The Nucleation of the Spin Spiral in Epitaxial / Exchange Spring Trilayers

Recently, exchanged coupled hard/soft magnetic trilayers based on the hard magnetic SmCo 5 phase and a high moment soft Fe phase reached largely improved energy densities, (BH) max , of above 300 kJ/m 3 by a combination of fully epitaxial growth and an optimized soft layer thickness. In order to evaluate quantitatively the effect of Fe-layer thickness on the nucleation field H n in these epitaxial trilayers, micromagnetic calculations were performed in a one-dimensional model using OOMMF assuming strong coupling at the interface. Calculating the hysteresis on the basis of the nominal layer architecture the experimental saturation polarization and remanence J r are underestimated, the nucleation field is overestimated and the onset of the spin spiral is too sharp. On the other hand, considering a texture spread via a tilt angle of 3° reduces H n slightly and leads to a realistic smoothening of the transition. Including diffusion of Co into the Fe layer with a diffusion zone of 2 nm at each interface increases the soft layer thickness by 4 nm on cost of the SmCo 5 layer thickness. Such a layer architecture results in a further reduction of H n together with an increase of J r and leads to a very reasonable agreement with the experimental data. The modified model furthermore reproduces the shape of the reversible demagnetizing branch of the hysteresis in the exchange-spring regime and is thus suitable for describing the magnetic behavior of existing exchange-coupled multilayer.