Interlayer exchange coupling mediated by a nonmagnetic spacer layer with a large electron-electron exchange interaction

Abstract

We derive an expression for the exchange coupling between two ferromagnetic layers separated by a nonmagnetic spacer metal having large exchange enhancement in the paramagnetic susceptibility. The theory of the exchange coupling between two magnetic spins embedded in a nonmagnetic metal, in which the random-phase approximation was used for the electron-electron exchange interaction, is extended to the exchange coupling between ferromagnetic layers in magnetic superlattices. The exchange coupling decreases with an increasing interlayer thickness in a nontrivial manner. An oscillatory behavior superimposed on the exponentially decreasing background is clearly shown. The effect due to the electron-electron exchange interaction in the nonmagnetic spacer metal produces a ferromagnetic bias to the oscillatory exchange coupling for low spacer thicknesses. The regions of the antiferromagnetic exchange coupling may disappear because of the ferromagnetic bias depending on the degree of the exchange enhancement. However, the traces of the disappeared regions can be recognized as dips or easy slopes in the region of the ferromagnetic exchange coupling. It is shown from the numerical results that in the case of magnetic superlattices with Pd interlayers, the first and second antiferromagnetic exchange couplings disappear and the third antiferromagnetic one appears, and the theoretical result agrees well with the experimental result. The ferromagnetic bias is less expected for the Pt interlayer than for the Pd interlayer because of a relatively weak electron-electron exchange interaction.