Inverted anisotropy of the partially screened magnetic impurity

Abstract

We investigate a single magnetic impurity in the presence of strong spin-orbit coupling and single-ion anisotropy. We show that at sufficiently strong coupling there exists a finite temperature window, before the moment is completely screened, where the magnetic anisotropy of the system flips: the hard-axis becomes the easy-axis or vice versa. We derive this rigorously for a single impurity using numerical renormalization group calculations as well as Nozieres' strong-coupling limit and discuss its relevance to heavy-fermion compounds which order magnetically along the hard-direction. We show that the coexistence of Curie-like response and Kondo fluctuations is stabilized along the initially hard direction leading to the anisotropy switch.

Figures (3)

• Figure 1: The anisotropy of the magnetic susceptibility of the impurity, $\Delta\chi$ in the strong coupling limit $J/W \gg 1$, calculated using finite $B=10^{-4}J$. Note the logarithmic scale on both axes. Inset: actual $\chi_z$ and $\chi_x$ as functions of $T$.
• Figure 2: (a) The susceptibility of the impurity (times temperature) in a uniform field in the case of the CS model with $j=3/2$ and $\rho J=3$ (light lines indicate $\rho J =1$ case), calculated by NRG. Inset: actual $\chi$ as function of $T$. (b) Corresponding difference $\Delta\chi$.
• Figure 3: Impurity contribution to entropy, $\Delta S^{\rm imp}$, as a function of $T$ for indicated values of $J$ and $D=J/10$ (solid lines) and $D=-J/10$ (dashed lines). Vertical lines indicate positions of sign-changes of $\Delta\chi$ for $\rho J=3$ (thick line).