SOMPOLINSKY IRREVERSIBILITY ORDER-PARAMETER IN RANDOM MAGNETIC-ANISOTROPY TBXY1-XAL2 CUBIC INTERMETALLICS

The Sompolinsky or irreversibility order parameter Delta has been dete rmined for the series of random magnetic anisotropy (RMA) spin glasses (SG's) TbxY1-xAl2 (x=0.10, 0.15, 0.25), from zero-field-cooled (ZFC) and field-cooled (FC) low-field susceptibility measurements, and low-f ield-low-frequency susceptibility (ac) measurements. A model is presen ted which, within the mean-field approximation, extends to RMA systems , with ferromagnetic exchange interactions, the dynamical Sompolinsky' s model for random exchange SG's. An expression for the low-field susc eptibility of RMA SG's, which includes irreversibility effects, is obt ained from this model. The thermal dependence of the susceptibilities chi(ZFC), chi(FC), and chi(ac) for the above compounds are well reprod uced by our model predictions, allowing a separate determination of th e RMA crystal field D and ferromagnetic exchange J parameters. Our mod el allows us to correctly explain the thermal dependence of the Sompol insky order parameter, Delta(T), at the SG-ordered regime for the abov e intermetallics.