Study on magnetic hard-direction ordering in anisotropic Kondo systems

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. Furthermore, we apply mean-field decoupling on the Coqblin-Schrieffer model with an additional on-site anisotropy term in the density and magnetic channels. By solving the self-consistent equations we show that the susceptibility crossing can also happen for a range of parameters.