Jr. Cullen et Kb. Hathaway, TEMPERATURE-DEPENDENCE OF THE EXCHANGE ENERGY AND CRITICAL-FIELD FOR ANTIFERROMAGNETICALLY COUPLED THIN-FILMS AND SUPERLATTICES, Physical review. B, Condensed matter, 47(22), 1993, pp. 14998-15006
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.