The Nucleation of the Spin Spiral in Epitaxial SmCo₅/ Fe/SmCo₅ Exchange Spring Trilayers

Recently, exchanged coupled hard/soft magnetic trilayers based on the hard magnetic SmCo5phase and a high moment soft Fe phase reached largely improved energy densities, (BH)max, of above 300 kJ/m3by 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 Hnin 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 Jrare 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 Hnslightly 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 SmCo5layer thickness. Such a layer architecture results in a further reduction of Hntogether with an increase of Jrand 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.