Iron oxides antiferromagnetic ordering

Lepidocrocite is paramagnetic at room temperature. The Neel temperature of 77 K is much lower than that of the other iron oxides and is the result of the layer-like structure of this mineral. The sheets of Fe(0,0H)6 octahedra are linked by weak hydrogen bonds, hence magnetic interactions are relatively weak. The saturation hyperfine field is also lower than for any other iron oxide (Tab. 6.2). In the antiferromagnetic state, the spins are ordered parallel to the c-axis with spins in alternate layers having opposite signs. A decrease of T by 5 K was observed for Al-lepidocrocites with an Al/(Fe-i-Al) ratio of 0.1 (De Grave et al., 1995). 

The oxide Ca2Fe20s contains one octahedral and one tetrahedral iron site. It is antiferromagnetic and, despite an earlier report of different ordering temperatures for the two sites (i.e. a four-sublattice ordering) [151], it is now known that all sites order together at 730 K [152]. The internal magnetic fields have been measured from 80 K up to this temperature [153]. Substitution of the type Ca2Fe2 has been studied for M = AP (0 < x < 1 -4),... 

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