Single-crystal neutron diffraction study of Nd magnetic ordering in NdFeO3 at low temperature

The temperature variation of the (100) and (010) neutron diffraction peak intensities, related only to the Nd magnetic moments, have been measured on a NdFeO3 single crystal, at temperatures down to 70 mK. The (100) peak becomes noticeable below 25 K while the (010) peak only gives an appreciable contribution below 1 K. Above T-N2 approximate to 1 K the (100) 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 (100) 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 similar to 0.4 K another mechanism, polarization of Nd nuclear moments by hyperfine field, contributes to the intensity of the (100) and (010) 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 = 1.0 +/- 0.15 MOe and of the Nd electronic magnetic moment mu(Nd) = 0.9 mu(B) are deduced, the ratio H-hf/mu(Nd) being the same as in other Nd compounds.