Hematite spin structures

The spin structures of hematite are shown in Figure 6.8. Above Tm, the Fe ions are antiferromagnetically coupled across the shared octahedral faces along the c-axis. 

E. De Grave, R.E. Vandenberghe, Mossbauer effect study of the spin structure in natural hematites. Phys. Chem. Miner. 17, 344—352 (1990)... 

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). 

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