Sound-velocity anomalies in cobalt and manganese aluminosilicate glasses

Abstract

Broad sound-velocity minima were found in concentrated cobalt aluminosilicate and manganese aluminosilicate glasses at low temperatures. Both the longitudinal and shear velocities showed these anomalies and there was good correlation between the anomaly and the magnetic susceptibility peak which was also found in each of the samples. Using the monodomain model of Verhelst and de Graaf which was also used to explain the susceptibility data for these glasses, we have been able to explain the temperature dependence of the velocity quite well. The model postulates that the samples contain small (\ensuremath{\sim}50-\AA{}) domains of magnetic ions (Co or Mn) separated by Co or Mn poor paramagnetic regions. Each monodomain is assumed to have a randomly oriented local anisotropy direction. Owing to this anisotropy, the net moments of the monodomains are found to align with the anisotropy direction at low temperatures. This alignment gives rise to a peak in the magnetic part of the specific heat which in turn is related to the sound velocity. Good quantitative agreement is found for the anisotropy energies used to fit the sound-velocity and susceptibility data.