Spin flop in goethite

A spin-flop transition is observed when a field of 20 T is applied parallel to the b axis of a specimen of natural goethite at 4.2 K. The goethite orders in a four-sublattice antiferromagnetic structure with the sublattice magnetizations inclined at +or-13 degrees to b, and an iron moment of 3.90 mu B . There is a small net ferromagnetic moment of 0.004 mu B along b which has the effect of more than doubling the threshold field for spin flop. The anisotropy field is deduced to be 0.11 T, corresponding to a uniaxial anisotropy energy K=6*10 4 J m -3 . The origin of the weak moment is discussed, and it is suggested that mode superparamagnetism may arise in non-collinear antiferromagnets when two transverse spin configurations are effectively degenerate.