Exchange coupled multilayers
Exchange coupling between hard and soft magnetic layers is a largely discussed approach to tailor materials with properties not achievable in single phase samples. In the area of high performance permanent magnets, exchange coupling can lead to enhanced remanences and improved energy densities, (BH)max, by coupling a soft magnetic phase with high polarization to a permanent magnet phase with high anisotropy.
Fully epitaxial SmCo5(25 nm)/Fe(x nm)/SmCo5(25 nm) trilayer films were prepared, in which the easy magnetization axis of both hard magnetic layers is aligned along one single direction. The interlayer exchange coupling is studied by hysteresis measurements and by dc-demagnetizing loops for different thicknesses of the soft Fe layer. The good coupling together with the optimum texture provided by the epitaxial growth lead to excellent magnetic properties.
Fully coupled trilayers are achieved for small Fe thicknesses (x < 8 nm). They maintain a square hysteresis with high coercivity of µ0Hc > 1.0 T and a measurable remanence enhancement over a single hard magnetic layer. For thicker Fe layers the exchange coupling leads to reversible rotation processes within the soft phase before the trilayer switches irreversibly. The still acceptable coercive field and the further improved remanence lead to a maximum energy density of 312 kJ/m3 (39 MGOe) for the trilayer with x = 12.6 nm. The specific epitaxial relation between SmCo5 and Fe with its unique orientation of the SmCo5 easy axis throughout the whole layer stack presents the essential step in the realization of thick, textured, exchange coupled multilayers with high energy density.
Figure: Magnetic hysteresis of epitaxial SmCo5(25nm)/Fe(x)/SmCo5(25nm) trilayers with x = 0, 6.3, 12.6 and 19 nm measured parallel to the SmCo5 c-axis. See S. Sawatzki et al. JAP 109 (2011).
- V. Neu, S. Sawatzki, M. Kopte, Ch. Mickel, L. Schultz, Fully epitaxial, exchange coupled SmCo5/Fe multilayers with energy densities above 400 kJ/m3, IEEE Trans. Magn. 48, 3599 (2012).
- M. Kopte, S. Sawatzki, L. Schultz, V. Neu, The nucleation of the spin spiral in exchange-coupled SmCo5/Fe/SmCo5 trilayers, IEEE Trans. Magn. 47, 3736 (2011).
S. Sawatzki, Ch. Mickel, M. Seifert, R. Heller, L. Schultz, V. Neu, Largely enhanced energy product in epitaxial SmCo5/Fe/SmCo5 trilayers, J. Appl. Phys. 109, 123922 (2011).
- V. Neu, R. Biele, A. Singh, L. Schultz, Modeling of intergrain exchange coupling for quantitative predictions of dm plots, IEEE Trans. Magn. 46, 2331 (2010).
R. Pellicelli, M. Solzi, V. Neu, K. Häfner, C. Pernechele, and M. Ghidini, Characterization and modeling and of the demagnetization processes in exchange-coupled SmCo5/Fe/SmCo5 trilayers, Phys. Rev. B 81, 184430 (2010).
- V. Neu, K. Häfner, L. Schultz, Dynamic coercivity and thermal stability of epitaxial exchange spring trilayers, J. Magn. Magn. Mater. 322, 1613 (2010)