Magnetic-cluster description of spin glasses in amorphous La-Gd-Au alloys

The bulk magnetic properties of splat-cooled amorphous alloys of composition ${\mathrm{La}}_{80-\mathrm{x}}{\mathrm{Gd}}_x{\mathrm{Au}}_{20}$ ($0\le x\le 80$) have been studied. Zero-field susceptibility, high-field magnetization (up to 75 kOe), and saturated remanence have been measured for temperatures ranging from 1.8 to 290°K. Detailed analysis of the data based on a magnetic-cluster description of the spin glass and mictomagnetic alloys ($x\le 56$) is presented. Our concentrated spin glasses are represented by rigid ferromagnetic clusters as individual spin entities interacting via random forces. Scaling laws similar to those of Blandin, Souletie, and Tournier for the magnetization are obtained and presented graphically for the $x\le 32$ alloys in which $\frac{M}{x}=g(\frac{H}{x^{*}}, \frac{T}{x})$, where $x^{*}$ is the concentration of clusters. Saturation remanent magnetization is interpreted in terms of the dipolar anisotropy model of Tholence and Tournier. The strength of the Ruderman-Kittel-Kasuya-Yosida interaction $V_o$ between clusters (or single spins in the dilute alloys) is determined from high-field magnetization data using the Larkin-Smith approach. The freezing temperatures $T_M$ (defined by susceptibility maxima) of dilute spin glasses in which $T_m\propto x$ are accounted for rather well, using the experimentally determined values of $V_0$ An attempt is made to explain the freezing temperatures of more-concentrated spin glasses in which $T_M\propto x^{1.3}$ ($12\le x\le 40$). It is also shown that for the $x\le 24$ alloys, the size of the clusters can be correlated to the structural short-range order in the amorphous state. More-concentrated alloys are marked by the emergence of cluster percolation.