Relaxationsphänomene in Mössbauerspektren von Magnetisierten paramagnetischen Substanzen. I [(Fe x,Al1-x )NH4(SO4)2· 12H2O]

The Fe³⁺-spin in alums of type (Fe _x , A1_1-x )NH₄(SO₄)₂ · 12 H₂O interacts (i) with the crystal lattice viaLS-coupling, and (ii) with the spins of the adjacent Fe³⁺-ions via magnetic dipole-dipole interaction. These interactions lead to a time fluctuation of the spin direction, characterized by correlation times τ _c and τ′ _c of increasing order. The times may be deduced from the⁵⁷Fe-Mössbauer spectra of the alums, τ _c from the width, and τ′ _c ≈τ _c from the position of the hyperfine structure lines. The theoretical interpretation of the Mössbauer spectra is relatively simple, when (i) the spin-lattice interaction gets frozen in, and (ii) a strong applied magnetic fieldH _a decouples the spins of the Fe³⁺-ion and the⁵⁷Fe-nucleus. The spectra were taken, therefore, at 4.2 °K and 8 kOe≦H _a ≦ 54 kOe. According to the 1/r ³-dependence of the magnetic dipole-dipole interaction τ _c should be related tox, the Fe-concentration, τ _c ·x≈τ₀=const. Forx≧0.5 our experimental results are in agreement with this rule when τ₀=(1.5±0.5) · 10^?9 s. For an alum withx=0.26, however, the observed spectra cannot be explained in terms of temporal spin fluctuations, at least not in the framework of the models which are available now. Here, presumedly, the electron spins of adjacent Fe³⁺-ions are coupled to more or less isolated and, consequently, relatively stationary spin clusters of various sizes, leading to many time independent internal magnetic fields. A treatment of this proposal is in preparation.