Exchange stiffness and magnetic anisotropies in bcc Fe1−xCox alloys

Brillouin scattering measurements from several bcc Fe1−xCox epitaxial films are reported. The magnetic-field dependence of the spin-wave spectrum as well as the in-plane directional dependence of the surface magnon are determined. The saturation magnetization of the alloys was measured independently through superconducting quantum interference device magnetometry. Self-consistent fitting of the magnon dispersions determined from the separate Brillouin experiments provide values for the concentration dependence of the exchange stiffness D, the fourth-order crystalline cubic anisotropy K1, and surface anisotropies Ks and Ks,p. In agreement with ferromagnetic resonance data, K1 is found to change sign for x beyond about 35 at. % while D steadily increases with cobalt concentration up to x∼0.5 and subsequently decreases in two Co-rich samples.