Thermal hysteresis of spin reorientation at Morin transition in alkoxide derived hematite nanoparticles

Goya, Gerardo F.; Veith, Michael; Rapalavicuite, R; Shen, Hao; Mathur, Sanjay

doi:10.1007/s00339-003-2381-4

Cited by 32 publications

(18 citation statements)

References 18 publications

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“…Different values of T M in heating and cooling have been reported by Flanders [1969], Labushkin et al [1976], Nininger and Schroeer [1978], Muench et al [1985], Vlasov et al [1986], Williamson et al [1986], Zysler et al [2003], Goya et al [2005], and Bercoff et al [2007]. Williamson et al 's [1986] Mössbauer spectra of hematite powders shocked at pressures of 8 to 27 GPa showed an increase in thermal hysteresis from Δ T M = 12 K for unshocked powder to Δ T M = 45 K for the 17‐Gpa sample.…”

Section: Discussionmentioning

confidence: 88%

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Morin transition in hematite: Size dependence and thermal hysteresis

Özdemir

Dunlop

Berquó

2008

Geochem Geophys Geosyst

171

110

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[1] Hematite is a frequently used mineral in paleomagnetic and environmental magnetic studies. Just below room temperature, it undergoes a magnetic phase transition, the Morin transition, whose nature is an important part of our basic understanding of hematite's magnetism and magnetic memory. We have determined the temperature T M of the Morin transition from saturation remanence warming curves to be 250-261 K for 0.5-6 mm hematite natural single crystals, 257-260 K for 45-600 mm sieved crystal fractions, and 241-256 K for submicron synthetic hematites with grain sizes between 120 and 520 nm. The variation must be due to differences in crystal morphology, lattice strain, and crystal defects common in both synthetic and natural crystals. Our results are compatible with published data for 100 nm to 10 mm hematites and show that T M is nearly size independent, decreasing very gradually as particle size decreases over this broad range, which includes both multidomain (MD) and single-domain (SD) structures. However, T M decreases sharply between 90 and 30 nm. Below 20 nm, the transition disappears entirely as near-surface spins deviate strongly from the antiferromagnetic easy axis. Our SD and MD hematites exhibit a thermal hysteresis in the Morin transition: the values of T M in cooling and in heating are different. For the same cooling/warming rate, ÁT M is much greater for submicron hematites than for larger crystals. We attribute the lag in the transition in both cooling and heating to crystal imperfections and resulting internal stresses, and speculate that defects may serve to pin and stabilize surface spins. Preventing spin rotation in a region large enough to trigger the phase transition would inhibit destabilization of the weakly ferromagnetic phase in cooling and the antiferromagnetic phase in heating. The wide distribution of particle sizes in our submicron samples may also play a role.Components: 6959 words, 8 figures.

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Section: Discussionmentioning

confidence: 88%

“… Goya et al [2005] observed thermal hysteresis in both structural and magnetic hyperfine parameters when 200 nm synthetic hematite crosses the Morin transition during heating or cooling. They appealed to magnon softening at the particle surface as the trigger for the phase transition.…”

Section: Discussionmentioning

confidence: 94%

Morin transition in hematite: Size dependence and thermal hysteresis

Özdemir

Dunlop

Berquó

2008

Geochem Geophys Geosyst

171

110

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“…The net magnetic moment is lost in this process, and bulk hematite transforms into an antiferromagnet. In contrast, magnetization studies on nanocrystalline and mesoporous samples of hematite show that no spin reorientation occurs upon cooling [14][15][16][17][18][19][20][21], and ferromagnetic behavior persists as low as T = 2 K. Many studies also show that hematite nanoparticles display superparamagnetic properties [22][23][24].…”

Section: Introductionmentioning

confidence: 82%

Size-dependence of the heat capacity and thermodynamic properties of hematite (α-Fe2O3)

Snow

Lee

Shi

et al. 2010

The Journal of Chemical Thermodynamics

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“…As with the MD crystals, the Morin transition of our SD hematites has a thermal hysteresis, with different values of T M in cooling and warming cycles. Hysteresis of T M is a well‐known property of hematite [ Lin , 1960; Nininger and Schroeer , 1978; Vlasov et al , 1986; Goya et al , 2005]. Chow and Keffer [1974] suggested that magnons at the surface (where the spin rotation initiates) soften differently as T M is approached from below or above, depending on the local surface anisotropy.…”

Section: Low‐temperature Cycling Of Sirmmentioning

confidence: 99%

Magnetic memory and coupling between spin‐canted and defect magnetism in hematite

Özdemir

Dunlop

2006

J. Geophys. Res.

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[1] Saturation isothermal remanent magnetization (SIRM) has been studied on submicron hematite powders with grain sizes between 0.12 and 0.52 mm and on 2 to 5 mm natural hematite single crystals before and after zero-field cycling through the Morin transition (T M ). SIRM cooling and warming curves for single-domain (SD) crystals are similar to those of multidomain (MD) hematites. Both have similar remanence losses at T M (97-98% of the original SIRM), a similar defect moment below T M (2-3% of initial SIRM), and similar memory (30-40% of initial SIRM). Regardless of grain size, higher SIRM memory ratios are associated with higher defect moments below T M . In SD and MD hematites alike, room temperature magnetic memory seems to be an amplification of residual weak ferromagnetism that persists even at very low temperatures, much below T M . Applying a strong field to initially demagnetized SD and MD hematites at 20 K produced a substantial SIRM, which spontaneously increased by a factor 10-28 upon crossing the transition at T M . These observations imply that some spins do not participate in the general rotation from the ferromagnetic c plane to the antiferromagnetic c axis below T M . The defect moment of these spins serves to restore preferred directions of spins and ferromagnetic domains during zero-field warming through the Morin transition and is thus responsible for the memory phenomenon. The existence of a weak ferromagnetic moment in antiferromagnetic hematite below T M is also indicated by colloid patterns observed by Gallon. We propose that the mechanism of memory is clusters of spins pinned magnetoelastically by lattice defects. These spins rotate only partially out of the basal plane during cooling through T M . Some basal plane anisotropy, also magnetoelastic in origin, must remain below T M in order to explain the existence of low-temperature SIRM and also to guide spin nuclei into preferred orientations above T M on rewarming through the transition in zero field.Citation: Ö zdemir, Ö ., and D. J. Dunlop (2006), Magnetic memory and coupling between spin-canted and defect magnetism in hematite,

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Thermal hysteresis of spin reorientation at Morin transition in alkoxide derived hematite nanoparticles

Cited by 32 publications

References 18 publications

Morin transition in hematite: Size dependence and thermal hysteresis

Morin transition in hematite: Size dependence and thermal hysteresis

Size-dependence of the heat capacity and thermodynamic properties of hematite (α-Fe2O3)

Magnetic memory and coupling between spin‐canted and defect magnetism in hematite

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