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

Contrary to the earlier reports, a detailed ferromagnetic resonance (FMR) study of amorphous alloys with in the critical region shows that the critical exponents and for spontaneous magnetization and initial susceptibility, which characterize the ferromagnetic (FM) - paramagnetic (PM) phase transition at the Curie temperature , 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 . In the critical region, the Landau - Lifshitz - Gilbert relaxation mechanism dominantly contributes to the `peak-to-peak' FMR linewidth and hence , where is the saturation magnetization. Consistent with the results obtained in a wide temperature range, which embraces the critical region, the Landé splitting factor g has a temperature- and composition-independent value of while the Gilbert damping parameter , although temperature independent, decreases with increasing Co concentration. The angular dependence of the resonance field 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 increases with increasing Co concentration and scales with . That the uniaxial anisotropy has its origin in the pseudo-dipolar atomic pair ordering is vindicated by the finding that .