Thermoremanence and Néel temperature of goethite

<jats:p>We have measured thermoremanence (TRM) and the temperature dependence of high‐field susceptibility χ both parallel and perpendicular to the crystallographic c‐axis, for a sample of well crystallized natural goethite (αFeOOH). Susceptibility χ<jats:sub>⟂</jats:sub> measured perpendicular to the c‐axis was almost temperature independent between 50 and 300 K, while χ<jats:sub>∥</jats:sub> 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 χ<jats:sub>⟂</jats:sub> and χ<jats:sub>∥</jats:sub> data trends to their point of intersection gives an estimate for the AFM Néel temperature T<jats:sub>N</jats:sub> of (120±2)°C. TRM's produced by cooling in a weak field applied either parallel or perpendicular to the c‐axis had intensities of 2.4 × 10<jats:sup>−4</jats:sup> Am²/kg and 1.2 × 10<jats:sup>−5</jats:sup> Am²/kg, respectively. Since (M<jats:sub>TRM</jats:sub>)<jats:sub>⟂</jats:sub> is only 5% of (M<jats:sub>TRM</jats:sub>)<jats:sub>∥</jats:sub>, the weak ferromagnetism of goethite must be parallel to the AFM spin axis, not perpendicular to it as in the case of hematite. The ferromagnetism is very hard: TRM was unaffected by AF demagnetization to 100 mT and by thermal demagnetization to 90°C. Above 90°C, TRM decreased sharply, reaching zero at (120±2)°C. Thus the ferromagnetic Curie point T<jats:sub>C</jats:sub> coincides with T<jats:sub>N</jats:sub>, as in hematite. However, the weak ferromagnetism cannot be due to spin canting, as it is in hematite, because canting of the sublattices would produce a net moment perpendicular to the c‐axis, rather than parallel to the c‐axis as observed.</jats:p>