On the comparison of the polarisation behaviour of exchange-biased AF/F NiMn/Fe37Co48Hf15 bi-layer and multi-layer films with increased ferromagnetic cut-off frequencies

Antiferromagnetic/ferromagnetic (AF/F) NiMn/Fe₃₇Co₄₈Hf₁₅ films were investigated with respect to their exchange bias, in-plane unidirectional anisotropy, polarisation and high frequency behaviour. After deposition, carried out by r.f. magnetron sputtering, the films were post-annealed for 4 h at 300 °C in a static magnetic field, in order to induce exchange-bias, which results in a unidirectional anisotropy. Dependent on the presence of a bi-layer or multi-layer sandwich structure the films show a different exchange-bias field-ferromagnetic inter-layer thickness behaviour with exchange-bias fields μ₀?H_eb between 2 and 10 mT. The in-plane uniaxial (single film) or unidirectional anisotropy fields μ₀*H_UF were between 4 and 18 mT. This results in a significant increase of the cut-off frequency in the GHz range in comparison to a single Fe₃₇Co₄₈Hf₁₅ film, which is shown by frequency-dependent permeability plots. High damping in the imaginary part of the permeability, i.e., high resonance line broadening could be observed for films with high coercivity μ₀*H_c of around 7 mT in the easy axis of magnetisation.