PHASE-TRANSITIONS AND CRITICAL PHENOMENA IN ALLOYS WITH RANDOM ANISOTROPY

The effects of random magnetic anisotropy on ferromagnetic critical be havior in four rare-earth-based amorphous alloys, Gd65Co35, Nd50Co50, Nd65Co35, and Tb65Co35, are reported. The ratio of anisotropy to excha nge, D/J0, ranges from 0.004(1) to 0.323(35) in these alloys. The zero -field state of the anisotropic alloys (Nd and Th based) is spin-glass -like below T(c). In order to determine the extent of the critical reg ime as a function of reduced temperature, epsilon = (T - T(c))/T(c), a nd to determine if the asymptotic critical regime is probed, a wide ra nge of epsilon is covered. Evidence for crossovers as a function of ap plied field and as a function of D/J0 are observed. As a function of a pplied field, the alloys containing anisotropic rare-earth atoms cross over with increasing field from a state that shows the characteristic s of a speromagnet to a state where the magnetic isotherms follow a fe rromagnetic-scaling equation of state. The field above which good ferr omagnetic scaling is observed increases monotonically with D/J0. No de viations from scaling were found at small epsilon giving confidence th at the asymptotic critical regime is probed. Deviations from scaling w ere seen at large values of epsilon (epsilon > 0.06) which gives the l imit of the critical regime. We find systematic variations in the meas ured ferromagnetic critical exponents gamma and delta with D/J0 indica ting a crossover in critical behavior presumably from an isotropic to a random anisotropy class with increasing D/J0. Finally, we point out that there are some theoretical hints of a ferromagneticlike state bel ow T(c) in random-anisotropy models.