Twisted magnetization phases in orbital-dominant rare-earth nitrides

Abstract

<p>In this thesis we investigate the magnetic properties of NdN and SmN, members of the rare-earth nitrides, a series of intrinsic ferromagnetic semiconductors. In rare-earth systems, the strong spin-orbit coupling of the partially filled 4ƒ shell ensures that there is a substantial orbital contribution to the ferromagnetic moment, in contrast to many transition metal systems where the orbital moment is usually quenched. In SmN and NdN the orbital moment actually exceeds the spin moment, and the resulting orbital dominant magnetization allows for the fabrication of a magnetic heterostructures showing novel behavior. We report a new theoretical study of the magnetic properties on both SmN and NdN by considering the atomic-like 4ƒ electrons. These calculations incorporate spin-orbit coupling, the exchange interaction in a self-consistent mean-field approach, and crystal field interactions in an arbitrary-multiplet point-charge model. Our findings show excellent agreement with the experimentally measured ferromagnetic moments of SmN and NdN, representing an advance from previous theoretical studies. We also report an experimental study on SmN/GdN heterostructures using the element-resolved method of x-ray magnetic circular dichroism (XMCD) to probe the magnetism. The competition between the orbital-dominant Zeeman coupling in SmN and the ferromagnetic spin-based interface exchange with GdN, which has purely a spin moment, results in a twisted magnetization profile. The depth profile of the magnetization derived from XMCD measurements showed good agreement with an analytical model developed to describe the competing interactions. In a second study, a superlattice of NdN/GdN was investigated via XMCD and standard magnetometry techniques. A twisted magnetization was shown to be present due to the same mechanism as in the SmN/GdN system. By varying the maximum applied field and temperature, twisted phases were shown to develop in both GdN and NdN layers. These twisted phases in orbital-dominant ferromagnetic semiconductors represent a departure from previously explored spin-dominant metallic systems displaying similar twisted phases.</p>