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Magnetic inhomogeneities in electron-doped Ca1−xLaxMnO3
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 246–248
Magnetic inhomogeneities in electron-doped Ca1xLaxMnO3 C.D. Linga,*, E. Granadob, J.J. Neumeierc, J.W. Lynnd, D.N. Argyrioue a Institut Laue-Langevin, 6 rue J. Horowitz, BP 156, 38042 Grenoble Cedex 9, France ! Laborotorio Nacional de Luz S!ıncrotron, Caixa Postal 6192, CEP 13084-971, Campinas, SP, Brazil c Department of Physics, Montana State University, Bozeman, Montana 59717, USA d NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA e Hahn-Maitner-Institut, Glienicker str. 100, Berlin D-14109, Germany b
Abstract We present the results of a neutron scattering investigation of Ca1xLaxMnO3 in the electron-doped (xB0) regime. ( in diameter are identified in a liquid-like distribution within the G-AFM For light electron-doping, FM clusters B10 A matrix. This mirrors behavior in the hole-doped (xB1) regime. These clusters are not long-range correlated for xp0:05; however, for 0:05pxp0:10; a spontaneous long-range FM moment is observed. This seems to be due to the formation of a ‘spin glass’ of overlapping clusters, as opposed to either the long-range delocalization of doped electrons or longrange phase separation into electron-rich FM and electron-poor AFM regions. r 2003 Elsevier B.V. All rights reserved. PACS: 61.12.q; 75.25.+z; 61.25.f; 75.70.kw Keywords: Neutron scattering; Colossal magnetoresistance; Manganite; Clusters
The application of simple double-exchange (DE) to 3þ perovskites such as Ca1x Lax Mn4þ 1x Mnx O3 predicts that the introduction of doped carriers (eg -electrons on Mn3+) to the system with x will lead to the homogeneous ferromagnetic (FM) canting of the G-type antiferromagnetic (G-AFM) state throughout the phase diagram [1]. The rich magnetic phase diagram that is actually observed [2] is a consequence of the degeneracy of the dx2 y2 and d3z2 r2 eg -orbitals. This leads to static and dynamic Jahn–Teller (J–T) distortions that mediate the coupling of charge, spin and lattice degrees of freedom lying at the heart of the colossal magnetoresistance (CMR) effect. For light electron-doping (xB0), a spontaneous and ostensibly homogeneous FM-canting of the G-AFM state has been observed by neutron powder diffraction (NPD) (e.g. Refs. [3–7]). At the holedoped end of the phase diagram (xB1), however, an analogous FM component has recently been shown to be locally inhomogeneous, consisting of a liquid *Corresponding author. Tel.: +33-4-76207388; fax: +33-476207648. E-mail address: [email protected] (C.D. Ling).
( clusters emdistribution of strongly canted B10 A bedded in a lightly canted matrix [8]. We synthesized polycrystalline Ca1xLaxMnO3, 0:0pxp0:2 using conventional ceramic techniques. DC magnetization measurements confirmed that an FM moment could be induced for xo0:2 [9]. Rietveldrefinement of NPD patterns collected at the Intense Pulsed Neutron Source (IPNS), as well as polarized neutron scattering experiments performed at the NIST Center for Neutron Research (NCNR), showed that this moment was (at least partly) spontaneous and long-range for 0:06pxp0:16: In the same range of x; the canted-GAFM state was found to co-exist at low temperature with the C-AFM state [2], identified by its characteristic magnetic Bragg reflections and also by a lowering of symmetry due to a co-operative J–T distortion. Careful Rietveld-refinements [10], focusing on the relationships among magnetic and structural phase transitions, support the assertion [4,11,12] that mesoscopic G-AFM/CAFM phase separation is intrinsic in these systems, and not an indication of sample inhomogeneity. Elastic neutron scattering experiments at NCNR [13] found magnetic scattering at low-Q similar to that seen
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.102
ARTICLE IN PRESS C.D. Ling et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 246–248
247
Fig. 2. Low-temperature magnetic field-dependence of the intensity of characteristic G-AFM and FM+nuclear Bragg reflections for x ¼ 0:09: Open symbols represent increasing field, filled symbols represent decreasing field. The inset shows a separate run focusing more closely on FM+nuclear intensity [13].
Fig. 1. Elastic magnetic neutron scattering cross-section (scattering at 200 K subtracted from that at 10 K) vs. Q for x ¼ 0:02 and 0.09, fit to a liquid-like distribution of magnetic clusters [13].
in the hole-doped regime [8]. Magnetic scattering, obtained by subtracting scattering at 200 K from that at 10 K, is shown in Fig. 1 for x ¼ 0:02 and 0.09. The fit shown is to a model for a liquid-like distribution of rigid magnetic clusters, fit variables being cluster diameter D; minimum separation dmin and concentration NV [8]. For x ¼ 0:02; where only an induced long-range FM ( moment is observed, we obtain D ¼ 10:4ð1:8Þ A, ( and NV ¼ 6:6ð1:4Þ 106 A ( 3 (1 cluster dmin ¼ 41ð3Þ A
per B59(12) doped electrons). For x ¼ 0:09; where a significant long-range FM moment appears sponta( dmin ¼ 24(2) A ( and neously, we obtain D ¼ 10:6(1.6) A, ( 3 (1 cluster per B63(14) doped NV ¼ 28(6) 106 A electrons). The difference between clusters in the regimes of induced and spontaneous long-range FM therefore appears only to be their separation. This is consistent with the hypothesis that spontaneous long-range FM at higher electron-doping is due to a ‘cluster-glass’ of discrete DE entities [14], as opposed to either a longrange delocalization of DE carriers [1] or a mesoscopic separation into electron-rich FM and electron-poor AFM regions (for a recent review of phase separation issues see Ref. [15]). This interpretation of the phase diagram was further supported by a field-dependent NPD experiment at NCNR [13]. Characteristic AFM and FM (coincident with structural) Bragg reflections were monitored as
functions of a magnetic field H applied perpendicular to the scattering plane (Fig. 2). For x ¼ 0:09; the FM intensity increased in the range 0pHp1 T, consistent with the re-orientation of FM domains parallel to H (i.e. perpendicular to the scattering vector, maximizing scattering from the palletized powder sample). At the same time, the G-AFM reflection decreased in intensity by 34(3) %, consistent with a reorientation of G-AFM domains perpendicular to H (i.e. a decrease from 23 of maximum scattering in a 3D averaged powder to 12 of maximum scattering in a 2D averaged powder). No such re-orientation of the G-AFM moment was observed for x ¼ 0:02: This implies that the FM moment is decoupled from the G-AFM matrix at light electron-doping but strongly coupled to it at higher electron-doping, consistent with the development from isolated clusters to a cluster-glass and inconsistent with FM/AFM phase separation. The electron-doped regime of this CMR manganite perovskite phase diagram therefore appears to mirror the hole-doped regime for very light doping, with the presence of similar FM clusters. Above only B10% electron-doping, however, the J–T asymmetry of the phase diagram is evident as the purely FM state is not achieved, the co-operatively J–T distorted C-AFM state appearing instead.
References [1] [2] [3] [4] [5] [6] [7]
P.-G. de Gennes, Phys. Rev. 118 (1960) 141. E.O. Wollan, et al., Phys Rev. 100 (1955) 545. K. Hagdorn, et al., Eur. Phys. J. B 11 (1999) 243. P.N. Santhosh, et al., Phys. Rev. B 62 (2000) 14928. C. Martin, et al., J. Magn. Magn. Mater. 205 (1999) 184. M. Respaud, et al., Phys. Rev. B 63 (2001) 144426. C. Martin, et al., Phys. Rev. B 62 (2000) 6442.
ARTICLE IN PRESS 248 [8] [9] [10] [11]
C.D. Ling et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 246–248 M. Hennion, et al., Phys. Rev. Lett. 81 (1998) 1957. J.J. Neumeier, et al., Phys. Rev. B 62 (2000) 14319. C.D. Ling, et al., Phys. Rev. B 68 (2003) 134439. R. Mahendiran, et al., Phys. Rev. B 62 (2000) 11642.
[12] [13] [14] [15]
P.H. Algarabel, et al., Phys. Rev. B 65 (2002) 104437. E. Granado, et al., cond-mat 0303311, 2003. A. Maignan, et al., Phys. Rev. B 58 (1998) 2758. E. Dagotto, et al., Phys. Rep. 344 (2001) 1.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 246–248
Magnetic inhomogeneities in electron-doped Ca1xLaxMnO3 C.D. Linga,*, E. Granadob, J.J. Neumeierc, J.W. Lynnd, D.N. Argyrioue a Institut Laue-Langevin, 6 rue J. Horowitz, BP 156, 38042 Grenoble Cedex 9, France ! Laborotorio Nacional de Luz S!ıncrotron, Caixa Postal 6192, CEP 13084-971, Campinas, SP, Brazil c Department of Physics, Montana State University, Bozeman, Montana 59717, USA d NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA e Hahn-Maitner-Institut, Glienicker str. 100, Berlin D-14109, Germany b
Abstract We present the results of a neutron scattering investigation of Ca1xLaxMnO3 in the electron-doped (xB0) regime. ( in diameter are identified in a liquid-like distribution within the G-AFM For light electron-doping, FM clusters B10 A matrix. This mirrors behavior in the hole-doped (xB1) regime. These clusters are not long-range correlated for xp0:05; however, for 0:05pxp0:10; a spontaneous long-range FM moment is observed. This seems to be due to the formation of a ‘spin glass’ of overlapping clusters, as opposed to either the long-range delocalization of doped electrons or longrange phase separation into electron-rich FM and electron-poor AFM regions. r 2003 Elsevier B.V. All rights reserved. PACS: 61.12.q; 75.25.+z; 61.25.f; 75.70.kw Keywords: Neutron scattering; Colossal magnetoresistance; Manganite; Clusters
The application of simple double-exchange (DE) to 3þ perovskites such as Ca1x Lax Mn4þ 1x Mnx O3 predicts that the introduction of doped carriers (eg -electrons on Mn3+) to the system with x will lead to the homogeneous ferromagnetic (FM) canting of the G-type antiferromagnetic (G-AFM) state throughout the phase diagram [1]. The rich magnetic phase diagram that is actually observed [2] is a consequence of the degeneracy of the dx2 y2 and d3z2 r2 eg -orbitals. This leads to static and dynamic Jahn–Teller (J–T) distortions that mediate the coupling of charge, spin and lattice degrees of freedom lying at the heart of the colossal magnetoresistance (CMR) effect. For light electron-doping (xB0), a spontaneous and ostensibly homogeneous FM-canting of the G-AFM state has been observed by neutron powder diffraction (NPD) (e.g. Refs. [3–7]). At the holedoped end of the phase diagram (xB1), however, an analogous FM component has recently been shown to be locally inhomogeneous, consisting of a liquid *Corresponding author. Tel.: +33-4-76207388; fax: +33-476207648. E-mail address: [email protected] (C.D. Ling).
( clusters emdistribution of strongly canted B10 A bedded in a lightly canted matrix [8]. We synthesized polycrystalline Ca1xLaxMnO3, 0:0pxp0:2 using conventional ceramic techniques. DC magnetization measurements confirmed that an FM moment could be induced for xo0:2 [9]. Rietveldrefinement of NPD patterns collected at the Intense Pulsed Neutron Source (IPNS), as well as polarized neutron scattering experiments performed at the NIST Center for Neutron Research (NCNR), showed that this moment was (at least partly) spontaneous and long-range for 0:06pxp0:16: In the same range of x; the canted-GAFM state was found to co-exist at low temperature with the C-AFM state [2], identified by its characteristic magnetic Bragg reflections and also by a lowering of symmetry due to a co-operative J–T distortion. Careful Rietveld-refinements [10], focusing on the relationships among magnetic and structural phase transitions, support the assertion [4,11,12] that mesoscopic G-AFM/CAFM phase separation is intrinsic in these systems, and not an indication of sample inhomogeneity. Elastic neutron scattering experiments at NCNR [13] found magnetic scattering at low-Q similar to that seen
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.102
ARTICLE IN PRESS C.D. Ling et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 246–248
247
Fig. 2. Low-temperature magnetic field-dependence of the intensity of characteristic G-AFM and FM+nuclear Bragg reflections for x ¼ 0:09: Open symbols represent increasing field, filled symbols represent decreasing field. The inset shows a separate run focusing more closely on FM+nuclear intensity [13].
Fig. 1. Elastic magnetic neutron scattering cross-section (scattering at 200 K subtracted from that at 10 K) vs. Q for x ¼ 0:02 and 0.09, fit to a liquid-like distribution of magnetic clusters [13].
in the hole-doped regime [8]. Magnetic scattering, obtained by subtracting scattering at 200 K from that at 10 K, is shown in Fig. 1 for x ¼ 0:02 and 0.09. The fit shown is to a model for a liquid-like distribution of rigid magnetic clusters, fit variables being cluster diameter D; minimum separation dmin and concentration NV [8]. For x ¼ 0:02; where only an induced long-range FM ( moment is observed, we obtain D ¼ 10:4ð1:8Þ A, ( and NV ¼ 6:6ð1:4Þ 106 A ( 3 (1 cluster dmin ¼ 41ð3Þ A
per B59(12) doped electrons). For x ¼ 0:09; where a significant long-range FM moment appears sponta( dmin ¼ 24(2) A ( and neously, we obtain D ¼ 10:6(1.6) A, ( 3 (1 cluster per B63(14) doped NV ¼ 28(6) 106 A electrons). The difference between clusters in the regimes of induced and spontaneous long-range FM therefore appears only to be their separation. This is consistent with the hypothesis that spontaneous long-range FM at higher electron-doping is due to a ‘cluster-glass’ of discrete DE entities [14], as opposed to either a longrange delocalization of DE carriers [1] or a mesoscopic separation into electron-rich FM and electron-poor AFM regions (for a recent review of phase separation issues see Ref. [15]). This interpretation of the phase diagram was further supported by a field-dependent NPD experiment at NCNR [13]. Characteristic AFM and FM (coincident with structural) Bragg reflections were monitored as
functions of a magnetic field H applied perpendicular to the scattering plane (Fig. 2). For x ¼ 0:09; the FM intensity increased in the range 0pHp1 T, consistent with the re-orientation of FM domains parallel to H (i.e. perpendicular to the scattering vector, maximizing scattering from the palletized powder sample). At the same time, the G-AFM reflection decreased in intensity by 34(3) %, consistent with a reorientation of G-AFM domains perpendicular to H (i.e. a decrease from 23 of maximum scattering in a 3D averaged powder to 12 of maximum scattering in a 2D averaged powder). No such re-orientation of the G-AFM moment was observed for x ¼ 0:02: This implies that the FM moment is decoupled from the G-AFM matrix at light electron-doping but strongly coupled to it at higher electron-doping, consistent with the development from isolated clusters to a cluster-glass and inconsistent with FM/AFM phase separation. The electron-doped regime of this CMR manganite perovskite phase diagram therefore appears to mirror the hole-doped regime for very light doping, with the presence of similar FM clusters. Above only B10% electron-doping, however, the J–T asymmetry of the phase diagram is evident as the purely FM state is not achieved, the co-operatively J–T distorted C-AFM state appearing instead.
References [1] [2] [3] [4] [5] [6] [7]
P.-G. de Gennes, Phys. Rev. 118 (1960) 141. E.O. Wollan, et al., Phys Rev. 100 (1955) 545. K. Hagdorn, et al., Eur. Phys. J. B 11 (1999) 243. P.N. Santhosh, et al., Phys. Rev. B 62 (2000) 14928. C. Martin, et al., J. Magn. Magn. Mater. 205 (1999) 184. M. Respaud, et al., Phys. Rev. B 63 (2001) 144426. C. Martin, et al., Phys. Rev. B 62 (2000) 6442.
ARTICLE IN PRESS 248 [8] [9] [10] [11]
C.D. Ling et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 246–248 M. Hennion, et al., Phys. Rev. Lett. 81 (1998) 1957. J.J. Neumeier, et al., Phys. Rev. B 62 (2000) 14319. C.D. Ling, et al., Phys. Rev. B 68 (2003) 134439. R. Mahendiran, et al., Phys. Rev. B 62 (2000) 11642.
[12] [13] [14] [15]
P.H. Algarabel, et al., Phys. Rev. B 65 (2002) 104437. E. Granado, et al., cond-mat 0303311, 2003. A. Maignan, et al., Phys. Rev. B 58 (1998) 2758. E. Dagotto, et al., Phys. Rep. 344 (2001) 1.