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 magnet ic multilayers consisting of a hard (h) phase with large anisotropy, exchan ge coupled to a soft (s) phase with large magnetization. Starting from a tr ilayer made of 50h/100s/50h layers, and using realistic Hamiltonian paramet ers pertinent to epitaxial Sm-Co/Fe films, we separately investigate the ef fect of (i) increasing nanostructuration, while maintaining constant the ov erall 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-b ias field and the coercive field strongly increase upon increasing nanostru cturation and that the maximum energy product (BH)(max) rapidly tends to th e ideal value (2 pi M-sat)(2). In contrast, upon reducing the thickness of the hard phase in a trilayer, the exchange-bias field and the coercive fiel d remain nearly constant. Thus, combining both effects in an opportune way, we are able to determine the most convenient composition in order to obtai n a permanent exchange-spring magnet with high performance, i.e., very high (BH)(max). [S0163-1829(99)02729-0].