Colossal magnetoresistive manganites

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Abstract

Magnetoelectronic features of the perovskite-type manganites are overviewed in the light of the mechanism of the colossal magnetoresistance (CMR). The essential ingredient of the CMR physics is not only the double-exchange interaction but also other competing interactions, such as ferromagnetic/antiferromagnetic superexchange interactions and charge/orbital ordering instabilities as well as their strong coupling with the lattice deformation. In particular, the orbital degree of freedom of the conduction electrons in the near-degenerate 3d eg state plays an essential role in producing the unconventional metal–insulator phenomena in the manganites via strong coupling with spin, charge, and lattice degrees of freedom. Insulating or poorly conducting states arise from the long or short-range correlations of charge and orbital, but can be mostly melted or turned into the orbital-disordered conducting state by application of a magnetic field, producing the CMR or the insulator–metal transition.

Introduction

The particular magnetoresistance (MR) phenomena described here are the gigantic decrease of resistance by application of a magnetic field that is observed for the manganese oxides (manganites) with perovskite or related structures [1]. Such a gigantic negative MR is now termed “colossal magnetoresistance” (CMR) to distinguish it from the giant magnetoresistance (GMR) observed in transtion metal systems in multilayer or granular forms. The MR in the perovskite manganites near the Curie temperature (TC) seems to have already been known at the very early stage of the study on transition metal oxides. For example, the paper in 1969 by Searle and Wang [2] reported thoroughly the magnetic field dependence of the resistivity for a flux-grown crystal of La1−xPbxMnO3; in particular the large MR near TC, as well as the phenomenological analysis. Soon after, Kubo and Ohata [3] have given a theoretical account for this phenomenon using the so-called double-exchange Hamiltonian (or the s–d model with the on-site ferromagnetic exchange interaction), that includes the essential ingredient of the double-exchange mechanism elaborated by Zener [4], Anderson and Hasegawa [5], and de Gennes [6]. Interest in the MR of those manganites has revived more lately since the rediscovery of the CMR or even more astonishing magnetic field induced insulator–metal [7], [8], [9], [10], [11] and/or lattice-structural [12] transitions. On the one hand, the highly sensitive and electrically readable magnetic-field sensors have recently been in industrial demand for the read-head of the magnetic memory and it is anticipated that the CMR oxides may be one such candidate material.

The “renaissance” of the CMR manganites was due partly to the revived interest in the barely metallic state of the transition metal oxides with strong electron–electron and/or electron–lattice interaction since the discovery of the high-temperature superconductivity in copper oxide (cuprate) compounds. After the experience of the research fever for superconducting cuprates, there has been much progress in the preparative method of oxide specimens, e.g., growth of single crystals and fabrication of epitaxial thin films, as well as in comprehensive understanding of the electronic and magnetic properties of such correlated-electron systems [13]. This circumstance has expedited the research on the CMR oxides, as of nowadays.

In perovskite-related structures (Fig. 1), the two important parameters, i.e., the band filling (or doping level) and the bandwidth (or electron hopping interaction), can be controlled to a considerable extent by modifying the chemical composition of the perovskite. Both the parameters control the kinetic energy of the conduction electrons which not only governs the metal–insulator phenomena but also the competing magnetic interactions, i.e. ferromagnetic vs. antiferromagnetic, in the perovskite manganites. We will see ample examples of the chemical control of the electronically important parameters in the following.

In this article, we first overview fundamental electronic and lattice features (Section 2) for perovskite manganites. After describing the electronic configuration and double-exchange interaction of Mn ions placed in the perovskite lattice, we focus on the experimentally observed features of magnetoresistance and insulator–metal transition for La1−xSrxMnO3 crystals that are viewed as the most typical double-exchange systems although many important deviations from the simple model already show up there. Section 3 is devoted to the description on the compositional tuning of the CMR effect in perovskite manganites. We present electronic phase daigrams in the plane of temperature and hole-doping level for representative manganites and specify the compositional region where the CMR is most pronounced. As a most remakable example of the CMR, we describe in Section 4 the magnetic field-induced melting of the charge/orbital ordered states which are widely observed in heavily hole-doped manganites (around x=0.5). We present the systematic electronic phase diagrams relevant to the charge/orbital ordering with variation of the electronic bandwidth at x=1/2 as well as of the hole doping level away from x=1/2, which are the bases of the magnetic or photonic metal-insulator phase control of these manganites. In Section 5 we briefly review late advances in study on the layered-structure manganites which show many unconventional magnetoelectronic and magnetoelastic properties, including the CMR effect.

Section snippets

Electronic features

The manganese (Mn) ion in the CMR manganite is surrounded by the oxygen octahedron. The 3d orbitals on the Mn-site placed in such an octahedral coordination are subject to the partial lifting of the degeneracy into lower-lying t2g states and higher-lying eg sates. In the Mn3+-based compounds, the Mn site shows the electronic configuration of t2g3eg1 (total spin number S=2). The t2g electrons, less hybridized with 2p states and stabilized by the crystal field splitting, are viewed as always

Variation of electronic phase diagrams

We show in Fig. 6 electronic phase diagrams in the temperature (T) vs. hole concentration (x) plane for prototypical compounds [48]; (a) La1−xSrxMnO3, (b) Nd1−xSrxMnO3 and (c) Pr1−xCaxMnO3. As the tolerance factor or equivalently the averaged ionic radius of the perovskite A-site decreases from (La,Sr) to (Pr,Ca) through (Nd,Sr), the orthorhombic distortion of GdFeO3-type increases, resulting in the bending of the Mn–O–Mn bond and hence in the decrease of the one-electron bandwidth (W) of the eg

Charge/orbital ordering at x=1/2

In Fig. 10 are shown typical examples of the charge/orbital ordering transitions observed for Nd1−xSrxMnO3(x=0.5) [61] and Sm1−xCaxMnO3(x=0.5) [62]. As shown in the temperature dependencies of resistivity and magnetization, the charge/orbital ordering transitions manifest themselves as decrease in magnetization and increase in resistivity, which locate at TCO(=TN) ≈160 K in Nd1/2Sr1/2MnO3 (left) and TCO≈270 K in Sm1/2Ca1/2MnO3 (right), respectively. Changes in lattice parameters are also observed

CMR of the layered-structure manganites La2−2xSr1+2xMn2O7

The title compounds correspond to the n=2 layered perovskite structures (see the inset to Fig. 14) with the MnO2 bilayer in a repeated unit. Taking the Mn3+-based compound, La2Sr1Mn2O7 (x=0), as the parent insulator, the composition x stands for the nominal hole number per Mn site. Fig. 14 shows the temperature dependence of the ρab in the series of Ruddlesden–Popper phases [(La,Sr)n+1MnnO3n+1; n=1,2 and infinite] with the nominal hole concentration fixed at x=0.3 [73]. In the pseudo-cubic (n

Concluding remarks

Fundamental features of the colossal magnetoresistive (CMR) manganites with perovskite-related structures have been described. The main advantage of the perovskites is the capability of controlling the band filling and the bandwidth, which are both important physical parameters for the strongly correlated electron system or strongly coupled electron-lattice system like the CMR manganites. First, we have seen a prototypical example of the double-exchange (DE) manganite, that is the case of La1−x

Acknowledgements

The authors would like to thank T. Kimura, A. Asamitsu, T. Okuda, and E. Saitoh for their help in preparation of the manuscript. The work was in part supported by NEDO and by a Grant-In-Aid for the COE Project from the Ministry of Education, Japan.

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