HYPERFINE-ENHANCED NUCLEAR POLARIZATION IN NDGAO3

The three-dimensional magnetic ordering of the ${\mathrm{Nd}}^{3+}$ ions in ${\mathrm{NdGaO}}_{3}$ has been studied by means of neutron powder diffraction. The transition to the antiferromagnetic state takes place at ${\mathit{T}}_{\mathit{N}}$\ensuremath{\approxeq}1 K. Below this temperature, the ${\mathrm{Nd}}^{3+}$ magnetic moments are oriented along the c axis in a ${\mathit{c}}_{\mathit{z}}$ mode. The value of the saturated magnetic moment (\ensuremath{\approxeq}1.1${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) is drastically reduced below the free ion value (3.27${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) due to crystal-field effects. The main contribution to the Nd-Nd magnetic interaction appears to be superexchange, the classical dipole-dipole interaction being too small to account for the observed N\'eel temperature. Below 300 mK, we observe an additional enhancement of the magnetic reflections. The analysis of the Q dependence of the neutron powder-diffraction intensities allowed us to interpret this increase as due to the polarization of the nuclear moments of the $^{143}\mathrm{Nd}$ and $^{145}\mathrm{Nd}$ isotopes in the hyperfine field created by the ${\mathrm{Nd}}^{3+}$ electronic moments. The particular conditions which lead to the observation of this phenomenon in ${\mathrm{NdGaO}}_{3}$ are briefly discussed.