Morin temperature

Hematite is paramagnetic above 956 K (Tc). At room temperature it is weakly ferromagnetic and at 260 K (the Morin temperature, Tm), it undergoes a phase transition to an antiferromagnetic state. Particles smaller than about 8 nm display superpara-magnetic relaxation at room temperature. A plot of the dependence of the B f (Hi) of hematite on temperature is shown in Figure 6.7 the plot follows an approximate Brillouin curve. 

Fig. 6.7 Temperature dependence of the magnetic properties of hematite. Tc = Curie temperature,Tm = Morin temperature, pm = paramagnetic region, wfm = weakly ferromagnetic region afm = antiferromagnetic region. The insets show simulated Mossbauer spectra of hematite in the paramagnetic, weakly ferromagnetic and antiferromagnetic states (Murad, 1988, with permission).

Fig. 6.9 Left Relationship between the Curie (Tc) and Morin (Tm) temperature of hematite and the extent of structural substitution of Fe " by Al, Ga, Cr and In (M) (Svab Kren, 1979, with permission). Right Relationship between the Morin temperature and the inverse mean crystal dimension (MCD) for different hematites ( ) natural from Elba ( ) heating lepido-...

Amhamdi, FI. Dumont, F. Buez-FIerman, C. (1997) Effect of urea on the stability of ferric oxide hydrosols. Colloids Surfaces 125 1-3 Amin, N. Arajs, S. (1987) Morin temperature of annealed submicronic a-Fe20j particles. Phys. Rev. B35 4810-4811 Amonette, J.E. Workman, D.J. Kennedy, D.W Fruchter, J.S. Gorby Y.A. (2000) Dechlorination of carbon tetrachloride by Fe(II) associated with goethite. Environ. Sci. Techn. 34 4606-4613... 

Amin N, Arajs S (1987) Morin temperature of annealed submieronie a-Fe203 partieles. Phys Rev B 35 4810-1811... 

Fig. 10.1 Spectra of a-Fc203 at 162 0 K (below Morin temperature) and at 371-3 K (above Morin temperature). Note the reversal in sign of the quadrupole perturbation as shown by the spacings of the outer lines. [Ref. 3, Fig. 1]...

Fig. 10.2 The line shape of the —-> + i> transition in a-FciOa near the Morin temperature. Note the presence of two distinct components, indicating that the spin-flip is not a continuous process. [Ref. 4, Fig. 6]...

There is a small apparent drop in the quadrupole interaction above the Morin temperature which suggests a small departure from axial symmetry and an asymmetry parameter of rj 0 03 [e(r < Tt /e T > Tm) = 1-94, not 2 00 as predicted]. The quadrupole splitting increases above the Neel temperature due to a small contraction along the trigonal axis. The chemical isomer shift does not show any irregularities at the Morin transition (Fig. 10.3). 

Schematic magnetic phase diagram of a-Fe203 as a function of particie size where P stands for the paramagnetic phase, WF for the weakly ferromagnetic phase, AF for the antiferromagnetic phase, SP for the superparamag-netic phase, and Tm for the Morin temperature. The dashed areas represent region of coexistence of AF and WF phase and region of coexistence of WF and SP phase (from left to right). (Adapted from Ref. 9 with permission of Springer.)...

N. Amin, S. Arajs, Morin temperature of annealed submicronic a-F203 particles. Phys. Rev. B 35, 4810- 811 (1987)... 

Table 14.3 Size dependence of the Morin temperature (Tm) and the spin-flop transition field at r = 0 (77s-F,o) for the heminatite nanosolids [165]...

Morin (hydrate) (2, 3,4, 5,7-pentahydroxyfiavone) [480-16-0] M 302.2, m 289-292", pKj 5.3, pKi 8.74. Stirred at room temperature with ten times its weight of absolute EtOH, then left overnight to settle. Filtered, and evaporated under a heat lamp to one-tenth its volume. An equal volume of water was added, and the ppted morin was filtered off, dissolved in the minimum amount of EtOH and again ppted with an equal volume of water. The ppte was filtered, washed with water and dried at 110° for Ih. (Yield ca 2.5%.) [Perkins and Kalkwarf Anal Ghent 28 1989 1956.] Complexes with W and Zr. 

As shown in Fig. 7.6, the Mossbauer data show a reduction in Morin transition temperature with increasing shock severity. At temperatures below the transition, increasing shock severity causes greater retention of the higher temperature, weak ferromagnetic contribution. The measure of weak ferromagnetic (WF) fraction (the high temperature form) is a sensitive indication of shock modification. 

An interesting situation arises in the spectra of a-Fe203 where the angle

Morin transition). At temperatures below TM the parameter . equals +0.20 mm/s whereas, at the transition temperature, c changes sign and magnitude to a value of -0.10 mm/s [15], in agreement with Expression (5-5). 

The superparamagnetic properties of -Fe203 have also been studied via the Mossbauer effect. Nakamura et al, (32) have investigated the temperature dependence of the internal field in -Fe203 particles of approximately 50 A. diameter. At 120 °K. they obtain a spectrum which is almost identical to the bulk material, except that no Morin transition has occurred and the spins still lie in the basal plane. At room temperature the magnetic hyperfine spectrum collapses (even though the bulk... 

The transition in this material from a low-temperature semiconductor to a metallic phase at 340 K was first observed by Morin (1959) more recently, jumps... 

Nappom, T. W., Morin, F., and Meunier, M. Evaluation of the Actual Working Temperature of a Single-chamber SOFC, Electrochem. and Solid-State Letters, 1, A60 (2004). 

Morin, P., Jimenez-Flores, R., Pouliot, Y. 2004. Effect of temperature and pore size on the fractionation of fresh and reconstituted buttermilk by microfiltration. J. Dairy Sci. 87, 267-273. 

Fig. 70. Electrical conductivity vs. reciprocal temperature for several metallic oxides. (After Morin (448).)...

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