- Email: [email protected]
Jahn–Teller distortion and cluster-glass like behavior in La0.875Ca0.125MnO3
Journal of Physics and Chemistry of Solids 63 (2002) 939±942
www.elsevier.com/locate/jpcs
Jahn±Teller distortion and cluster-glass like behavior in La0.875Ca0.125MnO3 S. Begum a,*, Y. Ono a, Y. Tomioka b, Y. Tokura b,c, Y. Ishii d, Y. Morii d, T. Kajitani a a
Department of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan b Joint Research Center for Atom Technology (JRCAT), Tsukuba 305-0046, Japan c Department of Applied Physics, University of Tokyo, Tokyo 113-0033, Japan d Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokaimura, Naka-Gun, Ibaraki 319-1195, Japan
Abstract Neutron diffraction and magnetization measurements were carried out on both powder and single crystal samples of La0.875Ca0.125MnO3. At room temperature (RT), this sample is paramagnetic and single phase, with crystallographic symmetry Pnma (the longest axis is b-axis). Neutron powder diffraction shows that the average magnetic moment of Mn ions is 3.0 (1) m B/ Mn at 10 K, which is signi®cantly less than an averaged moment estimated from the ratio of Mn 31/Mn 41 which amounts to 3.875 m B for 12.5% Mn 41, an indication of spin canting. The crystal structure is characterized by the ordering of two eg orbitals, being evidenced by the anomalous change of Mn±O bond lengths which starts from 100 K with decreasing temperature. Neutron diffraction measurement was carried out at 8 K and RT. Diffuse scattering at around 1/2 0 1/2 point was observed at 8 K but disappeared in the paramagnetic region. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Magnetic materials; C. Neutron scattering; D. Crystal structure; D. Magnetic properties
1. Introduction The La12xAxMnO3 (where A refers to Ca, Sr, etc.) has currently been the subject of intense theoretical, experimental, and applied interest. The low temperature phase of La12xAxMnO3 with x 0:125 is assigned to the insulating ferromagnetic (FM) phase below Curie temperature Tc [1]. Recently, the small FM polarons were observed by X-ray and neutron diffraction measurements [2] in the La/Casystem. It was suggested that the co-existence of FM and anti-ferromagnetic (AF) domains is essential for the CMR behavior in the manganites [3]. In the present work, we focus our attention on the Jahn±Teller (J±T) distortion which induces the ordering of eg orbitals. In the conventional pseudo-tetragonal unit cell, it is believed that d3x2 2r2 or d3y2 2r2 orbitals are dominantly populated in the basal plane and the d3z2 2r2 orbitals of eg symmetry become less populated in the ordered phase (orbital ordered phase) [4]. This orbital ordering (OO) increases the AF contribution and reduces the interlayer * Corresponding author. Fax: 181-22-263-9836. E-mail address: [email protected] (S. Begum).
FM exchange [4]. We present the strong magnetic diffuse scattering at around 1/2 0 1/2 reciprocal lattice point at low temperature for La12xCaxMnO3 with x 0:125 single crystal (SC), but disappearing in the paramagnetic region, being an evidence of relatively large AF clusters. We also carried out neutron powder diffraction measurement for this sample with x 0:125 and observed weak re¯ection at the same reciprocal lattice point at 10 K, i.e. at 1/2 0 1/2 which strengthened the existence of AF-type magnetic ordering as well as AF-type clusters observed in the SC sample of the same Ca-concentration. Our magnetization measurement in the ®eld of 0.01 T shows the cluster-glass like behavior at low temperatures. 2. Experimental Powder sample (PS) of La0.875Ca0.125MnO3 was prepared by a standard solid state reactions from the stoichiometric mixture of powders La2O3 (99.9%), Mn2O3 (99.9%), and CaCO3 (99.9%). The mixture was calcined in air at 1000 8C for 48 h. With one intermediate grinding, pressed into pellet and sintered at 1500 8C for 36 h in air. The room temperature (RT) X-ray diffraction analysis indicated that
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(02)00141-5
940
S. Begum et al. / Journal of Physics and Chemistry of Solids 63 (2002) 939±942
Fig. 2. Neutron powder diffraction pattern of La0.875Ca0.125MnO3 at 10 and 293 K. sample measured with HRPD l 1:8227 A For 10 K, the y scale is shifted by 90 counts. 1/2 0 1/2, 100 and 1/ 2 0 3/2 are the forbidden peaks in the Pnma symmetry.
Fig. 1. Contour maps of neutron diffraction intensities of La0.875Ca0.125MnO3 in h0l reciprocal plane at 8 and 293 K.
the sample has a single phase orthorhombic perovskite structure. The SC sample was grown using the traveling solvent ¯oating zone (TSFZ) method by one of the present authors, Tomioka. The synthesis procedure for SC sample is brie¯y described elsewhere [5]. The neutron powder diffraction measurements were carried out at temperatures from The SC neutron 10 K to RT using HRPD l 1:8227 A: diffraction measurements were carried out using HERMES [6], at temperatures 8 and 293 K. Measure l 1:8196 A ments were made in the a p ±c p scattering plane. The temperature dependence of zero ®eld cooled (ZFC) and ®eld cooled (FC) magnetization measurements were carried out in a warming process under a magnetic ®eld of 0.01 T using a quantum design SQUID magnetometer.
3. Results and discussion Fig. 1 shows the contour maps of neutron diffraction intensities of La0.875Ca0.125MnO3 in 010 plane at 8 K and 293 K. Well de®ned diffuse scattering is represented at about 1/2 0 1/2 reciprocal point at 8 K but almost vanishes in the paramagnetic region, i.e. at 293 K. We can observe strong intensity at around 1/2 0 1/2 and additional intensities at 100, 001, 1/2 0 3/2 and 3/2 0 1/2 reciprocal points where nuclear re¯ections are forbidden. These additional peaks
seem to be intense at low temperature but the peak position of 1/2 0 3/2 and 3/2 0 1/2 reciprocal points seemed to shift to lower angles, an indication of an incommensuration. Murakami et al. [7,8] observed the forbidden (h00) re¯ections (h , k odd) and 1/2 1/2 0 re¯ection (in Pbnm setting, the longest axis is c-axis) by synchroton X-ray diffraction measurement. Their observed intensities of those re¯ections increase with decreasing temperature, showing a similar behavior with our present and previous observation (for sample with x 0:15) [9]. They assigned 100 and 001 re¯ections to the OO, and 1/2 1/2 0 to the charge ordering (CO). Powder neutron diffraction intensities are shown in Fig. 2, weak re¯ections were observed at 2u 13.448, 19.108 and 30.418, indexed as 1/2 0 1/2, 001 and 1/2 0 3/2, respectively. Similar intensities were also observed in our previous studies for the SC sample with x 0:15 [9]. We observed strong diffuse scattering at FM Bragg point 101 only in the PM phase [9], but we did not observe similar diffuse scattering at the same Bragg point 101 for sample with x 0:125: This difference could be due to the enhanced correlation length of the ferromagnetic ordering at low temperature for sample with x 0:125 rather than the sample with x 0:15: Diffuse scattering at around 000 points in 293 K, i.e. the small angle scattering, provides the evidences for the presence of magnetic clusters. The Ê at 293 K (FM cluster size can be estimated to be 3±4 A cluster). That means, for the FM cluster, the correlation is limited to the ®rst neighbor of Mn-sites. The AF-type cluster size can also be estimated from the FWHM of the diffuse scattering at 1/2 0 1/2 position at 8 K. The AF-cluster size is Ê. about 20 A Temperature dependence of magnetization under a magnetic ®eld of 0.01 T is shown in Fig. 3, as measured with increasing temperature after the sample is ZFC and FC down to 4.2 K. A cusp is observed at about 100 K. It is evident that the FC curve coincide with the ZFC curve at temperature 130 K or above, but it starts to separate from ZFC curve below 130 K. We observed 26% decrease of ZFC magnetization than that of FC at 5 K, i.e. the value of
S. Begum et al. / Journal of Physics and Chemistry of Solids 63 (2002) 939±942
941
Fig. 3. Temperature dependence of ZFC and FC magnetization measured in a warming process under a magnetic ®eld of 0.01 T.
magnetizations are 0.50 and 0.37 emu/mol for FC and ZFC magnetization, respectively. This irreversibility nature is a remarkable evidence of the cluster-glass behavior which does not have simple long range FM order, being consistent with the observation of diffuse scattering around 1/2 0 1/2 which is also the indication of short-range magnetic order at low temperature. Furthermore, the FC curve shows some unusual bent below the cluster-glass temperature Tg( 130 K) where FC and ZFC curves start to deviate. Similar observation is reported by Maignan et al. [10]. To characterize the crystal structure, we have performed Rietveld analysis using RIETAN'97 [11] for neutron powder diffraction patterns observed at selected temperatures. A satisfactory agreement between the observed and calculated intensities were obtained with space symmetry Pnma, for example, Rwp 12:04%; RI 4:01% for 10 K and Rwp 11:79%; RI 4:08% for 293 K, respectively. b Lattice constants are determined as, a 5:481 3 A; at 10 K and a 7:7564 4 A; c 5:5087 3 A b 7:7651 4 A; c 5:5190 3 A at 293 K, 5:4877 3 A; respectively. A ferromagnetic contribution was observed below Curie temperature Tc 170 K as an additional increase of the Bragg intensities at low 2u 's. Main contribution is seen at 200 peak at low temperature, e.g. the increase of intensities at 10 K at 101 Bragg peak is 104% but 414% at 200, relative to those at the ambient. The average magnetic moment of Mn ion is 3.0 (1) m B/Mn at 10 K, which is well below than 3.875 m B/Mn the simple average moment of the mixed Mn 31/Mn 41 lattice at x 0:125: The reduced magnetic moment of Mn ion can be understood in terms of the canting of Mn spins. Temperature variation of Mn±O bond lengths is shown in Fig. 4(a). A large J±T distortion is observed at 10 K. With decreasing temperature from the ambient, the Mn±O octahedrons do not show any J±T distortion down to 100 K, but start to deviate from this temperature, i.e. equatorial Mn±O(2 0 ) bonds became short while the other Mn±O(2) elongated from 100 to 10 K and Mn±O(1) distances remain practically unchanged with the variation of temperature. The transition between 50 and 100 K seems to be secondorder type since x 0:10 and x 0:15 samples (unpublished) also show consistently smooth temperature alteration
Fig. 4. (a) Mn±O bond lengths in La0.875Ca0.125MnO3 as a function of temperature obtained from neutron powder diffraction data (The lines are guide to the eyes) and (b) a schematic picture of MnO6 at 10 K.
of Mn±O distances. The contraction of Mn±O(2 0 ) bonds Ê , 0.31% and the elongation between 10 and 100 K is 0.006 A Ê of Mn±O(2) bonds is 0.0057 A, 0.29%. This behavior can be understood in terms of partial ordering of d3x2 2y2 and/or d3z2 2r2 orbitals in the Pnma lattice. The J±T distortion occurs due to the degeneracy of the eg state, which induces orbital polarization. Strong eg-O-2P hybridization makes the eg states of Mn ions itinerant along the longer Mn±O bond which has a lower energy and thus will be more populated than the other case. In the present system, it is not clear which one of the d3x2 2r2 and d3z2 2r2 orbitals, is more stabilized. If d3z2 2r2 orbital is to be stabilize, it is necessary that the c axis should be longer than the a axis [12]. Present lattice parameter data show consistency and we may suggest that d3z2 2r2 type orbital is a little dominant rather than the d3x2 2r2 : Fig. 4(b) shows a schematic picture of MnO6, at 10 K. In conclusion, the observation of diffuse scattering from SC neutron diffraction and cluster-glass behavior from magnetization measurements suggest the existence of short-range anti-ferromagnetism which may correlates with the mixture of d3x2 2r2 and d3z2 2r2 -type OO (a little dominant of the d3z2 2r2 -type orbital) below 100 K for La12xCaxMnO3 with x 1=8: The AF clusters may become frozen easier for this special Ca-concentration x 1=8 than the previous sample with x 0:15 [9]. References [1] H. Fujishiro, T. Fukase, M. Ikebe, T. Kikuchi, J. Phys. Soc. Jpn 68 (1999) 1469. [2] M. De Teressa, M.R. Ibara, P.A. Algarabel, C. Ritter, C. Marquina, J. Blasco, J. Carcia, A. del Moral, Z. Arnold, Nature (London) 386 (1997) 256. [3] A. Moreo, S. Yunoki, E. Dagotto, Science 283 (1999) 2034. [4] H. Sawada, Y. Morikawa, K. Terakura, N. Hamada, Phys. Rev. B 56 (1997) 12154.
942
S. Begum et al. / Journal of Physics and Chemistry of Solids 63 (2002) 939±942
[5] Y. Okuda, Y. Tomioka, A. Asamitsu, Y. Tokura, Phys. Rev. B 61 (2000) 8009. [6] K. Ohoyama, T. Kanouchi, K. Nemoto, M. Ohashi, T. Kajitani, Y. Yamaguchi, Jpn. J. Appl. Phys. 37 (1998) 3314. [7] Y. Murakami, J.P. Hill, D. Gibbs, M. Blume, I. Koyama, M. Tanaka, H. Kawata, T. Arima, Y. Tokura, K. Hirota, Y. Endoh, Phys. Rev. Lett. 81 (1998) 582. [8] Y. Murakami, H. Kawada, H. Kawata, M. Tanaka, T. Arima, Y. Moritomo, Y. Tokura, Phys. Rev. Lett. 80 (1998) 1932.
[9] S. Begum, Y. Ono, Y. Miyazaki, Y. Tomioka, Y. Tokura, T. Kajitani, J. Phys. Soc. Jpn 70 (2001) 241. [10] A. Maignan, C. Martin, F. Damay, B. Raveau, J. Hejtmanek, Phys. Rev. B 58 (1998) 2758. [11] F. Izumi, in: R.A. Young (Ed.), The Rietveld Method, Oxford University Press, Oxford, 1993 Chapter 3. [12] R. Maezono, S. Ishihara, N. Nagaosa, Phys. Rev. B 57 (1998) R13993.
www.elsevier.com/locate/jpcs
Jahn±Teller distortion and cluster-glass like behavior in La0.875Ca0.125MnO3 S. Begum a,*, Y. Ono a, Y. Tomioka b, Y. Tokura b,c, Y. Ishii d, Y. Morii d, T. Kajitani a a
Department of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan b Joint Research Center for Atom Technology (JRCAT), Tsukuba 305-0046, Japan c Department of Applied Physics, University of Tokyo, Tokyo 113-0033, Japan d Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokaimura, Naka-Gun, Ibaraki 319-1195, Japan
Abstract Neutron diffraction and magnetization measurements were carried out on both powder and single crystal samples of La0.875Ca0.125MnO3. At room temperature (RT), this sample is paramagnetic and single phase, with crystallographic symmetry Pnma (the longest axis is b-axis). Neutron powder diffraction shows that the average magnetic moment of Mn ions is 3.0 (1) m B/ Mn at 10 K, which is signi®cantly less than an averaged moment estimated from the ratio of Mn 31/Mn 41 which amounts to 3.875 m B for 12.5% Mn 41, an indication of spin canting. The crystal structure is characterized by the ordering of two eg orbitals, being evidenced by the anomalous change of Mn±O bond lengths which starts from 100 K with decreasing temperature. Neutron diffraction measurement was carried out at 8 K and RT. Diffuse scattering at around 1/2 0 1/2 point was observed at 8 K but disappeared in the paramagnetic region. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Magnetic materials; C. Neutron scattering; D. Crystal structure; D. Magnetic properties
1. Introduction The La12xAxMnO3 (where A refers to Ca, Sr, etc.) has currently been the subject of intense theoretical, experimental, and applied interest. The low temperature phase of La12xAxMnO3 with x 0:125 is assigned to the insulating ferromagnetic (FM) phase below Curie temperature Tc [1]. Recently, the small FM polarons were observed by X-ray and neutron diffraction measurements [2] in the La/Casystem. It was suggested that the co-existence of FM and anti-ferromagnetic (AF) domains is essential for the CMR behavior in the manganites [3]. In the present work, we focus our attention on the Jahn±Teller (J±T) distortion which induces the ordering of eg orbitals. In the conventional pseudo-tetragonal unit cell, it is believed that d3x2 2r2 or d3y2 2r2 orbitals are dominantly populated in the basal plane and the d3z2 2r2 orbitals of eg symmetry become less populated in the ordered phase (orbital ordered phase) [4]. This orbital ordering (OO) increases the AF contribution and reduces the interlayer * Corresponding author. Fax: 181-22-263-9836. E-mail address: [email protected] (S. Begum).
FM exchange [4]. We present the strong magnetic diffuse scattering at around 1/2 0 1/2 reciprocal lattice point at low temperature for La12xCaxMnO3 with x 0:125 single crystal (SC), but disappearing in the paramagnetic region, being an evidence of relatively large AF clusters. We also carried out neutron powder diffraction measurement for this sample with x 0:125 and observed weak re¯ection at the same reciprocal lattice point at 10 K, i.e. at 1/2 0 1/2 which strengthened the existence of AF-type magnetic ordering as well as AF-type clusters observed in the SC sample of the same Ca-concentration. Our magnetization measurement in the ®eld of 0.01 T shows the cluster-glass like behavior at low temperatures. 2. Experimental Powder sample (PS) of La0.875Ca0.125MnO3 was prepared by a standard solid state reactions from the stoichiometric mixture of powders La2O3 (99.9%), Mn2O3 (99.9%), and CaCO3 (99.9%). The mixture was calcined in air at 1000 8C for 48 h. With one intermediate grinding, pressed into pellet and sintered at 1500 8C for 36 h in air. The room temperature (RT) X-ray diffraction analysis indicated that
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(02)00141-5
940
S. Begum et al. / Journal of Physics and Chemistry of Solids 63 (2002) 939±942
Fig. 2. Neutron powder diffraction pattern of La0.875Ca0.125MnO3 at 10 and 293 K. sample measured with HRPD l 1:8227 A For 10 K, the y scale is shifted by 90 counts. 1/2 0 1/2, 100 and 1/ 2 0 3/2 are the forbidden peaks in the Pnma symmetry.
Fig. 1. Contour maps of neutron diffraction intensities of La0.875Ca0.125MnO3 in h0l reciprocal plane at 8 and 293 K.
the sample has a single phase orthorhombic perovskite structure. The SC sample was grown using the traveling solvent ¯oating zone (TSFZ) method by one of the present authors, Tomioka. The synthesis procedure for SC sample is brie¯y described elsewhere [5]. The neutron powder diffraction measurements were carried out at temperatures from The SC neutron 10 K to RT using HRPD l 1:8227 A: diffraction measurements were carried out using HERMES [6], at temperatures 8 and 293 K. Measure l 1:8196 A ments were made in the a p ±c p scattering plane. The temperature dependence of zero ®eld cooled (ZFC) and ®eld cooled (FC) magnetization measurements were carried out in a warming process under a magnetic ®eld of 0.01 T using a quantum design SQUID magnetometer.
3. Results and discussion Fig. 1 shows the contour maps of neutron diffraction intensities of La0.875Ca0.125MnO3 in 010 plane at 8 K and 293 K. Well de®ned diffuse scattering is represented at about 1/2 0 1/2 reciprocal point at 8 K but almost vanishes in the paramagnetic region, i.e. at 293 K. We can observe strong intensity at around 1/2 0 1/2 and additional intensities at 100, 001, 1/2 0 3/2 and 3/2 0 1/2 reciprocal points where nuclear re¯ections are forbidden. These additional peaks
seem to be intense at low temperature but the peak position of 1/2 0 3/2 and 3/2 0 1/2 reciprocal points seemed to shift to lower angles, an indication of an incommensuration. Murakami et al. [7,8] observed the forbidden (h00) re¯ections (h , k odd) and 1/2 1/2 0 re¯ection (in Pbnm setting, the longest axis is c-axis) by synchroton X-ray diffraction measurement. Their observed intensities of those re¯ections increase with decreasing temperature, showing a similar behavior with our present and previous observation (for sample with x 0:15) [9]. They assigned 100 and 001 re¯ections to the OO, and 1/2 1/2 0 to the charge ordering (CO). Powder neutron diffraction intensities are shown in Fig. 2, weak re¯ections were observed at 2u 13.448, 19.108 and 30.418, indexed as 1/2 0 1/2, 001 and 1/2 0 3/2, respectively. Similar intensities were also observed in our previous studies for the SC sample with x 0:15 [9]. We observed strong diffuse scattering at FM Bragg point 101 only in the PM phase [9], but we did not observe similar diffuse scattering at the same Bragg point 101 for sample with x 0:125: This difference could be due to the enhanced correlation length of the ferromagnetic ordering at low temperature for sample with x 0:125 rather than the sample with x 0:15: Diffuse scattering at around 000 points in 293 K, i.e. the small angle scattering, provides the evidences for the presence of magnetic clusters. The Ê at 293 K (FM cluster size can be estimated to be 3±4 A cluster). That means, for the FM cluster, the correlation is limited to the ®rst neighbor of Mn-sites. The AF-type cluster size can also be estimated from the FWHM of the diffuse scattering at 1/2 0 1/2 position at 8 K. The AF-cluster size is Ê. about 20 A Temperature dependence of magnetization under a magnetic ®eld of 0.01 T is shown in Fig. 3, as measured with increasing temperature after the sample is ZFC and FC down to 4.2 K. A cusp is observed at about 100 K. It is evident that the FC curve coincide with the ZFC curve at temperature 130 K or above, but it starts to separate from ZFC curve below 130 K. We observed 26% decrease of ZFC magnetization than that of FC at 5 K, i.e. the value of
S. Begum et al. / Journal of Physics and Chemistry of Solids 63 (2002) 939±942
941
Fig. 3. Temperature dependence of ZFC and FC magnetization measured in a warming process under a magnetic ®eld of 0.01 T.
magnetizations are 0.50 and 0.37 emu/mol for FC and ZFC magnetization, respectively. This irreversibility nature is a remarkable evidence of the cluster-glass behavior which does not have simple long range FM order, being consistent with the observation of diffuse scattering around 1/2 0 1/2 which is also the indication of short-range magnetic order at low temperature. Furthermore, the FC curve shows some unusual bent below the cluster-glass temperature Tg( 130 K) where FC and ZFC curves start to deviate. Similar observation is reported by Maignan et al. [10]. To characterize the crystal structure, we have performed Rietveld analysis using RIETAN'97 [11] for neutron powder diffraction patterns observed at selected temperatures. A satisfactory agreement between the observed and calculated intensities were obtained with space symmetry Pnma, for example, Rwp 12:04%; RI 4:01% for 10 K and Rwp 11:79%; RI 4:08% for 293 K, respectively. b Lattice constants are determined as, a 5:481 3 A; at 10 K and a 7:7564 4 A; c 5:5087 3 A b 7:7651 4 A; c 5:5190 3 A at 293 K, 5:4877 3 A; respectively. A ferromagnetic contribution was observed below Curie temperature Tc 170 K as an additional increase of the Bragg intensities at low 2u 's. Main contribution is seen at 200 peak at low temperature, e.g. the increase of intensities at 10 K at 101 Bragg peak is 104% but 414% at 200, relative to those at the ambient. The average magnetic moment of Mn ion is 3.0 (1) m B/Mn at 10 K, which is well below than 3.875 m B/Mn the simple average moment of the mixed Mn 31/Mn 41 lattice at x 0:125: The reduced magnetic moment of Mn ion can be understood in terms of the canting of Mn spins. Temperature variation of Mn±O bond lengths is shown in Fig. 4(a). A large J±T distortion is observed at 10 K. With decreasing temperature from the ambient, the Mn±O octahedrons do not show any J±T distortion down to 100 K, but start to deviate from this temperature, i.e. equatorial Mn±O(2 0 ) bonds became short while the other Mn±O(2) elongated from 100 to 10 K and Mn±O(1) distances remain practically unchanged with the variation of temperature. The transition between 50 and 100 K seems to be secondorder type since x 0:10 and x 0:15 samples (unpublished) also show consistently smooth temperature alteration
Fig. 4. (a) Mn±O bond lengths in La0.875Ca0.125MnO3 as a function of temperature obtained from neutron powder diffraction data (The lines are guide to the eyes) and (b) a schematic picture of MnO6 at 10 K.
of Mn±O distances. The contraction of Mn±O(2 0 ) bonds Ê , 0.31% and the elongation between 10 and 100 K is 0.006 A Ê of Mn±O(2) bonds is 0.0057 A, 0.29%. This behavior can be understood in terms of partial ordering of d3x2 2y2 and/or d3z2 2r2 orbitals in the Pnma lattice. The J±T distortion occurs due to the degeneracy of the eg state, which induces orbital polarization. Strong eg-O-2P hybridization makes the eg states of Mn ions itinerant along the longer Mn±O bond which has a lower energy and thus will be more populated than the other case. In the present system, it is not clear which one of the d3x2 2r2 and d3z2 2r2 orbitals, is more stabilized. If d3z2 2r2 orbital is to be stabilize, it is necessary that the c axis should be longer than the a axis [12]. Present lattice parameter data show consistency and we may suggest that d3z2 2r2 type orbital is a little dominant rather than the d3x2 2r2 : Fig. 4(b) shows a schematic picture of MnO6, at 10 K. In conclusion, the observation of diffuse scattering from SC neutron diffraction and cluster-glass behavior from magnetization measurements suggest the existence of short-range anti-ferromagnetism which may correlates with the mixture of d3x2 2r2 and d3z2 2r2 -type OO (a little dominant of the d3z2 2r2 -type orbital) below 100 K for La12xCaxMnO3 with x 1=8: The AF clusters may become frozen easier for this special Ca-concentration x 1=8 than the previous sample with x 0:15 [9]. References [1] H. Fujishiro, T. Fukase, M. Ikebe, T. Kikuchi, J. Phys. Soc. Jpn 68 (1999) 1469. [2] M. De Teressa, M.R. Ibara, P.A. Algarabel, C. Ritter, C. Marquina, J. Blasco, J. Carcia, A. del Moral, Z. Arnold, Nature (London) 386 (1997) 256. [3] A. Moreo, S. Yunoki, E. Dagotto, Science 283 (1999) 2034. [4] H. Sawada, Y. Morikawa, K. Terakura, N. Hamada, Phys. Rev. B 56 (1997) 12154.
942
S. Begum et al. / Journal of Physics and Chemistry of Solids 63 (2002) 939±942
[5] Y. Okuda, Y. Tomioka, A. Asamitsu, Y. Tokura, Phys. Rev. B 61 (2000) 8009. [6] K. Ohoyama, T. Kanouchi, K. Nemoto, M. Ohashi, T. Kajitani, Y. Yamaguchi, Jpn. J. Appl. Phys. 37 (1998) 3314. [7] Y. Murakami, J.P. Hill, D. Gibbs, M. Blume, I. Koyama, M. Tanaka, H. Kawata, T. Arima, Y. Tokura, K. Hirota, Y. Endoh, Phys. Rev. Lett. 81 (1998) 582. [8] Y. Murakami, H. Kawada, H. Kawata, M. Tanaka, T. Arima, Y. Moritomo, Y. Tokura, Phys. Rev. Lett. 80 (1998) 1932.
[9] S. Begum, Y. Ono, Y. Miyazaki, Y. Tomioka, Y. Tokura, T. Kajitani, J. Phys. Soc. Jpn 70 (2001) 241. [10] A. Maignan, C. Martin, F. Damay, B. Raveau, J. Hejtmanek, Phys. Rev. B 58 (1998) 2758. [11] F. Izumi, in: R.A. Young (Ed.), The Rietveld Method, Oxford University Press, Oxford, 1993 Chapter 3. [12] R. Maezono, S. Ishihara, N. Nagaosa, Phys. Rev. B 57 (1998) R13993.