Magnetic susceptibility of a dilute magnetic alloy with magnetic interactions: ZnMn

Abstract

Magnetic-susceptibility measurements at low temperatures (1.2-4.2 K) on a series of ZnMn alloys (20-2500 ppm Mn) indicate that $\ensuremath{\chi}$ follows a Curie-Weiss law, $\ensuremath{\chi}=\frac{C}{(T+\ensuremath{\Theta})}$, in which both $C$ and $\ensuremath{\Theta}$ vary as a function of concentration $n$ of Mn impurities. As a function of $n$, $\ensuremath{\Theta}$ goes through a minimum (${\ensuremath{\Theta}}_{min}=0.3$ K) at about $n=100$-ppm Mn. We interpret this minimum in $\ensuremath{\Theta}$, here observed for the first time in a dilute magnetic alloy system, as separating two distinct concentration regions. For $nl100$ ppm we have $n{V}_{0}l{k}_{B}{T}_{K}$, where ${V}_{0}$ is the strength of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two Mn impurities and ${T}_{K}$ is the Kondo temperature. In this region the single-impurity Kondo effect is the dominant interaction and the RKKY interactions reduce the Kondo temperature as $n$ increases up to 100 ppm. Above 100 ppm where $n{V}_{0}g{k}_{B}{T}_{K}$, RKKY interactions between Mn impurities dominate the measured magnetic susceptibility and we find $\ensuremath{\Theta}$ increasing proportional to $n{V}_{0}$, in agreement with theoretical predictions. For $n=2500$-ppm Mn, we find $\ensuremath{\Theta}$ equal to zero to within experimental error, thus indicating a change in character of the impurity-impurity interactions for $ng1000$-ppm Mn. From the concentration dependence of the Curie constant $C$, the effective magnetic moment per Mn impurity, ${\ensuremath{\mu}}_{\mathrm{eff}}$, is also found to vary with $n$. Like $\ensuremath{\Theta}$, ${\ensuremath{\mu}}_{\mathrm{eff}}$ goes through a minimum at about $n=100$-ppm Mn. From $\ensuremath{\Theta}(n)$ we determine ${V}_{0}$ to be (2.25\ifmmode\pm\else\textpm\fi{}0.20)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}36}$ erg ${\mathrm{cm}}^{3}$, in satisfactory agreement with values for ${V}_{0}$ obtained from recent specific-heat and magnetization measurements on these same ZnMn alloys.