Nmr Evidence For F-Electron Excitations In The Multipolar Ground State Of Npo2

An analysis is presented of the Np-237-O-17 cross-relaxation (CR) process at temperatures below T = 10 K that was reported [Tokunaga et al., Phys. Rev. B 74, 064421 (2006)] in O-17 NMR studies of NpO2 . The CR evidently results from Np-237 spin fluctuations driven by indirect spin-spin interactions analogous to the Suhl-Nakamura [Nakamura, Prog. Theor. Phys. 20, 542 (1958)]effect in conventional magnets. Here, however, there is a body of evidence that the interacting Np4+ ions are in a magnetic, rank-5 dotriakontadipolar ground state [Santini et al., Phys. Rev. Lett. 97, 207203 (2006); Magnani et al., Phys. Rev. B 78, 104425 (2008)]. Owing to the observed exponential form of the fluctuation spectrum, it is possible to evaluate explicitly the CR bandwidth in terms of any specified Np-237-Np-237 (spin-spin) coupling Hamiltonian. We model the latter as an enhanced dipolar coupling. The enhancement is concluded to be compound, with the "enlarged nuclear moment" mechanism identified in the paramagnetic state [Tokunaga et al., Phys. Rev. B 74, 064421 (2006)] responsible for a factor similar to 22, while the indirect Np-237-Np-237 coupling effect is found to be similar to 240 times the amplitude of the classical dipolar interaction. We suggest that these results are consistent with the identification of a magnetic rank 5 multipolar ground state [Magnani et al., Phys. Rev. B 78, 104425 (2008)] for NpO2.