The effects of random magnetic anisotropy on ferromagnetic critical behavior in four rare-earth-based amorphous alloys, ${\mathrm{Gd}}_{65}$${\mathrm{Co}}_{35}$, ${\mathrm{Nd}}_{50}$${\mathrm{Co}}_{50}$, ${\mathrm{Nd}}_{65}$${\mathrm{Co}}_{35}$, and ${\mathrm{Tb}}_{65}$${\mathrm{Co}}_{35}$, are reported. The ratio of anisotropy to exchange, D/${\mathit{J}}_0$, ranges from 0.004(1) to 0.323(35) in these alloys. The zero-field state of the anisotropic alloys (Nd and Tb based) is spin-glass-like below ${\mathit{T}}_{\mathit{c}}$. In order to determine the extent of the critical regime as a function of reduced temperature, ε=(T-${\mathit{T}}_{\mathit{c}}$)/${\mathit{T}}_{\mathit{c}}$, and to determine if the asymptotic critical regime is probed, a wide range of ε is covered. Evidence for crossovers as a function of applied field and as a function of D/${\mathit{J}}_0$ are observed. As a function of applied field, the alloys containing anisotropic rare-earth atoms cross over with increasing field from a state that shows the characteristics of a speromagnet to a state where the magnetic isotherms follow a ferromagnetic-scaling equation of state. The field above which good ferromagnetic scaling is observed increases monotonically with D/${\mathit{J}}_0$. No deviations from scaling were found at small ε giving confidence that the asymptotic critical regime is probed. Deviations from scaling were seen at large values of ε (ε>0.06) which gives the limit of the critical regime. We find systematic variations in the measured ferromagnetic critical exponents γ and δ with D/${\mathit{J}}_0$ indicating a crossover in critical behavior presumably from an isotropic to a random anisotropy class with increasing D/${\mathit{J}}_0$. Finally, we point out that there are some theoretical hints of a ferromagneticlike state below ${\mathit{T}}_{\mathit{c}}$ in random-anisotropy models.

• Received 19 July 1993

DOI:https://doi.org/10.1103/PhysRevB.48.13614