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It is shown that a quasi-two-dimensional ͑layered͒ Heisenberg antiferromagnet with fully frustrated interplane couplings ͑e.g., on a body-centered tetragonal lattice͒ generically exhibits two thermal phase transitions with lowering temperature-an upper transition at T TO ͑"order from disorder without order"͒ in which the lattice point-group symmetry is spontaneously broken, and a lower Néel transition at T N at which spin-rotation symmetry is broken. Although this is the same sequence of transitions observed in La 2 CuO 4 , in the Heisenberg model ͑without additional lattice degrees of freedom͒ ͑T TO − T N ͒ / T N is much smaller than is observed. The model may apply to the bilayer cuprate La 2 CaCuO 6 , in which the transitions are nearly coincident.

Mossbauer spectra of (NH4)2FeCl5.H2O have been measured over the temperature range 1.5-297 K for powder and various single-crystal absorbers. This antiferromagnet has shown complicated magnetic behaviour but, contrary to a previous suggestion, the spectra show that below the ordering temperature the Fe spins are aligned along the alpha axis. The antiferromagnetic spectra may be fitted with a small number of components coupled with relaxation broadening due to critical fluctuations. The data suggest two closely spaced ordering temperatures. Details of the hyperfine electrostatic interaction have been deduced from the paramagnetic spectra. No evidence was found for a reported structural phase change in the paramagnetic region.

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57Fe Mossbauer absorption spectra have been recorded at 4.2 K for various single-crystal samples of the rare-earth orthoferrite YbFeO3 in applied magnetic fields up to 10 T. A field applied along the easy antiferromagnetic (c) axis induced, as expected, the Fe spins to 'flop' to being perpendicular to the field. Fields applied along the a and b crystal axes induced reorientations of the spins respectively towards and around the applied-field direction, the spins rotating towards the a axis in both cases. An explanation of these unusual reorientations is offered in terms of Fe-Yb spin decoupling.