Microstrip line ferromagnetic resonance and Brillouin light scattering investigations of magnetic properties of Co2MnGe Heusler thin films

Abstract

Co2MnGe films of different thicknesses (34, 55, and 83 nm) were grown by rf sputtering at 400 °C on single-crystal Al2O3 corundum substrates showing an in-plane c axis. Their dynamic magnetic properties were studied using conventional and microstrip line (MS) ferromagnetic resonances (FMRs), as well as Brillouin light scattering (BLS) techniques. The effective magnetizations and gyromagnetic factors are first deduced from the resonance spectra involving the uniform magnetic mode under in-plane and out-of-plane magnetic applied fields. The angular dependence of the frequency, measured under a weak in-plane magnetic applied field, then allows deriving of the parameters describing the in-plane magnetic anisotropy. In the 34- and 55-nm-thick films, its behavior is described assuming a magnetic energy density showing an orthorhombic symmetry with a twofold axis normal to the film and planar anisotropy axes at ±pi/4 of the c axis of the substrate; however, due to the comparative deduced values of the pertinent coefficients, this energy density is predominantly tetragonal. In the thickest film (83 nm), one of the planar anisotropy axes is parallel to c and approximate tetragonal symmetry is no more observed. Moreover, the orthorhombic symmetry is not completely fulfilled and a small misalignment between the principal directions connected to the uniaxial and the fourfold energy terms appears. Finally, the perpendicular surface standing modes, which are observed in MS-FMR and in BLS spectra, allow evaluation of the exchange stiffness constant. Good agreement between BLS and MS-FMR measurements has been found.