Evolution of the Electronic Phase Separation with Magnetic Field in

Sep 4, 2011 - The magnetic properties of the bulk and nanosized Pr0.67Ca0.33MnO3 particles were investigated. For the bulk materials, the obvious fiel...
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Evolution of the Electronic Phase Separation with Magnetic Field in Bulk and Nanometer Pr0.67Ca0.33MnO3 Particles T. Zhang,* X. P. Wang, and Q. F. Fang Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, China 230031 ABSTRACT: The magnetic properties of the bulk and nanosized Pr0.67Ca0.33MnO3 particles were investigated. For the bulk materials, the obvious field dependence of the electronic phase separation, i.e., with increase in the field, the ferromagnetic (FM) fraction increases and the antiferromagnetic (AFM) fraction shrinks and then disappears finally at 60 kOe was observed. The exchange bias field was found due to the existence of the FM and AFM cabbage sheets. While for nanosized Pr0.67Ca0.33MnO3 particles with average diameter of 100 nm, the sharp transition from AFM to FM does not occur even up to 60 kOe. For nanoparticles the exchange bias field significantly increases compared with the bulk one. This behavior can be attributed to a change of the spin configuration from collinear AFM to the coexistence of disordered surface spins, canted AFM, and collinear AFM arrangement which are induced by the surface effect and the coupling among the shell and interface spins.

’ INTRODUCTION In earlier studies, the phase separated (PS) state in manganites was proposed to explain the colossal magnetoresistance (CMR) found in this class of materials.1 Recently, a lot of attentions have been focused on the charge-ordered compositions of Pr1-xCaxMnO3 due to their importance as possible prototypes for the electronic PS models.2 11 The PS models are qualitatively different from the double exchange model12 14 or those based on strong Jahn Teller polarons.15,16 According to the PS model the ground state of CMR materials is comprised of coexisting nanosize clusters of metallic ferromagnetic (FM) and insulating antiferromagnetic (AFM) nature.7 A number of experimental results, including small-angle neutron scattering, indicates the existence of phase separation; their size varies from nano- to mesoscopic scales.9,10 Nanosized stripes of a FM phase were reported in Pr0.67Ca0.33MnO3.17 This compound also attracted much attention due to the existence of a nearly degenerate ferromagnetic metallic state and a charge ordered AFM insulating state with a field-induced phase transition possible between them, which make the chare- orbital-ordered Pr0.67Ca0.33MnO3 be a prototype for the PS scenario. It is well-known that finite-size effects induce a plethora of new phenomena in the solid-state magnetism.18 24 In particular, it is believed that the reduction of a sample size down to the nanometer scale is capable of influencing the magnetic order in doped mixed valence manganites R1 xAxMnO3 (R = La and rare earths, A = Ca, Sr, Ba, etc.) by changing the coupling between the spin subsystem (spins of both Mn ions and carriers and the lattice. The point of special interest in the case of these materials is characterized usually by a stable AFM ground state and charge ordering. Recently, the experimental evidence of PS on the r 2011 American Chemical Society

surface, i.e., coexistence of FM and AFM/CO state, suppression of the charge ordering state and exchange bias effect in nanosized Pr0.5Ca0.5MnO3,24 La0.25Ca0.75MnO3,21 Nd0.5Ca0.5MnO3,19 and La0.2Ca0.8MnO325 particles due to the uncompensated surface spins and surface pressure. However, for the lamellar PS Pr0.67Ca0.33MnO3 with irreversible switching from the AFM sheets to the FM ones, transforming the system to the complete FM state at 60 kOe. How do the FM and AFM phases evolve with the magnetic field in nanosize Pr0.67Ca0.33MnO3 is still an open question. Also, in the bulk Pr0.67Ca0.33MnO3, the coexistence of lamellar FM and AFM phases should lead to the exchange bias effect; however, value of exchange bias field and its evolution with particle size are still unknown. In the present work, we studied the evolution of magnetic properties of the bulk and 100 nm Pr0.67Ca0.33MnO3 nanoparticles with magnetic fields, respectively. It was found that for the bulk Pr0.67Ca0.33MnO3, the magnetic properties is very similar to that reported by ref 18; that is, the AFM transforming to FM phase with increasing field gradually disappears at 50 kOe below TC; at about 40 kOe field, the system reaches the percolation of the metallic FM phase. Although for nanosized particles, the charge ordering transition still exists but the percolation of the metallic FM phase and the complete FM transition do not occur up to 60 kOe, the AFM transition disappears, and the exchange bias field increases compared with that of bulk. All these can be ascribed to the surface pressure and uncompensated surface spins. Received: May 12, 2011 Revised: September 3, 2011 Published: September 04, 2011 19482

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Figure 2. Room temperature X-ray diffraction patterns for bulk and nanosized Pr0.67Ca0.33MnO3.

Figure 1. (Top) SEM image of the Pr0.67Ca0.33MnO3 nanopowder annealed at 800 °C. (Bottom) The corresponding lattice stripe image obtained by a high resolution TEM.

’ EXPERIMENT SECTION The Pr0.67Ca0.33MnO3 nanopowders with mean grain size of around 100 nm determined by transmission electron microscopy (TEM) and X-ray diffraction (XRD) were prepared by a sol gel method21 and subsequent crystallization was observed at 800 °C. The bulk sample was synthesized in air atmosphere by a standard solid-state reaction at 1350 °C. The crystallinity, composition, and stoichiometry of the nano and bulk samples were measured by high resolution transmission electron microscopy (HRTEM), the X-ray diffraction electron dispersive analysis (EDS), inductively coupled plasma atomic emission spectroscopy techniques and chemical titration.26 The temperature T and magnetic field (H) dependence of dc magnetization M were measured using a superconducting quantum interference device (SQUID) magnetometer in the temperature range of 10 300 K and under H up to 60 kOe. The relaxation was measured after the sample was rapidly cooled down in zero-field from room temperature to 20 K and was kept for different waiting time tw. Then the relaxation of the magnetization was recorded as a function of the time elapsed after an application of a probing field H = 20 Oe. ’ RESULTS AND DISCUSSIONS The size and crystalline quality of the nanoparticles (NP) was confirmed by TEM and high resolution TEM. Figure 1 shows the bright field (upper) TEM and high resolution TEM (down) images for nanosized sample. It was found that the average size of Pr0.67Ca0.33MnO3 NP is about 100 nm and size distribution is

narrow. The clear lattice strips implies that the good crystalline and good quality of a nanoparticle. The EDS analysis confirmed the composition and homogeneous distribution of the constituent elements with nominal atomic values Pr:Ca:Mn = 0.67:0.33:1.0. The approximate value of oxygen content determined by chemical titration is 2.98 + 0.02. The room temperature XRD pattern shown in Figure 2 indicates a single perovskite phase with Pnma space group for both bulk and nanosized Pr0.67Ca0.33MnO3. The grain size, obtained by Scherrer formulation D = kλ/(β cos 2θ), where D is the diameter of the grain, k is a constant (0.9), λ is the wavelength of the X-ray, and β is the full width of the halfmaximum of a peak, is consistent with that acquired by TEM in limits of errors. All of above characterization results indicate that NP particles are the correct formation of the desired Pr0.67Ca0.33MnO3 phase with average size of 100 nm. Temperature dependence of dc magnetization of bulk measured at different fields after zero-field-cooling (ZFC) and fieldcooling (FC) procedures are shown in Figures 3. The magnetization has a history dependence, i. e, with increasing magnetic fields, the difference between ZFC and FC data becomes increasingly large or small, and the onset temperature of the difference shifts toward high or low temperatures, with a bifurcation between ZFC and FC data at an irreversibility temperature Tirr defined as the point where (MFC MZFC)/MZFC and [∂(MFC MZFC)/ ∂T]/[∂MZFC/∂T] deviates below the noise (which was always less than 0.1%).11 According to the phase diagram of Pr1 xCaxMnO327 and the magnetic measurement results reported by Zhang et al24 as well as integrating with our magnetic data, in a low magnetic field (50 Oe), four phase transitions when the sample is cooled from room temperature can be confirmed: first a charge ordering transition defined as the peak at about TCO = 230 K in ZFC and FC curve,21 then a AFM transition corresponding to the small peak at about TN = 150 K,10 and followed a FM transition due to the existence of FM clusters at about TC = 80 K10 defined as the inflection in dM/dT T curve, finally a spin glass transition occurs at a lower temperature Tf ≈ 23 K, taken as the temperature corresponding to a small peak in ZFC curve while which is disappears in FC curve, due to the competition between FM and AFM interaction. With increasing field, the charge ordering and AFM phase transition becomes more and more obvious, because the AFM and charge ordering transition characters are covered by the FM moment at low fields, the AFM moments increase 19483

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Figure 3. Field cooled (closed symbols) and zero field cooled (open symbols) magnetization of bulk Pr0.67Ca0.33MnO3 as a function of temperature, measured in the field ranges 100 Oe to 60 kOe.

linearly with increasing field which leads to the more obvious AFM character in high field. The FM transition becomes flat from 50 to 10 Oe and then increases sharply from 20 to 60 kOe. In order to better understand the nature of the phase evolution with field, we plot the field dependence of the analyzed data in Figure 4, showing the low temperature relative difference between the ZFC and FC magnetization ΔM = (MFC MZFC)/ MZFC in Figure 4a, the irreversibility temperature Tirr in Figure 4b, the difference between Tirr and the temperature of the maximum in the ZFC magnetization (Tmax) in Figure 4c, and the low temperature magnetization in Figure 4d. The data support the existence of four different stage of phase separation with increasing magnetic field, just like the small angle magnetic neutron scattering results.18 For H e 10 kOe, the FM moment is strongly dependent on the magnetic history in that the difference between MZFC and MFC is relatively large. Additionally, in this regime ΔT = Tirr Tmax is relatively small and increases slightly with increasing field. The strong history dependence implies that either the intrinsic anisotropy of the clusters or the intercluster interactions dominate the effects of the external magnetic field, and the relatively small ΔT indicates that the FM clusters at such low fields are relatively uniform in size and blocking field.28,29 The small change of ΔT with increasing field to 1 kOe suggests that the applied field does not change the size of FM clusters in this field range, but the continuing increasing field gradually overcomes anisotropy energy of the frozen spins and thereby align the moments, which leads to the smooth increase in magnetization and the sharp decrease in Tirr and ΔM. The decrease in ΔM is related with the increase in Zeeman energy relative to the anisotropy energy of the clusters, possibly due to the cluster becoming more spherical. For 1 kOe e H e 4 kOe, all Tirr, ΔT, ΔM, and M increase with increasing field. The sharp increase in ΔT and the second increase in Tirr and ΔM presumable correspond to an increasing

Figure 4. Field dependence of the analyzed magnetization data for bulk Pr0.67Ca0.33MnO3. (a) The relative history dependence of the magnetization ΔM = (MFC MZFC)/MZFC at 5 K. (b) The irreversibility temperature Tirr. (c) The difference between the irreversibility temperature and the temperature of the maximum in the ZFC magnetization ΔT = Tirr Tmax. (d) MZFC and MFC at 5 K.

size distribution among the FM clusters, implying that they are growing in size in this regime rather than simply reorienting their moments,28 31 which leads to the phase transition from AFM to FM. In detail, during the FC process, the applied field favors the growth of the FM phase within the PM phase and leads to a larger FM phase fraction after the PM phase disappears. However, after ZFC, the AFM and FM phases are “frozen” due to the energy barriers between them, and the application of magnetic field only slightly increases the FM phase fraction at low temperatures through moving the interfaces between the FM and AFM phases toward the AFM phase. Obviously, it results in a difference of the FM phase fractions at low temperatures between the ZFC and FC modes and thereby the history dependence of magnetization. With increasing temperature, the influence of the applied field becomes more and more strong, driving more AFM phase into the FM phase. As a result, the difference of the FM phase fractions between the ZFC and FC modes is gradually reduced on warming, which causes the decrease in the difference between the ZFC and FC magnetization. When the sample is heated to Tirr, the FM phase fraction under the ZFC mode becomes the same as that under the FC mode, and therefore the magnetization under the ZFC and FC modes become superposed. Further, since the effect of the applied field on the FM phase in the FC process is stronger than that during the warming process after ZFC, the difference of the FM phase fractions between the FC and ZFC modes becomes larger with increasing magnetic field, and a higher temperature is needed to eliminate this larger 19484

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Figure 5. ZFC and FC magnetic hysteresis loop measured at 10 K for bulk Pr0.67Ca0.33MnO3.

difference. As a result, the differences between the ZFC and FC magnetization expand and Tirr gradually shifts toward high temperatures with increasing fields. For H g 40 Oe, Tirr, ΔT, and ΔM sharply decrease again and approaches zero, while M increase step-likely and tends to saturate. There is a sharp FM transition in M T curve. We presume that this feature corresponds to the sample undergoing the insulator to metal transition and system transforms itself smoothly to pure FM systems (above 50 kOe), which is demonstrated clearly by the M T plotting at H = 50 and 60 kOe. The present result is consistent with the results of ref.18 Figure 5 shows the field dependence of magnetization measured in ZFC and FC modes, respectively. In the FC process the hysteresis loops shift toward the negative fields and the positive magnetization, exhibiting the exchange bias (EB) behavior, the EB field Heb defined as Heb = |H1 + H2|/2 is about 36 Oe. In contrast, the loops are still centered about the origin in the ZFC case. The coexistence of the FM and AFM phase leads to the natural FM/AFM interfaces, as the sample is cooled down to T = 10 K at H = 20 kOe external field. The spins of the FM cluster align parallel to the external magnetic field. The interfacial FM spins on the exterior surface of the AFM or canted AFM inner core tend to be coupled with AFM spins at the interface as the temperature is below TN; this leads to the EB behavior. The coercivity is HC,ZFC = 184 Oe and HC,FC = 406 Oe, respectively. It is noted that Heb is very small, which could be explained as follows: one of necessary conditions for appearance of exchange bias is TC > TN,32 but here TC < TN as indicated in Figure 3. It must be clarified that TN only represent a beginning of AFM appearance in paramagnetic phase. With decreasing temperature, AFM fraction increases gradually. When temperature reaches a certain value, FM clusters start appearance. Therefore, only a small part of AFM phase adjacent to FM cluster and appearing at temperature below TC can induce exchange bias, which leads to a small Heb, similar to ref 33. As we know that when the particle size is reduced to nanoscale, the magnetic properties of the manganites will change significantly. In order to investigate the evolution of the phase separation in nanosized particles, we measured the magnetic properties of the nanosized Pr0.67Ca0.33MnO3 particles with average grain size of 100 nm. The character of the charge ordering transition still keeps, but the AFM transition is invisible, as plotted in Figure 6. The FC transition is much flatter than that for bulk. As shown in Figure 7, with increasing field, evolution tend of ΔM is a little different

Figure 6. Field cooled (closed symbols) and zero field cooled (open symbols) magnetization of Pr0.67Ca0.33MnO3 nanoparticles as a function of temperature, measured in the field ranges 100 Oe to 60 kOe.

with that of bulk, i.e., first a sharp decrease and then an increase slowly, no decrease up to 60 kOe. However, Tirr demonstrating a very different change tend compared with that of the bulk, sharply decreases from 100 to 5000 Oe and then continually rises. The magnetization MZFC and MFC smoothly increases with increasing filed and no step-like increase like that in bulk up to 60 kOe. For H e 5000 Oe, the first sharp decrease of the ΔM, Tirr and weak decline of ΔT is attributed to the gradual conquest of the anisotropy of frozen spin and alignment with field, because the field is not large enough to induce the growth of the FM cluster. Due to the surface effect, the FM-like surface spins contribute additional moment, which leading to a large magnetization compared with that of bulk. So there is another phase, i. e., surface spin glass or surface FM phase besides AFM and inside FM phase. With increasing field, as the bulk, the FM cluster size increase and FM fraction increase. In the FC process, as mentioned above, the FM phase fraction is larger than in ZFC process, because the field cooling can select a surface spin configuration with some spins aligning with external field which favors the particle being magnetized. However, no complete FM transition and saturated magnetization appear. This interesting behavior may be closely related to following factor: due to strong coupling among the surface spins and interface spins which also deviate from AFM arrangement and strongly couple with surface spins, the field required to force transitions between surface spin configuration can be very large since the exchange fields are approximately 5  106 Oe.34 Therefore, the field even up to 60 kOe, although which can align the AFM core spins like for bulk, is not large enough to force transition of surface spins and interface spins to full FM configuration,34 which leads to the slow increase of the ΔM and Tirr. It is reported that the time dependence of the ZFC relaxation rate S(t)=dM/dln(t) can give more credibility to the glass idea, because the S(t) is known to be strongly dependent on the age of 19485

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Figure 9. ZFC and FC magnetic hysteresis loop measured at 10 K for Pr0.67Ca0.33MnO3 nanoparticles.

Figure 7. Field dependence of the analyzed magnetization data for Pr0.67Ca0.33MnO3 nanoparticles. (a) The relative history dependence of the magnetization ΔM = (MFC MZFC)/MZFC at 10 K. (b) The irreversibility temperature Tirr. (c) The difference between the irreversibility temperature and the temperature of the maximum in the ZFC magnetization ΔT = Tirr Tmax. (d) MZFC and MFC at 10 K.

Figure 8. ZFC relaxation rate S(t) of measured at T = 10 K with different waiting time for Pr0.67Ca0.33MnO3 nanoparticles. The data were recorded at a field of H = 10 Oe.

system, i.e., the time elapsed (waiting time tw) since the system was quenched,35 37 while the FM state or blocking transition arising from size effects do not exhibit a strong aging dependence.38 So in order to verify above analysis and the surface spin glass behavior, the history-dependent behavior of these samples was measured at tw = 10, 100, and 1000 s, respectively, as shown in Figure 8.

The relaxation rate S(t) is almost strongly dependent on the waiting time, which confirms the surface spin glass and supports the foregoing analysis. Figure 9 shows the magnetic field dependence of magnetization in ZFC and FC models for nanosized Pr0.67Ca0.33MnO3 particles. Like that of bulk counterpart, the ZFC loop is still centered about the origin, while FC loop exhibits more obvious exchange bias effect than that of the bulk. The EB field Heb of about 312 Oe is much larger than the bulk one of 36 Oe. This behavior is caused by the surface phase separation. For nanosized particles, the surface spin act as FM on AFM core. So due to the coupling between surface FM and AFM at the surface, the FM spins will drag AFM spins to the external fields and produce additional exchange bias fields besides that in the core, which results in a larger Heb. The coercivity HC,ZFC = 312 Oe and HC,FC = 1292 Oe, respectively, which are much larger than those of bulk. As mentioned above, the FM region increases with size shrink leading to rise of total anisotropy energy; meanwhile, the FM domain gradually becomes single-domain tendency, the two factors may cause the enhancement of coercivity.39

’ CONCLUSION The evolution of the magnetic and electronic phase in bulk and nanosized Pr0.67Ca0.33MnO3 particles with magnetic field was studied. For bulk, with increase in field from at 1 to 10 kOe, the continuing increasing field gradually overcomes anisotropy energy of the frozen spins and align the moments; 10 kOe e H < 40 kOe, the FM clusters become large and FM region increases; at about 40 kOe, system reaches the percolation of the metallic ferromagnetic phase; 50 kOe field induces a transformation corresponds to a gradual irreversible switching from the antiferromagnetic sheets to the ferromagnetic ones, transforming the system to the complete ferromagnetic state. However, for nanosized Pr0.67Ca0.33MnO3 particles with diameter of 100 nm, due to the surface spin configuration and spin coupling, a smooth increase in ferromagnetism with increasing field is observed, but no sharp transition from AFM to FM phase appears even up to 60 kOe. In addition, an enhanced exchange bias effect compared with bulk is found. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. 19486

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’ ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China under the Grant No. 10904146. ’ REFERENCES (1) Dagotto, E; Hotta, T; Moreo, A Phys. Rep. 2001, 344, 1–3. (2) Deac, I. G.; Mitchell, J. F.; Schiffer, P. Phys. Rev. B 2001, 63, 172408. (3) Fisher, L. M.; Kalinov, A. V.; Voloshin, I. F.; Babushkina, N. A.; Kugel, K. I.; Khomskii, D. I. Phys. Rev. B 2003, 68, 174403. (4) Kajimoto, R.; Mochizuki, H.; Yoshizawa, H.; Okamoto, S.; Ishihara, S. Phys. Rev. B 2004, 69, 054433. (5) Nagapriya, K. S.; Raychoudhuri, A. K.; Bansal, B.; Venkataraman, V.; Parashar, S.; Rao, C. N. R. Phys. Rev. B 2005, 71, 024426. (6) Moreo, A.; Yunoki, S.; Dagatto, E. Science 1999, 283, 2034. (7) Moreo, A.; Mayr, M.; Feiguin, A.; Yunoki, S.; Dagotto, E. Phys. Rev. Lett. 2000, 84, 5568. (8) Yunoki, S.; Hu, J.; Malvezzi, A. L.; Moreo, A.; Furukawa, N.; Dagotto, E. Phys. Rev. Lett. 1998, 80, 845. (9) Sarma, D. D.; Topwal, D.; Manju, U.; Krishnakumar, S. R.; Bertolo, M.; La Rosa, S.; Cautero, G.; Koo, T. Y.; Sharma, P. A.; Cheong, S.-W.; Fujimori, A. Phys. Rev. Lett. 2004, 93, 097202. (10) Radaelli, P. G.; Ibberson, R. M.; Argyriou, D. N.; Casalta, H.; Andersen, K. H.; Cheong, S.-W.; Mitchell, J. F. Phys. Rev. B 2001, 63, 172419. (11) Deac, I. G.; Mitchell, J. F.; Schiffer, P. Phys. Rev. B 2001, 63, 172408. (12) Zener, C. Phys. Rev. 1951, 82, 403. (13) Anderson, P. W.; Hasegawa, H. Phys. Rev. 1955, 100, 675. (14) de Gennes, P.-G. Phys. Rev. 1960, 118, 141. (15) Millis, A. J.; Littlewood, P. B.; Shraiman, B. I. Phys. Rev. Lett. 1995, 74, 5144. (16) Millis, A. J.; Shraiman, B. I.; Mueller, R. Phys. Rev. Lett. 1996, 77, 175. (17) Simon, Ch.; Mercone, S.; Guiblin, N.; Martin, C.; Brulet, A.; Andre, G. Phys. Rev. Lett. 2002, 89, 207202. (18) Mercone, S.; Hardy, V.; Martin, C.; Simon, C.; Saurel, D.; Brulet, A. Phys. Rev. B 2003, 68, 094422. (19) Rao, S. S.; Tripathi, S.; Pandey, D.; Bhat, S. V. Phys. Rev. 2006, B74, 144416. (20) Lu, C. L.; Dong, S.; Wang, K. F.; Gao, F.; Li, P. L.; Lv, L. Y.; Liu, J.-M. Appl. Phys. Lett. 2007, 91, 032502. (21) Zhang, T.; Zhou, T. F.; Qian, T.; Li, X. G. Phys. Rev. B 2007, 76, 174415. (22) Markovich,1, V.; Fita, I.; Wisniewski, A.; Mogilyansky, D.; Puzniak, R.; Titelman, L.; Gorodetsky, G. J. Appl. Phys. 2010, 108, 063918. (23) Rozenberg, E.; Auslender, M.; Shames, A. I.; Mogilyansky, D.; Felner, I.; Sominskii, E.; Gedanken, A.; Mukovskii, Ya. M. Phys. Rev. B 2008, 78, 052405. (24) Zhang, T.; Dressel, M. Phys. Rev. B 2009, 80, 014435. (25) Markovich, V.; Fita, I.; Wisniewski, A.; Mogilyansky, D.; Puzniak, R.; Titelman, L.; Martin, C.; Gorodetsky, G. Phys. Rev. B 2010, 81, 094418. (26) Zhang, T.; Fang, Y. Z.; Dressel, M.; Wang, X. P.; Fang, Q. F. J. Appl. Phys. 2010, 108, 113901. (27) Tomioka, Y.; Asamitsu, A.; Kuwahara, H.; Moritomo, Y.; Tokura, Y. Phys. Rev. B 1996, 53, R1689. (28) Saurel, D.; Simon, Ch.; Brulet, A.; Heinemann, A.; Martin, C. Phys. Rev. B 2007, 75, 184442. (29) Nagamine, L. C. C. M.; Mevel, B.; Dienty, B.; Rodmacq, B.; Regnard, J. R.; Revenant-Brizard, C.; Manzini, I. J. Magn. Magn. Mater. 1999, 195, 437. (30) Chikuzami, S. Physics of Ferromagnetism; Clarendon: Oxford, U. K., 1997. (31) Qian, T.; Li, G.; Zhang, T.; Zhou, T. F.; Kang, X. W.; Li, X. G. Phys. Rev. B 2007, 76, 014433. (32) Nogues, J.; Schuller, K. J. Magn. Magn. Mater. 1999, 192, 203.

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