Quantum criticality in the Itinerant Ferromagnets Zr$_{1-x}$Nb$_{x}$Zn$_{2}$

We report the results of magnetization measurements performed on the family itinerant ferromagnets Zr$_{1-x}$Nb$_{x}$Zn$_{2}$, (0 $\leq x \leq 0.14)$. Nb doping reduces the moment M$_{0}$ and also the Curie temperature T$_{c}$, which simultaneously disappear at the critical Nb concentration x$_{c}$=0.084. We find that T$_{c}$ $\propto$ (x-x$_{c}$)$^{3/4}$, as predicted for a 3d ferromagnet, while M$_{0}$ $\propto$ T$_{c}$ (x), as expected for a Stoner ferromagnet. For all Nb concentrations and for temperatures which approach 100 K, the extrapolated zero field susceptibility $\chi$ can be expressed with a modified Curie Weiss expression $\chi=C/(T^\gamma +\theta)$. $\theta$ is finite in the paramagnetic state (x$>$x$_{C}$), but vanishes as the system becomes critical at x=x$_{C}$, evidenced by the T=0 divergence of $\chi$ in this system. We find that $\gamma$ is near one in paramagnetic regimes for x$<$x$_{c}$ (T$>$T$_{c}$), and for x $\gg$ x$_{c}$. However, $\gamma$ is substantially enhanced in the vicinity of the quantum critical point (0.08$<$x$<$0.09), indicating the breakdown of the conventional Stoner theory.