Ferromagnetic resonance in coupled ultrathin films

Ferromagnetic resonance (FMR) is known to be one of the most informative techniques to measure basic physical quantities such as magnetic anisotropy energies, the g tensor in solids or the interlayer exchange coupling J_inter. We investigate prototype Cu/Ni/Cu/Ni/Cu(001) and Ni/Cu/Co/Cu(001) trilayers as well as Fe_n/V_m superlattices. We show for the case of trilayers how in situ ultrahigh vacuum FMR can be used to determine J_inter in absolute energy units in a straightforward way: we first prepare and measure the bottom magnetic layer together with the Cu spacer in situ and then evaporate the second magnetic film on top. Thus, it is possible to investigate the FMR signal before and after the two magnetic films become coupled. We discuss results, showing that the temperature dependence of J_inter follows a T^3/2 law over a wide temperature range. This indicates that thermally excited spin waves at the interface of the ferromagnetic layers dominate the temperature dependence of J_inter. The second part focuses on the measurement of the g value. From the g value, the ratio of orbital to spin magnetic moment can be obtained via the relation μ_L /μ_S = (g − 2)/2. We show for Fe_n/V_m superlattices how μ_L /μ_S increases with decreasing Fe-layer thickness.