  Using ferromagnetic-resonance spectroscopy (FMR), we investigate the
anisotropic properties of epitaxial 3 nmPt/2 nmAg/10 nmFe/10 nmAg/GaAs(001)
films in fully saturated meta-stable states at temperatures ranging from 70 K
to 280 K. By comparison to spin-wave theory calculations, we identify the role
of thermal fluctuation of magnons in overcoming the energy barrier associated
with these meta-stable states. We show that the energy associated with the size
of the barrier that bounds the meta-stable regime is proportional to the heat
stored in the magnonic bath. Our findings offer the possibility to measure the
magnonic contribution to the heat capacity by FMR, independent of other
contributions at temperatures ranging from 0 K to ambient temperature and
above. The only requirement being that the selected sample exhibits magnetic
anisotropy, here, magnetocrystalline anisotropy.
