J. Ph.vs Chem Sol& Vol59. No. 4. pp 487-502. 1998 Pergamon PII: SOO22-3697(97)00203-S 0 1998 Pubbshcd by Elsevtcr Scmce Ltd Pnnted in Gnat Bntrn All nghts reserved 0022-36976% $19.00 + 0 00 zyxwvuts GIANT MAGNETORESISTANCE, CHARGE ORDERING AND OTHER NOVEL PROPERTIES OF PEROVSKITE MANGANATES C.N.R. RAO+*, R. MAHESH+, A.K. RAYCHAUDHURI’ and R. MAHENDIRAN$ ‘Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India *Department of Physics, Indian Institute of Science, Bangalore 560012, India Abstract-Giant magnetoresistance (GMR) and related properties of manganate perovskites of the general formula Ln ,_,A,Mn03 (Ln = rare earth; A = divalent ion) are discussed in detail. There is a fine interplay of magnetic exchange, structural properties and electronic transport in these materials which gives rise to several novel properties. The manganates am ferromagnetic at or above a certain value ofx (or Mn4+content) and become metallic at temperatures below the curie temperature. T,. This behavior is attributed to double-exchange. GMR is generally a maximum close to T, or the insulator-metal (I-M) transition temperature, T,,. The T, and %MR are markedly affected by the size of the A site cation, < rA > , thereby affording a useful electronic phase diagram when T, or T,, is plotted against < rA > or pressure. The commonalities and correlations found in the properties of manganates are examined along with certain unusual features in the electron-transport properties of these materials. Some of the Ln ,_,A,MnOr compositions exhibit charge-ordering and related effects. Charge ordering is crucially dependent on < rA > or the er band width and the charge-ordered insulating state transforms to a metallic ferromagnetic state on the application of a magnetic field, charge-ordering and double-exchange being competing interactions. 8 1998 Published by Elsevier Science Ltd. All rights reserved. Keywords: D. electrical properties, D. magnetic properties 1. INTRODUCTION Magnetoresistance (MR) is the relative change in the electrical resistance or resistivity of a material produced on the application of a magnetic field. It is defined by, MR = [&b(O)1 = W-0 - P(WP(‘~ (1) where p(H) and p(O) are the resistances or resistivities at a given temperature in the presence and absence of a magnetic field, H, respectively. MR can be negative or positive. All metals show MR, but only of a few percent. Large magnetoresistance, referred to as giant magnetore- sistance (GMR), was first observed on the application of magnetic fields to atomically engineered magnetic super- lattices (e.g. Fe/Cr) [ 1] and in magnetic semiconductors [2]. Several bimetallic or multimetallic layers, containing ferromagnetic and antiferromagnetic or non-magnetic metals, have since been found to exhibit GMR. GMR has also been found in ferromagnetic granules dispersed in paramagnetic metal films (e.g. Co/Cu) [l]. The phenomenon is of great interest because of its potential technological applications in magnetic recording, actuators and sensors. GMR in magnetic layered and granular materials arises from the ability of magnetic fields to change and control the scattering of conduction electrons through the modification of the electron-orbit and spin-orbit interactions. GMR is the extra resistance due to the scattering of electrons by the non-aligned ferromagnetic components in zero magnetic field. *Corresponding author. The discovery of GMR in rare-earth manganates [3,4], Lnl_JA,MnOs (Ln = rare earth; A = a divalent cation such as an alkaline earth) with the perovskite structure, has attracted wide attention. Since 1993, a variety of manganese oxides have been investigated in the form of polycrystalline powders, thin films and single crystals. These studies have provided valuable insight into the GMR phenomenon in these oxides. In this article, we present some of the highlights of the results pertaining to GMR and related properties of the manganates obtained hitherto and point out the significant general features. We also examine certain novel aspects of the manganates, in particular, the unusual resistivity behavior and charge- ordering exhibited by them. 2. THE ROLE OF Mn’+-0-Mn’+ INTERACTION IN BARE EARTH MANGANATES LaMnO is an insulator with an orthorhomobically dis- torted perovskite structure (b > a > c”, Pbnm) and typically contains some Mn4+ as prepared by the usual solid state reactions. LaMnO with a small proportion of Mn*+ ( 5 5%) becomes antifetromagnetically ordered at low temperatures (TN-150 K). When the La3+ in LaMnO, is progressively substituted by a divalent cation as in La l_,A,Mn03 (A = Ca, Sr or Ba), it becomes ferromagnetic with a well-defined Curie temperature, T,, and also becomes metallic below T, [5]. The saturation moment is typically = 3.8 pa, which is close to the theoretical estimate based on localized spin-only 487