Rare-earth—gallium—iron glasses. I. Magnetic ordering and hysteresis in alloys based on Gd, Tb, and Er

Results of magnetic measurements are presented on the new metallic glass systems ${\left(R_{80}{\mathrm{Ga}}_{20}\right)}_{100-x}{\mathrm{Fe}}_x$ and ${\left(R_{80}{\mathrm{Ga}}_{20}\right)}_{90}{\mathrm{B}}_{10}$, where $R$ denotes the heavy rare earths Gd, Tb, and Er, for $x=10, 20, \mathrm{and} 30$, respectively. High-field magnetization (to 80 kOe) and ac and dc susceptibility measurements were made from 1.4 to 300 K. The Gd-Fe glasses are simple ferrimagnets with opposing Gd and Fe moments. The Gd-B glass exhibits two apparent transitions (paramagnetic—ferromagnetic—like, and ferromagnetic spin-glass) as the temperature is lowered. As Fe is added, the low-temperature transition disappears. The data can be understood in terms of Sherrington-Kirkpatrick—type models for the phase diagram with competing ferromagnetic and spin-glass order. In the Tb and Er glasses a strong local random anisotropy prevents saturation of the magnetization in high fields, and the ac susceptibility shows sharp peaks indicating speromagnetic (spin-glass-like) order. In high fields the moment approaches that expected for a hemisphere of randomly oriented spins ($\frac{1}{2}g\frac{J}{\mathrm{spin}}$), so that an asperomagnetic structure is produced by the applied field. For small values of $x$ (10 and 20), the Tb and Er glasses exhibit coercive force, $H_c$, and hysteresis characteristics consistent with coherent-rotation processes. However, for large Fe concentrations ($x=30\mathrm{}$), the ac susceptibility peaks are considerably smeared, suggesting increased chemical short-range order and increased heterogeneity. The observed increase of $H_c$ in these alloys seems to be consistent with the theory of Paul, although the choice of parameters is not well defined. The temperature dependence of $H_c$ for the $x=30\mathrm{}$ alloys indicates that domain-wall motion or nucleation processes appear to dominate magnetization reversal.