THE MAGNETIC STRUCTURE OF FeMn LAYERS ACROSS A Cu SPACER

Following the convincing experimental evidence of oscillatory exchange interaction between antiferromagnetic FeMn layers across a Cu spacer recently found by Cai et al., we have performed ab-initio spin-polarized density functional theory calculations on (FeMn)_n/Cu/(FeMn)_n with n = 1, 2, 3 in the (001) crystallographic face. For n = 1 we have investigated all possible magnetic configurations in the self-consistent procedure and we end up with five solutions. The ground state is found to be of ferromagnetic type: i) there is an intrinsic ferromagnetic configuration in the FeMn plane, and ii) there is a ferromagnetic coupling between the two FeMn planes separated by the Cu spacer. At 20.17 mRyd/cell a solution with complex magnetic behavior is observed: ferromagnetic coupling between Fe and Mn, in one FeMn plane, and antiferromagnetic coupling between Fe and Mn in the second FeMn plane separated by Cu. Another solution at 41.26 mRyd above the ferromagnetic ground state presents in-plane antiferromagnetic configurations in each FeMn layer. For n = 2 the ground state presents in-plane ferromagnetism for the two FeMn layers adjacent to Cu; the other two FeMn layers are clearly of in-plane antiferromagnetic type. Contrary to n = 1, the ferromagnetic coupling through the Cu spacer is now absent. We report on this ground state and various metastable states with small differences of energy with the ground state. For n = 3 the magnetic map is more complex. Explanation of the experimental results of Cai et al. is tentatively made.