In-plane anomalies of the exchange bias field in Ni80Fe20/Fe50Mn50 bilayers on Cu(110)

We report on the exchange bias effect as a function of the in-plane direction of the applied field in twofold symmetric, epitaxial ${\mathrm{Ni}}_{\mathrm{80}}{\mathrm{Fe}}_{\mathrm{20}}{\mathrm{/Fe}}_{\mathrm{50}}{\mathrm{Mn}}_{\mathrm{50}}$ bilayers grown on Cu(110) single-crystal substrates. An enhancement of the exchange bias field, $H_{\mathrm{eb}},$ up to a factor of 2 is observed if the external field is nearly, but not fully aligned perpendicular to the symmetry direction of the exchange bias field. From the measurement of the exchange bias field as a function of the in-plane angle of the applied field, the unidirectional, uniaxial and fourfold anisotropy contributions are determined with high precision. The symmetry direction of the unidirectional anisotropy switches with increasing NiFe thickness from [11̄0] to [001].