Twisted phase of the orbital-dominant ferromagnet SmN in a GdN/SmN heterostructure

The strong spin-orbit interaction in the rare-earth elements ensures that even within a ferromagnetic state there is a substantial orbital contribution to the ferromagnetic moment, in contrast to more familiar transition metal systems in which the orbital moment is usually quenched. The orbital-dominant magnetization that is then possible within rare-earth systems facilitates the fabrication of entirely new magnetic heterostructures, and here we report a study of a particularly striking example comprising interfaces between GdN and SmN. Our investigation reveals a twisted magnetization arising from the large spin-only magnetic moment in GdN and the nearly zero, but orbital-dominant, moment of SmN. The unusual twisted phase is driven by (i) the similar ferromagnetic Gd-Gd, Sm-Sm, and Gd-Sm exchange interactions, (ii) a SmN Zeeman interaction 200 times weaker than that of GdN, and (iii) the orbital-dominant SmN magnetic moment. The element specificity of x-ray magnetic circular dichroism is used in separate modes probing both bulk and surface regions, revealing the depth profile of the twisting magnetization.