Magnetic Properties of Hematite Single Crystals. I. Magnetization Isotherms, Antiferromagnetic Susceptibility, and Weak Ferromagnetism of a Natural Crystal

Abstract

Two sets of magnetization isotherms of pure natural hematite single crystals from Elba have been obtained in the temperature range from 488\ifmmode^\circ\else\textdegree\fi{}K down to liquid helium temperatures. The first set of curves, along a certain direction in the basal plane, support N\'eel's magnetic model of a superposition of a weak ferromagnetism on a normal antiferromagnetism. The second set of curves, along the ternary axis, display very unusual form. The analysis of the isotherms shows that the antiferromagnetic susceptibility-temperature curves, $\ensuremath{\chi}\ensuremath{-}T$, are in good agreement with those obtained by N\'eel and Pauthenet but the weak ferromagnetic properties are apparently contradictory to their interpretations. The spontaneous magnetization-temperature curves, ${\ensuremath{\sigma}}_{0}\ensuremath{-}T$, indicate that there is no isotropic ferromagnetism, and that the weak anisotropic ferromagnetism in the basal plane above transition and along the ternary axis below transition seems to have the same nature and origin. The $\ensuremath{\chi}\ensuremath{-}T$ and ${\ensuremath{\sigma}}_{0}\ensuremath{-}T$ curves show that the wide transition takes place gradually and continuously. A general magnetic model of canted antiferromagnetism with unequal sublattice moments has been proposed which explains all the experimental data satisfactorily. From the present model Haigh's data of remanent magnetization of hematite powder seems to be explained naturally.