Indirect exchange coupling in Co/Nb and Co/NbHx multilayers

The indirect exchange coupling of Co through pure and hydrogenated Nb spacer layers has been investigated as a function of the spacer layer thickness and temperature. It is found that these systems exhibit significant biquadratic and bilinear coupling in the temperature range studied between 10 and 300 K. The biquadratic term appears to decay exponentially with spacer layer thickness, increases rapidly with decreasing temperature and is insensitive to the addition of hydrogen. For both sets of samples a peak in the antiferromagnetic bilinear term is observed, with the hydrogenated samples displaying a larger maximum coupling strength at a smaller spacer layer thickness. The data are consistent with Slonczewski's 'loose spins' model for biquadratic coupling. Resistance, magnetoresistance, magnetization and X-ray diffraction data suggest that the multilayers have a highly disordered structure.