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This work employs Green's-function theory to compute the temperature dependence of effective spin-model parameters in two-sublattice ferrimagnets, revealing distinct thermal renormalizations of intra- and intersublattice exchange interactions and sublattice anisotropies. The unequal renormalization of these parameters leads to a strongly nonmonotonic and polarization-dependent long-wavelength spin-wave response, notably an enhanced spin-wave stiffness in one polarization near the angular momentum compensation region. These Green's-function predictions are quantitatively validated by atomistic spin-dynamics simulations, establishing a direct link between temperature-dependent atomistic interactions and parameters crucial for multiscale modeling of ferrimagnetic materials.
Ferrimagnets exhibit a counterintuitive enhancement of spin-wave stiffness near compensation temperatures, defying simple monotonic decay of underlying exchange interactions.
We compute the temperature dependence of effective spin-model parameters in two-sublattice ferrimagnets with antiparallel and inequivalent sublattice magnetizations using Green's-function theory. The effective intra- and intersublattice exchange interactions, together with the sublattice anisotropies, exhibit distinct thermal renormalizations arising from the different magnetic interactions and correlations of the two sublattices. Although these effective interaction parameters decrease monotonically with increasing temperature, their unequal renormalization produces a strongly nonmonotonic and polarization-dependent long-wavelength spin-wave response. In particular, the spin-wave exchange stiffness exhibits a strongly branch-dependent and nonmonotonic temperature dependence. One polarization develops an enhanced spin-wave stiffness in the vicinity of the angular momentum compensation region despite the monotonic thermal reduction of the underlying exchange interactions. The Green's-function predictions are in quantitative agreement with atomistic spin-dynamics simulations, which provide a numerical benchmark including thermally induced spin correlations within the underlying atomistic spin model. These results establish a direct connection between temperature-dependent atomistic interactions, long-wavelength spin-wave stiffness, and the effective parameters required for finite-temperature micromagnetic and multiscale modeling of ferrimagnetic materials.