MAGNETOSTRICTION IN MIXED VALENT MAGNETIC OXIDES

摘要

Up to now, most of the magnetostriction measurements reported on magnetic oxides were performed on ferrites (spinels, garnets, hexaferrites). The observed anisotropic magnetostriction was small and the volume effect negligible. Magnetostriction studies in magnetic oxides seemed to be a closed chapter. However, recent research in our group has shown the relevance of the magnetostriction in manganese and cobalt mixed valent oxides to explain a broad phenomenology. In this review, we have summarised thc main results, which open a new field in magnetostriction in order to explain the interplay among basic interactions derived from the attributes of the electron: charge, orbit and spin. We have found huge spontaneous and field induced magnetovolume effects in mixed valent magnetic oxides. One of the most relevant results was the explanationof the intrinsic CMR in manganites. The existence of dynamic electronic phase segregation as magnetic polarons gives rise to a huge and unusual magnetovolume effect in the paramagnetic phase. By narrowing of the e_g electron bandwidth, a strong effect on magnetostriction was observed and interpreted in terms of the static electronic phase segregation. The existence of two characteristic volume thermal expansions related to the insulator and metallic states were systematically observed a and are the key for the prediction of low temperature electronic inhomogeneous states. All the magnetostriction results were found to be closely related to the magnetostransport properties. Huge magnetostriction phenomena were also observed as a consequence of structural or magnetic instabilities. The existence of a field induced intermediate spin state in cobalt mixed valent oxides was an interesting conclusion from the huge anisotropic magnetostriction. Large anisotropic lattice effects were found in layered manganites. The isotopic effect on magnetostriction is very important as reviewed in this paper and constitutes a novel piece of information never explored in the field of magnetoelasticity.