A spin-flop transition is observed when a field of 20 T is applied parallel to the h axis of a specimen of natural goethite at 4.2 K. The gwthite orders in a four-sublattice antiferromagnetic structure with the sublattice magnetizations inclined at i I 3 ' to b, and an iron moment of 3.90 pg. There is a small net ferromagnetic moment of 0.004 pg along b which has the eRect of more than doubling fhe threshold field for spin flop. The anisotropy field is deduced to be 0.1 1 T. cotresponding to a uniaxial anisotropy energy K = 6 x IO4 J d. The origin of the weak moment is discussed. and it is suggested that mode superparmgnefism may arise in non-collinear antiferromagnets when two transverse spin configurations xe effectively degenerate.

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Tenofovir disoproxil fumarate (TDF) is a nucleotide reverse transcriptase inhibitor used worldwide to treat AIDS. The aim of this study was to investigate the structural and dynamic properties of the polymorphic form I of the drug by thermal analysis, Fourier transform infrared spectroscopy, solid-state NMR, and powder X-ray diffraction. A full assignment of 13 C NMR resonances was achieved for the first time. Variable-temperature studies were conducted, and the results were related to changes in the TDF structure and dynamics of specific molecular fragments, especially of the fumarate portion of the molecule. It was found that, under heating, the 13 C NMR signals related to the carbon atoms of the disoproxil fumarate moiety split into two resonances. The region of the phosphonate vicinity also changes. This can reflect the lack of symmetry of the two molecules in the asymmetric unit after heating. This solid−solid transformation was related to DSC and XRD observations.

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