Microspectroscopic study of the magnetic textures in a permalloy/hematite bilayer across the Morin transition

Interfaces between antiferromagnetic (AFM) and ferromagnetic (FM) materials have long been studied. They exhibit complex interactions which are the source of new phenomena. Aside from the exchange bias, the control of the coercivity is another significant topic. Coercivity in FM materials is often determined through grain size, composition, density of defects, etc., which is set during material growth and offers limited room for modification after growth. Hematite is an AFM material that undergoes a temperature-controlled spin-flop transition, the Morin transition. This transition gives an extra degree of freedom making hematite an intriguing component to study the exchange coupling when interfaced with a FM material. We have observed, in very recent experiments, changes in the magnetic properties of soft magnetic permalloy thin films grown on hematite: they show a remarkable change in coercivity across the Morin transition. We attribute this effect to the magnetic domain mixture of hematite during the Morin transition while heating, where a fraction of the moments still point out-of-plane while other lie in-plane. A complex interaction of these perpendicular domains with the permalloy domain Neél boundaries might explain the increase in the coercivity. This behaviour reveals the existence of complex interactions in the system, which include the Dzyaloshinskii–Moriya interaction in hematite, crystal and shape anisotropies or magnetic dipolar interactions. In this proposal we want to study the configuration of the magnetic domains in permalloy as a result of these interactions across the Morin transition. In these conditions, special stable magnetic textures, with a non-collinear alignment of the magnetic moments, are expected to emerge. We aim at visualizing the new textures using X-ray Magnetic Circular and Linear Dichroism (XMCD, XMLD). CIRCE beamline at Alba, because of its
experimental set-up and proven experience, is a most convenient place to carry out this kind of studies.