starting state of the magnetic electrode as an ideal maghemite. The rationale behind this assumption stems from all the experimental results: the XRD pattern of the as-prepared sample shows typical maghemite inverse spinel crystal structure (Figure S1); the saturation magnetization (MS) is close to the ideal value of maghemite (Figure S5); and Mössbauer spectroscopy (MS) indicates 97% ions being at the Fe3+ valence state with a negligible 3% having Fe2+ state (Figure 3a, Table 1). The lithium intercalation-driven magnetization response data show two clearly discernible regimes. At the initial stage of lithium intercalation, when the discharge potential is decreased from the OCV to 1.7 V, the magnetization MS rises from ∼67 Am2/kg to ∼74 Am2/kg. This is followed by a monotonic decrease in MS to a value of ∼54 Am2/kg at the 1.1 V terminal discharge potential (Figure 2a,b). As mentioned previously, a quantitative understanding would require knowledge of the charge states and distribution of the Fe ions. Firstly, the XRD analysis reveals a nominal decrease in the tetrahedral 8a-site iron occupancy with a corresponding increase at the octahedral 16d-site within the initial lithiation period (down to 1.7 V) (refer to Figure S1; Table S1). At this stage, the inserted Li+ reduces a small fraction of tetrahedral (8a) iron ions from Fe3+ to Fe2+ state, and then the larger Fe2+ ions jump to the neighboring octahedral (16d) site that offers more space than the tetrahedral position. Thus, the net ferromagnetism increases as the occupancy of the B sublattice grows at the expense of the A sublattice. The chemical reduction of the Fe3+ to Fe2+ at the octahedral (B) site may also have immediately started upon lithiation; however, clearly until 1.7 V the decrease of the tetrahedral site Fe occupation dominates the resulting magnetization, which increases by only a few percent. The tendency of further reduction of Fe3+ to Fe2+ at the tetrahedral (A) site and migration to the octahedral (B) site may also have continued down to the discharge potential of 1.1 V (by comparing Figure S1b with Figure S1c and from Table S1). Nevertheless, upon further lithiation below the discharge potential
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Figure 2. (a) Fully-reversible variation in magnetic response (measured with an applied magnetic field of 1 Tesla) of the spinel iron oxide (γ-Fe2O3) nanoparticles with respect to the lithiation (discharging) and delithiation (charging) cycles. The discharge and charge cycles are carried out with a potentiometric constant current of 115 mA/g and cut-off potentials are set at 1.1 V and 3.5 V vs. Li+/Li, during lithiation and delithiation steps, respectively. (b) Magnetic hysteresis measured at different stages of lithiation; blue, green and red symbols stand for the as-prepared maghemite sample and the samples that are discharged down to 1.7 V and down to 1.1 V, respectively. After reaching the desired discharge potential, the electrochemical cell has been electrically disconnected prior to the hysteresis measurements. (c) Schematic of the typical electrochemical cell with the primary cathodic and anodic reactions; the respective electrodes are physically isolated by a polymer separator membrane, the electrode at the left (black, cathode) is the magnetic electrode, coated with maghemite nanoparticles, while the electrode at the right (shinny, anode) hand side is pure Li metal.
of 1.7 V, the chemical reduction of a large fraction of the octahedral Fe3+ to Fe2+ controls the overall magnetic response. The chemical reduction of a quite large number of the octahedral Fe, from the high-spin Fe3+ (5 μB) to the high-spin Fe2+ (4 μB) state, during this period, overwhelms any Fe3+ reduction at the tetrahedral site and causes a significant decrease in the
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net magnetic moment (Figure 2a,b, Figure 3b,c). Overall, the majority of the intercalated Li (refer to the large intercalation peak at 1.6 V, Figure 1a) reduces the octahedral magnetic component which, being the stronger counterpart, diminishes the resulting magnetization (Figure 3c). Such a complete understanding (involving rearrangements of magnetic cations) of
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Figure 3. Mössbauer spectra (measured with external field, Hext = 5 T, applied parallel to the sample) of (a) the as-prepared and (b) the lithiated (discharged down to 1.1. V vs. Li+/Li) maghemite samples, respectively; the black spheres correspond to the experimental data points and the dark blue lines represent the fitted data; the red, the green and the light blue lines correspond to the tetrahedral Fe3+, octahedral Fe3+ and octahedral Fe2+ components, respectively. (c) Schematic presentation showing the rationale behind the observed intercalation-driven large reduction in magnetization in the spinel iron oxide. Inserted lithium donates one electron to the octahedral iron ions resulting in a chemical reduction from Fe3+ to Fe2+ states and thereby also causing a decrease in the octahedral spin states (green); considering the tetrahedral component to remain unaltered (red), this ensures a decrease in the resultant magnetic moment, ΔM = Mmeasured (blue).
the lithiation-dependent variation in magnetization can only be inferred from a rigorous analysis of the Mössbauer data (Figure 3a,b). Using their spectral fingerprint in MS, the distribution of iron ions of different valence states (Fe2+, Fe3+) at various interstitial sites (tetrahedral and octahedral) can be clearly differentiated. The external magnetic field applied parallel to
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Area (%)
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Table 1. Summary of the fitted Mössbauer data (shown in Figure 3) of the as-prepared and lithiated (1.1 V with respect to Li+/Li) iron spinel sample measured at T = 4.2 K, with an external field of Hext = 5 T, applied parallel to the sample. Bhf, δis, Γ represent hyperfine field, isomer shift and quadruple splitting of the components, respectively.
the sample splits the Mössbauer spectra of the as-prepared iron spinel into two main components; outer sextet (colored in red, in Figure 3a) associates with the tetrahedral 8a-site Fe and the inner one (colored in green, in Figure 3a) with the octahedral 16d-site. The small admixture of 3% Fe2+ present in the as-prepared sample grows to reach a value as high as 33% of the total number of Fe ions after the lithiation process to the discharge potential of 1.1 V (Figure 3b, Table 1). This value corresponds to ∼0.9 mole lithium per formula unit of maghemite, which is comparable to the amount (∼0.94 Li per formula unit) obtained from the electrochemical data (i.e., reversible electrochemical capacity in Figure 2a). In this work, the concept of dynamic and fully reversible control over bulk magnetism via electrochemical processes is proposed, tested, and interpreted. Broadly speaking, the whole concept rests on the nondestructive intercalation and de-intercalation of mobile ions in a magnetic material, resulting in a significant change in its magnetic state. In principle, the concept can be applied to any intercalation-friendly magnetic material with a sufficient concentration of intercalation sites. In the future, the most suitable material to study would be the one that displays a magnetic phase transition near room temperature, so that a complete ferro-to-paramagnetic transition could be induced. Moreover, in order to accelerate the kinetics of the process, it would be highly desirable to obtain such magnetic transitions with the least amount of intercalated ions (Li or other). Additionally, the selection of the magnetic material and its microstructure (grain size, texture, additives, etc.) to enhance Li (or other) ion diffusivity would be of interest to gain faster magnetic switching. As an example of potential device, intercalation-driven electromagnetic actuators (e.g., in microfluidics[15–19] or micro- (macro) robotics,[20–22] may replace the standard micro-electromagnets as an easy-to-fabricate and energy-efficient alternative, not requiring a continuous power supply. On a more speculative level, one may aim to realize arrays of such switchable magnets by using roll-to-roll printing technologies.
Experimental Section Synthesis of Spinel Iron Oxide Nanoparticles: Single-phase spinel iron oxide nanoparticles were synthesized at relatively low temperature (90 °C), by a simple wet-chemical precipitation method. For the synthesis of nanoparticles, 0.2 M FeCl3 and 0.2 M FeSO4.7H2O were dissolved
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www.MaterialsViews.com in deionized water separately and stirred at 90 °C for 30 min. The two solutions were then mixed together in a beaker under continuous stirring and then ammonium hydroxide was added to the mixture, drop wise, in order to precipitate iron ferrite. The crystallite size of γ-Fe2O3 nanoparticles was controlled by adjusting the pH of the solution. Finally, the precipitate was washed with copious amount of deionized water and dried at 80 °C. Electrode Preparation and Cell Fabrication: The electrodes for electrochemical studies were prepared by mixing the active material (γ-Fe2O3), graphite (electronically conducting additive; davg. ∼10 nm) and PVDF binder in the weight ratio of 60:30:10. N-methyl-pyrrolidinone (NMP) was used as the solvent for the PVDF binder to prepare the slurry. The doctor-blade technique was used to coat the slurry onto an etched Al-foil (20 micron thick, Hohsen Co. Ltd., Japan) to form a layer of approx. 50 micron thickness. It was then cut into small square size pieces of about 0.25 cm2 for in-situ lithiation dependent magnetization measurement in PPMS. Prior to cell assembly, the coated electrodes were again dried at 80 °C for 10–20 minutes. A Celgard separator was used to physically separate the electrodes. A thin cylindrical glass tube was used to enclose the cell and also to hold the electrolyte. The cell was aged for 2–3 hours prior to the start of measurements in order to ensure complete percolation of electrolyte into the electrode material. Characterization: (a) Structural: X-ray powder diffraction measurements were carried out for the as-prepared iron ferrite nanopowders and for the powders collected at different lithiation/delithiation stages. The as-prepared and lithiated/ delithaited spinel oxide powders were filled into 0.5 mm diameter quartz capillaries and sealed with vacuum grease inside an Ar glove box. The capillaries were then taken out and further sealed by gas lighter. A STOE STADI P diffractometer equipped with a Dectris Mythen 1 K linear silicon strip detector and Ge (111) double crystal monochromator (with Mo-Kα1 radiation; λ = 0.70926 Å) was used in Debye-Scherrer mode (0.5 mm Ø capillaries). Data analysis was performed by Rietveld refinements using the software package FULLPROF. (b) Spectroscopic: For the Mössbauer measurements, the nanopowders, either as-prepared or collected from the electrochemical cells following respective discharge (lithiation) process, were distributed homogeneously inside the specially designed plexiglass sample holders to form a thin and uniform layer. The plexiglass sample holders were then sealed with the high vacuum epoxy glue to ensure complete airtightness; the whole process was carried out inside an Ar glove box. The spectra were collected with the standard spectrometer configured in the transmission geometry. To circumvent any problems with the data interpretation, often caused by the spin-dynamic relaxation processes observed at room temperature, the measurements were done in a helium-cooled cryostat at 4.2 K. Magnetic Measurements: All the magnetic measurements were carried out at room temperature (300 K) using a Quantum Design physical property measurement system (PPMS) magnetometer. The magnetic measurements were performed in-situ simultaneously with the electrochemical discharging/ charging processes. Prior to the magnetic hysteresis measurements, the cell was electrically disconnected following the respective electrochemical process.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
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Acknowledgements The authors would like to thank Dr. Tessy Baby for the synthesis of the maghemite nanoparticles. Help from Dipl.-Ing. U. von Hörsten, Universität Duisburg-Essen in performing the Mössbauer experiments is gratefully acknowledged. RK and HH also acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG) under contract HA1344/28–1. Received: December 2, 2013 Revised: January 23, 2014 Published online: March 3, 2014 [1] H. J. A. Molegraaf, J. Hoffman, C. A. F. Vaz, S. Garigilo, D. van der Marel, C. H. Ahn, J.-M. Triscone, Adv. Mater. 2009, 21, 3470. [2] C. A. F. Vaz, J. Hoffman, C. H. Ahn, R. Ramesh, Adv. Mater. 2010, 22, 2900. [3] F. Xiu, Y. Wang, J. Kim, A. Hong, J. Tang, A. P. Jacob, J. Zou, K. L. Wang, Nature Mater. 2010, 9, 337. [4] M. Weisheit, S. Fähler, A. Marty, Y. Souche, C. Poinsignon, D. Givord, Science 2007, 315, 349. [5] T. Maruyama, Y. Shiota, T. Nozaki, K. Ohta, N. Toda, M. Mizuguchi, A. A. Tulapurkar, T. Shinjo, M. Shiraishi, S. Mizukami, Y. Ando, Y. Suzuki, Nature Nanotechnol. 2009, 4, 158. [6] F. Xiu, Y. Wang, J. Kim, P. Upadhyaya, Y. Zhou, X. Kou, W. Han, R. K. Kawakami, J. Zou, K. L. Wang, ACS Nano 2010, 4, 4948. [7] Y. Yamada, K. Ueno, T. Fukumura, H. T. Yuan, H. Shimotani, Y. Iwasa, L. Gu, S. Tsukimoto, Y. Ikuhara, M. Kawasaki, Science 2011, 332, 1065. [8] D. Chiba, S. Fukami, K. Shimamura, N. Ishiwata, K. Kobayashi, T. Ono, Nature Mater. 2011, 10, 853. [9] H. Huang, S.-C. Yin, L. F. Nazarz, Electrochim. Sol. State Lett. 2001, 4, A170. [10] G.-N. Zhu, H.-J. Liu, J.-H. Zhuang, C.-X. Wang, Y.-G. Wang, Y.-Y. Xia, Energy Environ. Sci. 2011, 4, 4016. [11] C. Sun, S. Rajasekhara, J. B. Goodenough, F. Zhou, J Am. Chem. Soc. 2011, 133, 2132. [12] J. Fontcuberta, J. Rodriguez, M. Pernet, G. Longworth, J. B. Goodenough, J. Appl. Phys. 1986, 59, 1918. [13] S. Komaba, T. Mikumo, N. Yabuuchi, A. Ogata, H. Yoshida, Y. Yamada, J. Electrochem. Soc. 2010, 157, A60. [14] Y. S. Kang, S. Risbud, J. F. Rabolt, P. Stroeve, Chem. Mater. 1996, 8, 2209. [15] N. Pamme, Lab Chip 2006, 6, 24. [16] J. R. Basore, L. A. Baker, Anal. Bioanal. Chem. 2012, 403, 2077. [17] S. Dubus, Anal. Chem. 2006, 78, 4457. [18] J. P. Desai, A. Pillarisetti, A. D. Brooks, Annu. Rev. Biomed. Eng. 2007, 9, 35. [19] J. Castillo, M. Dimaki, W. E. Svendsen, Integr. Biol. 2009, 1, 30. [20] Q. Ramadan, C. Yu, V. Samper, D. P. Poenar, Appl. Phys. Lett. 2006, 88, 032501. [21] C. S. Lee, H. Lee, R. M. Westervelt, Appl. Phys. Lett. 2001, 79, 3308. [22] L. F. Zanini, N. M. Dempsey, D. Givord, G. Reyne, F. Dumas-Bouchiat, Appl. Phys. Lett. 2011, 99, 232504.
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Subho Dasgupta,* Bijoy Das, Michael Knapp, Richard. A. Brand, Helmut Ehrenberg, Robert Kruk, and Horst Hahn Magnetoelectric coupling refers to the change of magnetic state (i.e., control of magnetism) in a material by the application of an electric field. So far, however, magnetoelectric effects in artificially structured composites have only been limited to extremely small material volumes, adjacent to the surfaces or interfaces.[1–8] To go beyond the interfacial effects, we propose an extension of the definition of magnetoelectric phenomena to include intercalation-driven electrochemical approaches which can also be pertinent to bulk materials. The concept is demonstrated for ferromagnetic iron oxide by a judicious control over the reversible lithium chemistry. The insertion of lithium ions results in a valence change and partial redistribution of the Fe3+ cations in the spinel structure, thus yielding a large (up to 30%) and fully reversible change in magnetization at room temperature. Considering the availability of a large number of intercalation-friendly magnetic materials, ample opportunities for increasing the effect size towards a complete on-and-off magnetic switching can be foreseen, thereby paving ways for applications involving micro-magnetic actuation. The control of magnetic properties in composite nanostructures by means of an electric field, known as magnetoelectric coupling, is presently among the most thriving research areas.[1–8] While magnetoelectric effects have widely been reported at low temperatures, recent results, put in the application context,
Dr. S. Dasgupta, Dr. B. Das, Dr. R. A. Brand, Dr. R. Kruk, Prof. H. Hahn Institute for Nanotechnology Karlsruhe Institute of Technology (KIT) D-76344, Eggenstein-Leopoldshafen, Germany E-mail: [email protected] Dr. B. Das, Dr. M. Knapp, Prof. H. Ehrenberg, Prof. H. Hahn Helmholtz Institute Ulm for Electrochemical Energy Storage (HIU) Albert-Einstein Allee 11, D-89081, Ulm, Germany Dr. M. Knapp, Prof. H. Ehrenberg Institute for Applied Materials (IAM) Karlsruhe Institute of Technology (KIT) D-76344, Eggenstein Leopoldshafen, Germany Dr. R. A. Brand Department of Physics Universität Duisburg-Essen Duisburg Campus, Duisburg, Germany Prof. H. Hahn KIT-TUD Joint Research Laboratory Nanomaterials Technische Universität Darmstadt (TUD) Institute of Materials Science Jovanka-Bontschits-Str. 32 D-64287, Darmstadt, Germany
DOI: 10.1002/adma.201305932
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Intercalation-Driven Reversible Control of Magnetism in Bulk Ferromagnets
show that the control over magnetic properties (e.g., magnetocrystalline anisotropy, KU; Curie temperature, TC) is also possible at room temperature.[4–8] Therefore, the burst of research activities in this area is not only curiosity-driven but also motivated by the potential for a large number of technological applications, for example, spin-based, remarkably energy-efficient electronics. However, beyond the prospective super-fast spintronics, there may be other application areas, such as on-chip micro-magnetic actuation or micro-mechanics, where the sheer magnitude of the magnetic response becomes a decisive factor. The very nature of the magnetoelectric coupling mechanisms, either elastic or purely electronic, limits the change in magnetization to only few monolayers adjacent to the surfaces or interfaces. Therefore an extension of the controllable magnetic volume beyond the surface atoms poses a significant scientific challenge that may require a completely new approach. Searching for ways to maximize the magnetic response of a material to an electrical stimulus, one can obtain radical variation of magnetism in a bulk material by an electrically-driven ion insertion process. The apparent difficulty arises from the fact that such changes in chemical states are usually either not at all or only partially reversible. However, encouragement can be sought from advanced electrochemical systems, where reversible ion intercalation process (e.g., reversible lithium chemistry) has been demonstrated for thousands of charge-discharge cycles.[9–11] Therefore, in order to display the feasibility of a chemistry-controlled, yet fully reversible, change in magnetic response of a material, even in the bulk form, we have thoroughly investigated the lithium intercalation and de-intercalation processes in iron oxide spinel. The large and reproducible switching between distinctly different magnetic states have been achieved which can be related to the change in the valence state of the iron ions. We show that by a judicious selection of the discharge potential, one can simultaneously insert a surprisingly large amount of Li (up to 1 mole per formula unit) into the iron ferrite lattice while maintaining the spinel crystal structure intact; the associated highly reversible variation in the magnetization comes as a direct consequence of this nondestructive, electrochemical process. The importance of this critical control over the electrochemical process becomes clearer by referring to the pertinent literatures. Without a careful control over the degree of lithiation, typically irreversible transformation to non-magnetic rock-salt type crystal structure has been reported, where upon de-lithiation the original spinel structure could not be recovered.[12,13] The selected ferromagnetic material for this study was the archetypical defective spinel iron oxide, γ-Fe2O3, i.e., maghemite. To facilitate Li ion migration, maghemite was prepared
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in the form of nanoparticles (davg ∼ 21 nm), following a wet chemical precipitation technique.[14] Maghemite crystallizes in a simple cubic (inverse spinel) structure. In this inverse spinel, Fe ions occupy some of the interstitial sites between the oxygen ions which assume a close-packed, fcc-type, cubic structure. In maghemite, Fe is found in two different nearest-neighbors surroundings: on the A-site (Wyckoff 8a-site) neighbored by an oxygen tetrahedron and on the B-site (Wyckoff 16d-site) in the center of an oxygen octahedron. The chemical formula for maghemite is Fe2O3, however, the crystallographic non-primitive unit cell is represented as (Fe3+)8[Fe3+5/6 + 䊐1/6]16O32. Thus, eight Fe ions occupy the Wyckoff 8a-site; while the remaining nominal 40/3 Fe ions occupy the Wyckoff 16d-site, leaving 8/3 vacancies also on the Wyckoff 16d-site. A consideration of the structural and electronic aspects of maghemite brings attention to the following points: (1) an ideal maghemite contains only trivalent Fe3+ ions; (2) the Fe3+ ions on the B-site are antiferromagnetically coupled to those on the A-site; and (3) the net ferromagnetic moment results from the higher number of Fe3+
ions on the B-site. Evidently, the other interstitial sites in the fcc oxygen lattice of γ-Fe2O3 are left unoccupied and potentially available for Li insertion. The magnetic electrodes (cathode) were prepared by thoroughly mixing γ-Fe2O3 nanopowders (davg = 21 nm) with nanometer-size graphite (davg ∼ 10 nm) particles and polyvinylidene fluoride (PVDF) as binder; 1 M LiPF6 dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) in 1:1 volume ratio (Merck), was used as the electrolyte, while pure Li metal served as the anode. It had been previously observed that lithium insertion into the spinel lattice renders the lithiated spinel oxide unstable in air;[12] hence, the electrochemical cells were assembled inside an Ar-filled glove box (O2, H2O < 0.5 ppm) where they were carefully sealed for the subsequent structural (X-ray diffraction, XRD), spectroscopic (Mössbauer) and in-situ magnetic measurements. At the beginning, the most subtle and critical experimental step was to combine cyclic voltammetry (CV) (Figure 1a) with XRD (Figure 1b) to determine the suitable discharge potential
Figure 1. (a) Cyclovoltammogram of nanocrystalline maghemite vs. Li+/Li counter electrode, a pronounced lithium intercalation (around 1.6 V) and the spinel structure destruction peak (around 1.0 V) are marked with green and red circles, respectively, the blue dotted line shows the discharge potential limit (1.1 V vs. Li+/Li) used for the reversible chemistry-controlled magnetization switching. (b) Changes in the characteristic (311) Bragg reflection of maghemite, as obtained from high resolution XRD, while operating within the safe potential range (green zone in the CV) that avoids structural collapse. (c) Schematic presentation of a section of the as-prepared maghemite unit cell, i.e., the magnetic electrode at position I in the CV, a vacant octahedral (16d) site can be seen at the right hand side. (d) Illustration of the same section of crystal lattice after lithiation down to the discharge potential of 1.1 V, i.e., position II in the CV. While, the crystal structure remains intact, new Li atoms can be found to occupy tetrahedral 8a-site, octahedral 16d-site and previously vacant octahedral 16c-site, respectively. (e) A complete conversion of the spinel lattice to α-LiFeO2 type rock-salt structure upon lithiation down to discharge potential of 0.9 V, i.e., position III in the CV.
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Adv. Mater. 2014, 26, 4639–4644
ingly, upon pulsing the electrode potential directly between the lithiated and delithiated states, we have obtained very similar magnetic states at the two terminal potentials (Figure S4), as has been observed during the in-situ continuous sweep measurements (Figure 2a). From the combination of the structural, electronic and magnetic response to the nondestructive lithium insertion, a consistent physical picture of the whole process can be gleaned. First of all, every Li ion intercalated into the γ-Fe2O3 lattice leads to the reduction of one Fe3+ ion to the Fe2+ state (Figure 2c). The overall electrochemical reactions occurring at the respective electrodes during the intercalation/de-intercalation processes are as follows: at cathode : γ − Fe 2O3 + xLi + + xe − ↔ Li x Fe 2O3 ; at anode : Li ↔ (1 − x )Li + xLi + + xe −
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limit that would allow maximum lithium insertion but avoid any structural damage to the magnetic electrode. The CV measurement identifies the electrochemical processes occurring at the different electrode potentials. At the open circuit potential (OCV) of ∼3.5 V, the crystal structure of the magnetic electrode is analogous to as-prepared maghemite, as shown in Figure 1c. Upon discharging (lithiation), first a pronounced lithium intercalation peak appears at around 1.6 V against Li+/Li (marked with green circle), followed by the first peak indicative of structural destruction, as verified later by XRD, around 1 V (marked with red circle). The large current peak, at the end, at around 0.6 V denotes a complete collapse of the oxide lattice, resulting in the formation of metallic iron particles. Powder XRD performed on the sample, for which the discharge process was interrupted just before (at 1.1 V vs. Li+/Li) the first sign of a severe structural change at 1 V, shows a well-preserved spinel structure (Figure 1d, Figure S1c). In contrast, the XRD pattern recorded on samples that are lithiated beyond this critical potential (down to 0.9 V vs. Li+/Li) confirms that a nearly complete conversion to α-LiFeO2-type, rock-salt crystal structure has taken place with a shift of all the previously tetrahedrally coordinated (8a-site) Fe atoms to the 16c-site (Figure 1e, Figure S1d). Figure 1B compares the position of the characteristic Bragg reflection (311) of the spinel oxide (at around 2θ = 16.15°) for the as-prepared γ-Fe2O3, and at different stages of lithiation (discharge) and delithiation (charge) processes. Interestingly, the position of the characteristic diffraction peak remains nearly constant for the as-prepared sample, the sample discharged down to 1.7 V, and for the sample charged back to 3.5 V, respectively, while only a nominal shift in the peak position is noted for the sample discharged to 1.1 V. The details of all the structural parameters are summarized in Table S1. Therefore, during the in-situ magnetic measurements, in order to retain the spinel structure, the discharge (Li insertion) and charge (Li extraction) processes were always reversed at the cut-off potential of 1.1 V and 3.5 V, respectively (marked as the green zone, in Figure 1a). Consequently, a highly reversible variation in magnetic response accompanies the electrochemically-driven intercalation process (Figure 2a). Similarly, reversibility in terms of intercalation/de-intercalation redox couple (redox peaks) has also been observed when CV measurements are conducted within this defined potential window (1.1 – 3.5 V vs. Li/Li+) (Figure S2). In contrast, discharging down to 0.9 V has resulted in a complete transition to a rocksalt type crystal structure (Figure S1d), thereby producing a nearly paramagnetic state and in reference to both the initial magnetic state and the spinel structure, they cannot be recovered upon charging (delithiation) (Figure S3). The lithiation dependent magnetization variation was measured with a potentiometric constant current (115 mA/g, unless otherwise specified) discharge and charge processes for many consecutive Li insertion and extraction cycles (as shown in Figure 2a), the magnetic response was simultaneously recorded. Noticeably, the discharge-charge cycles and the observed magnetization response were found fully reversible and can always be correlated to the electrode potential (Figure 2a,b). In fact, after completion of each charging cycle at an upper cut-off potential of 3.5 V, the measured magnetization matches exactly with the magnetic state of the as-prepared (delithiated) sample. Interest-
(1)
(when, x mole of Li ions are considered to be intercalated or de-intercalated). However, for an overall understanding, one also needs to correlate, at every critical lithiation state, the total magnetization with the charge states of the Fe ions (here, it also means Fe spin states) and their distribution over the unit cell. To simplify the situation, we consider the starting state of the magnetic electrode as an ideal maghemite. The rationale behind this assumption stems from all the experimental results: the XRD pattern of the as-prepared sample shows typical maghemite inverse spinel crystal structure (Figure S1); the saturation magnetization (MS) is close to the ideal value of maghemite (Figure S5); and Mössbauer spectroscopy (MS) indicates 97% ions being at the Fe3+ valence state with a negligible 3% having Fe2+ state (Figure 3a, Table 1). The lithium intercalation-driven magnetization response data show two clearly discernible regimes. At the initial stage of lithium intercalation, when the discharge potential is decreased from the OCV to 1.7 V, the magnetization MS rises from ∼67 Am2/kg to ∼74 Am2/kg. This is followed by a monotonic decrease in MS to a value of ∼54 Am2/kg at the 1.1 V terminal discharge potential (Figure 2a,b). As mentioned previously, a quantitative understanding would require knowledge of the charge states and distribution of the Fe ions. Firstly, the XRD analysis reveals a nominal decrease in the tetrahedral 8a-site iron occupancy with a corresponding increase at the octahedral 16d-site within the initial lithiation period (down to 1.7 V) (refer to Figure S1; Table S1). At this stage, the inserted Li+ reduces a small fraction of tetrahedral (8a) iron ions from Fe3+ to Fe2+ state, and then the larger Fe2+ ions jump to the neighboring octahedral (16d) site that offers more space than the tetrahedral position. Thus, the net ferromagnetism increases as the occupancy of the B sublattice grows at the expense of the A sublattice. The chemical reduction of the Fe3+ to Fe2+ at the octahedral (B) site may also have immediately started upon lithiation; however, clearly until 1.7 V the decrease of the tetrahedral site Fe occupation dominates the resulting magnetization, which increases by only a few percent. The tendency of further reduction of Fe3+ to Fe2+ at the tetrahedral (A) site and migration to the octahedral (B) site may also have continued down to the discharge potential of 1.1 V (by comparing Figure S1b with Figure S1c and from Table S1). Nevertheless, upon further lithiation below the discharge potential
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Figure 2. (a) Fully-reversible variation in magnetic response (measured with an applied magnetic field of 1 Tesla) of the spinel iron oxide (γ-Fe2O3) nanoparticles with respect to the lithiation (discharging) and delithiation (charging) cycles. The discharge and charge cycles are carried out with a potentiometric constant current of 115 mA/g and cut-off potentials are set at 1.1 V and 3.5 V vs. Li+/Li, during lithiation and delithiation steps, respectively. (b) Magnetic hysteresis measured at different stages of lithiation; blue, green and red symbols stand for the as-prepared maghemite sample and the samples that are discharged down to 1.7 V and down to 1.1 V, respectively. After reaching the desired discharge potential, the electrochemical cell has been electrically disconnected prior to the hysteresis measurements. (c) Schematic of the typical electrochemical cell with the primary cathodic and anodic reactions; the respective electrodes are physically isolated by a polymer separator membrane, the electrode at the left (black, cathode) is the magnetic electrode, coated with maghemite nanoparticles, while the electrode at the right (shinny, anode) hand side is pure Li metal.
of 1.7 V, the chemical reduction of a large fraction of the octahedral Fe3+ to Fe2+ controls the overall magnetic response. The chemical reduction of a quite large number of the octahedral Fe, from the high-spin Fe3+ (5 μB) to the high-spin Fe2+ (4 μB) state, during this period, overwhelms any Fe3+ reduction at the tetrahedral site and causes a significant decrease in the
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net magnetic moment (Figure 2a,b, Figure 3b,c). Overall, the majority of the intercalated Li (refer to the large intercalation peak at 1.6 V, Figure 1a) reduces the octahedral magnetic component which, being the stronger counterpart, diminishes the resulting magnetization (Figure 3c). Such a complete understanding (involving rearrangements of magnetic cations) of
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As-prepared
Lithiated
Bhf (T)
δis (mm/s)
Γ (mm/s)
Fe3+ (T)
56.8
0.25
0.43
35
Fe3+ (O)
48.6
0.40
0.54
62
Fe2+ (O)
38
1.16
1.1
3
Fe3+ (T)
56.2
0.26
0.33
29
Fe3+ (O)
48.4
0.46
0.60
38
43.0
0.90
1.2
33
2+
Fe (O)
Figure 3. Mössbauer spectra (measured with external field, Hext = 5 T, applied parallel to the sample) of (a) the as-prepared and (b) the lithiated (discharged down to 1.1. V vs. Li+/Li) maghemite samples, respectively; the black spheres correspond to the experimental data points and the dark blue lines represent the fitted data; the red, the green and the light blue lines correspond to the tetrahedral Fe3+, octahedral Fe3+ and octahedral Fe2+ components, respectively. (c) Schematic presentation showing the rationale behind the observed intercalation-driven large reduction in magnetization in the spinel iron oxide. Inserted lithium donates one electron to the octahedral iron ions resulting in a chemical reduction from Fe3+ to Fe2+ states and thereby also causing a decrease in the octahedral spin states (green); considering the tetrahedral component to remain unaltered (red), this ensures a decrease in the resultant magnetic moment, ΔM = Mmeasured (blue).
the lithiation-dependent variation in magnetization can only be inferred from a rigorous analysis of the Mössbauer data (Figure 3a,b). Using their spectral fingerprint in MS, the distribution of iron ions of different valence states (Fe2+, Fe3+) at various interstitial sites (tetrahedral and octahedral) can be clearly differentiated. The external magnetic field applied parallel to
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Area (%)
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Table 1. Summary of the fitted Mössbauer data (shown in Figure 3) of the as-prepared and lithiated (1.1 V with respect to Li+/Li) iron spinel sample measured at T = 4.2 K, with an external field of Hext = 5 T, applied parallel to the sample. Bhf, δis, Γ represent hyperfine field, isomer shift and quadruple splitting of the components, respectively.
the sample splits the Mössbauer spectra of the as-prepared iron spinel into two main components; outer sextet (colored in red, in Figure 3a) associates with the tetrahedral 8a-site Fe and the inner one (colored in green, in Figure 3a) with the octahedral 16d-site. The small admixture of 3% Fe2+ present in the as-prepared sample grows to reach a value as high as 33% of the total number of Fe ions after the lithiation process to the discharge potential of 1.1 V (Figure 3b, Table 1). This value corresponds to ∼0.9 mole lithium per formula unit of maghemite, which is comparable to the amount (∼0.94 Li per formula unit) obtained from the electrochemical data (i.e., reversible electrochemical capacity in Figure 2a). In this work, the concept of dynamic and fully reversible control over bulk magnetism via electrochemical processes is proposed, tested, and interpreted. Broadly speaking, the whole concept rests on the nondestructive intercalation and de-intercalation of mobile ions in a magnetic material, resulting in a significant change in its magnetic state. In principle, the concept can be applied to any intercalation-friendly magnetic material with a sufficient concentration of intercalation sites. In the future, the most suitable material to study would be the one that displays a magnetic phase transition near room temperature, so that a complete ferro-to-paramagnetic transition could be induced. Moreover, in order to accelerate the kinetics of the process, it would be highly desirable to obtain such magnetic transitions with the least amount of intercalated ions (Li or other). Additionally, the selection of the magnetic material and its microstructure (grain size, texture, additives, etc.) to enhance Li (or other) ion diffusivity would be of interest to gain faster magnetic switching. As an example of potential device, intercalation-driven electromagnetic actuators (e.g., in microfluidics[15–19] or micro- (macro) robotics,[20–22] may replace the standard micro-electromagnets as an easy-to-fabricate and energy-efficient alternative, not requiring a continuous power supply. On a more speculative level, one may aim to realize arrays of such switchable magnets by using roll-to-roll printing technologies.
Experimental Section Synthesis of Spinel Iron Oxide Nanoparticles: Single-phase spinel iron oxide nanoparticles were synthesized at relatively low temperature (90 °C), by a simple wet-chemical precipitation method. For the synthesis of nanoparticles, 0.2 M FeCl3 and 0.2 M FeSO4.7H2O were dissolved
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www.MaterialsViews.com in deionized water separately and stirred at 90 °C for 30 min. The two solutions were then mixed together in a beaker under continuous stirring and then ammonium hydroxide was added to the mixture, drop wise, in order to precipitate iron ferrite. The crystallite size of γ-Fe2O3 nanoparticles was controlled by adjusting the pH of the solution. Finally, the precipitate was washed with copious amount of deionized water and dried at 80 °C. Electrode Preparation and Cell Fabrication: The electrodes for electrochemical studies were prepared by mixing the active material (γ-Fe2O3), graphite (electronically conducting additive; davg. ∼10 nm) and PVDF binder in the weight ratio of 60:30:10. N-methyl-pyrrolidinone (NMP) was used as the solvent for the PVDF binder to prepare the slurry. The doctor-blade technique was used to coat the slurry onto an etched Al-foil (20 micron thick, Hohsen Co. Ltd., Japan) to form a layer of approx. 50 micron thickness. It was then cut into small square size pieces of about 0.25 cm2 for in-situ lithiation dependent magnetization measurement in PPMS. Prior to cell assembly, the coated electrodes were again dried at 80 °C for 10–20 minutes. A Celgard separator was used to physically separate the electrodes. A thin cylindrical glass tube was used to enclose the cell and also to hold the electrolyte. The cell was aged for 2–3 hours prior to the start of measurements in order to ensure complete percolation of electrolyte into the electrode material. Characterization: (a) Structural: X-ray powder diffraction measurements were carried out for the as-prepared iron ferrite nanopowders and for the powders collected at different lithiation/delithiation stages. The as-prepared and lithiated/ delithaited spinel oxide powders were filled into 0.5 mm diameter quartz capillaries and sealed with vacuum grease inside an Ar glove box. The capillaries were then taken out and further sealed by gas lighter. A STOE STADI P diffractometer equipped with a Dectris Mythen 1 K linear silicon strip detector and Ge (111) double crystal monochromator (with Mo-Kα1 radiation; λ = 0.70926 Å) was used in Debye-Scherrer mode (0.5 mm Ø capillaries). Data analysis was performed by Rietveld refinements using the software package FULLPROF. (b) Spectroscopic: For the Mössbauer measurements, the nanopowders, either as-prepared or collected from the electrochemical cells following respective discharge (lithiation) process, were distributed homogeneously inside the specially designed plexiglass sample holders to form a thin and uniform layer. The plexiglass sample holders were then sealed with the high vacuum epoxy glue to ensure complete airtightness; the whole process was carried out inside an Ar glove box. The spectra were collected with the standard spectrometer configured in the transmission geometry. To circumvent any problems with the data interpretation, often caused by the spin-dynamic relaxation processes observed at room temperature, the measurements were done in a helium-cooled cryostat at 4.2 K. Magnetic Measurements: All the magnetic measurements were carried out at room temperature (300 K) using a Quantum Design physical property measurement system (PPMS) magnetometer. The magnetic measurements were performed in-situ simultaneously with the electrochemical discharging/ charging processes. Prior to the magnetic hysteresis measurements, the cell was electrically disconnected following the respective electrochemical process.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
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Acknowledgements The authors would like to thank Dr. Tessy Baby for the synthesis of the maghemite nanoparticles. Help from Dipl.-Ing. U. von Hörsten, Universität Duisburg-Essen in performing the Mössbauer experiments is gratefully acknowledged. RK and HH also acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG) under contract HA1344/28–1. Received: December 2, 2013 Revised: January 23, 2014 Published online: March 3, 2014 [1] H. J. A. Molegraaf, J. Hoffman, C. A. F. Vaz, S. Garigilo, D. van der Marel, C. H. Ahn, J.-M. Triscone, Adv. Mater. 2009, 21, 3470. [2] C. A. F. Vaz, J. Hoffman, C. H. Ahn, R. Ramesh, Adv. Mater. 2010, 22, 2900. [3] F. Xiu, Y. Wang, J. Kim, A. Hong, J. Tang, A. P. Jacob, J. Zou, K. L. Wang, Nature Mater. 2010, 9, 337. [4] M. Weisheit, S. Fähler, A. Marty, Y. Souche, C. Poinsignon, D. Givord, Science 2007, 315, 349. [5] T. Maruyama, Y. Shiota, T. Nozaki, K. Ohta, N. Toda, M. Mizuguchi, A. A. Tulapurkar, T. Shinjo, M. Shiraishi, S. Mizukami, Y. Ando, Y. Suzuki, Nature Nanotechnol. 2009, 4, 158. [6] F. Xiu, Y. Wang, J. Kim, P. Upadhyaya, Y. Zhou, X. Kou, W. Han, R. K. Kawakami, J. Zou, K. L. Wang, ACS Nano 2010, 4, 4948. [7] Y. Yamada, K. Ueno, T. Fukumura, H. T. Yuan, H. Shimotani, Y. Iwasa, L. Gu, S. Tsukimoto, Y. Ikuhara, M. Kawasaki, Science 2011, 332, 1065. [8] D. Chiba, S. Fukami, K. Shimamura, N. Ishiwata, K. Kobayashi, T. Ono, Nature Mater. 2011, 10, 853. [9] H. Huang, S.-C. Yin, L. F. Nazarz, Electrochim. Sol. State Lett. 2001, 4, A170. [10] G.-N. Zhu, H.-J. Liu, J.-H. Zhuang, C.-X. Wang, Y.-G. Wang, Y.-Y. Xia, Energy Environ. Sci. 2011, 4, 4016. [11] C. Sun, S. Rajasekhara, J. B. Goodenough, F. Zhou, J Am. Chem. Soc. 2011, 133, 2132. [12] J. Fontcuberta, J. Rodriguez, M. Pernet, G. Longworth, J. B. Goodenough, J. Appl. Phys. 1986, 59, 1918. [13] S. Komaba, T. Mikumo, N. Yabuuchi, A. Ogata, H. Yoshida, Y. Yamada, J. Electrochem. Soc. 2010, 157, A60. [14] Y. S. Kang, S. Risbud, J. F. Rabolt, P. Stroeve, Chem. Mater. 1996, 8, 2209. [15] N. Pamme, Lab Chip 2006, 6, 24. [16] J. R. Basore, L. A. Baker, Anal. Bioanal. Chem. 2012, 403, 2077. [17] S. Dubus, Anal. Chem. 2006, 78, 4457. [18] J. P. Desai, A. Pillarisetti, A. D. Brooks, Annu. Rev. Biomed. Eng. 2007, 9, 35. [19] J. Castillo, M. Dimaki, W. E. Svendsen, Integr. Biol. 2009, 1, 30. [20] Q. Ramadan, C. Yu, V. Samper, D. P. Poenar, Appl. Phys. Lett. 2006, 88, 032501. [21] C. S. Lee, H. Lee, R. M. Westervelt, Appl. Phys. Lett. 2001, 79, 3308. [22] L. F. Zanini, N. M. Dempsey, D. Givord, G. Reyne, F. Dumas-Bouchiat, Appl. Phys. Lett. 2011, 99, 232504.
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