Size-Dependent Structural and Magnetic Properties of LaCoO3

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Size-Dependent Structural and Magnetic Properties of LaCoO3 Nanoparticles Shiming Zhou,* Laifa He, Shuangyi Zhao, Yuqiao Guo, Jiyin Zhao, and Lei Shi* Hefei National Laboratory for Physical Science at Microscale, UniVersity of Science and Technology of China, Hefei 230026, P. R. China ReceiVed: January 12, 2009; ReVised Manuscript ReceiVed: June 11, 2009

The structural and magnetic properties of LaCoO3 nanoparticles with the particle size (D) ranging from ∼60 to 450 nm prepared by a sol-gel method are investigated in this paper. It is found that all the nanoparticles have rhombohedral structure as the bulk, while the volume of unit cell monotonically increases with the decrease of the particle size. Magnetic measurements reveal that in all the nanoparticles a weakly ferromagnetic behavior appears below ∼85 K, in agreement with recent studies on single crystals, powders, epitaxially strained thin films, and particles of this compound, and that the magnetic moment increases with reduction in particle size. In particular, both the unit cell volume and ferromagnetic moment show a nearly linear relation with 1/D, which allows us to assign the enhancement of the ferromagnetic moment in the nanoparticles to the lattice expansion. Moreover, from the linear relation, a significant but size-independent ferromagnetic moment can be obtained by extrapolating 1/D to zero, which is very close to the saturated magnetic moment previously reported for the single-crystal samples in the literatures. We propose that the ferromagnetic behavior usually observed in the single crystal and bulk polycrystalline LaCoO3 at low temperatures may be an intrinsically magnetic property of this material. Additionally, a paramagnetic phase is found to coexist with the ferromagnetic phase at low temperatures for all the nanoparticles and to show a similar dependence on the particle size as the ferromagnetic phase, which suggests that the paramagnetism arises from the higher spin-state Co3+ ions and may also be an intrinsic property of this material. 1. Introduction Cobaltates have recently received significant interest due to their various intriguing physical properties such as magnetoresistance,1 superconductivity,2 and large thermoelectric effect.3 Among those compounds, the perovskite-type cobalt oxide LaCoO3 shows its unique magnetic transitions: a steep transition from nonmagnetic to paramagnetic behavior at about 100 K and a broad transition at around 500 K accompanied by an insulator-metal transition. Both magnetic transitions have been attributed to the temperature-induced spin state transition of Co3+ 6 0 ions from the low-spin (LS) (t2g eg, S ) 0) state to higher spin 5 1 eg, S ) 1) or high-spin state, i.e., the intermediate-spin (IS) (t2g (HS) (t42ge2g, S ) 2) state. Despite numerous studies on structural, magnetic, and electronic properties, the nature of the thermally excited spin state transitions is still under extensive debate.4-14 For instance, experimentally, recent inelastic neutron scattering,9 X-ray absorption spectroscopy (XAS), and magnetic circular dichroism10 suggested the transition at about 100 K to the HS state, while the electron energy spectroscopy11 distinguished this exited spin state as the IS state. Theoretically, generalized gradient approximation plus Hubbard calculations suggested that the HS state is more stable than the IS state, and the first magnetic transition is from LS to HS state,12 whereas local density approximation plus Hubbard13 and ab initio band structure calculations14 provided the reverse results. Besides the controversial excited spin state, recently, a shortor long-range ferromagnetic (FM) order at low temperatures observed in LaCoO3 single crystals,15-17 powders,15,18 thin films,19,20 and nanoparticles17,21-23 has also attracted much attention. Since the ground state of LaCoO3 is generally believed * Authors to whom the correspondence should be addressed. Electronic mail: [email protected]; [email protected].

to be the nonmagnetic LS state, the observed FM behavior is very interesting but puzzling. Furthermore, although those samples were prepared by different methods and in much different forms and have the drastically different contents of FM phase, they show almost the same value of TC ∼ 85 K. This strongly suggests that the nature of the FM behavior in those samples should be identical. However, the study on this issue is still a matter of some contention.15-23 Thus far, there mainly remain two open questions: (i) What is the origin of the FM coupling in LaCoO3? Several scenarios have been considered to account for this coupling. One is the FM exchange interaction between the Co3+ and Co4+ ions,17,18,21 analogous to the cases reported in hole-doped LaCoO3.24 The existence of Co4+ ions was attributed to La vacancies18 or oxygen chemisorption at the surface.17 On the other hand, because no obvious variation of TC was found for different oxygen treatments for the ground single crystals and epitaxial films, respectively, Yan et al.15 and Fuchs et al.19 excluded the coupling between the Co3+ and Co4+ ions as the origin of the ferromagnetism and brought out another possibility. Yan et al. supposed that the superexchange interactions between the surface Co3+ ions, having higher spin states, most probably an IS state, due to their reduced oxygen coordination, provides a possible FM coupling.15 Fuchs et al. suggested that in the epitaxially strained films the tensile strain results in not only a stabilizing of higher spin state but also a suppression of the Jahn-Teller (JT) splitting of the eg orbitals of IS Co3+ ions and the latter would drive the orbitals to favor the FM ordering.19 Moreover, the studies of X-ray absorption spectroscopy on the strained LaCoO3 films had demonstrated that a significant and temperature-independent fraction of higher spin states is present down to 30 K, which identifies an important prerequisite for the FM coupling.27 The third explanation for the FM behavior

10.1021/jp9003032 CCC: $40.75  2009 American Chemical Society Published on Web 07/14/2009

Structural and Magnetic Properties of LaCoO3 NPs was proposed by Giblin et al.,16 who investigated the magnetic properties of the polycrystalline and single-crystal LaCoO3 samples by muon spin spectroscopy and attributed the observed FM behavior to a formation of magnetic excitons due to the presence of oxygen vacancies. (ii) Are the FM exchange interactions just local at the surface or also present in the bulk? Yan et al. compared the magnetic properties of several samples with different surface morphology and found an increase in remanent magnetization and coercivity with increasing surface to volume ratio of the different samples.15 They suggested that the FM coupling between the surface Co ions is responsible for “surface ferromagnetism”. Using scanning superconducting quantum interference devices (SQUID) microscope, Harada et al.17 demonstrated that the ferromagnetism exists at the surface of the single crystal. Moreover, they proposed that their observed linear relation between the spontaneous magnetization and the inverse of size for LaCoO3 particles further supports the idea of the surface ferromagnetism. However, in the epitaxially strained thin films, Fuchs et al.19 found that the saturated magnetic moment linearly increases with the increase of the film thickness, which proves that the FM ordering extends over the complete film and is not simply restricted to the surface region. In our previous work, we have investigated the structural and magnetic properties of the LaCoO3 nanoparticles with the average particle size of ∼80 nm, in which an FM order below ∼85 K is also found.22 Our infrared spectra have given evidence for a stabilizing of higher spin state, most possible IS state, and reduced JT distortions in the nanosized sample with respect to the bulk, which is consistent with the recent reports in the strained films.19 The superexchange interaction between the stabilized higher spin state Co3+ ions with reduced JT distortion has been supported as the origin of the observed ferromagnetism in LaCoO3. To get further insight into the intriguing nature of the FM order, especially to try to address the second question mentioned above, in this work, we prepared the LaCoO3 nanoparticles with different particle sizes, i.e., with different ratios of surface to volume, and investigate their magnetic properties in detail. 2. Experimental Details A sol-gel method was used to synthesize the LaCoO3 nanoparticles with different particle sizes. The stoichiometric amounts of La(NO3)3 · 6H2O and Co(NO3)2 · 6H2O were dissolved in deionized water. Then, an amount of citric acid and ethylene glycol in a molar ratio to the total metal ions of 4:1 was added to the solution. Subsequently, the obtained transparent solution was slowly evaporated to get a gel, which was decomposed at about 400 °C for 4 h to result in a dark brown powder. The precursor powder was divided into four parts and further annealed at 600, 700, 800, and 900 °C for 6 h, respectively, to obtain the LaCoO3 nanoparticles with different particle sizes. The phase purity and crystal structure of the samples were determined by an 18 kW rotating anode X-ray diffractometer (type MXP18AHF, MAC Science) with graphite monochromatized Cu KR(λ ) 1.5418 Å) radiation in the Bragg-Brentano geometry at room temperature. The particle sizes and morphology were determined by a JEOL (model JSM6700F) field emission scanning electron microanalyzer (SEM). The measurements of infrared absorption spectra were recorded with a Nicolet model Magna 750 Fourier transform spectrometer using KBr as a carrier. The magnetic measurements were carried out with a MPMS SQUID magnetometer.

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Figure 1. Room-temperature XRD patterns of the LaCoO3 nanoparticles annealed at different temperatures.

3. Results and Discussion Figure 1 shows the room-temperature X-ray diffraction (XRD) patterns of the LaCoO3 nanoparticles annealed at different temperatures. The sharp diffraction peaks indicate good crystallinity for each specimen and can be indexed by a single phase with rhombohedral crystal structure (space group R-3C) as that of the bulk LaCoO3.7,22 The SEM studies show that for each sample the size of the crystallized particles is homogeneous and increases with increasing annealing temperature as shown in Figure 2. The lattice parameters obtained from the XRD patterns and the average particle size estimated from the SEM images for the nanoparticles with different annealing temperatures are summarized in Table 1. One can see that comparing to the bulk [a ) 5.442(1), c ) 13.088(1) Å],22 the nanoparticles have larger lattice parameters. Furthermore, the lattice parameters monotonically increase as the particle size decreases, indicating the presence of a size-induced expansion of the unit cell. Figure 3 shows the temperature dependence of zero-fieldcooling (ZFC) and field-cooling (FC) magnetization below 150 K under a measured field of 500 Oe for the LaCoO3 nanoparticles with different particle sizes. The magnetization for all the nanoparticles rises sharply below about 85 K and the FC magnetization continuously increases with further decrease in the temperature, which are usually typical characteristics of ferromagnetic order. All the nanoparticles have the almost same value of TC, ∼85 K, determined from the onset of the FC magnetization, which is consistent with the reported values in refs 15-23. Below TC, a large bifurcation between ZFC and FC magnetization and a broad cusp in the ZFC curve were observed for all the nanoparticles, which may be correlated with the presence of the spin freezing at low temperatures.21-23 Figure 4 shows the field-dependent magnetization at 5 K for the nanoparticles with different particle sizes. The clear hysteresis loops further confirm the existence of the ferromagnetic order at low temperatures in these samples. The coercivity HC and the remanent magnetization Mr of these nanoparticles are listed in Table 1. One can see that, as the particle size decreases, i.e., as the surface to volume ratio increases, HC monotonically decreases, which is inconsistent with the results

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Figure 2. SEM micrographs of the LaCoO3 nanoparticles annealed at 600 (a), 700 (b), 800 (c), and 900 (d).

TABLE 1: Particle Size (D), Lattice Parameters, Coercivity (HC), and Remanent Magnetization (Mr) for the LaCoO3 Nanoparticles Annealed at Different Temperatures (Tan) Tan 600 700 800 900

°C °C °C °C

D (nm)

a (Å)

c (Å)

V (Å3)

60 80 200 450

5.444(1) 5.445(1) 5.444(1) 5.442(1)

13.122(2) 13.109(1) 13.098(1) 13.092(1)

336.83(8) 336.53(4) 336.21(2) 335.78(6)

HC Mr (kOe) (µB/Co) 3.0 4.3 6.1 6.4

0.018 0.016 0.011 0.008

Figure 4. Field-dependent magnetization at 5 K for the LaCoO3 nanoparticles with different particle sizes.

Figure 3. ZFC (solid) and FC (open) magnetization for the LaCoO3 nanoparticles with different particle sizes between 5 and 150 K.

in ref 15. However, Mr shows a monotonic increase with reduction in the particle size, which indicates that the magnetic moment is enhanced by the decrease in the particle size. The enhancement of magnetic moment is also supported by the ZFC magnetization and the field-dependent magnetization as shown in Figures 3 and 4, respectively, in both of which the smaller particles have larger magnetization. These results strongly indicate the existence of size-induced spin-state transition from LS to higher spin state in the LaCoO3 nanoparticles. In our previous work, the infrared spectra have revealed that the ∼80 nm sample shows an increased population of higher spin state and a reduced JT distortion with respect to the bulk. The infrared studies on the other three samples with different particle sizes

also give similar results (not shown here). Moreover, it is found that as the particle size decreases the population of the higher spin state increases and the magnitude of JT distortion decreases, which further supports the origin of FM ordering in LaCoO3due to the FM superexchange coupling between the higher spin state Co3+ ions. It is noticeable that the field-dependent magnetization at 5 K for all the samples does not saturate even up to 50 kOe, which indicates the coexistence of the FM and paramagnetic (PM) phases in the nanoparticles. Approximately, the field-dependent magnetization can be described as: M(H) ) Mnl(H) + χlH, where the first nonlinear component Mnl(H) corresponds to the FM moment, which saturates above ∼35 kOe, and the second linear term χlH originates from the PM phase. From the fielddependent magnetization above 35 kOe, the saturated ferromagnetic moment MS and the paramagnetic susceptibility χl can be approximated. Figure 5 shows the particle size dependent MS and χl. It can be clearly seen that a nearly linear dependence of MS on the inverse value of size (1/D) is found as shown in Figure 5a, which agrees well with the recent results reported by Harada et al.17 and Fita et al.23 Since the surface to volume ratio is proportional

Structural and Magnetic Properties of LaCoO3 NPs

Figure 5. Particle size dependence of MS and χl at 5 K for the LaCoO3 nanoparticles. The solid lines are the linear fitting results. The inset shows the particle size dependence of the unit cell volume and the linear fitting result for the nanoparticles.

to 1/D, such behavior undoubtedly reflects the significant role of the surface on the ferromagnetism. At first glance, this behavior may support the idea of surface ferromagnetism in LaCoO3.17 However, as mentioned in the Introduction, strong evidence against surface ferromagnetism in LaCoO3 has been provided by Fuchs et al.19 A linear increase in the saturated FM moment with the increase of the film thickness, i.e., with the increase of the surface to volume ratio, indicates that the FM behavior extends over the complete film and is not only restricted at the surface. Moreover, the studies on the structural and magnetic properties of the films deposited on different substrates show that the magnetic moment increases almost linearly as a function of the mean lattice parameter. In a very recent work,23 Fita et al. found that the unit cell of the LaCoO3 nanoparticles expands proportionally to the increase in 1/D and that an applied hydrostatic pressure strongly suppresses the FM phase. They attributed the increase in the FM moment with the decrease of the particle size to the expansion of the unit cell and proposed a model of “expansion-induced ferromagnetism”. Taking into account that the ferromagnetism originates from the superexchange coupling between the higher spin state Co3+ ions and that the lattice expansion promotes the stabilization of the higher spin states in cobaltates,7,32,33 the model can get rid of the seeming contradiction between data obtained the films and nanoparticles. In the present work, it is also found that the unit cell volume of the LaCoO3 nanoparticles monotonically increases with decreasing particle size as listed in Table 1. Specially, as shown in the inset of Figure 5a, a similar linear relation between the volume of unit cell and 1/D is obtained, too. Therefore, the enhancement of the FM moment with decreasing particle size in our LaCoO3 nanoparticles should be due to the expansion of the unit cell. Actually, an expansion of unit cell volume with reducing particle size has been observed in many partly covalent oxides.28-30 A possible explanation discussed by Ayyub et al. suggests that unpaired electronic orbitals at the particle surface in small solids would repel each

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13525 other and result in a larger value of the equilibrium lattice constant than in the bulk crystals.28 Since our results together with the findings reported in refs 19 and 23 indicate that the enhanced FM moment in the nanoparticles and strained films is closely related with the structural distortion induced by the size and strain, respectively, and do not support the previously simple idea of surface ferromagnetism,15,17 they are naturally followed by a question: How do we understand the usually observed FM behavior in the bulk polycrystalline and single crystal samples? We notice that if extrapolating 1/D to zero MS still remains a significant value of MS0 ∼ 7.8 × 10-3 µB/Co. This means that a sizeindependent term is included in the linear relation of the FM moment with 1/D. Assuming that the case as 1/D is extrapolated to zero corresponds to the ideal bulk sample, i.e., the bulk polycrystalline or single crystal specimen, the obtained considerable value of MS0 implies that the ferromagnetism is also present in the bulk. In our recent study on the size-dependent magnetic properties of La1.96Sr0.04CuO4 nanoparticles,31 a similar extrapolation has been carried out to reveal a size-independent term in the linear relation of the Curie constant with the inverse of the particle size, which successfully confirms that the previously observed very small effective spin density in the single crystal and bulk polycrystalline samples is an intrinsically magnetic behavior for La1.96Sr0.04CuO4. Moreover, in the same way, a size-independent FM moment close to MS0 can also be found in ref 17. Most importantly, based on the reported data of the field-dependent magnetization at 5 K in refs 15 and 17, we estimated the value of MS to be about 5.0 × 10-3 µB/Co for the single crystals, which is very close to the extrapolated MS0 in the present work. Those results further suggest that the observed FM behavior at low temperatures in the single crystal and bulk polycrystalline LaCoO3 may be not just from the surface but also present in the bulk, i.e, an intrinsically magnetic property of the compound. This suggestion is supported by the recent low-energy muon relaxation measurements on a large bulk polycrystalline LaCoO3 sample, where the surface is revealed to behave as the bulk in the magnetic properties because a constant depolarization rate was found at various depths.16 Furthermore, the high-resolution synchrotron single-crystal X-ray diffraction data had given evidence for a monoclinic distortion even down to 20 K, meaning that the higher spin states of Co3+ ions still exist in LaCoO3 single crystal at low temperatures.8 Since the ferromagnetism is related with the higher spin state Co3+ ions, the diffraction data are also in support of our suggestion. On the other hand, we find that the paramagnetic susceptibility χl also exhibits an approximate linear dependence on 1/D as shown in Figure 5b. Moreover, by extrapolating 1/D to zero, we get a considerable but size-independent value of χ0l ∼ 1.3 × 10-7 µBOe1-/Co, too. Practically, the PM phase at low temperatures has been frequently reported in the single crystal and bulk polycrystalline LaCoO3,4,6,15,34 and also found in the strained films19 and nanoparticles.17,23 This component was previously ascribed to impurities6 or to localized spins associated with the surface and/or lattice defects.4 Recently, Yan et al.34 observed a similar magnetic behavior also in PrCoO3 and NdCoO3 up to relatively higher temperatures and proposed that the behavior should be an intrinsic properties arising from surface cobalt, possibly a LS ground state bearing some IS character caused by virtual excitation to the IS state. In the strained films, Fuchs et al. found that the unsaturated magnetic moment at 5 K shows a linear increase with increasing film thickness like the saturated magnetic moment, indicating that

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the paramagnetism is a magnetic behavior of the total volume of film and does not support the surface-related scenario. Our results clearly show that the PM phase exhibits the same dependence on the particle size as the FM one, which suggests that the paramagnetism derives from the higher spin state Co3+ ions and is enhanced by the lattice expansion in the LaCoO3 nanoparticles. Moreover, the considerable value of χ0l implies that the paramagnetism previously observed in the single crystal and bulk polycrystalline samples at low temperatures may also be an intrinsic property of this material like the ferromagnetism as discussed above. The coexistence of the FM and PM phases at low temperatures unequivocally indicates the presence of phase separation in the LaCoO3 nanoparticles. The presence of phase separation in LaCoO3 at low temperature had been proposed in refs 8, 16, and 23. Maris et al. found that the IS states are not uniformly distributed and the JT-distorted IS-rich and non-JT-distorted LSrich regions coexist in the compound.8 Giblin et al. proposed that the formation of magnetic excitons supports the idea of magnetic phase separation at low temperatures and predicted that the exciton may be a precursor of magneto-electronic phase separation that is observed in the hole doped cobaltites.16 Our studies demonstrate that there are at least three phases in LaCoO3 at low temperatures, i.e., nonmagnetic, PM, and FM phases. The LS-rich regions are responsible for the nonmagnetic phase and the higher spin state-rich regions for the PM and FM phases, respectively. The variable spin state of Co3+ ions may be an additional source of the phase separation.23 Moreover, since our results imply that both the ferromagnetism and paramagnetism are intrinsic magnetic properties of LaCoO3, we propose that an intrinsic phase separation may be present. Of course, more experimental and theoretical research is urgently required to completely understand the microscopic nature of these complex magnetic properties at low temperatures in this compound. 4. Conclusion In conclusion, we have investigated the structural and magnetic properties of the LaCoO3 nanoparticles with different particle sizes. We find that the volume of the unit cell for the nanoparticles expands with the decrease of the particle size. Magnetic measurements show that all the nanoparticles exhibit weak ferromagnetism below ∼85 K and the magnetization monotonically increases with decreasing particle size. Detailed analysis of the size-dependent unit-cell volume and magnetization reveals that both the volume and the FM moment exhibit a linear relation with 1/D. Moreover, by extrapolating 1/D to zero from the relation between the FM moment and 1/D, we obtain a significant but size-independent value of MS0, which is very close to the saturated magnetic moment reported in the single crystal samples. These results imply that the ferromagnetism observed in the single crystal and bulk polycrystalline samples at low temperatures may be an intrinsic property of the bulk of LaCoO3 and is enhanced in the nanoparticles by the surface effect through the expansion of the unit cell, which is consistent with the recent studies on the LaCoO3 tensile thin films. Additionally, a PM phase is found to coexist with the FM phase at low temperatures and to show a similar dependence on the particle size, which indicates that the paramagnetism arises from the higher spin state Co3+ ions and may also be an intrinsic property of this material. We suggest that the present work will shed light toward understanding the nature of the complex and puzzling magnetic properties of LaCoO3 at low temperatures.

Zhou et al. Acknowledgment. This project was financially supported by the National Basic Research Program of China (973 program, 2009CB939901), and the National Science Foundation of China, grant no.10874161. References and Notes (1) Briceno, G.; Chang, H. Y.; Sun, X. D.; Schultz, P. G.; Xiang, X. D. Science 1995, 270, 273. (2) Takada, K.; Sakurai, H.; Takayama-Muromachi, E.; Izumi, F.; Dilanian, R. A.; Sasaki, T. Nature 2003, 422, 53. (3) Terasaki, I.; Sasago, Y.; Uchinokura, K. Phys. ReV. B 1997, 56, R12685. (4) Sen˜arı´s-Rodrı´guez, M. A.; Goodenough, J. B. J. Solid State Chem. 1995, 116, 224. (5) Thornton, G.; Tofield, B. C.; Hewat, A. W. J. Solid State Chem. 1986, 61, 301. (6) Yamaguchi, S.; Okimoto, Y.; Tokura, Y. Phys. ReV. B 1997, 55, R8666. (7) Radaelli, P. G.; Cheong, S. W. Phys. ReV. B 2002, 66, 094408. (8) Maris, G.; Ren, Y.; Volotchaev, V.; Zobel, C.; Lorenz, T.; Palstra, T. T. M. Phys. ReV. B 2003, 67, 224423. (9) Podlesnyak, A.; Streule, S.; Mesot, J.; Medarde, M.; Pomjakushina, E.; Conder, K.; Tanaka, A.; Haverkort, M. W.; Khomskii, D. I. Phys. ReV. Lett. 2006, 97, 247208. (10) Haverkort, M. W.; Hu, Z.; Cezar, J. C.; Burnus, T.; Hartmann, H.; Reuther, M.; Zobel, C.; Lorenz, T.; Tanaka, A.; Brookes, N. B.; Hsieh, H. H.; Lin, H.-J.; Chen, C. T.; Tjeng, L. H. Phys. ReV. Lett. 2006, 97, 176405. (11) Klie, R. F.; Zheng, J. C.; Zhu, Y.; Varela, M.; Wu, J.; Leighton, C. Phys. ReV. Lett. 2007, 99, 047203. (12) Plakhty, V. P.; Brown, P. J.; Grenier, B.; Shiryaev, S. V.; Barilo, S. N.; Gavrilov, S. V.; Ressouche, E. J. Phys.: Condens. Matter 2006, 18, 3517. (13) Korotin, M. A.; Ezhov, S. Yu.; Solovyev, I. V.; Anisimov, V. I.; Khomskii, D. I.; Sawatzky, G. A. Phys. ReV. B 1996, 54, 5309. (14) Pandey, S. K.; Kumar, A.; Patil, S.; Medicherla, V. R. R.; Singh, R. S.; Maiti, K.; Prabhakaran, D.; Boothroyd, A. T.; Pimpale, A. V. Phys. ReV. B 2008, 77, 045123. (15) Yan, J.-Q.; Zhou, J.-S.; Goodenough, J. B. Phys. ReV. B 2004, 70, 014402. (16) Giblin, S. R.; Terry, I.; Clark, S. J.; Prokscha, T.; Prabhakaran, D.; Boothroyd, A. T.; Wu, J.; Leighton, C. Europhys. Lett. 2005, 70, 677. (17) Harada, A.; Taniyama, T.; Takeuchi, Y.; Sato, T.; Kyoˆmen, T.; Itoh, M. Phys. ReV. B 2007, 75, 184426. (18) Androulakis, J.; Katsarakis, N.; Giapintzakis, J. Phy. ReV. B 2001, 64, 174401. (19) Fuchs, D.; Pinta, C.; Schwarz, T.; Schweiss, P.; Nagel, P.; Schuppler, S.; Schneider, R.; Merz, M.; Roth, G.; Lo¨hneysen, H. v. Phys. ReV. B 2007, 75, 144402. (20) Fuchs, D.; Arac, E.; Pinta, C.; Schuppler, S.; Schneider, R.; Lo¨hneysen, H. v. Phys. ReV. B 2008, 77, 014434. (21) Armelao, L.; Barreca, D.; Bottaro, G.; Maragno, C.; Tondello, E.; Caneschi, A.; Sangregorio, C.; Gialanella, S. J. Nanosci. Nanotechnol. 2006, 6, 1060. (22) Zhou, S. M.; Shi, L.; Zhao, J. Y.; He, L. F.; Yang, H. P.; Zhang, S. M. Phys. ReV. B 2007, 76, 172407. (23) Fita, I.; Markovich, V.; Mogilyansky, D.; Puzniak, R.; Wisniewski, A.; Titelman, L.; Vradman, L.; Herskowitz, M.; Varyukhin, V. N.; Gorodetsky, G. Phys. ReV. B 2008, 77, 224421. (24) Wu, J.; Leighton, C. Phys. ReV. B 2003, 67, 174408. (25) Mizokawa, T.; Fujimori, A. Phys. ReV. B 1996, 54, 5368. (26) Zhou, J.-S.; Yin, H. Q.; Goodenough, J. B. Phys. ReV. B 2001, 63, 184423. (27) Pinta, C.; Fuchs, D.; Merz, M.; Wissinger, M.; Arac, E.; Lo¨hneysen, H. v.; Samartsev, A.; Nagel, P.; Schuppler, S. Phys. ReV. B 2008, 78, 174402. (28) Ayyub, P.; Palkar, V. R.; Chattopadhyay, S.; Multani, M. Phys. ReV. B 1995, 51, 6135. (29) Selbach, S. M.; Tybell, T.; Einarsrud, M.; Grande, T. Chem. Mater. 2007, 19, 6478. (30) Gaur, A.; Varma, G. D. J. Phys.:Condens. Matter 2006, 18, 8837. (31) Zhou, S. M.; Zhao, J. Y.; Chu, S. N.; Shi, L. Physica C 2007, 451, 38. (32) Phelan, D.; Louca, D.; Kamazawa, K.; Hundley, M. F.; Yamada, K. Phys. ReV. B 2007, 76, 104111. (33) Takami, T.; Zhou, J.-S.; Goodenough, J. B.; Ikuta, H. Phys. ReV. B 2007, 76, 144116. (34) Yan, J.-Q.; Zhou, J.-S.; Goodenough, J. B. Phys. ReV. B 2004, 69, 134409.

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