SINGLE-CRYSTAL NEUTRON-DIFFRACTION STUDY OF ND MAGNETIC-ORDERING IN NDFEO3 AT LOW-TEMPERATURE

The temperature variation of the (100) and (010) neutron diffraction p eak intensities, related only to the Nd magnetic moments, have been me asured on a NdFeO3 single crystal, at temperatures down to 70 mK. The (100) peak becomes noticeable below 25 K while the (010) peak only giv es 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) reflectio n intensity, demonstrating the crossover from the above polarization o f 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 intensit y of the (100) and (010) peaks. A simple mean-field model explains con sistently the observed temperature dependence of the diffraction inten sities as well as earlier specific-heat data. The main feature of this model is allowance for Van Vleck susceptibility, which appears to pla y an important role in the overall polarization of Nd. The values of t he 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 rat io H-hf/mu(Nd) being the same as in other Nd compounds.