The isothermal $M-H$ curves of magnetite in the principal crystallographic directions have been measured from room temperature to the Curie point. These curves give the anisotropy energy and spontaneous magnetization as a function of temperature. Near the Curie point the magnetizing field required to create a single domain is found to be <100 amp/m, giving an unambiguous experimental determination of the spontaneous magnetization in this critical region. Temperature increments of 0.1°C define the Curie point with an uncertainty of <0.05°C. In the neighborhood of the Curie point the $M-H$ curves are very nonlinear; replotting the data in the form of $H-T$ curves at constant magnetization shows that in this region the magnetic energy can be expressed as $U_M=-\frac{{\mu }_{0}W{M}^2}{2}$, where $W$ is a constant.

• Received 20 January 1956

DOI:https://doi.org/10.1103/PhysRev.102.959