Magnetism and the Trimeron Bond

Abstract

A review of progress in understanding the Verwey transition in magnetite (Fe 3 O 4 ) over the past decade is presented. This electronic and structural transition at T V ≈ 125 K was reported in 1939 and has since been a contentious issue in magnetism. Long range Fe 2+ /Fe 3+ charge ordering has been confirmed below the transition from crystal structure refinement, and Fe 2+ orbital ordering and formation of trimerons through weak bonding of Fe 2+ states to two Fe neighbors has been discovered. This model has accounted for many spectroscopic observations such as the 57 Fe NMR frequencies. The trimeron lifetime has been measured, and trimeron soft modes have been observed. The origin of the first to second order crossover of Verwey transitions in doped magnetites has been revealed by a nanoparticle study. Electronic and structural fluctuations are found to persist to temperatures far above T V and local structural distortions track the bulk magnetization, disappearing at the 850 K Curie transition. New binary mixed-valent iron oxides discovered at high pressure are found to have electronic transitions and orbital molecule ground states similar to those of magnetite.

Dressed jeff-1/2 objects in mixed-valence lacunar spinel molybdates

“…The M sites define a pyrochlore lattice, each of those ions being encapsulated within an X 6 octahedral cage of nearest-neighbor chalcogenide ligands. Magnetite, for example, belongs to this class of systems, with Fe ions at both A and M sites 1 . In lacunar spinels, the A sites have half-integer occupation.…”

“…Mixed valency, the presence of more than one oxidation state for a particular chemical element, is associated with very rich physics, in both molecular and solid-state systems. It can be intrinsic, as for example in stoichiometric magnetite Fe 3 O 4 1 , or obtained through doping as in the colossal magneto-resistance manganese oxides ( R 1- x A x )MnO 3 ( R = rare-earth cation, A = alkali or alkaline-earth cation) 2 . It can also imply quasi-localized ligand-hole states, i. e., ligand mixed valency, as shown for underdoped cuprates 3 , 4 or for the spin-Peierls compound NaV 2 O 5 5 , 6 .…”

Oxidation-State Dynamics and Emerging Patterns in Magnetite

“…Allowing for swaps between all iron sites, similar to what was observed by Elnaggar et al, tetrahedral iron sites are increasingly occupied by Fe 2+ with increasing temperature. In particular, the amount of Fe tet 2+ depends on T MC and follows the relationship given in ref : (Fe 1– x 3+ Fe x 2+ ) tet [Fe 1+ x 3+ Fe 1– x 2+ ] oct O 4 . A T MC of 4000 K corresponds to the experimental temperature range between 330 and 880 K (called Regime II in ref ) with a value of x = 0.125 at 840 K .…”

“…In particular, the amount of Fe tet 2+ depends on T MC and follows the relationship given in ref : (Fe 1– x 3+ Fe x 2+ ) tet [Fe 1+ x 3+ Fe 1– x 2+ ] oct O 4 . A T MC of 4000 K corresponds to the experimental temperature range between 330 and 880 K (called Regime II in ref ) with a value of x = 0.125 at 840 K . Note that T MC here is typically an order of magnitude larger than the experimentally observed temperatures.…”

Multi-order and structural mechanism of atom nanocluster formation in rhombohedral Cu-Zr-thiospinels

“…Geometric frustration causes the degeneracy of the ground state of the crystals, which leads to the fact that even a slight external perturbation can affect the properties of matter and, in particular, lead to the formation of exotic phases with unusual properties, nontrivial atomic, charge and orbital ordering. One of the degeneracy removal mechanisms is the formation of metal clusters, called "orbital molecules", in the form trimerons in Fe 3 O 4 [14], octamers in CuIr 2 S 4 [9,15], dimers in MgTi 2 O 4 [16][17][18], and metal clusters in many other substances [19][20][21][22].…”