Probing boundary magnetization through exchange bias in heterostructures with competing anisotropy

Cr$_{2}$O$_{3}$ (chromia) is a magnetoelectric antiferromagnet with a bulk T$_{\mathrm{N}}$ of 307 K. It has been utilized for electrically controlled exchange bias (EB) by taking advantage of voltage-controllable boundary magnetization (BM) occurring as a generic property in magnetoelectric single domain antiferromagnets.\footnote{Xi He, et al., Nature Mater.\textbf{9}, 579-585 (2010)} In the perpendicular Cr$_{2}$O$_{3}$(0001)/CoPd EB system the EB-field shows an order parameter type T-dependence close to T$_{\mathrm{N}}$ reflecting the T-dependence of the BM. At about 150 K a decrease of the EB-field sets in with decreasing temperature suggesting canting of the BM. To evidence this mechanism we use EB as a probe. Specifically, we investigate EB in Permalloy(5nm)/Cr$_{2}$O$_{3}$ (0001)(100nm) with Permalloy and chromia having competing anisotropies. We measure easy axis magnetic hysteresis loops via longitudinal magneto-optical Kerr effect for various temperatures after perpendicular and in-plane magnetic field-cooling. The T-dependence of the EB field supports the canting mechanism. In addition to the all thin film EB system, we explore a Permalloy(10nm)/Cr$_{2}$O$_{3}$(0001 single crystal) heterostructure where magnetoelectric annealing allows selecting Cr$_{2}$O$_{3}$ single domain states. Here the effect of T-dependent canting of the BM is compared with findings in the complementary perpendicular EB system.