Magneto-optical Kerr effect and nuclear resonant scattering study of uni-directional anisotropy in hard-soft magnetic bilayers

Abstract

The present work reports the unconventional exchange bias (EB) phenomena in an exchange-coupled hard and soft magnetic bilayer system and the tunability of EB. The EB phenomena, i.e., shifting of the hysteresis loop of the soft (Fe) layer is observed when the hard magnetic (L10 FePt) layer is under the remanent state indicating the development of unidirectional anisotropy. The nuclear resonant scattering measurements clearly reveal the development of unidirectional anisotropy in the soft magnetic (Fe) layer, when the hard magnetic layer is under the remanent state. The magnetization reversal process is investigated by measuring two in-plane orthogonal components of magnetization, i.e., parallel (M∥) and perpendicular (M⊥) to the applied field using the magneto-optical Kerr effect (MOKE). When the magnetic field is applied parallel (antiparallel) to the biasing field direction, (HSAT) magnetization reversal is nonuniform, and on the other hand, the rotation of magnetization is observed when the magnetic field is applied away from the HSAT direction. In addition, the sign inversion of the M⊥ component is observed when the magnetic field is applied at the same angle on either side of the HSAT direction, which clearly imply the change in handedness of the chirality of spin structure during the magnetization reversal of the soft layer. Further, it is observed that the EB decreases with the increase of soft magnetic (Fe) layer thickness, demonstrating the tunable nature of EB phenomena even in these unconventional systems.