Noncollinear Fe spin structure in (Sm-Co)/Fe exchange-spring bilayers: Layer-resolved 57Fe Mossbauer spectroscopy and electronic structure calculations

Magnetization reversal in nanoscale (Sm-Co)/Fe (hard/soft) bilayer exchange-spring magnets with in-plane uniaxial magnetic anisotropy was investigated by magnetometry, conversion-electron Mossbauer spectroscopy (CEMS) and atomistic Fe spin-structure calculations. Magnetization loops along the easy direction exhibit signatures typical of exchange-spring magnets. In-field CEMS at inclined gamma-ray incidence onto thin (2 nm) Fe-57 probe layers embedded at various depths in the 20-nm-thick natural (soft) Fe layer provides depth-dependent information (via the line-intensity ratio R-23 as a function of the applied field H) about the in-plane rotation of Fe spins. A minimum in the R-23-vs-H dependence at (H-min, R-min) determines the field where Fe magnetic moments roughly adopt an average perpendicular orientation during their reversal from positive to negative easy-axis orientation. A monotonic decrease of H-min with distance from the hard/soft interface is observed. Rotation of Fe spins takes place even in the interface region in applied fields far below the field of irreversible switching, H-irr, of the hard phase. Formation of an Fe-Co alloy is detected in the interface region. For comparison, the noncollinear Fe spin structure during reversal and the resulting R-23 ratio were obtained by electronic-structure calculations based on a quantum-mechanical Hamiltonian for itinerant electrons. The coupling at the hard/soft interface is described by the uniaxial exchange-anisotropy field, h(int), as a parameter. Our calculated R-23 ratios as a function of the (reduced) applied field h exhibit similar features as observed in the experiment, in particular a minimum at (h(min), R-min). R-min is found to increase with h(int), thus providing a measure of the interface coupling. Evidence is provided for the existence of fluctuations of the interface coupling. The calculations also show that the Fe spin spiral formed during reversal is highly inhomogeneous. In general, our simulation of the Fe spin structure is applicable for the interpretation of experimental results on layered exchange-spring magnets.