Optimization study of the nanostructure of hard/soft magnetic multilayers

We present numerical simulations, based on a mean-field calculation of the equilibrium configuration, for the magnetization reversal process in magnetic multilayers consisting of a hard (h) phase with large anisotropy, exchange coupled to a soft (s) phase with large magnetization. Starting from a trilayer made of $50h/100s/50h$ layers, and using realistic Hamiltonian parameters pertinent to epitaxial Sm-Co/Fe films, we separately investigate the effect of (i) increasing nanostructuration, while maintaining constant the overall hard/soft ratio, and (ii) decreasing the thickness of the hard phase, while keeping constant that of the soft phase. We find that the exchange-bias field and the coercive field strongly increase upon increasing nanostructuration and that the maximum energy product (BH)ₘₐₓ rapidly tends to the ideal value (2πMₛₐₜ)². In contrast, upon reducing the thickness of the hard phase in a trilayer, the exchange-bias field and the coercive field remain nearly constant. Thus, combining both effects in an opportune way, we are able to determine the most convenient composition in order to obtain a permanent exchange-spring magnet with high performance, i.e., very high (BH)ₘₐₓ.