Magnetic properties of cobalt and manganese aluminosilicate glasses

Susceptibility measurements over a wide range of temperatures on concentrated amorphous transition-metal aluminosilicate glasses (11.8-33.5-at.% Co, 12.3-41.7-at.% Mn) are reported. In the high-temperature regime (>\ensuremath{\sim}50 K), these glasses show Curie-Weiss behavior with large negative paramagnetic Curie temperatures, indicating the presence of strong antiferromagnetic exchange interactions. The slope of ${\ensuremath{\chi}}^{\ensuremath{-}1}$ vs $T$ curves shows a slight dependence on the transition-metal concentration, implying that the local environment of the magnetic ions is concentration dependent. The magnitude as well as the concentration dependence of the magnetic moments, as determined from the high-temperature data, can be understood for the cobalt glasses by appealing to the solidus surface diagram (which gives information concerning the local environment of the cobalt ions) of the glass system. In the low-temperature regime ($1<T<50$ K), the susceptibility of both systems is enhanced and furthermore exhibits an anomalously sharp peak below \ensuremath{\sim}10 K. The magnitude of this peak decreases with increasing magnetic-ion concentration, while the position at which it occurs shifts to higher temperatures. This low-temperature behavior can be explained satisfactorily by assuming that these glasses contain small (\ensuremath{\sim}50 \AA{}) regions of relatively high cobalt or manganese concentration (monodomains) which are ordered antiferromagnetically, and which are separated from one another by paramagnetic areas of lesser magnetic-ion content. Each monodomain has a net magnetic moment which freezes in the direction of its anisotropy field at the so-called blocking temperature, giving rise to the peak in the susceptibility. By calculating the susceptibility of the monodomains and properly averaging over the random directions of their anisotropy fields, remarkable agreement with the low-temperature data is obtained. The values of the anisotropy constants needed to fit the data are consistent with values found in various cobalt and manganese compounds.