Magnetic and Electric Properties of Natural and Synthetic Single Crystals of Magnetite

Abstract

When a crystal of magnetite has been cooled through the low temperature transition (near -164\ifmmode^\circ\else\textdegree\fi{}C) in a demagnetized state, the magnetization undergoes a large and abrupt increase in the three principal crystallographic directions [100], [110], and [111] upon warming. The behavior of the magnetization upon cooling in a magnetic field is influenced by crystallographic orientation as well as by the strength of the applied field. The form of the magnetization curve at temperatures below the transition depends on the crystallographic direction and on the magnetic treatment of the specimen while cooling through the transition. Barkhausen experiments lead to tentative ideas concerning domain orientation. Significant information concerning the nature of the transition was also obtained from measurements of thermal expansion and saturation magnetostriction.The magnetic effects in the low temperature transition region appear to be influenced by the establishment of internal stresses which cause a "freezing-in" of preferential domain directions below the transition region. The stress and domain pattern resulting appears to depend on the magnetic treatment of the crystal during its passage through the transition region.The temperature dependence of the electrical resistivity is in agreement with the results of Verwey and de Boer for sintered magnetite. Some preliminary observations are reported on the effects of magnetic fields on the resistivity.