THE INADEQUACY OF APPLIED-FIELD MOSSBAUER-SPECTROSCOPY AS A MEANS OF PROVING THE EXISTENCE OF SPEROMAGNETISM

We reply to the suggestion that a two-sublattice collinear spin model employed by us to analyse applied-field Mossbauer spectra of various i nsulators yields impossibly large values for the uniaxial anisotropy c onstant and should therefore be disregarded in favour of a model of sp eromagnetism. We note that the value of the anisotropy constant obtain ed for antiferromagnetic goethite (alpha-FeOOH) using the collinear mo del, K1 congruent-to 3 x 10(5) J m-3, is consistent with both spin-flo p measurements on bulk goethite and with independent theoretical calcu lations. The anisotropy constants of samples of the disordered iron ox yhydroxide feroxyhite (delta'-FeOOH), obtained with the collinear mode l, may be as high as approximately 10 X 10(5) j m-3. The anisotropy co nstants of three reputedly speromagnetic insulators, calculated using the collinear model, fall within this range, and are therefore not imp ossibly large. We reaffirm that there remains a fundamental misconcept ion in the belief that applied-field Mossbauer spectroscopy may be emp loyed to distinguish between speromagnetism and antiferromagnetism in randomly aligned particles and disordered materials.