Critical phenomena and magnetic properties of an amorphous ferromagnet: Gadolinium-gold

Magnetization was measured between 4.2 and 290°K in fields up to 70 kOe on liquid-quenched ${\mathrm{Gd}}_{80}$ ${\mathrm{Au}}_{20}$ amorphous alloys. The Curie temperature and critical exponents $\beta$, $\gamma$, and $\delta$ are found to be 149.45±0.2°K, 0.44±0.02, 1.29±0.05, and 3.96±0.03, respectively. The data are fitted to an equation of state previously derived for a second-order phase transition in fluid systems. It is found that the magnetization exponent $\beta$ of amorphous ferromagnets studied so far has a value ∼0.4, slightly enhanced over those observed in corresponding crystalline elements, and the estimated specific-heat exponent $\alpha$ is negative. Both are in qualitative agreement with theories on the critical behavior of random systems. The effects of structural disorder on the magnetic properties studied are compared and discussed with recent theories on amorphous magnetism. A comparison of the magnetization data with Handrich's theory for amorphous ferromagnets suggests that in our amorphous alloys the average fluctuation in the exchange constant ($J$) can be an appreciable fraction of $J$ itself. The effective magnetic moment in the paramagnetic state has a value of $(8.9\mathrm{}\pm 0.1){\mu }_B$ per gadolinium atom. The saturation moment extrapolated to 0°K equals $(7.0\mathrm{}\pm 0.25){\mu }_B$ per Gd atom. The low-temperature saturation magnetization follows the $T^{\frac{3}{2}}$ law from 0.13 $T_c$ to 0.80 $T_c$. The mean exchange integrals $J$ determined from the Rushbrooke-Wood formula and spinwave theory are found to be 2.28±0.15°K and 1.34±0.08°K, respectively. The exchange constant of the Ruderman-Kittel-Kasuya-Yosida interaction as estimated from the de Gennes model ($J_{\mathrm{sf}}\simeq 0.19$ eV) is not drastically reduced in this amorphous matrix. Finally, the Curie temperature of pure amorphous Gd is estimated and its value compared with those obtained from experimental extrapolation.