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 degrees 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.