EPR linewidth (T >~ Tg) in amorphous transition-metal-metalloid spin glasses: Theory.

Abstract

A microscopic theory of the paramagnetic linewidth and line shift for an amorphous transition-metal--metalloid alloy spin glass has been given in the Mori-Kawasaki formalism. The linewidth has been calculated as being due to random single-ion anisotropy and the magnetic dipole-dipole anisotropic interaction, which are considered to be small perturbations. The contribution of the single-ion anisotropy has been found to be smaller. While it is noted that for T>${T}_{g}$ there is no static component of the random dipolar magnetic field the dynamic component of this field does exist. Taking the point of view of Souletie and Tholence [Phys. Rev. B 32, 516 (1985)] that the autocorrelation time of the dynamic component increases as the temperature is lowered to ${T}_{g}$ and attains the saturation value of 2\ensuremath{\pi}/w at T=${T}_{g}$(w), we predict that at this temperature the EPR response should be an inhomogeneously broadened Voigt line shape. The very limited data available have been compared with a rough numerical estimate of the linewidth at ${T}_{g}$(w) and reasons have been suggested for the lack of good agreement. At higher temperature the dynamic component will be partially averaged out by the EPR probe and the linewidth is expected to be less and the line shape more Lorentzian. It has been found that the critical part of the linewidth and line shift for all frequencies should obey dynamic scaling down to and including ${T}_{g}$(w). Also their temperature and frequency dependence shows trends similar to those for the canonical spin glasses. It is noted that more detailed data on the line shape close to ${T}_{g}$(w) and accurate determination of the single-ion anisotropy constant are needed for a check on the above predictions. A justification has been given for the applicability of the Mori-Kawasaki formalism at least for the higher frequencies. The regime of applicability of the theory is expected to be about T\ensuremath{\gtrsim}1.25${T}_{g}$.