TEMPERATURE-DEPENDENCE OF THE EXCHANGE ENERGY AND CRITICAL-FIELD FOR ANTIFERROMAGNETICALLY COUPLED THIN-FILMS AND SUPERLATTICES

We calculate the temperature dependence of the antiferromagnetic excha nge energy and critical magnetic field for a model ferromagnet/paramag net/ferromagnet (FM/PM/FM) sandwich and a model (FM/PM/FM)infinity sup erlattice. The temperature dependence is assumed to originate from flu ctuations in the spin angular momentum of the FM components, rather th an from electronic effects in the PM components. The spins are taken t o be localized and coupled via near-neighbor ferromagnetic exchange wi thin the ferromagnets, while spins on outermost layers are coupled to their counterparts across the paramagnetic spacer by a weaker antiferr omagnetic exchange. The latter is shown to renormalize with temperatur e T through a surface-spin-surface-spin correlation function. This cor relation function is calculated in the spin-wave region of temperature . For the superlattice, the fractional decrease in critical field and in antiferromagnetic exchange is proportional to T In T except at extr emely low T, where there is a T3/2 dependence. For the sandwich, a lin ear dependence on T is found, with a coefficient that depends logarith mically on the dipole energy. Again, a T3/2 dependence is found as T-- >0. We compare these results with related calculations and discuss the experimental implications.