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TP61: Low energy nuclear spin excitations in NdGaO₃

T. Chatterji, B. Frick (; Institut Laue Langevin)

We have investigated the low energy excitations in NdGaO₃ in the μeV range by a back scattering neutron spectrometer. The energy scans on a NdGaO₃ single crystal revealed inelastic peaks at E = 1.650 μeV at T = 50 mK on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and finally merge with the elastic peak at the electronic magnetic ordering temperature T_N = 975 mK. We interpret the inelastic signal observed in NdGaO₃ to be due to the excitations of the Nd nuclear spins I = 7/2 of the ¹⁴³Nd and ¹⁴⁵Nd isotopes. In a first approximation one can consider these inelastic peaks to arise due to the transitions between the hyperfine-field-split nuclear levels. This is the single-nucleus effect. However the nuclear spins are coupled through Suhl-Nakamura interaction [1,2]. So one expects nuclear spin wave excitations (cooperative lattice effect) discussed by de Gennes et al. [3] . The nuclear spin waves should show dispersions at small momentum transfers. Word et al. [4] have discussed the possibility of measuring nuclear spin waves by inelastic neutron scattering. They have calculated the differential scattering cross section and polarization of neutrons scattered by nuclear spin systems described by Suhl-Nakamura Hamiltonian in the formalism of van Hove correlation function. In our experiment, however, we failed to detect so far the expected dispersion of the nuclear spin waves. This is perhaps due to the insufficient Q resolution of the back-scattering spectrometer. The dispersion of the nuclear spin waves can probably be measured by a neutron spin echo (NSE) spectrometer using a PG-analyzer at a strong spallation neutron source.

1. H. Shul, Phys. Rev. 109, 606 (1958)
2. T. Nakamura, Progr. Theotet.Phys. (Kyoto) 20, 542 (1958)
3.P.G. de Gennes et al., Phys. Rev. 129, 1105 (1963)
4. R. Word, A. Heidemann and D. Richter, Z. Phys. B 28, 23 (1977)

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