The temperature dependence of the bilinear and biquadratic exchange coupling in Fe/Cu, Ag/Fe(001) structures

Abstract

The exchange coupling through non-magnetic metallic layers is described by bilinear (− J 1 cos( θ)) and biquadratic ( J 2 cos 2( θ)) exchange coupling terms. In simple metals such as Cu(001) and Ag(001) the bilinear exchange coupling exhibits strong short wavelength oscillations (∼ 2 ML). The exchange coupling through non-ferromagnetic interlayers in this case is strongly affected by the interface roughness. Realistic interfaces consist of finite terraces which results in variations of the interlayer thickness. Slonczewski showed that in this case the biquadratic exchange term can be produced by frustrations of the local magnetic moments. In the presented studies we used the MOKE and FMR techniques to investigate the temperature dependence of the bilinear and biquadratic exchange coupling in Fe/Cu/Fe(001) and Fe/Ag/Fe(001) structures. Quantitative studies of the exchange coupling were carried out in the temperature range 77–400 K. By growing the first Fe layer at various substrate temperatures one can prepare samples possessing interfaces with a variable atomic terrace width. That way we can controlle the strength of the biquadratic exchange coupling. Two limits were investigated: (a) J 2 ⪡ J 1 and (b) J 2 ∼ J 1. It was found that the biquadratic coupling scales linearly with the strength of the bilinear exchange coupling. The results will be discussed within the framework of existing theories.