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

Abstract

The exchange coupling through nonmagnetic metallic interlayers is described by bilinear and biquadratic 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 is strongly affected by the interface roughness. Realistic interfaces consist of finite atomic terraces that result in variations of the interlayer thickness. Slonczewski showed that in this case the biquadratic exchange term can be produced by frustration of local magnetic moments. Recent band structure calculations showed that the biquadratic exchange coupling can be intrinsic. In this case its strength is usually found to be significantly weaker than that of bilinear coupling. The temperature dependence of biquadratic exchange coupling is expected to be different for the Slonczewski and intrinsic mechanisms. The intrinsic biquadratic exchange coupling is expected to vary with temperature faster than that predicted by the Slonczewski mechanism. In the present studies using the FMR and MOKE techniques we have investigated extensively the temperature dependence of the bilinear and biquadratic exchange coupling in Fe/Cu/Fe and Fe/Ag/Fe(001) structures. The 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 at the Fe/Cu, Ag interface. That way one can control the strength of the biquadratic exchange coupling. Two limits were investigated: (a) The strength of the biquadratic term in the exchange coupling was much smaller (10%) compared to the bilinear term; (b) the strength of the biquadratic term was comparable to the bilinear exchange coupling. The results are discussed within the framework of existing theories.