Anomalous magnetic and dynamic behavior in magnetoresistive compounds: origin of bulk colossal magnetoresistivity

Abstract

We present the results of our extensive Mössbauer effect studies carried out on a wide variety of mixed valence manganites as well as other types of magnetoresistive materials, including pyrochlore Tl₂Mn₂O₇ and the chalcospinels Fe₀.₅Cu₀.₅Cr₂S₄ and FeCr₂S₄ with absolutely different natures of the magnetism, in a search for similarities linked to their magnetoresistive behavior. The double exchange electron transfer and coupling between the electrons and Jahn–Teller lattice distortions invoked by most theories to explain the colossal magnetoresistivity and associated metal–insulator transition in manganites are not applicable to pyrochlore nor to chalcospinels. Nevertheless, we find intriguing similarities in the anomalous magnetic and dynamic behavior among these widely different systems at, above, and below the Curie temperature Tc, which shed light on the origin of bulk magnetoresistivity in general. All these compounds share the following features. The long-range ferromagnetic order breaks down even below the Curie temperature, with the formation of nano-size spin clusters. Softening of the lattice was observed near Tc. The short-range interactions in these spin clusters survive well above Tc. When an external magnetic field is applied, the spin clusters coalesce to form large clusters, with considerable lowering of the resistivity. There is a strong evidence that the existence of nano-size spin clusters with superparamagnetic-like behavior near Tc is a prerequisite for the occurrence of bulk magnetoresistivity.