Thermal stability of micro- and nanoscale magnetite by thermomagnetic analysis data

Ponomar, Vitalii; Dudchenko, N.O.; Брик, А. Б.

doi:10.1109/nap.2017.8190407

Cited by 4 publications

(5 citation statements)

References 8 publications

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“…f V was fitted to first/second/third unit cell ranges of the X-ray PDF, which describe short/medium/long-range electronic orders. The first unit cell values show that substantial local structural distortions persist up to T C and closely match the reported variation of the bulk magnetization . This demonstrates that the structural and electronic fluctuations responsible for the Verwey transition are a direct result of the long-range magnetic order.…”

Section: Resultssupporting

confidence: 80%

“… 38 , 39 However, short-range structural correlations, on the length scale of an individual trimeron, remain present above T V and decrease to zero near the Curie transition at T C ≈ 850 K, following a similar temperature dependence to the reported bulk magnetization. 47 This also matches the thermal transfer of extra electrons (Fe 2+ states) from octahedral to tetrahedral sites seen by X-ray spectroscopy. 36 The weak bonding Fe–Fe interactions in a trimeron require ferromagnetic alignment of the three core S = 5/2 spins so that the extra minority spin electron can be delocalized over the three Fe ions, as shown in Figure 1 c. Hence, magnetization is coupled to local Fe displacements to which the X-ray PDF is particularly sensitive.…”

Section: Resultssupporting

confidence: 76%

“…The SWA model was used to fit the average degree of local distortion over short, medium, and long-range length scales in an X-ray PDF study covering a wide range of temperatures (90–923 K) . The resulting plots in Figure show that long-range structural distortions fall sharply to zero just above T V , while medium range distortions persist up to 250–300 K, which matches Raman observations of modes associated with the electronic order. , However, short-range structural correlations, on the length scale of an individual trimeron, remain present above T V and decrease to zero near the Curie transition at T C ≈ 850 K, following a similar temperature dependence to the reported bulk magnetization . This also matches the thermal transfer of extra electrons (Fe 2+ states) from octahedral to tetrahedral sites seen by X-ray spectroscopy .…”

Section: Resultsmentioning

confidence: 52%

“…This variation demonstrates that the Verwey transition is suppressed not only by the doping effect from the added oxygen but also by inhomogeneous strains from the concentration gradient developed between the oxygen-rich exterior and oxygen-poor interior of the nanoparticles during oxidation, and this was confirmed by quantitative modeling. Observation of the persistent value of T V = 95 K close to the T V ≈ 100 K crossover between first and second-order Verwey transitions ( Figure 4 ) 47 , 48 shows that the crossover corresponds to the intrinsic lower temperature limit of the Verwey transition in homogeneously doped magnetite. Lower T V values down to 70 K in the second-order regime result from additional effects of strain gradients on the transition.…”

Section: Resultsmentioning

confidence: 82%

“…The first unit cell values show that substantial local structural distortions persist up to T C and closely match the reported variation of the bulk magnetization. 47 This demonstrates that the structural and electronic fluctuations responsible for the Verwey transition are a direct result of the long-range magnetic order. Material reprinted with permission from ref ( 46 ).…”

Section: Resultsmentioning

confidence: 88%

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Magnetism and the Trimeron Bond

Attfield

2022

Chem. Mater.

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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.

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Section: Resultssupporting

confidence: 80%

Section: Resultssupporting

confidence: 76%

Section: Resultsmentioning

confidence: 52%

Section: Resultsmentioning

confidence: 82%

Section: Resultsmentioning

confidence: 88%

See 3 more Smart Citations

Magnetism and the Trimeron Bond

Attfield

2022

Chem. Mater.

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Co-emergence of magnetic order and structural fluctuations in magnetite

et al. 2019

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The nature of the Verwey transition occurring at T V ≈ 125 K in magnetite (Fe 3 O 4 ) has been an outstanding problem over many decades. A complex low temperature electronic order was recently discovered and associated structural fluctuations persisting above T V are widely reported, but the origin of the underlying correlations and hence of the Verwey transition remains unclear. Here we show that local structural fluctuations in magnetite emerge below the Curie transition at T C ≈ 850 K, through X-ray pair distribution function analysis. Around 80% of the low temperature correlations emerge in proportion to magnetization below T C . This confirms that fluctuations in Fe-Fe bonding arising from magnetic order are the primary electronic instability and hence the origin of the Verwey transition. Such hidden instabilities may be important to other spin-polarised conductors and orbitally degenerate materials.

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Optimal ferrofluids for magnetic cooling devices

Pattanaik

Varma

Cheekati

et al. 2021

Sci Rep

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Superior passive cooling technologies are urgently required to tackle device overheating, consequent performance degradation, and service life reduction. Magnetic cooling, governed by the thermomagnetic convection of a ferrofluid, is a promising emerging passive heat transfer technology to meet these challenges. Hence, we studied the performance metrics, non-dimensional parameters, and thermomagnetic cooling performance of various ferrite and metal-based ferrofluids. The magnetic pressure, friction factor, power transfer, and exergy loss were determined to predict the performance of such cooling devices. We also investigated the significance of the magnetic properties of the nanoparticles used in the ferrofluid on cooling performance. γ-Fe2O3, Fe3O4, and CoFe2O4 nanoparticles exhibited superior cooling performance among ferrite-based ferrofluids. FeCo nanoparticles had the best cooling performance for the case of metallic ferrofluids. The saturation magnetization of the magnetic nanoparticles is found to be a significant parameter to enhance heat transfer and heat load cooling. These results can be used to select the optimum magnetic nanoparticle-based ferrofluid for a specific magnetic cooling device application.

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Thermal stability of micro- and nanoscale magnetite by thermomagnetic analysis data

Cited by 4 publications

References 8 publications

Magnetism and the Trimeron Bond

Magnetism and the Trimeron Bond

Co-emergence of magnetic order and structural fluctuations in magnetite

Optimal ferrofluids for magnetic cooling devices

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