Magnetocrystalline anisotropy of magnetite

Abstract

The spin reorientation temperature T(SR) of stoichiometric Fe(3)O(4), as well as of magnetite with a small number of vacancies and magnetite containing a low concentration of Ti, Zn, Al and Ga was measured on single-crystal samples using the ac susceptibility. In the same experiment the temperature T(V) of the Verwey transition was also found. The results show that a correlation between T(SR) and T(V) exists. The electronic structure of the compounds studied was determined using the density-functional-based GGA + U method. For stoichiometric magnetite the first and second cubic anisotropy constants were calculated, while for magnetite with defects the distribution of electron density using the 'atoms in molecules' approach was determined. Based on a combination of experimental results with the electronic structure calculations an explanation of the temperature dependence of the magnetocrystalline anisotropy of magnetite is suggested.

The dipole moment of the spin density as a local indicator for phase transitions

“…the high-field endstation at beamline UE46-PGM1 at the electron storage ring BESSY II, various transitions between 4 K and 300 K have been found at the known transition temperatures. Therefore we believe that the enhanced absolute values of the magnetic dipole moments of the monoclinic structure persist above the VT temperature until a temperature between 150 K and 175 K. This is in agreement with the unusual temperature dependence of the magnetocrystalline anisotropy that can be explained by a local charge and orbital ordering well above the VT as suggested by Řezníček et al34. In this sense, our work gives the first experimental evidence for this interpretation of changes in the electronic structure above the VT.…”

“…In addition, spin-orbit coupling is larger for 3d oxides like magnetite than for 3d metals due to a stronger localization of the 3d electrons. The large spin-orbit coupling becomes noticeable by the large magnetic anisotropy constant in the monoclinic phase of magnetite34. Moreover, it causes that the 3d charge distribution is no longer independent from the spin direction5.…”

“…For polycrystalline samples it seems to be impossible to distinguish between the two transitions. As pointed out by Řezníček et al34, there exists a correlation between the spin reorientation and the VT. In this work, the authors connected the contribution to K 1 which leads to an anomalous increasing behavior with decreasing temperature between 250 K and T SR to a local charge and orbital ordering that becomes long-range and stable for temperatures below the VT.…”

“…The spin reorientation is characterized by a vanishing magnetocrystalline anisotropy constant K 1 , which is negative at room temperature. With decreasing temperature it decreases, acquiring a minimum at a temperature of about 250 K34. With further decreasing temperature, it increases and becomes positive at T SR .…”

Spin and orbital magnetic moment of reconstructed2×2R45∘magnetite(001)

“…At a temperature of 128 K the sample undergoes the spin-reorientation transition exhibited by the cubic phase, detected by measurements (not shown) of the surface magnetization in spin-polarized electron microscopy. But the nonstoichiometry cannot be large, given that the Verwey transition is still first order [31], and that the spin-reorientation transition is not much lower than for stoichiometric samples [32]. Using the experimental Verwey transition temperature as an estimate of the nonstoichiometry and assuming it arises from iron vacancies, the crystal should have a nonstoichiometry Fe 3(1−δ) O 4 of δ ∼ 0.005 [31,32].…”