Mössbauer Effect in α‐FeOOH

The antiferromagnetic compound α‐FeOOH is studied between 90 and 440°K by the Mössbauer technique. Analysis of the data gives a Néel temperature T N of 393.3°K and an effective magnetic field, extrapolated to 0°K, of 510 kOe. From the quadrupole splitting above and below T N and from the field gradient, calculated from the lattice positions, it is concluded that the direction of the spins of the iron ions lies along the crystallographic z ‐axis. This is in agreement with susceptibility measurements. The shape of the peaks in the Mössbauer spectra is discussed and possible explanations of the shapes in the vicinity of T N are given. The temperature dependence of the sublattice magnetization is compared with the predictions of the two‐particle cluster theory for a Heisenberg magnet. If the reduced sublattice magnetization is represented as m ( T ) = D (1 − T/T N ) f it is found that f = 0.32. Finally, a comparison is made with other experimental Mössbauer data on goethite and the discrepancies are explained.