Current understanding of low temperature phase transition of magnetite, particularly in relation to the behavior of magnetocrystalline anisotropy

Abstract

Recent experiments have revised old concepts of the low temperature phase of magnetite, such as the Verwey order scheme, orthorhombic crystal symmetry etc. Neutron diffraction has revealed that doubly degenerate Δ5 phonon modes tend to condense towards a point 0.3 K below the transition point, Tv. At T<Tv, however, one of the degenerate modes, Δ5(1), which has ionic shifts mainly parallel to the monoclinic b‐axis, appears. It seems that there is no ionic ordering (charge ordering) of the wave‐length of 2c, associated with the Δ5(1) mode.Below Tv, the crystal symmetry is monoclinic, and its single phase was realized by the application of a magnetic field parallel to [001] and at the same time by squeezing the crystal about [1̄11] while cooling through Tv. The magnetocrystalline anisotropy was measured on a single‐phase specimen by using a computerized fully‐automatic torque magnetometer. The anisotropy energy is expressed by Ea=Kaα2a+Kaaα4a +Kbα2b+Kbbα4b +Kabα2aαα2b−Kuα21 ̄01 where αa, αb and α1̄01 are direction cosines of the angle of spin from monoclinic a‐ and b‐axes and [1̄01], respectively. It was found that Ka, Kb, Ku exhibit temperature dependences of the activation‐type with activation energies of about 0.02 eV.