Local spin-flip transitions induced by magnetic quantum impurities in two-dimensional magnets

We predict a general local spin-flip transition mechanism caused by magnetic quantum impurities in (partially) polarized phases of quantum magnets in the absence of conservation laws. This transition arises when a magnon bound state crosses zero energy as function of the magnetic field. As application, we study 2D van der Waals magnets described by the Kitaev-Heisenberg honeycomb model which applies to the transition metal trihalides CrI$_3$ and $\alpha$-RuCl$_3$. We consider adatom and substitutional impurity positions, and show how spin-flip transitions can be detected in scanning tunneling spectroscopy.