In situ modification of the magnetocrystalline anisotropy in cobalt-ferrite microcrystals

We present the in situ modification of the magnetocrystalline anisotropy of iron-rich cobalt-ferrite epitaxial islands by changing their composition. Crystalline cobalt-ferrite spinel islands have been grown by oxygen- assisted molecular beam epitaxy on a Ru(0001) single crystal. The micro-crystals are several micrometers wide with heights of tens of nanometers. Their spatially-resolved vector magnetization is mapped by x-ray magnetic circular dichroism in photoemission microscopy at the L3 absorption edges of Co and Fe. Thick islands present a closure-like magnetic domain structure with the magnetization directions predominantly aligned along the projections of the easy magnetic axes onto the (111) surface plane. Thinner islands are more affected by growth defects and present amore complex domain structure. Upon deposition of additional Fein an oxygen background pressure, Co is found to segregate out of the spinel islands, while their Fe content increases. This produces a reduction in the magnetocrystalline anisotropy, which manifests as a reorientation of the magnetization directions towards the edges of the islands, reflecting an increased contribution of the shape anisotropy.