Magnetic properties of Ni-Rh alloys near the critical composition for ferromagnetism

The magnetizations of Ni-Rh alloys on either side of the critical composition for ferromagnetism ($c_{\mathrm{crit}}\approx 63-\mathrm{a}\mathrm{t}.\%\mathrm{N}\mathrm{i}$) were measured between 4.2 and 250°K in fields up to 56 kOe. The initial paramagnetic susceptibility of each alloy is shown to be resolvable into a Curie-Weiss component and a weakly temperature-dependent component of the exchange-enhanced Pauli type. The latter component, which is essentially equal to the high-field differential susceptibility at low temperatures, is maximum near $c_{\mathrm{crit}}$. The onset of ferromagnetism, however, is more directly related to divergence of the Curie-Weiss susceptibility component and is attributed to interactions between superparamagnetic clusters. The average moment per cluster is deduced to be $\sim \mathrm{}(20-24)\mathrm{}{\mu }_B$ over the composition range investigated (65-51-at.% Ni). The concentration of the magnetic clusters, though remaining dilute, increases rapidly as $c_{\mathrm{crit}}$ is approached from the paramagnetic side; moreover, it correlates closely with the statistical occurrence of extremely Ni-rich local regions in these atomically disordered alloys. Comparisons are made with the Ni-Cu and Ni-V systems near $c_{\mathrm{crit}}$, for which recent magnetization studies have revealed the analogous existence of dilute concentrations of giant magnetic clusters.