Influence of structural instabilities on the spontaneous magnetization of amorphous ferromagnets

Abstract

In this letter we point out that the temperature dependence of the spontaneous magnetization in amorphous ferromagnets can be interpreted by taking into account the presence of structural instabilities for magnetic atoms giving rise to strong localized fluctuations in magnetic-exchange interactions. The effects of these fluctuations are then accounted for by means of a mean field calculation. A theory relating the form of the M(T) curve in amorphous systems to a spatially distributed, homogeneous disorder has been proposed by HAI~DRICH (1). In this case, the effect of structural disorder on magnetization is related to the presence of a distribution of exchange integrals J~ with mean value ( J ) . The influence of disorder on the spin wave spectrum at low temperatures has been roughly accounted for by SIMPSO~ (8). More refined approaches to the calculation of the spin wave spectrum have been proposed (8.5). However, these models are generally highly idealized ones, hardly providing a clue to the interpretation of reduced magnetization curves experimentally studied in several materials. Anyway, it seems that the simple hypothesis of a distribution of exchange integrals due to structural disorder does not explain the behaviour of re(T) at low temperatures (6). This mismatch can be partially accounted for by the obvious consideration that a mean field theory does not describe the magnetization behaviour at low temperatures. However, the observed discrepancies between Handrieh's curves and the experimental ones are generally too large to be wholly interpreted in this way. We have shown elsewhere (7) that even within a simple mean field theory the presence of thermally activated magnetic structural instabilities plays a significant role in the temperature dependence of magnetization.