Spin Dynamics of the Low-Temperature Magnetic Relaxation in Disordered Fe35Al50B15 Alloys

An alloy of Fe₃₅Al₅₀B₁₅ has been prepared by milling under vacuum for 840 h. The X-ray diffraction pattern shows only two broad peaks stemming from a highly disordered structural state. The magnetic state was characterized by AC susceptibility. It displays sharp maxima around 20 K (21 K) in the real (complex) components. The maxima are due to the onset of a spin frozen disordered arrangement, and shift to higher temperature with increasing frequency. The magnetic dynamics is accounted for a power-law in the vicinity of the transition. Values of zv = 9.0(3) and T₀ = 19.5(1) K are obtained. Complex susceptibility dynamic scaling results in a large β = 1.3(1). The values extracted for the critical exponents are close to those of glassy magnets and are an indication of a nonconventional transition. The nonlinear susceptibility reveals a peak at the transition, which is affected by the oscillating h <; 5 Oe and biasing (H_DC ≤ 40 Oe) fields. The results are interpreted in terms of the freezing of very fine Fe-rich magnetic particles which are present as the result of an incomplete compositional homogenization despite the long milling time.