Observations of Exchange Bias in Patterned Bilayers using Three-Axis Torque Magnetometry.

We report studies of exchange bias in circular disk bilayers of 20nm Permalloy/20nm cobalt oxide, lithographically patterned on nanomechanical resonators. Torque magnetometry is well-suited to thin film studies of exchange bias since the measurements are sensitive to magnetic behavior at the bilayer interface¹. The nanomechanical resonators employed here are of unique design whereby three orthogonal components of field induced torque, and thus three orthogonal components of anisotropy, can be measured simultaneously². The three axis technique provides a platform for obtaining standard hysteresis curves while permitting measurement of rotational hysteresis with no requirement to reorient the sample or the field generators. Shifted hysteresis loops and rotational hysteresis are observed at cryogenic temperatures thus confirming exchange bias at the Permalloy/cobalt oxide interface. A strongly temperature-dependent and magnetic history-independent contribution to out-of-plane torque that may be an indicator of disordered exchange bias will be discussed. Micromagnetic simulations of mechanical torque lend further insight through qualitative and quantitative comparisons with the measured magnetic behavior.
¹ da Silva OE, et al., JMMM 451, 507 (2018).
² Fast KR, et al., arXiv:2011.01463.