Annealing temperature and thickness dependencies of structural and magnetic properties of Co2FeAl thin films

Co2FeAl (CFA) thin films, of various thicknesses (3 nm <= t <= 50 nm), have been grown by sputtering on (001) MgO single-crystal substrates and annealed at different temperatures (RT <= T-a <= 600 degrees C, where RT is the room temperature). The influence of the CFA thickness (t), as well as ex situ annealing temperature (T-a), on the magnetic and structural properties has been investigated by x-ray diffraction (XRD), vibrating sample magnetometry, and broadband microstrip ferromagnetic resonance (MS-FMR). The XRD revealed an epitaxial growth of the films with the cubic [001] CFA axis normal to the substrate plane and that the chemical order varies from the B2 phase to the A2 phase when decreasing t or T-a. The deduced lattice parameters showed an in-plane tetragonal distortion and in-plane and out-plane strains that increase with T-a and 1/t. For all T-a values, the variation of the effective magnetization, deduced from the fit of MS-FMR measurements, shows two different regimes separated by a critical thickness, which is T-a dependent. It decreases (increases) linearly with the inverse thickness (1/t) in the first (second) regime due to the contribution of the magnetoelastic anisotropy to surface (to volume) anisotropy. The observed behavior has been analyzed through a model allowing for the separation of the magnetocrystalline, magnetoelastic, and Neel-type interface anisotropy constants to the surface and the volume anisotropies. Similar behavior has been observed for the effective fourfold anisotropy field which governs the in-plane anisotropy present in all the samples. Finally, the MS-FMR data also allow one to conclude that the gyromagnetic factor remains constant and that the exchange stiffness constant increases with T-a.