Morin transition

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

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... 

As with Al, the Morin transition was suppressed for hematites containing Ga, Cr,... 

G.M. De Grave, E. (2001) About the Morin transition in hematite in relation with particle size and aluminum substitution. Czech J. 

Both plane wave and LCCO methods can now reproduce the band gap and nature of the valence and conduction bands but only if recent improvements are used. Neither have yet managed to correctly predict both the band gap and the equilibrium structure. A further point to note is that the calculations in Table 1 refer to a-Fe203 at low temperatures. At the Morin transition the alignment of the spins with respect to the crystal axes changes. This effect can only be modelled by including spin-orbit coupling. 

Mossbauer spectroscopy is also able to give local moment orientations, with respect to the crystalline lattice, or the correlations between moment orientations and local distortion axis orientations in a chemically disordered or amorphous material. This arises from the interplay between the structural (electric field gradient) hyperfine parameters and the magnetic hyperfine parameters. In this way, the spin flop Morin transition of hematite, for example, is easily detected and characterized (e.g., Dang et al. 1998). The noncollinear magnetic structures of nanoparticles can also be characterized. 

A material such as hematite, that can host a WF magnetic structure, also is able to host a classic AF structure with magnetic sublattices along a different crystalline axis. A spin flop transition, known as the Morin transition in hematite, can occur where the AF axis abruptly changes from one crystal orientation to another, at a certain transition temperature. Such spin flop transitions are sensitive to sample features such as impurity chemistry and particle size and shape, as discussed below (Dang et al. 1998). 

Bharati S, Pathasarathy R, Rao KJ, Rao CNR (1983) Mossbauer studies of inorganic glasses through their glass transition temperatnres. Sol State Comm 46 457-460 Bruzzone CL, Ingalls RL (1983) Mossbauer-effect study of the Morin transition and atomic positions in hematite nnder pressnre. Phys Rev B 28 2430-2440... 

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). 

The system (Rh Fei x)203 has been examined for x = OT1,0-22,0-25,0-41, and 0-81 [114]. It has the homogeneous haematite structure with randomisation of the cations. The Neel temperature falls with increasing x while the Morin transition rises. For a-Fc203 the Morin transition is 257 K, but at X = OTl it is 507 K and at x = 0-22 it is 544 K. Significant collapse of the hyperfine splitting is seen below the Neel temperatures. 

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