Magnetic hard-direction ordering in anisotropic Kondo systems

Abstract

We present a generic mechanism that explains why many Kondo materials show magnetic ordering along directions that are not favored by the crystal-field anisotropy. Using a renormalization-group (RG) analysis of single-impurity Kondo models with single-ion anisotropy, we demonstrate that strong fluctuations above the Kondo temperature drive a moment reorientation over a wide range of parameters, e.g., for different spin values $S$ and number of Kondo channels $N$. In tetragonal systems, this can happen for both easy-plane or easy-axis anisotropy. The characteristic crossing of magnetic susceptibilities is not an artifact of the weak-coupling RG treatment but can be reproduced in brute-force perturbation theory. Employing the numerical renormalization group (NRG), we show that for an underscreened moment $(S=1,$ $N=1)$ with easy-plane anisotropy, a crossing of magnetic susceptibilities can also occur in the strong-coupling regime (below the Kondo temperature). This suggests that collective magnetic ordering of such underscreened moments would develop along the magnetic hard axis.