Relaxationsphänomene in Mössbauerspektren von magnetisierten paramagnetischen Substanzen

The Mössbauer spectra of partly magnetized FeNH₄(SO₄)₂·12 H₂O show a broadening and a shift of the hyperfine structure lines, reflecting the paramagnetic spin fluctuations. These fluctuations and their influence on the γ-spectrum may be treated in a spin wave model without introducing phenomenological parameters. By means of a simple diagram technique we get a line broadening γ and line shift δ, proportional to second and third order polynomials of the magnetization and to ∫g ² dΩ and ∫g ³ dΩ, respectively.g(Ω) is the frequency spectrum of spin waves. The values of the two frequency integrals, as deduced from the measured Mössbauer data γ and δ of ferric alum, are in reasonable agreement with the results obtained from the calculated spin wave spectrum, assuming pure magnetic dipole-dipole coupling (long wave length approximation of Holstein-Primakoff). A small contribution of non-magnetic dipole-dipole interaction (van Vleck) cannot be excluded.