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Metamagnetic transition and recovery of isotropic transport phenomena in A-type antiferromagnet Pr0.45Sr0.55MnO3
Journal of Physics and Chemistry of Solids 63 (2002) 925±928
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Metamagnetic transition and recovery of isotropic transport phenomena in A-type antiferromagnet Pr0.45Sr0.55MnO3 T. Hayashi a,*, N. Miura a, Y. Tomioka b, Y. Tokura b,c a
Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan b Joint Research Center for Atom Technology (JRCAT), 1-1-4 Higashi, Tsukuba, Ibaraki 305-0046, Japan c Department of Applied Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Abstract Magnetization and magnetoresistance of A-type antiferromagnet Pr0.45Sr0.55MnO3 have been measured by means of pulsed magnetic ®elds up to 45 T. Below TN , 215 K; metamagnetic transitions and attendant discontinuous decreases in resistivity were observed. Above the transition ®elds, the transport anisotropy is reduced compared with that in the antiferromagnetic state. The observed ferromagnetic transition and resistivity change indicate the simultaneous collapse of spin and orbital order by magnetic ®eld. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; D. Magnetic properties; D. Phase transitions
1. Introduction Perovskite manganites R12xAxMnO3 (R and A are the trivalent and divalent ions, respectively) have attracted much attention since the rediscovery of a colossal magnetoresistance effect. Beyond the classical double-exchange (DE) mechanism, it has been recognized that the interplay of spin, charge, lattice, and orbital is important in these compounds. Especially the role of orbital is becoming one of the most important issues not only in manganites but also in other transition metal oxides [1]. The orbital takes an essential role in a variety of magnetic structures of manganites which are controllable by doping level x, and crystal structure controlled by chemical substitution, and external parameters such as temperature, magnetic ®eld, and pressure. In the non-doped or slightly doped region, the orbital structure has directly been observed, and the ordering is now recognized as an actual presence [2]. On the other hand, in the heavily doped region, a number of experimental phenomena have been reported, and theoretical explanations are being constructed to explain the observed phenomena. Experimentally, Kawano et al. clari®ed the magnetic structure of A-type AF in Pr0.5Sr0.5MnO3 and Nd0.45Sr0.55MnO3 [3]. Akimoto et al. proposed the metallic conduction in A-type AF (La12zNdz)12xSrxMnO3 [4]. Kuwahara et al. * Corresponding author. Fax: 181-471-363338. E-mail address: [email protected] (T. Hayashi).
reported the anisotropic transport phenomena of Nd0.45Sr0.55MnO3 and suggested the d x2 2 y2 -type orbital ordering in the AF region [5]. In Nd0.45Sr0.55MnO3, the complete destruction of the A-type AF ordering under high magnetic ®elds has been reported. Magnetic ®elds suppress the AF ordering as a ®rst-order transition, that can be thought as a signature of the simultaneous collapse of spin and orbital ordering [6]. Some theoretical models have succeeded in reproducing the magnetic structure as a function of doping level x by taking account of the orbital degeneracy [7±9]. In addition to the ground state, Okamoto et al. considered the spin and orbital states under ®nite temperature and magnetic ®elds [9], which consistently explained recent experiments for Nd0.45Sr0.55MnO3. Pr0.45Sr0.55MnO3 has an A-type AF structures as well as Nd0.45Sr0.55MnO3 [10,11]. Corresponding to the magnetic transition, the crystal structure changes to the monoclinic one as in the case of Pr0.5Sr0.5MnO3, which also has the A-type AF structure. Although the ®eld effect in an A-type AF manganite has been investigated in Nd0.45Sr0.55MnO3, it is important to check the universality of the transition and clarify the transport anisotropy to investigate the related materials. In the present work, we report the magnetic and transport properties of Pr0.45Sr0.55MnO3 in high magnetic ®elds up to 45 T. We observed a ®rst-order ferromagnetic transition accompanied by a reduction of the anisotropic transport properties. These results can be interpreted as signatures of a
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(02)00111-7
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T. Hayashi et al. / Journal of Physics and Chemistry of Solids 63 (2002) 925±928
Fig. 1. M±H curves of Pr0.45Sr0.55MnO3 at different temperatures.
simultaneous diminish of spin and orbital ordering as discussed in Nd0.45Sr0.55MnO3.
Fig. 2. Phase diagram of Pr0.45Sr0.55MnO3 determined by magnetization measurements. Circles and squares stand for transition ®elds of ®eld-increasing and -decreasing process, respectively. The schematic spin con®guration is illustrated in each phase. The inset shows the phase diagram of Nd0.45Sr0.55MnO3 [6].
2. Experiment Single crystals of Pr0.45Sr0.55MnO3 were grown by the ¯oating-zone method. For transport measurements, we used the small samples cut with the edges along the pseudocubic principal directions to investigate the anisotropy. For magnetization measurements, we used the multi-domain ingot. (To obtain magnetization, we need a large sample size of about 2 £ 2 £ 2 mm 3, which is much larger than the domain size.) Therefore the ®eld direction cannot be de®ned with respect to the crystallographic axis. Pulsed magnetic ®elds were generated in a duration time of about 40 ms using a capacitor bank of 900 kJ maximum stored energy. Magnetization was measured by means of an induction method using a couple of coaxial pickup coils. Magnetoresistance was measured by the standard fourprobe method with a dc current. Keeping the ®eld direction in the ferromagnetic (F) plane, both the resistivities for a current in plane (r k) and for a current perpendicular plane (r ') have been measured. 3. Result and discussion Magnetization processes of a Pr0.45Sr0.55MnO3 crystal at temperatures between 4.2 and 250 K is shown in Fig. 1. For each curve in this ®gure, the origin is shifted by 1 m B/f.u. for clarity. It is clearly shown that there are two anomalies in the curve below TN. Around 10 T, there is a small kink below 200 K. High ®eld susceptibility (dM/dH) above the kink (Hsf) is about twice of that below Hsf while the curve above Hsf still shows antiferromagnetic character. Taking account of the behavior and the multi-domain structure, the kink is probably due to spin±¯op transition. Above
Hsf, metamagnetic transition appears. This behavior is quite similar to that observed in Nd0.45Sr0.55MnO3, which also has A-type AF structure as the ground state. The saturation moment M0 determined by an extrapolation of the M±H curve from the ferromagnetic state to the zero-®eld is 3.4 ^ 0.1 m B/f.u. at 4.2 K, which value is nearly equal to the saturation moment of an average Mn-ion moment (3.45 m B). Fig. 2 shows the magnetic phase diagram obtained from the magnetization measurements. All kinds of the transition ®elds were assigned by an in¯ection point of the M±H curve at each temperature. Circles and squares represent the ferromagnetic transition ®elds in the ®eld-increasing and -decreasing processes, respectively. Because the phase above TN is the paramagnetic one at zero-®eld [10,11], there should be a phase boundary between the paramagnetic state and the ferromagnetic state at high temperatures, although it could not be identi®ed in this study. As for the ferromagnetic transition, the phase boundary is also qualitatively the same as that of Nd0.45Sr0.55MnO3 (see the inset of Fig. 2), which indicates the same mechanism of the transition. In addition, spin±¯op transition has also been observed in this study. Namely, the antiferromagnetic state (AF1) undergoes a transition to a spin±¯op state (AF2). Note that in the usual antiferromagnet which has spin±¯op transition there is no metamagnetic transition in the usual magnetic substance. Therefore, it is needed to introduce some additional mechanism to explain the metamagnetic transition. In the inset of Fig. 3, temperature-dependence of the resistivity for a current in plane (r k) and for a current perpendicular plane (r ') are shown. Corresponding to the
T. Hayashi et al. / Journal of Physics and Chemistry of Solids 63 (2002) 925±928
Fig. 3. Magnetoresistance of Pr0.45Sr0.55MnO3. The inset shows temperature-dependence of resistivity. The ®eld direction is parallel to the F-plane.
antiferromagnetic transition, r ' is discontinuously enhanced while r k decreases, as a result the transport anisotropy enhanced by the AF ordering. The complicated temperature dependence of r ' below 170 K is probably due to remaining multi-domain effect. Fig. 3 shows the ®eld-dependence of r ' at various temperatures. The direction of the ®eld was parallel to the F-plane. The main feature of this ®gure is the behavior corresponding to the metamagnetic transition. Just after the ferromagnetic transition, the complicated features completely diminish and r ' shows a simple metallic resistance with a positive temperature-dependence. The drastic change of the transport properties at the transition implies the dimensional crossover of transport as inferred in Nd0.45Sr0.55MnO3. To con®rm the change of the anisotropy, we have done more direct measurements. Fig. 4 shows the ®elddependence of r '/r k at several temperatures. The ®eld direction is the same as the case of Fig. 3. Above TN, there is no signi®cant change in the anisotropy, although the magnetoresistance is quite large (see Fig. 3). Below TN, the anisotropy at zero-®eld is much enhanced as shown in the inset of Fig. 3. By the ferromagnetic transition, the anisotropy is discontinuously dropped, and nearly isotropic transport is recovered. This behavior strongly suggests that the system changed from the orbital-ordered twodimensional metal to a nearly isotropic three-dimensional metal, where the planer orbital structure diminishes. We note that the observed anisotropy change can be explained even by the pure DE model without orbital order±disorder transition: with the nearly isotropic orbital structure, carriers in the AF state can move only in the ferromagnetic layers while after the transition carriers become mobile to any directions. In this interpretation, however, we cannot explain the ferromagnetic transition itself as mentioned before. Furthermore, in such a classical
927
Fig. 4. Field dependence of transport anisotropy r' =rk : The ®eld direction is parallel to the F-plane.
DE model the presence of spin-canted phase is predicted [12], which is inconsistent with the experiments. On the contrary, if we consider the complete d x2 2 y2 orbital unrelated to the magnetic structure, we cannot explain the isotropic conduction even in the ferromagnetic state because the hopping is still quenched in the z direction. Therefore, the simultaneous diminish of the AF spin and orbital ordering is necessary to explain the observed ferromagnetic transition and additional phenomena. 4. Summary We have measured magnetization and magnetoresistance for Pr0.45Sr0.55MnO3 using pulsed high magnetic ®elds. We have observed a ferromagnetic transition accompanied by a discontinuous decrease of resistivity as observed in Nd0.45Sr0.55MnO3. We have also clari®ed that the transport anisotropy is reduced in the ferromagnetic state. The behavior indicates that the transition is accompanied by the collapse of the orbital ordering. Acknowledgements We would like also to thank Y.H. Matsuda and K. Uchida for their experimental help. References [1] Y. Tokura, N. Nagaosa, Orbital physics in transition-metal oxides, Science 288 (2000) 462±468. [2] Y. Murakami, J.P. Hill, D. Gibbs, M. Blume, I. Koyama, M. Tanaka, H. Kawata, T. Arima, Y. Tokura, K. Hirota, Y. Endoh, Resonant X-ray scattering from orbital ordering in LaMnO3, Phys. Rev. Lett. 81 (1998) 582±585.
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T. Hayashi et al. / Journal of Physics and Chemistry of Solids 63 (2002) 925±928
[3] H. Kawano, R. Kajimoto, H. Yoshizawa, Y. Tomioka, H. Kuwahara, Y. Tokura, Magnetic ordering and relation to the metal±insulator transition in Pr12xSrxMnO3 and Nd12xSrxMnO3 with x , 1/2, Phys. Rev. Lett. 78 (1997) 4253±4256. [4] T. Akimoto, Y. Maruyama, Y. Moritomo, A. Nakamura, K. Hirota, K. Ohoyama, M. Ohashi, Antiferromagnetic metallic state in doped manganites, Phys. Rev. B 57 (1998) R5594±R5597. [5] H. Kuwahara, T. Okuda, Y. Tomioka, A. Asamitsu, Y. Tokura, Two-dimensional charge-transport and spin-valve effect in the layered antiferromagnet Nd0.45Sr0.55MnO3, Phys. Rev. Lett. 82 (1999) 4316±4319. [6] T. Hayashi, N. Miura, K. Noda, H. Kuwahara, Field-induced orbital order transition of Nd0.45Sr0.55MnO3 observed in
[7] [8] [9] [10] [11] [12]
high-®eld magnetization and resistivity , Physica B 294±295 (2001) 115±118. R. Maezono, S. Ishihara, N. Nagaosa, Phase diagram of manganese oxides, Phys. Rev. B 58 (1998) 11583±11596. J. van den Brink, D. Khomskii, Double exchange via degenerate orbitals, Phys. Rev. Lett. 82 (1999) 1016±1019. S. Okamoto, S. Ishihara, S. Maekawa, Phase transition in perovskite manganites with orbital degree of freedom, Phys. Rev. B 61 (2000) 14647±14655. Y. Tomioka et al., unpublished. C. Martin, A. Maignan, M. Hervieu, B. Raveau, Magnetic phase diagrams of L12xAxMnO3 manganites (L Pr, Sm; A Ca, Sr), Phys. Rev. B 60 (1999) 12191±12199. P.G. de Gennes, Effects of double exchange in magnetic crystals, Phys. Rev. 118 (1960) 141±154.
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Metamagnetic transition and recovery of isotropic transport phenomena in A-type antiferromagnet Pr0.45Sr0.55MnO3 T. Hayashi a,*, N. Miura a, Y. Tomioka b, Y. Tokura b,c a
Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan b Joint Research Center for Atom Technology (JRCAT), 1-1-4 Higashi, Tsukuba, Ibaraki 305-0046, Japan c Department of Applied Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Abstract Magnetization and magnetoresistance of A-type antiferromagnet Pr0.45Sr0.55MnO3 have been measured by means of pulsed magnetic ®elds up to 45 T. Below TN , 215 K; metamagnetic transitions and attendant discontinuous decreases in resistivity were observed. Above the transition ®elds, the transport anisotropy is reduced compared with that in the antiferromagnetic state. The observed ferromagnetic transition and resistivity change indicate the simultaneous collapse of spin and orbital order by magnetic ®eld. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; D. Magnetic properties; D. Phase transitions
1. Introduction Perovskite manganites R12xAxMnO3 (R and A are the trivalent and divalent ions, respectively) have attracted much attention since the rediscovery of a colossal magnetoresistance effect. Beyond the classical double-exchange (DE) mechanism, it has been recognized that the interplay of spin, charge, lattice, and orbital is important in these compounds. Especially the role of orbital is becoming one of the most important issues not only in manganites but also in other transition metal oxides [1]. The orbital takes an essential role in a variety of magnetic structures of manganites which are controllable by doping level x, and crystal structure controlled by chemical substitution, and external parameters such as temperature, magnetic ®eld, and pressure. In the non-doped or slightly doped region, the orbital structure has directly been observed, and the ordering is now recognized as an actual presence [2]. On the other hand, in the heavily doped region, a number of experimental phenomena have been reported, and theoretical explanations are being constructed to explain the observed phenomena. Experimentally, Kawano et al. clari®ed the magnetic structure of A-type AF in Pr0.5Sr0.5MnO3 and Nd0.45Sr0.55MnO3 [3]. Akimoto et al. proposed the metallic conduction in A-type AF (La12zNdz)12xSrxMnO3 [4]. Kuwahara et al. * Corresponding author. Fax: 181-471-363338. E-mail address: [email protected] (T. Hayashi).
reported the anisotropic transport phenomena of Nd0.45Sr0.55MnO3 and suggested the d x2 2 y2 -type orbital ordering in the AF region [5]. In Nd0.45Sr0.55MnO3, the complete destruction of the A-type AF ordering under high magnetic ®elds has been reported. Magnetic ®elds suppress the AF ordering as a ®rst-order transition, that can be thought as a signature of the simultaneous collapse of spin and orbital ordering [6]. Some theoretical models have succeeded in reproducing the magnetic structure as a function of doping level x by taking account of the orbital degeneracy [7±9]. In addition to the ground state, Okamoto et al. considered the spin and orbital states under ®nite temperature and magnetic ®elds [9], which consistently explained recent experiments for Nd0.45Sr0.55MnO3. Pr0.45Sr0.55MnO3 has an A-type AF structures as well as Nd0.45Sr0.55MnO3 [10,11]. Corresponding to the magnetic transition, the crystal structure changes to the monoclinic one as in the case of Pr0.5Sr0.5MnO3, which also has the A-type AF structure. Although the ®eld effect in an A-type AF manganite has been investigated in Nd0.45Sr0.55MnO3, it is important to check the universality of the transition and clarify the transport anisotropy to investigate the related materials. In the present work, we report the magnetic and transport properties of Pr0.45Sr0.55MnO3 in high magnetic ®elds up to 45 T. We observed a ®rst-order ferromagnetic transition accompanied by a reduction of the anisotropic transport properties. These results can be interpreted as signatures of a
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(02)00111-7
926
T. Hayashi et al. / Journal of Physics and Chemistry of Solids 63 (2002) 925±928
Fig. 1. M±H curves of Pr0.45Sr0.55MnO3 at different temperatures.
simultaneous diminish of spin and orbital ordering as discussed in Nd0.45Sr0.55MnO3.
Fig. 2. Phase diagram of Pr0.45Sr0.55MnO3 determined by magnetization measurements. Circles and squares stand for transition ®elds of ®eld-increasing and -decreasing process, respectively. The schematic spin con®guration is illustrated in each phase. The inset shows the phase diagram of Nd0.45Sr0.55MnO3 [6].
2. Experiment Single crystals of Pr0.45Sr0.55MnO3 were grown by the ¯oating-zone method. For transport measurements, we used the small samples cut with the edges along the pseudocubic principal directions to investigate the anisotropy. For magnetization measurements, we used the multi-domain ingot. (To obtain magnetization, we need a large sample size of about 2 £ 2 £ 2 mm 3, which is much larger than the domain size.) Therefore the ®eld direction cannot be de®ned with respect to the crystallographic axis. Pulsed magnetic ®elds were generated in a duration time of about 40 ms using a capacitor bank of 900 kJ maximum stored energy. Magnetization was measured by means of an induction method using a couple of coaxial pickup coils. Magnetoresistance was measured by the standard fourprobe method with a dc current. Keeping the ®eld direction in the ferromagnetic (F) plane, both the resistivities for a current in plane (r k) and for a current perpendicular plane (r ') have been measured. 3. Result and discussion Magnetization processes of a Pr0.45Sr0.55MnO3 crystal at temperatures between 4.2 and 250 K is shown in Fig. 1. For each curve in this ®gure, the origin is shifted by 1 m B/f.u. for clarity. It is clearly shown that there are two anomalies in the curve below TN. Around 10 T, there is a small kink below 200 K. High ®eld susceptibility (dM/dH) above the kink (Hsf) is about twice of that below Hsf while the curve above Hsf still shows antiferromagnetic character. Taking account of the behavior and the multi-domain structure, the kink is probably due to spin±¯op transition. Above
Hsf, metamagnetic transition appears. This behavior is quite similar to that observed in Nd0.45Sr0.55MnO3, which also has A-type AF structure as the ground state. The saturation moment M0 determined by an extrapolation of the M±H curve from the ferromagnetic state to the zero-®eld is 3.4 ^ 0.1 m B/f.u. at 4.2 K, which value is nearly equal to the saturation moment of an average Mn-ion moment (3.45 m B). Fig. 2 shows the magnetic phase diagram obtained from the magnetization measurements. All kinds of the transition ®elds were assigned by an in¯ection point of the M±H curve at each temperature. Circles and squares represent the ferromagnetic transition ®elds in the ®eld-increasing and -decreasing processes, respectively. Because the phase above TN is the paramagnetic one at zero-®eld [10,11], there should be a phase boundary between the paramagnetic state and the ferromagnetic state at high temperatures, although it could not be identi®ed in this study. As for the ferromagnetic transition, the phase boundary is also qualitatively the same as that of Nd0.45Sr0.55MnO3 (see the inset of Fig. 2), which indicates the same mechanism of the transition. In addition, spin±¯op transition has also been observed in this study. Namely, the antiferromagnetic state (AF1) undergoes a transition to a spin±¯op state (AF2). Note that in the usual antiferromagnet which has spin±¯op transition there is no metamagnetic transition in the usual magnetic substance. Therefore, it is needed to introduce some additional mechanism to explain the metamagnetic transition. In the inset of Fig. 3, temperature-dependence of the resistivity for a current in plane (r k) and for a current perpendicular plane (r ') are shown. Corresponding to the
T. Hayashi et al. / Journal of Physics and Chemistry of Solids 63 (2002) 925±928
Fig. 3. Magnetoresistance of Pr0.45Sr0.55MnO3. The inset shows temperature-dependence of resistivity. The ®eld direction is parallel to the F-plane.
antiferromagnetic transition, r ' is discontinuously enhanced while r k decreases, as a result the transport anisotropy enhanced by the AF ordering. The complicated temperature dependence of r ' below 170 K is probably due to remaining multi-domain effect. Fig. 3 shows the ®eld-dependence of r ' at various temperatures. The direction of the ®eld was parallel to the F-plane. The main feature of this ®gure is the behavior corresponding to the metamagnetic transition. Just after the ferromagnetic transition, the complicated features completely diminish and r ' shows a simple metallic resistance with a positive temperature-dependence. The drastic change of the transport properties at the transition implies the dimensional crossover of transport as inferred in Nd0.45Sr0.55MnO3. To con®rm the change of the anisotropy, we have done more direct measurements. Fig. 4 shows the ®elddependence of r '/r k at several temperatures. The ®eld direction is the same as the case of Fig. 3. Above TN, there is no signi®cant change in the anisotropy, although the magnetoresistance is quite large (see Fig. 3). Below TN, the anisotropy at zero-®eld is much enhanced as shown in the inset of Fig. 3. By the ferromagnetic transition, the anisotropy is discontinuously dropped, and nearly isotropic transport is recovered. This behavior strongly suggests that the system changed from the orbital-ordered twodimensional metal to a nearly isotropic three-dimensional metal, where the planer orbital structure diminishes. We note that the observed anisotropy change can be explained even by the pure DE model without orbital order±disorder transition: with the nearly isotropic orbital structure, carriers in the AF state can move only in the ferromagnetic layers while after the transition carriers become mobile to any directions. In this interpretation, however, we cannot explain the ferromagnetic transition itself as mentioned before. Furthermore, in such a classical
927
Fig. 4. Field dependence of transport anisotropy r' =rk : The ®eld direction is parallel to the F-plane.
DE model the presence of spin-canted phase is predicted [12], which is inconsistent with the experiments. On the contrary, if we consider the complete d x2 2 y2 orbital unrelated to the magnetic structure, we cannot explain the isotropic conduction even in the ferromagnetic state because the hopping is still quenched in the z direction. Therefore, the simultaneous diminish of the AF spin and orbital ordering is necessary to explain the observed ferromagnetic transition and additional phenomena. 4. Summary We have measured magnetization and magnetoresistance for Pr0.45Sr0.55MnO3 using pulsed high magnetic ®elds. We have observed a ferromagnetic transition accompanied by a discontinuous decrease of resistivity as observed in Nd0.45Sr0.55MnO3. We have also clari®ed that the transport anisotropy is reduced in the ferromagnetic state. The behavior indicates that the transition is accompanied by the collapse of the orbital ordering. Acknowledgements We would like also to thank Y.H. Matsuda and K. Uchida for their experimental help. References [1] Y. Tokura, N. Nagaosa, Orbital physics in transition-metal oxides, Science 288 (2000) 462±468. [2] Y. Murakami, J.P. Hill, D. Gibbs, M. Blume, I. Koyama, M. Tanaka, H. Kawata, T. Arima, Y. Tokura, K. Hirota, Y. Endoh, Resonant X-ray scattering from orbital ordering in LaMnO3, Phys. Rev. Lett. 81 (1998) 582±585.
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T. Hayashi et al. / Journal of Physics and Chemistry of Solids 63 (2002) 925±928
[3] H. Kawano, R. Kajimoto, H. Yoshizawa, Y. Tomioka, H. Kuwahara, Y. Tokura, Magnetic ordering and relation to the metal±insulator transition in Pr12xSrxMnO3 and Nd12xSrxMnO3 with x , 1/2, Phys. Rev. Lett. 78 (1997) 4253±4256. [4] T. Akimoto, Y. Maruyama, Y. Moritomo, A. Nakamura, K. Hirota, K. Ohoyama, M. Ohashi, Antiferromagnetic metallic state in doped manganites, Phys. Rev. B 57 (1998) R5594±R5597. [5] H. Kuwahara, T. Okuda, Y. Tomioka, A. Asamitsu, Y. Tokura, Two-dimensional charge-transport and spin-valve effect in the layered antiferromagnet Nd0.45Sr0.55MnO3, Phys. Rev. Lett. 82 (1999) 4316±4319. [6] T. Hayashi, N. Miura, K. Noda, H. Kuwahara, Field-induced orbital order transition of Nd0.45Sr0.55MnO3 observed in
[7] [8] [9] [10] [11] [12]
high-®eld magnetization and resistivity , Physica B 294±295 (2001) 115±118. R. Maezono, S. Ishihara, N. Nagaosa, Phase diagram of manganese oxides, Phys. Rev. B 58 (1998) 11583±11596. J. van den Brink, D. Khomskii, Double exchange via degenerate orbitals, Phys. Rev. Lett. 82 (1999) 1016±1019. S. Okamoto, S. Ishihara, S. Maekawa, Phase transition in perovskite manganites with orbital degree of freedom, Phys. Rev. B 61 (2000) 14647±14655. Y. Tomioka et al., unpublished. C. Martin, A. Maignan, M. Hervieu, B. Raveau, Magnetic phase diagrams of L12xAxMnO3 manganites (L Pr, Sm; A Ca, Sr), Phys. Rev. B 60 (1999) 12191±12199. P.G. de Gennes, Effects of double exchange in magnetic crystals, Phys. Rev. 118 (1960) 141±154.