The temperature variation of the ͑100͒ and ͑010͒ neutron diffraction peak intensities, related only to the Nd magnetic moments, have been measured on a NdFeO 3 single crystal, at temperatures down to 70 mK. The ͑100͒ peak becomes noticeable below 25 K while the ͑010͒ peak only gives an appreciable contribution below 1 K. Above T N2 Ϸ1 K the ͑100͒ peak intensity is accounted for by the electronic magnetic moments polarized by the Nd-Fe exchange field. Near T N2 a change of slope is observed in the temperature dependence of the ͑100͒ reflection intensity, demonstrating the crossover from the above polarization of Nd under the Nd-Fe exchange to proper long-range ordering due to Nd-Nd interaction. Below ϳ0.4 K another mechanism, polarization of Nd nuclear moments by hyperfine field, contributes to the intensity of the ͑100͒ and ͑010͒ peaks. A simple mean-field model explains consistently the observed temperature dependence of the diffraction intensities as well as earlier specific-heat data. The main feature of this model is allowance for Van Vleck susceptibility, which appears to play an important role in the overall polarization of Nd. The values of the hyperfine field at the Nd nuclei H hf ϭ1.0Ϯ0.15 MOe and of the Nd electronic magnetic moment Nd ϭ0.9 B are deduced, the ratio H hf / Nd being the same as in other Nd compounds.

The crystal and magnetic structures of the lanthanum cobaltites Lao.6Sro.4CoO 3, Lao.(,Sro.,~Coo.~Feo.~O 3 and Lao.6Bao.4Coo.gFeo.~O~ have been studied by neutron powder diffraction at temperatures of 2, 300 and 90(1 K. All compounds undergo a phase transition from cubic to rhombohedral structure. Below the room temperature La~.~,Sro.,~CoO 3 becomes ferromagnetic while for the components with 10% Fe substituted for Co, we found an antiferromagnetic order.

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