Magnetic anisotropy and reversal in epitaxial FeGa/IrMn bilayers with different orientations of exchange bias

Abstract

Epitaxial FeGa/IrMn bilayers with exchange biases along the FeGa[100] and [110] directions were prepared on MgO(001) single crystal substrates by magnetron sputtering with controlling the orientation of the external field in situ applied during growth. The effect of the exchange bias orientation on the magnetic switching processes and the magnetic switching field was studied. The X-ray φ-scan indicates that the FeGa layer is epitaxially grown with a 45°in-plane rotation on the MgO(001) substrate with a relationship of FeGa(001)110||MgO(001)100. The measurement of the angular dependence of the ferromagnetic resonance field and the corresponding fitting to the Kittel equation show that the samples have a superposition of fourfold symmetric magnetocrystalline anisotropy K1, unidirectional magnetic exchange bias anisotropy Keb, and uniaxial magnetic anisotropy Ku with configuration of Keb||[100] or Keb||[110]. The combined longitudinal and transverse magneto-optical Kerr effect measurements show that sample with Keb||[100] exhibit square loops, asymmetrically shaped loops, and one-sided two-step loops at different external magnetic field directions. In contrast, the sample with Keb||[110] exhibit one-sided two-step and two-sided two-step loops as the magnetic field orientation changes. Because the K1 is superimposed by Ku and Keb the in-plane fourfold symmetry of the magnetic anisotropy energy is broken. The local minima are no longer strictly along the in-plane 100 directions, but make a deviation angle which depends on the relative orientation and strength of magnetic anisotropies. A model based on the domain wall nucleation and propagation was proposed with considering the different orientations of Keb, which can nicely interpret the change of the magnetic switching route with the magnetic field orientation and fit the angular dependence of the magnetic switching fields, indicating a significant change of domain wall nucleation energy as the orientation of Keb changes.