THERMOREMANENCE AND NEEL TEMPERATURE OF GOETHITE

We have measured thermoremanence (TRM) and the temperature dependence of high-field susceptibility chi, both parallel and perpendicular to t he crystallographic c-axis, for a sample of well crystallized natural goethite (alpha FeOOH). Susceptibility chi(perpendicular to) measured perpendicular to the c-axis was almost temperature independent between 50 and 300 K, while chi(parallel to) measured parallel to the c-axis increased almost linearly with temperature over the same range. These are the dependences expected for an antiferromagnetic (AFM) substance with sublattice magnetizations along the c-axis. Extrapolation of the chi(perpendicular to) and chi(parallel to) data trends to their point of intersection gives an estimate for the AFM Neel temperature T-N of (120 +/- 2)degrees C. TRM's produced by cooling in a weak field applie d either parallel or perpendicular to the c-axis had intensities of 2. 4 x 10(-4) Am-2/kg and 1.2 x 10(-5) Am-2/kg, respectively. Since (M(TR M))(perpendicular to) is only 5% of (M(TRM))(parallel to), the weak fe rromagnetism of goethite must be parallel to the AFM spin axis, not pe rpendicular to it as in the case of hematite. The ferromagnetism is ve ry hard: TRM was unaffected by AF demagnetization to 100 mT and by the rmal demagnetization to 90 degrees C. Above 90 degrees C, TRM decrease d sharply, reaching zero at (120 +/- 2)degrees C. Thus the ferromagnet ic Curie point T-C coincides with T-N, as in hematite. However, the we ak ferromagnetism cannot be due to spin canting, as it is in hematite, because canting of the sublattices would produce a net moment perpend icular to the c-axis, rather than parallel to the c-axis as observed.