Artificial Skyrmion Lattices and Tunable Positive Exchange Bias via Oxygen Migration

The unique spin texture in magnetic skyrmions leads to a host of fascinating phenomena due to the topologically protected quantum state and emergent electromagnetic field, offering great potential for low dissipation magnetic information storage. We have demonstrated the realization of artificial Bloch skyrmion lattices over extended areas in their ground state at room temperature by patterning asymmetric magnetic nanodots with controlled circularity on a PMA underlayer [1]. The vortex structure is imprinted from the dots into the interfacial region of the underlayer via suppression of the PMA by a critical ion irradiation step. The imprinted skyrmion lattices are identified directly with polarized neutron reflectometry and confirmed by magnetoresistance measurements. These results illustrate an exciting platform to explore room temperature ground state skyrmion lattices. Time permitting, I will also discuss a recent study on ionic transport in metal/oxide heterostructures. We have directly observed oxygen migration across buried interfaces in bilayer films of GdxFe1-x/NiCoO, which leads to tunable positive exchange bias over a relatively small cooling field range. The tunability is shown to be the result of an interfacial layer of elemental NiCo, a few nanometers in thickness, substantially thicker than expected from uncompensated NiCoO moments. This interface layer is attributed to chemical reduction of the NiCoO by the Gd. Our findings demonstrate a new and effective path to control the interfacial exchange coupling in metal/oxide heterostructures. This work has been supported by the US-NSF (DMR-1008791, ECCS-1232275, and DMR-1543582). [1]. D. A. Gilbert, B. B. Maranville, A. L. Balk, B. J. Kirby, P. Fischer, D. T. Pierce, J. Unguris, J. A. Borchers, and Kai Liu, Nat. Commun. 6:8462, doi: 10.1038/ncomms9462 (2015).