Random-anisotropy ferromagnetism in amorphous U₂₇Fe₇₃ films

Abstract

We measured magnetic and transport properties of amorphous U 27Fe73 films deposited by magnetron sputtering. The films show a magnetic ordering temperature Tf =32 K, and a spin-glass regime below a temperature Tirr(H). The Arrott plot shows a characteristic change in concavity at the transition, as predicted by Aharony and Pytte (A-P) for random-anisotropy systems with D≪J. Although we find a finite susceptibility below Tf, with χ0=0.66 in units of the inverse of the demagnetizing factor, the critical exponents δ1 and δ2 are close to the values predicted by the A-P model. The electrical resistivity has a minimum at 31 K, resulting from the competition between exchange scattering and inelastic spin-flip processes. For T>200 K a linear regime with negative dρ/dT is found, characteristic of phonon scattering in amorphous materials. The isotropic magnetoresistance is positive and levels off below Tf reflecting freezing of spin correlations at this temperature. This behavior can be explained by the existence of small Imry and Ma domains and partial local magnetic order below Tf.