Magnetic resonance saturation in solids at finite temperatures

The present paper gives an extension of the Provotorov theory of magnetic resonance saturation to finite temperatures. Such an extension is not straightforward because at finite temperatures when line-shape and resonance frequency become temperature dependent the weak coupling expansion in a small h.f. field is no longer convergent. In this paper it is assumed that this expansion can be renormalized by means of spin temperature concepts. The results are formulated using a version of the Mori formalism without recourse to a linear response approximation.