Magnetocrystalline Anisotropy Energy of a Transition Metal Monolayer: A Non-perturbative Theory

The magnetocrystalline anisotropy energy $E_{anis}$ for a monolayer of Fe and Ni is determined using a fully convergent tight-binding calculation including $s$-$d$ hybridization. The spin-orbit interaction $\lambda_{so}$ is treated non-perturbatively. Remarkably, we find $E_{anis}\propto\lambda_{so}^2$ and important contributions to $E_{anis}$ due to the lifting of degeneracies near the Fermi-level. This is supported by the calculated decrease of the anisotropy energy with increasing temperature on a scale of several hundred K. Our results clarify the present debate on the origin of $E_{anis}$.