Colossal Magnetoresistive Materials

Abstract

Magnetoresistance (MR) is defined as the relative change in the electrical resistivity of a material upon the application of a magnetic field. MR is positive for most nonmagnetic metals, and its magnitude is limited to a few percent, whereas MR can be negative in magnetic materials because the magnetic field tends to reduce the spin disorder. MR is of considerable technological interest. IBM is using the Permalloy (composition: 80% Ni and 20% Fe) MR of about 3% in a small magnetic field at room temperature for the magnetic storage of information. More recently, larger magnetoresistance also called giant MR (GMR) was observed in thin films of magnetic superlattices (for instance, Fe, Cr) for which metallic layers of a ferromagnet and a nonmagnetic material (or an antiferromagnet) are alternately deposited on a substrate. By doing so, the MR magnitude is increased by an order of magnitude. Small ferromagnetic particles deposited on a paramagnetic thin film also provide an alternative way to obtain GMR devices. For both material classes, small magnetic field applications (a few oersteds) are sufficient to align the magnetizations ferromagnetically and thus to induce a resistivity decrease originating in decreased scattering. In hole‐doped perovskite manganites magnetoresistance values of ∼ −100% in large magnetic fields (several teslas) have been discovered. These effects are called CMR to distinguish them from GMR. CMR has motivated a large number of experimental studies of these oxides in bulk (ceramics and crystals) and in thin films and also of theoretical work to understand the origin of the phenomenon. In this article, several representative examples of perovskite manganites are given to illustrate the richness of their phase diagrams. More particularly, the chemical key factor governing the CMR of hole‐doped manganites that contain 30% Mn 4 + and have the Ln 0.7 AE 0.3 MnO 3 formula are reviewed. The existence of Mn 3 + /Mn 4 + charge ordering in the Mn lattice for half‐doped manganites and also for Mn 4 + }‐rich compositions (electron‐doped, x > 0.5) are discussed. Finally, the possibility of obtaining CMR properties in Mn 4 + ‐rich manganites is shown.