Coherently strained [Fe-Co(C)/Au-Cu]n multilayers: a path to induce magnetic anisotropy in Fe-Co films over large thicknesses

Among novel critical-element-free materials for permanent magnets, the nearly equiatomic Fe-Co alloy has recently attracted a great deal of attention as a large magneto-crystalline anisotropy can be induced by straining the Fe-Co unit cell. In thin film systems, the use of a suitable underlayer allows a tetragonal reconstruction of the Fe-Co to be triggered up to a critical thickness of few nanometers, above which the crystal structure relaxes to the magnetically soft cubic phase. Scaling-up the thickness of the metastable tetragonal Fe-Co phase is of crucial significance for different nanoscale applications, such as magnetic micro- and nano-electromechanical systems. To suppress the strain relaxation occurring at high thicknesses, we explored a novel approach based on Fe-Co(C)/Au-Cu multilayer films, where both Au-Cu interlayers and carbon (C) doping were used to stabilize the strained Fe-Co tetragonal phase over large thicknesses. Both doped and un-doped multilayer structures show a coherently strained regime, persisting up to a thickness of 60 nm, which leads, possibly in combination with the surface anisotropy induced at the Au-Cu interfaces, to the appearance of a large out-of-plane anisotropy (up to 0.4 MJ m-3), thus suggesting the potential of such an approach to develop critical-element-free thin film permanent magnets for a variety of nanoscale applications