Manifestation of the Verwey transition in the tunneling spectra of magnetite nanocrystals

Abstract

Tunneling transport measurements performed on single particles and on arrays of Fe 3 O 4 (magnetite) nanocrystals provide strong evidence for the existence of the Verwey metal-insulator transition at the nanoscale. The resistance measurements on nanocrystal arrays show an abrupt increase of the resistance around 100 K, consistent with the Verwey transition, while the currentvoltage characteristics exhibit a sharp transition from an insulator gap to a peak structure around zero bias voltage. The tunneling spectra obtained on isolated particles using a Scanning Tunneling Microscope reveal an insulator-like gap structure in the density of states below the transition temperature that gradually disappeared with increasing temperature, transforming to a small peak structure at the Fermi energy. These data provide insight into the roles played by long-and short-range charge ordering in the Verwey transition.( * )

Doping effects on the magnetic resonance of ZnFe2O4 and NiFe2O4

“…As shown in Fig. 2, DH of S1 increases from 2200 to 2612 Oe with increasing T; but has a turning point at 110 K. The linear increase above 110 K is consistent with the mechanism of eddy current and the turning point may be related to Verwey transition [3]. As for S2, DH slightly decreases from 1835 to 1654 Oe with increasing T from 80 to 300 K. According to literature [4] Apparently, the partial doping of Ni 2+ ion into A-site affects the strength of A-B interaction.…”

Magnetic susceptibility studies of the spin-glass and Verwey transitions in magnetite nanoparticles

“…This is observed as a little slope change in magnetic susceptibility curves at T V . Verwey transition is reported by several authors to have values around 120 K for bulk samples, 1,5,[8][9][10] but, for our nanoparticles, it decreases to 90 K. Using an elementary thermodynamic model, this reduction can be associated to an increment on surface energy due to the small crystallite size, leading to a reduction of the energy barrier for the transformation to occur; hence, a reduction on the transition temperature befalls because of the size of nanocrystals. 13 The second transition observed at lower temperature, T sg ¼ 35 K, was a spin-glass like, which could be seen in both the in-phase and out-phase susceptibility curves (Fig.…”

“…The origin of this drop in electrical resistance is still under discussion, but it is assumed that there occurs a charge order-disorder transition in crystal structure. [6][7][8] The other interesting transformation in magnetite is the so called spin-glass transition, where a transition from high temperature ferromagnetic state to low temperature paramagnetic state happens at the grain surfaces. This is, below temperature of spin-glass transition, T sg , grain-boundary spins freeze, and a drop in magnetization would take place.…”

Magnetoresistive telegraph noise in Langmuir-Blodgett films of colloidal magnetite nanocrystals as seen via scanning tunneling microscopy

“…I͑t͒ curves were measured 5 times on each selected NC, under a small bias voltage ͑ϳ20 mV͒. It was previously shown that the magnetite nanocrystals used here have a high conductivity 26 as well as maximal MR ͑Ref. 21͒ at low bias conditions.…”

“…26͒ and on multiple NC arrays 20 consistently showed that the tunneling density of states of these NCs is high around zero bias ͑the Fermi level͒ and that an insulating gap opens only below the Verwey transition, which occurred at ϳ100 K for these NCs. 26 In the present work we probed the local fluctuations in current passing through a close-packed Fe 3 O 4 NC film using a nonmagnetic STM tip.…”