Detailed ferromagnetic resonance study of amorphous Fe-rich Fe90-xCoxZr10 alloys .2. Critical behaviour and uniaxial anisotropy

Contrary to the earlier reports, a detailed ferromagnetic resonance (FMR) study of amorphous Fe90-xCoxZr10 alloys with 0 less than or equal to x less than or equal to 10 in the critical region shows that the critical exponents beta and gamma for spontaneous magnetization and initial susceptibility, which characterize the ferromagnetic (FM)-paramagnetic (PM) phase transition at the Curie temperature T-C, possess values that are independent of composition and close to those predicted for a three-dimensional isotropic nearest-neighbour Heisenberg ferromagnet. The fraction c of spins that participates in the FM-PM phase transition has a value of 11% for the alloy with x = 0 and increases with increasing Co concentration x as c(x) - c(0) similar or equal to ax(2). In the critical region, the Landau-Lifshitz-Gilbert relaxation mechanism dominantly contributes to the 'peak-to-peak' FMR linewidth Delta H-pp and hence Delta H-pp(T) proportional to [M(S)(T)](-1), where M(S) is the saturation magnetization. Consistent with the results obtained in a wide temperature range, which embraces the critical region, the Lande splitting factor g has a temperature- and composition-independent value of 2.07 +/- 0.02 while the Gilbert damping parameter lambda, although temperature independent, decreases with increasing Co concentration. The angular dependence of the resonance field H-res observed in both 'in-plane' and 'out-of-plane' sample geometries has been fitted to theoretical expressions that take into account the uniaxial anisotropy. The uniaxial anisotropy field H-k = 2K(u)/M(S) increases with increasing Co concentration and scales with M(S). That the uniaxial anisotropy has its origin in the pseudo-dipolar atomic pair ordering is vindicated by the finding that K-u proportional to M(S)(2).