Brillouin scattering studies inFe3O4across the Verwey transition

Abstract

Brillouin scattering studies have been carried out on high quality single crystals of Fe 3 O 4 with [100] and [110] faces in the temperature range of 300 to 30 K. The room temperature spectrum shows a surface Rayleigh wave (SRW) mode at 8 GHz and a longitudinal acoustic (LA) mode at 60 GHz. The SRW mode frequency shows a minimum at the Verwey transition temperature T V of 123 K. The softening of the SRW mode frequency from about 250 K to T V can be quantitatively understood as a result of a decrease in the shear elastic constant C 44 , arising from the coupling of shear strain to charge fluctuations. On the other hand, the LA mode frequency does not show any significant change around T V , but shows a large change in its intensity. The latter shows a maximum at around 120 K in the cooling run and at 165 K in the heating run, exhibiting a large hysteresis of 45 K. This significant change in intensity may be related to the presence of stress-induced ordering of Fe 3+ and Fe 2+ at the octahedral sites, as well as to stress-induced domain wall motion.

Anharmonicity due to Electron-Phonon Coupling in Magnetite

“…The key element, which stabilizes the CO order and drives the crystal symmetry change is the coupling between electronic and vibrational degrees of freedom. Such cooperative nature of the Verwey transition is supported by numerous experimental observations: oxygen isotope effect [17], critical softening of the c 44 elastic constant [18], critical diffuse scattering [19,20], phonon anomalies measured by the Brillouin [21], Raman [22], and nuclear inelastic scattering [23,24]. The instability of the electronic structure is intimately connected with a lattice deformation, with certain similarity to the Peierls model [25].…”

Mapping the lattice dynamical anomaly of the order parameters across the Verwey transition in magnetite

“…Precursor effects were found to anticipate the discontinuous transition over temperature intervals extending far above T V [12][13][14][15][16][17][18][19][20][21]. Diffuse scattering of neutrons and x-rays with local maxima at incommensurate points in reciprocal space was shown to survive to room temperature and attributed to the persistence of local and dynamical electronic and structural correlations in the cubic phase [13][14][15][16].…”

“…X-ray nuclear inelastic scattering studies indicated possible hardening of the low-energy modes at T V [5,24]. Ultrasonic measurements and Brillouin scattering evidenced progressive softening of the shear sound velocity associated with the c 44 elastic constant from room temperature to T V [18,19]. However, further experiments to directly determine the energy and momentum transfer of low-energy phonons in the critical region and thus gain a complete picture were so far limited by crystal microtwinning below T V .…”

Short-Range Correlations in Magnetite above the Verwey Temperature

“…Above the Verwey transition, where the inverse spinel structure, with Fe 3+ ions in A sites and B sites in a mixed-valence Fe 2.5+ state is realized, several observations indicate the existence of short-range order of polaronic character [22][23][24][25]. This correlated state is reflected in the critical softening, on cooling, of the c 44 elastic constant [26,27], softening of the surface phonons [28], critical diffuse scattering [29][30][31][32][33][34] and anomalous phonon broadening [35]. Furthermore, the quasi-elastic character of neutron scattering suggests low-energy fluctuations of the lattice distortions coupled to electrons [31,32].…”