NMR for magnetite

Abstract

Magnetite is the oldest magnet material known to mankind. It is getting attention again from solid state physics researchers now a days because it is one of the most strongly correlated electron systems. Spin, charge, and orbital orders are interplaying with lattice and involved in the Verwey transition where magnetization, conductivity, and structure changes suddenly. The peculiar ordering states above and below the transition temperature mainly originate from the coexistence of $Fe^{2+}$ and $Fe^{3+}$ ions in the B site of the inverse spinel structure. In particular, the state of the charge and orbital order was the oldest and most intriguing problem. NMR has made significant contribution to the investigation of this question. A. Abragam stated that there is no doubt that NMR is a very powerful tool for the study of ferromagnetic and antiferromagnetic materials. In this mini-review, a short history of NMR investigation of magnetite is presented, providing a support to Abragam's claim.

Figure 1. Resistivity vs. temperature of magnetite. Phase transition is observed around 120 K.²

Figure 2. Variation of the NMR frequency of Fe⁵⁷ in Fe₃O₄ with temperature. The frequency at which the most intense echo signal is observed is plotted for the inhomogeneously broadened line from B ion⁴.

Figure 3. Temperature dependence of the NMR frequency of lines corresponding to Fe on tetrahedral (A) and octahedral (B) sites⁵.

Figure 4. Anisotropic hyperfine field vs. external field direction for three different orbitals, as the spin direction changes from z to y axis, y to x, and x to back to z axis⁶.

Figure 5. Orbital ordering structure of B-site Fe ions on (111) plane and adjacent to it⁷.

Figure 6. Charge density of extra electron (α) of the Fe^{(3- α)+} ion in the wave function⁷. The colors are the same as those in Figure 5.