Nanoscale magnetism and exchange bias coupling at the antiferromagnetic/ferrimagnetic interface

The thermally activated spin relaxation of singly inverted FeO/Fe 3O4 core-shell nanocrystals and strongly interacting iron oxide nanocrystal assemblies formed by electrophoretic deposition is investigated by vibrating sample magnetometry. Modification of the thermally activated spin relaxation via interaction with an antiferromagnet is analyzed according to the general theory of exchange bias at the antiferromagnetic/ferromagnetic (AFM/FM) interface. Structural characterization of the iron oxide nanocrystals are conducted by transmission electron microscopy (TEM), x-ray diffractometry (XRD), atomic force microscopy, and absorption spectroscopy. The core-shell iron oxide nanocrystals are characterized as singly inverted, with an AFM FeO core that attains magnetic order below the Néel temperature (T N ≈ 198 K) and a ferrimagnetic (FIM) Fe3O4 shell that attains magnetic order below the Curie temperature (TC ≈ 860 K). Sintering of the core-shell nanocrystals leads to a spinel phase of iron oxide, as verified by XRD and TEM. Vibrating sample magnetometry indicates exchange bias at the AFM/FIM interface of the FeO/Fe3O4 core-shell nanocrystals. The exchange bias decreases in a non-linear fashion with increasing temperature and approaches zero near TN. Temperature dependent magnetization measurements verify induced anisotropy from the FeO core, resulting in a stable superparamagnetic transition near TN for applied fields ranging from 25 to 200 Oe. Consequently, the suppression of superparamagnetism in nanostructured magnetic storage media could possibly be realized with singly inverted systems consisting of an antiferromagnet with TN above 300 K. Finally, an investigation of a strongly interacting iron oxide nanocrystal assembly confirms an enhancement in the superparamagnetic transition temperature at an applied field of 25 Oe. An increase in the applied field leads to a monotonic decrease in the transition temperature, which is in agreement with current theoretical models.