SPIN-FLOP IN GOETHITE

A spin-flop transition is observed when a held 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 +/-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 held is deduced to be 0.11 T, corresponding to a uniaxial anisotropy energy K = 6 x 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.