Mossbauer spectra of mixed crystals: diamagnetically - substituted yttrium iron garnet in the critical region

Abstract

There is a range of temperature where the Mossbauer spectra of 57Fe3+ in diamagnetically substituted magnetically ordered iron oxides show broad lines and may be reproduced by local-molecular-field theory. The lines can exhibit a well-defined structure if the number of neighboring cation sites is less than six, and the spectra can then be used to determine whether the cation distribution is random. Such is the case for Y3(Fe1-xScx)2[Fe]3O12 with x=0.10 and 0.25. The site preferences of other trivalent cations in yttrium iron garnet (YIG) are measured, including Rh3+ which goes onto a sites. At higher temperatures, central peaks appear in the spectra of mixed oxides, and they grow at the expense of the magnetically split spectrum until it finally disappears at Tc. The spectra of Y3(Fe0.75Sc0.25)2[Fe]3O12 were studied in and around this broad critical region. An applied field produces increased hyperfine splitting, which is correlated with the static magnetic susceptibility. The mixed spectra cannot be explained by a range of ordering temperatures in different parts of the crystal, or by independent relaxation of the ionic spins. Short-range collective behavior is indicated, with coupled moments of (102-104)?B around Tc. A computer study of the geometry of the random lattice shows that there is no tendency to form localized clusters of the right size. The short-range order is probably evanescent.