Hematite Morin transition

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. 

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... 

It is well known, and also well documented, that iron oxides can be prepared in the form of nanoparticles. The majority of such studies [26-39] have concentrated on the size effects on the magnetic properties in different iron-oxide phases. Typically, it has been found that transition temperatures decrease with decreasing particle size. For example, in hematite the Morin transition shifts from Tm= 263 K in bulk to temperatures below 4K in particles smaller than 8-20 nm [33]. It is interesting to note that magnetic anisotropy rapidly increases for particles with diameters less than... 

Substitution of Fe " " ions by Cr + ions in hematite affected the reduction of HMF and broadening of Mossbauer lines [253,255], as well as lowering of the Neel temperature [252,261]. The Morin transition was observed only for low substituted Cr-hematites—in samples with 4.3 mol% of substituted Cr or more the Morin transition no longer occurred [261]. 

Mn-for-Fe substitution in hematite influenced a reduction in HMF and has a considerable effect on the Morin transition—a weak ferromagnetic state was observed at 80 K for the substitution >4mol% [264]. 

Fig. 3.12 Morin transition temperature vs. inverse average particle size for differently prepared hematite samples Black square prepared from decomposition of lepidocrocite for the other symbols, see Ref, [99])...

Somewhat more pronounced effect of A1 substitution is reflected in the behavior of the Morin transition. With increasing A1 content the transition temperature Tm decreases and the transition region becomes significantly broader [97]. Moreover the Morin transition is completely suppressed at about lOat % A1 in bulk hematite [99] and even at somewhat lower concentrations (8 at %) for less crystalline hematite [100]. On the other hand, the effect of A1 on the Morin transition temperature is smaller in the case of more homogeneous A1 substitution in samples prepared from oxinates [103]. Using the aforementioned definition, the Morin transition temperature for as-such obtained hematite species decreases by 8 K per at % Al. Because the spectral implications of A1 substitution are quite similar to those of morphological effects, the separation of both effects remains a major problem and additional techniques are necessary for the characterization of natural samples. 

T. Ericsson, A. Krishnamurthy, B. Srivastava, Morin-transition in Ti-substituted hematite A Mossbauer study. Phys. Scr. 33, 88-90 (1986)... 

E. De Grave, D. Chambaere, L.H. Bowen, Nature of the Morin transition in Al-substituted hematite. J. Magn. Magn. Mater. 30, 349-354 (1983)... 

G.M. da Costa, E. Van San, E. De Grave, R.E. Vandenberghe, V. Barron, L. Datas, A1 hematites prepared by homogeneous precipitation of oxinates material characterization and determination of the Morin transition. Phys. Chem. Miner. 29, 122-131 (2002)... 

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. 

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