57Fe Mössbauer effect study of well-crystallized goethite (α-FeOOH)

Well-crystallized, natural goethite has been investigated as a function of temperature. The saturation magnetic hyperfine field was found to be 507 kOe and the Néel temperatureT _N=(400±2) K. The antiferromagnetic structure is stable in external fields of up to 6O kOe and the spin direction is at right angles with respect to the EFG's principal axis. AtT/T _N<0.5, the reduced hyperfine field varies asT ² and in the range 0.80<T/T _N<0.97 asT ^1/3. From the temperature dependence of the isomer shift, the Debije temperature was found to be 440 K from which a relative Mössbauer fraction of 0.96 against hematite was evaluated.