Temperature dependent biquadratic exchange coupling in Co/Cu (0.94)Mn(0.06) multilayers

Abstract

Magnetic interlayer coupling phenomena in systems consisting of ferromagnetic films separated by non-magnetic spacer layers, such as Co/Cu multilayers, are heavily dependent on the thickness and composition of the spacer layer. Usual Co/Cu multilayers exhibit the normal oscillatory RKKY exchange coupling resulting in alternating ferromagnetic/antiferromagnetic magnetic alignment of adjacent layers with increasing Cu thickness. We report on a crucial temperature dependence of this coupling introduced as the non-magnetic spacer layer is substituted by a spin-glass material, such as CuMn. Kobayashi et al. first discovered the extraordinary behaviour of a ferromagnetic exchange coupled Co/CuMn multilayer structure showing an increasing antiferromagnetic component as the system is cooled below a critical temperature. In order to investigate the magnetic rearrangement throughout the transition temperature in more detail we used polarized neutron reflectometry in specular and off-specular geometry to resolve the orientation of the magnetization in the Co layers. Co/Cu(0.94)Mn(0.06) multilayer samples with a manganese concentration of 6at% in the spacer layer are grown using sputter deposition and quality checked with X-ray reflectivity and diffraction. The onset of a magnetic rearrangement into an antiferromagnetic constitution around 100K is confirmed by neutron reflectivity measurements, revealing the onset of biquadratic interlayer coupling. Simulations of the full two dimensional off-specular scattering points out each layer breaking into magnetic domains. Further magnetic characterisation of the sample using X-Ray magnetic circular dichroism and conventional magnetometry indicate the role of the Mn doping in the magnetic transition into a biquadratic coupling.