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

Abstract

The Verwey transition in nanostructured magnetite produced by a combination of chimie douce and spark plasma sintering J. Appl. Phys. 115, 17E117 (2014) Magnetite nanostructured powder samples were synthesized by aging chemical method. Phase, structural, and magnetic properties were characterized. X-ray diffraction patterns showed cubic magnetite pure phase, with average crystallite size, hDi, equal to 40 nm. Susceptibility measurements showed the well-known Verwey transition at a temperature of 90 K. The decrease of Verwey transition temperature, with respect to the one reported in literature (125 K) was attributed to the low average crystallite size. Moreover, the spin-glass like transition was observed at 35 K. Activation energy calculated from susceptibility curves, with values ranging from 6.26 to 6.93 meV, showed a dependence of spin-glass transition on frequency. Finally, hysteresis loops showed that there is not an effect of Verwey transition on magnetic properties. On the other hand, a large increase of coercivity and remanent magnetization at a temperature between 5 and 50 K confirmed the presence of a magnetic transition at low temperatures. V C 2013 American Institute of Physics.[http://dx

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“…Consistently, the ZFC curves showed no maximum in temperatures up to 300 K (i.e., the blocking temperature is not below 300 K) as previously reported on similar Fe 3 O 4 NPs with size ≥ 50 nm 31 . Two distinct ‘shoulders’ were observed in ZFC-mode curves at temperatures T 1 ≈ 45 K and T 2 ≈ 98 K. The shoulder at T 2 has been previously related to the Verwey transition, which occurs at T V = 122 K in bulk Fe 3 O 4 31 – 33 . The small bump observed in the FC branch at the same temperature supports this interpretation.…”

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“…Other low-temperature reports were assigned to a signicant amount of surface spin disorder at 15 K for 7 nm maghemite NPs 48 while spin-glass-like-transition at 35 K for 40 nm magnetite NPs. 19 Considering that non-interacting particles where the barrier energy is determined by the uniaxial anisotropy, the spin relaxation is a thermally activated process, as is proposed by the Néel-Arrhenius model, s ¼ s 0 exp(E a /k B T), where E a /k B is the activation energy. Assuming that the temperatures in c 00 (T,f) correspond to the T B and the low-temperature peak is best observed in dc 00 /dT vs. T curve (inset on see Fig.…”

“…16 In magnetite NPs of 40 nm a spin-glass like behavior with a freezing temperature of $35 K was determined and could be observed in both, the in-phase and the out-phase magnetic susceptibility curves. 19 These characteristics can be tuned during the synthesis process.…”

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“…It should be remarked that this material does not show any trace of the Verwey transition. As a matter of fact, this transition is a rather elusive effect in systems of magnetite nanoparticles: for instance, the recent literature indicates a strong reduction [28][29][30][31] or even a complete disappearance [32,33] of the Verwey transition temperature T V in small particles. Moreover, the particle shape plays an important role in the Verwey transition, at least in ultrasmall nanoparticles [34].…”