Detailed ferromagnetic resonance study of amorphous Fe-rich Fe_90-xCo_xZr₁₀ alloys. II: critical behaviour and uniaxial anisotropy

Abstract

Contrary to the earlier reports, a detailed ferromagnetic resonance (FMR) study of amorphous Fe_90-xCo_xZr₁₀ alloys with 0 ≤ x ≤ 10 in the critical region shows that the critical exponents β and γ 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) ≈ αx². In the critical region, the Landau - Lifshitz - Gilbert relaxation mechanism dominantly contributes to the 'peak-to-peak' FMR linewidth ΔH_pp and hence ΔH_pp (T) ∝ [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 λ, although temperature independent, decreases with increasing Co concentration. The angular dependence of the resonance field H_resobserved 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 ∝ M_s².