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Characterization of Monodisperse Wu¨stite Nanoparticles following Partial Oxidation Chih-Jung Chen,†,‡ Ray-Kuang Chiang,†,* Hsin-Yi Lai,‡ and Chun-Rong Lin§ Nanomaterials Laboratory, Department of Materials Science and Engineering, Far East UniVersity, Hsing-Shih, Tainan County 74448, Taiwan, Department of Mechanical Engineering, National Cheng Kung UniVersity, Tainan 701, Taiwan, and Department of Mechanical Engineering, Southern Taiwan UniVersity of Technology, Tainan County 710, Taiwan, Republic of China ReceiVed: August 24, 2009; ReVised Manuscript ReceiVed: February 6, 2010
Monodisperse Fe1-xO nanoparticles (NPs) with a mean size of 21.7 ( 2.1 nm were prepared by the thermal decomposition of iron(III) oleate complex at 380 °C using oleic acid as the solvent. Variation in their composition was monitored using XRD for a period of 120 days under ambient conditions, under which the dominant phase changed from wu¨stite to a spinel-type iron oxide phase. HR-TEM images and absorption spectra of the 10-day sample further revealed an FeO/spinel-type phase core-shell structure. Exchange-bias coupling on the interfaces between the wu¨stite and the spinel-type phase accompanied the variation in composition. The dependence of HE on temperature demonstrates that the HE onset temperature is approximately 200 K, which correlates with the TN of bulk FeO. 1. Introduction Monodisperse magnetic iron oxide nanoparticles (NPs) of Fe1-xO, R-Fe2O3, γ-Fe2O3, and Fe3O4, with quantifiable sizeand shape-dependent properties, have been intensively studied owing to their various potential applications, including in the separation of pollutants, as magnetic resonance imaging (MRI) agents, and as magnetic storage media, among others.1 Recently, much attention has been paid to wu¨stite Fe1-xO (x ) 0.05-0.17) (hereinafter referred to as FeO) NPs, because of not only their complex defect structure and defect-related physical properties but also their interesting phase transformation to multiphase nanostructures2 with modulated magnetic properties.3 For example, FeO/Fe3O4 core-shell nanostructures with ferromagnetic (FM)/ferrimagnetic (FIM) interfaces may entail interesting exchange anisotropy. Exchange bias was first discovered in Co/ CoO core-shell NPs by Meiklejohn and Bean in 1956.4 Since then exchange bias has been widely studied in nanostructures such as thin films and core-shell NPs and practically applied in areas such as read-head and sensor technologies.5 Recently the chance of “beating the superparamagnetic limit by exchange bias” has been exemplified in Co NPs embedded in a antiferromagnetic CoO matrix by Skumryev and co-workers.6 These have made exchange bias an appealing research topic. FeO is an antiferromagnet with a Ne´el temperature (TN) of approximately 200 K, below which the spins are antiparallel. However, bulk FeO is only stable above 560 °C and remains a metastable phase under ambient conditions when rapidly quenched from temperatures at which it is stable; this metastable phase can undergo a disproportionation reaction to Fe and Fe3O4 or oxidation to other magnetic phases, such as magnetite or maghemite.7 Traditionally, FeO NPs are prepared by the highenergy milling of a mixture of Fe2O3 + Fe or Fe3O4 + Fe powders at high temperature (>570 °C) under or not under high pressure.8 However, the product has certain limitations: it is * To whom correspondence should be addressed. E-mail: rkc.chem@ msa.hinet.net. † Far East University. ‡ National Cheng Kung University. § Southern Taiwan University of Technology.
irregularly shaped and exhibits polydispersity. However, colloidal FeO NPs are typically prepared by the thermal decomposition of soluble precursors in organic solvents at high temperature in the presence of capping agents. This approach can commonly yield NPs with controllable size, shape, and uniformity. Numerous precursors, such as Fe(CO)5,9 Fe(OAc)2, Fe(acac)2,10 Fe(acac)3,11 and iron oleate complex12 have been adopted in the synthesis of FeO NPs in various organic solvents, such as oleylamine, trioctylamine, 1-octadecene, and eicosane, among others. New approaches for preparing water-based FeO NPs have been developed, involving the low-temperature hydrolysis of the organometallic precursor {Fe[N(SiMe3)2]2}13 or the pulse laser ablation of pure iron in liquid media.14 The cited investigations have raised important issues such as the survival of the obtained metastable FeO NPs, and the evolution of the composition under ambient conditions. No quantitative work has yet addressed time-evolved FeO NP products under ambient conditions, even though time can have uncertain effects on the intrinsic properties of FeO NPs, including their magnetic, electric, and catalytic properties.3,15 In this study, monodisperse FeO NPs with a size of approximately 22 nm were initially synthesized and then adopted to study the time evolution of composition, structure, and magnetic properties under ambient conditions. An FeO/Fe3O4 core-shell nanostructure was observed in a 10-day sample, and its interesting exchange-bias properties were studied based on the core-shell model. 2. Experimental Section Chemicals. Goethite (R-FeO(OH), 99.9%, Strem) and oleic acid (OA, 90%, Showa) were used as received without further purification. Synthesis of Iron(III) Oleate from Goethite Powder. The synthesis of iron oleate by the dissolution of goethite in oleic acids has been demonstrated in our earlier works.16 Briefly, a mixture of goethite (FeO(OH), 3 mmol) and oleic acid (OA, 13.5 mmol) was loaded into a three-necked round-bottom flask, and then heated to 290 °C (∼15 °C/min) under flowing argon. The transformation was complete after 4 h, according to X-ray
10.1021/jp908153y 2010 American Chemical Society Published on Web 02/19/2010
Monodisperse Wu¨stite NPs after Partial Oxidation powder diffraction (XRD) and IR spectra. The brown and greaselike product, iron(III) oleate, was obtained with free OA. This product can be directly used as a precursor in the synthesis of iron oxide nanoparticles. Synthesis of Monodisperse FeO Nanoparticles. The greaselike precursor (which contained 3 mmol of iron(III) oleate and 4.5 mmol of free OA) was loaded with an additional 10 mL of OA in a three-necked round-bottom flask and heated to 365 °C for 30 min and then to 380 °C (∼15 °C/min) for 1.5 h under flowing nitrogen. The temperature was controlled using a heating mantel and a thermocouple. During the reaction, the color of the reactant mixture changed from brown to black, suggesting the formation of NPs. The resulting products were diluted with hexane and acetone and then centrifuged at 5000 rpm. The precipitate was washed two or three times in a mixture of acetone and ethanol. The precipitated NPs were redispersed in hexane. The phase of the NPs was observed by XRD and transmission electron microscopy (TEM). The TEM sample was prepared by dropping the hexane suspension onto a copper grid (200 mesh) that was coated with a carbon film. The size of each NP was determined by averaging the lengths of the major and minor axes, which were measured using a Sigmascan Pro 5. The standard deviation of the size of the NPs was calculated from more than 500 particles in the TEM micrographs. Characterization. The phases of the products were characterized by X-ray powder diffraction (Shimadzu XRD-6000) using Cu KR radiation. A transmission electron microscope (JEOL JEM 1200EX) with an accelerating voltage of 80 kV and a high-resolution transmission electron microscope (Philips Tecnai G2 F20 or JEOL JEM 2010) with an accelerating voltage of 200 kV were used for imaging. Fourier transform infrared spectra (Nicolet 5700) were obtained at a resolution of 4 cm-1. The content of iron oxide in the dried precipitate was determined using a thermogravimetric analyzer (TGA) under a constant flow of N2 gas. A vibrating sample magnetometer was used to measure the hysteresis loop and magnetization. Field-cooling (FC) measurements were made by cooling the samples from room temperature (T < TC) to a temperature below the Ne´el temperature (TN) in the presence of a magnetic filed, and then plotting the hysteresis loop. To make zero-field-cooling (ZFC) measurements, cooling was performed without a magnetic field. 3. Results and Discussion Synthesis of FeO NPs. Bulk goethite and oleic acid are used in the preparation of iron oleate at a temperature of about 290 °C. The process is cost effective and green, since the iron source is simple; the byproduct is water, and no time-consuming purification step is required.13 The as-formed iron oleate complex is mixed with excess oleic acid and brought to a boil at about 365 °C. Interestingly, after about 30 min at the boiling temperature, the reaction temperature increases further to 380 °C, probably because the oleic acids decompose and form highmolecular-weight materials. During this reaction, CO and H2 may be formed;17 they are responsible for the reduction of Fe3+ ions to Fe2+ and the formation of FeO NPs as the product. The oleic acid used herein has various roles, including as a solvent of hematite (which is the decomposition product of goethite), a high-temperature solvent for the decomposition reaction, and a protection ligand of NPs. According to other works, the concentration of oleic acid affects the size of NPs obtained.18 When the concentration is high, the nucleation of iron oxide is much more difficult because the as-formed iron oxide clusters dissolve immediately. Therefore, only sufficiently large nuclei can survive, and so the products are larger. In this work, the
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Figure 1. XRD patterns of as-formed FeO NPs stored under ambient conditions for various periods, showing change in dominant phase from wu¨stite to spinel-type iron oxide.
oleic acid is adopted as solvent, and the conditions herein are the harshest for nucleation: large 22 nm monodisperse FeO NPs are developed. Additionally, these large FeO NPs are formed because of the beneficial retardation of the dissolution of the iron oxide NPs by the decomposition of oleic acids at higher temperature. Composition of NPs. The phase of the evolving FeO samples was monitored using X-ray powder diffractometry (XRD), as presented in Figure 1. The XRD pattern of the as-prepared NPs is consistent with that of wu¨stite (Fm3m, a ) 4.239 Å, JCPDF No. 461312), which has a rock-salt-type structure. Both wu¨stite and spinel-type iron oxide phases, such as magnetite and maghemite, have an fcc oxygen sublattice. Therefore, these two compounds yield similar XRD patterns. Their major peaks have slightly different positions. From the 10-day sample, the major peaks of FeO are weak and broad. More importantly, some minor peaks are observed around the major one, which finding is consistent with the spinel-type structure (Fe3O4 or γ-Fe2O3). As the storage time increases, the spinel-type phase becomes dominant. This evidence reveals that the metastable wu¨stite NPs did not maintain their rock-salt structure under storage; rather, they slowly changed to the spinel-type phase. Experimental and theoretical studies have established that the modification of the composition in FeO is accompanied by the aggregation of a basic cluster in which four Fe2+ vacancies at octahedral sites surround each Fe3+ ion that is located in tetrahedral interstitials (4:1).19 The aggregation of these basic clusters by edge and corner sharing can generate clusters of various sizes, and the larger (16:5) clusters can be regarded as a partial Fe3O4 structure. Accordingly, the oxidation of some Fe2+ in FeO may promote the formation of the extended spinel-type Fe3O4 and γ-Fe2O3. This model is consistent with the observations herein. The gradual oxidation of FeO slowly changes the composition from wu¨stite to the spinel-type phase. The proportion of cation vacancies, x, in Fe1-xO can be evaluated using the x vs a relation, a ) 4.334 - 0.478x, where a is the cell parameter, measured from the time-evolved samples.8a The results give x values of 0.0539, 0.0693, and 0.0795 for the 0-, 5-, and 10-day samples, respectively. Based on these three points, an x-storage time relationship can be obtained by fitting a linear equation. The time for the full oxidation of these 22 nm FeO NPs can be determined by extrapolation to x ) 0.25 (Fe3O4), yielding a period of 78 days for the complete oxidation of FeO to magnetite. This value is on the order of that obtained from the XRD observations (∼120 days). The value is quite approximate because the x-storage time relationship in the equation may not be simply linear.
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Figure 3. HR-TEM image of single nanoparticle, showing core-shelllike contrast variation. Inset presents ED pattern of single particle. Figure 2. (a, b) TEM images of 10-day sample of FeO NPs under two magnifications, revealing uniform sizes, (c) size distribution plot, and (d) SAED pattern, which shows mixed wu¨stite and spinel-type iron oxide phase.
However, FeO NPs with a size of 22 nm are semiquantitatively determined to be able to survive for only approximately 100 days. TEM. Although the composition of FeO NPs changes slowly from wu¨stite to the spinel-type structure during storage, as revealed by the XRD results, knowing how these phases are distributed in the particles is more important. A 10-day sample was studied in detail. Figure 2a,b displays large-area TEM images indicating them with a mean size of 21.7 nm ( 2.1 nm. Electron diffraction (ED) was also performed, as shown in Figure 2d. Each diffraction ring was carefully measured, and their interplanar distance, dhkl, in angstroms, was calculated. The results show that the rings are comprised of FeO and the Fe3O4 or γ-Fe2O3 phases. The three relatively intense diffraction rings in this picture can be assigned to the (111), (200), and (220) planes of the rock-salt FeO structure, as the major phase of FeO, which finding is consistent with the XRD results for the 10day samples. Most of the spheres exhibit a contrast variation: the interiors are darker, suggesting compositional variation inside each NP. Therefore, a study of the composition variation within a single NP was performed using HR-TEM. Most of the observed lattice fringes are distributed consistently inside the single particle with dhkl ) 0.211 nm, assigned to the {200} form of FeO; however, this could include the contribution from the {400} form of Fe3O4, as shown in Figure 3. The minor interplanar distance, dhkl ) 0.290 nm, is observed exclusively near the edge of each particle, and is assigned to the {220} form of Fe3O4. Furthermore, to identify clearly the phase distribution in the NP, fast Fourier transform (FFT) approaches are adopted to reconstruct the lattice images from selected lattice planes. The real lattice image of a single particle, shown in Figure 4a, is transformed to the reciprocal lattice points, as shown in Figure 4b. The distribution of spots in this FFT pattern is similar to that obtained in the ED experiment, shown in Figure 3 as a projection along the 〈001〉 zone axis of the cubic structure. The spots indexed as the {200} form of FeO and {400} planes of Fe3O4 were chosen for the reconstruction of lattice patterns by inverse FFT. Figure 4c presents an almost uniform distribu-
Figure 4. FFT analysis of a single particle. (a) HR-TEM image of the selected NP. (b) FFT pattern from the lattice image in (a), the spots marked with 0 corresponding to (200) of FeO and (400) of Fe3O4; the spot marked with O corresponding to (220) of magnetite. (c) Image filtered including the spots marked with 0. (d) Image filtered including the spots marked with O exclusively from (220) of magnetite showing inverse FFT image distributed in the outer shell.
tion of the two phases in the particle. The spots indexed as {220} exclusively from Fe3O4 were chosen for the reconstruction of lattice patterns by inverse FFT. Conversely, Figure 4d indicates that Fe3O4 is limited to the outer shell. This evidence suggests the presence of an FeO/Fe3O4 core-shell structure, which is consistent with the slow oxidation of the FeO NPs from the outer shell to the inner core. Absorption Spectra. The similarity between the diffraction patterns of maghemite (γ-Fe2O3) and magnetite (Fe3O4) makes distinguishing them by XRD difficult. Therefore, UV to nearIR absorption spectroscopy was used to identify the presence of the Fe3O4 phase in the NPs. Figure 5 presents the adsorption spectra of the as-prepared and 10-day samples, which include a broad absorption hump centered at ∼1350 nm, whereas those of a γ-Fe2O3 sample reveal no absorption in this range. The
Monodisperse Wu¨stite NPs after Partial Oxidation
Figure 5. Absorption spectra of 0- and 10-day samples of FeO NPs during ambient storage. The inset presents magnified spectra between 300 and 800 nm. A spectrum of pure γ-Fe2O3 NPs is included for comparison.
Figure 6. Hysteresis loops of as-prepared sample and 10-day sample, showing superparamagnetic behavior at room temperature.
broad adsorption band centered at ∼1350 nm has been attributed to the Fe2+-Fe3+ intervalence charge transfer in the mixedvalence Fe3O4 structure.20 Its presence indicates that the NP samples contain a substantial amount of magnetite; however, the presence of maghemite phase cannot be completely excluded. The results of our earlier work18b indicated that NPs with the spinel-type phase, determined by XRD, actually comprise a mixture of Fe3O4 and γ-Fe2O3, according to the results of Mo¨ssbauer spectroscopy, because the formed Fe3O4 can further oxidize slowly to the more stable γ-Fe2O3. Interestingly, the intervalence-charge-transfer band is prominent in the as-prepared FeO sample, indicating that the defect clusters in FeO may also have absorption properties similar to those of the Fe2+-Fe3+ intervalence charge transfer. However, absorption in the range 600-800 nm is attributable to the absorption edge of FeO, which is characteristic of NPs with a high wu¨stite content.14 Magnetic Properties. Figure 6 presents room-temperature hysteresis loops of the as-prepared and 10-day FeO NP samples. Both loops exhibited superparamagnetic behavior21 at room temperature with saturated magnetizations of 18.1 emu/g (asprepared sample) and 24.0 emu/g (10-day sample). The stored sample had a higher saturation value because it has a higher ferrimagnetic phase content. This finding agrees with the structure revealed by XRD and TEM, which revealed that the amount of shell (ferrimagnetic) that was oxidized increased with storage time, surrounding the antiferromagnetic (AFM) FeO core. This particular structure will generate exchange-bias anisotropy below the TN of the FeO nanocore.22 Field cooling (FC) through TN of thin films or core-shell NPs that contain FM/AFM interfaces with TC > TN is well-known to yield
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Figure 7. Zero-field-cooled (ZFC) and field-cooled (FC; applied field of 10.8 kOe) hysteresis loops of the 10-day samples at 100 K.
hysteresis loop shift frequently accompanied by enhanced coercive fields.23 The complex spin structure at the FM/AFM interface, which is influenced by the intrinsic properties of both FM and AFM components, the interfacial structure, including its dimensions and roughness, interparticle interactions, and other factors, is the origin of the observed macroscopic peoperties.23 Notably, the structure in the current case differs from that of the conventional FMcore/AFMshell system. It is an inverted AFMcore/FIMshell magnetic structure.24,25 To study the exchange-bias interaction in this inverted magnetic structure, the zero-field-cooled (ZFC) and the fieldcooled (FC) hysteresis loops of the 10-day sample were obtained between room temperature and 100 K in steps of 25 K. Figure 7 plots the typical hysteresis loop measured at 100 K. (All other hysteresis loops are presented in Figure S1 in the Supporting Information.) The coercivity values and the horizontal displacement of the magnetization loop, the so-called exchange-bias field, HE, are given by the following formula:26
HC ) (HCRight - HCLeft)/2
and
HE ) -(HCRight + HCLeft)/2 (1)
where HCRight and HCLeft denote the right and left intercepts of the hysteresis loop on the field axis. The horizontal shift of HE has been interpreted as unidirectional anisotropy and its value is related to the strength of the interaction across the AFM/FM interface. It is generally accepted that the pinned uncompensated spins at the interface anchored to the AFM are responsible for the shifted loop.27 During cycling of the magnetic field, the anisotropy of AFM is high enough to fix the interfacial spin and induce the shifted hysteresis loop. As well as the shift in the field axis, the loop shifts significantly in the moment axis. However, this shift is known to depend on the cooling field and is also related to the pinned uncompensated spins at the interface that are coupled to the AFM.22 The results show slight enhancement of coercivity from 614 Oe (ZFC-HC) to 659.5 Oe (FC-HC) and a large HE of -1432 Oe. However, the saturation magnetization Ms ) 19.7 emu/g at 100 K is lower than Ms )24.0 emu/g at 300 K under ZFC. This result is consistent with the fact that the measured temperature of 100 K is below the TN of FeO. At this low temperature, the magnetic contribution of the FeO core is eliminated by antiferromagnetic coupling. Figure 8 plots the dependence on temperature of the exchange bias in the 10-day sample. Table 1 presents the values of HE (exchange-bias fields), ZFC-HC (ZFC coercivity), and FC-HC (FC coercivity). At temperatures of between 225 and 300 K, the FC and ZFC coercivities (FC-HC and ZFC-HC) are mutually
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Figure 8. Temperature dependence of exchange-bias field, HE, and the coercivity HC (ZF and ZFC), showing that the HE onset temperature is about 200 K.
TABLE 1: HE and HC after Indicated ZFC and FC Protocols temp (K)
HE (Oe)
HC for FC (Oe)
HC for ZFC (Oe)
100 125 150 175 200 225 250 275 300
1432 1314 1204 1086 795 0 0 0 0
660 564 511 543 795 290 97 58.5 0
614 560 428 447 726 300 93 31 0
Figure 9. Temperature dependence of zero-field-cooled (ZFC) and field-cooled magnetization (FC) of 10-day sample in a magnetic field of 100 Oe, showing T BSP at 292.3 K, obtained from the highest peak of the ZFC curve.
magnetic fields of 100 Oe. T BSP is measured from the highest peak of the ZFC magnetization curves,21 which is at 292.3 K. The shell may break up into multiple domains because of its shape, and this size effect will lower the T BSP of the shell. To exclude that this high T BSP is from the interparticle interaction, ZFC and FC magnetization of the 10-day sample diluted in a paraffin wax matrix was carried out. The result, Figure S2 in the Supporting Information, shows that T BSP is around 294.2 K. The obtained high T BSP of the shell is believed to be beneficial from the exchange anisotropy. 4. Conclusion
consistent, indicating no enhancement of coercivity. In the temperature range between 225 and 100 K, FC-HC exceeds ZFCHC; this enhancement in HC follows from the net spins at the AFM/FIM interfaces that are coupled to AFM weakly (unpinned) and can rotate freely with the applied external field. The peak in this plot also indicates the strong AFM anisotropy of FeO. Part of the interfacial spins of FIM decoupled around TN because it cannot drag the antiferromagnetic spins, which leads to a drop of HC. Also, the value of HE decreased abruptly with temperature between 200 and 225 K. The temperature above which HE is zero is generally referred to as T BEB. This HE onset temperature equals the temperature at which HC is enhanced. Those are signatures of the exchange bias. It has been reported that the reduction of T BEB with respect to the bulk TN on Co/CoO core-shell NPs is related to the thin shell thickness (less than 5 nm), although it has been attributed to different reasons such as the finite-size effect on TN, amorphous crystal structure of thin CoO layers, and lack of dipolar interaction.28 The HE onset temperature in the large core-shell NPs in this work is around the TN of bulk FeO.8a This fact may follow simply from the fact that the NPs have large FeO cores. This finding is consistent with the results recently reported by Kavich et al. for inverted FeO/Fe3O4 core-shell nanocrystals with a mean size of about 14 nm (core size of 7 nm, shell thickness 3.5 nm).21 For the AFM/FIM (TN ∼ 200 K, TC ) 860 K) core-shell NPs considered herein, the ferrimagnetic shell also play an important role. As the shell thickness is in the nanoscale range, the blocking temperature T BSP of the nanoshell that is the temperature of transition from the superparamagnetic state to a spin flop blocked state, only below which can the coercivity be detected, has to be considered. According to Figure 8, the blocking temperature, T BSP, of the ferrimagnetic iron oxide nanoshells is between 275 and 300 K. Figure 9 plots the measured temperature dependence of zero-field-cooled (ZFC) and field-cooled (FC) magnetization of the 10-day sample in
In this work, monodisperse wu¨stite (Fe1-xO) NPs with a mean size of 21.7 ( 2.1 nm were prepared via thermal decomposition of the iron(III) oleate complex at 380 °C, using oleic acid as the solvent. XRD, TEM, and magnetic measurements indicate that the partially oxidized sample had an AFM FeO/ferrimagnetic iron oxide core-shell structure. The nanostructure exhibited strong exchange-bias properties below the TN of bulk FeO, around 200 K, and a high T SP B of the ferromagnetic shell, because of the large FeO core. Acknowledgment. The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 95-2113-M-269-001-MY3 and an anonymous reviewer for valuable suggestions. Supporting Information Available: Supporting results mentioned in the text and XPS of as-prepared and 10-day samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Zeng, B. Z.; Sun, S. AdV. Funct. Mater. 2008, 18, 391. (b) Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Elst, L. V.; Muller, R. N. Chem. ReV. 2008, 108, 2064. (c) Jun, Y. W.; Seo, J. W.; Cheon, J. Acc. Chem. Res. 2008, 41, 179. (2) Minervini, L.; Grimes, R. W. J. Phys. Chem. Solids 1999, 60, 235. (3) (a) Lin, X.; Murthy, A. S.; Hadjipanayis, G. C. J. Appl. Phys. 1994, 76, 6543. (b) Mauvernay, B.; Presmanes, L.; Bonningue, C.; Tailhades, Ph. J. Magn. Magn. Mater. 2008, 320, 58. (4) Meiklejohn, W. H.; Bean, C. P. Phys. ReV. 1956, 105, 904. (5) Papaefthymiou, G. C. Nano Today 2009, 4, 438. (6) Skumryev, V.; Stoyan, S.; Zhang, Y.; Hadjipanayis, G.; Givord, D.; Norgue´s, J. Nature 2003, 432, 850. (7) Zhang, J.; Zhao, Y. Phys. Chem. Miner. 2005, 32, 241. (8) (a) Gheisari, M.; Mozaffari, M.; Acet, M.; Amighian, J. J. Magn. Magn. Mater. 2008, 320, 2618. (b) Emel’yanov, D. A.; Korolev, K. G.; Mikhailenko, M. A.; Knot’ko, A. V.; Oleinkov, N. N.; Tret’yakov, Y. D.; Boldyrev, V. V. Inorg. Mater. 2004, 40, 726.
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