3FTFBSDI

Novel Magnetic Ordering in Neptunium Dioxide : A Mystery for Half aCentury is Addressed using the Nuclear Magnetic Resonance Technique
The phase transition in Neptunium dioxide (NpO2) was originallydiscovered in 1953. Since this discovery, however, it has remained
a most-lasting mystery in the physics of actinide compounds. In
recent works, the possibility that this phase transition can be
described as one of a new class of phase transition associated withthe orbital degrees of freedom has been discussed. With this inmind, we have tackled this problem anew using the nuclearmagnetic resonance (NMR) technique.
Actinide compound are very exciting, because they have mostof the interesting effects that are being studied in condensed matterphysics nowadays. They are very complex systems to understand,and they are the least understood. One of the reasons that thesesystems have not been studied much is because of their radioactivity.
Therefore, they require special facilities and special places. There
are only a few places in the world where these compounds arestudied, and International Research Center for Nuclear Materials
Science, Institute for Materials Research Science is one of them.
A complexity of actinide compound arises from the fact that theorbital moment of f-electron is never quenched by the crystal field,
which differentiates the f-electron systems from the d-electronsystems. Furthermore, because of the strong spin-orbit interaction
in a f-electron shell, their spin and orbital degrees of freedoms aretightly coupled with each other. In such the case, the relevant
degrees of freedom for one ion are its magnetic and electric multipolemoments, that is, charge (rank-0), dipole (rank-1), quadrupole (rank2), and octupole (rank-3) etc. The interactions between these
multipole moments bring complex phase diagram to actinide
compounds, even if the f-electrons stay in the localized limit.
The phase transition in NpO2 was originally discovered throughspecific heat and magnetic susceptibility measurements, whichappeared to suggest a transition to a magnetic dipolar ordering state
at T0 =26 K. However, in the 1980s, such a dipole order scenario has
been unambiguously ruled out by both neutron elastic scattering andMösbauer spectroscopy measurements. In 2000, the possibility ofordering of higher rank of magnetic multipole, i.e. octupole wasproposed theoretically. Soon after that, resonant x-ray scattering
measurements suggested the occurrence of longitudinal triple-q typeantiferro-octupolar (AFO) order below T0 . This AFO ordered state has
also been suggested by recent microscopic calculations based on thej-j coupling scheme by K.Kubo and T. Hotta (Fig.1) [1].
In order to elucidate the microscopic properties of this exotic

Fig.1. Magnetic-octupolar ordering state of NpO2 calculated by K.
Kubo and T. Hotta [1]. Red and Blue colors on the surface indicate
the weight of up- and down-spin states.

52IMR KINKEN Research Highlights 2007

ordered state, we have initiated the first NMR studies in NpO2 [2,3].
Figure 2 shows the field angular (θ) dependence of the 17O NMRline splitting, which illustrates the angular variation of hyperfine (HF)
fields at 17O nuclei [3]. In the presence of longitudinal triple-q typeorder, the 17O NMR line splits into two inequivalent sites, i.e., theO(1) and O(3) sites [2]. The nearly flat curve for the O(1) site
corresponds to the isotropic nature of its HF field. On the other hand,
the two other curves for the O(3) sites show a strong θ-dependence
due to the anisotropic nature of their HF fields. Using the invariant
form of the HF field contributions derived by O.Sakai et al. (TMU) by
assuming the longitudinal triple-q AFO ordered state for NpO2, wecan deduce the angular dependence of HF field at O sites. As shownin Fig. 2, we have found that the angular dependences of HF field
are well reproduced by the model calculation for all three O sites.
This excellent agreement obtained with just three scaling parametersprovides microscopic evidence for the proposed longitudinal triple-q
AFO model for NpO2.
Compared with dipolar ordering, multipolar ordering is hard toinvestigate by conventional techniques. In the present work, wedemonstrate that microscopic investigation of multipole ordering ispossible by means of NMR through the HF interactions.
References[1] K. Kubo and T. Hotta, Phys. Rev. B 71 140404(R) (2005); ibid.
72, 132411 (2005).
[2] Y. Tokunaga, Y. Homma, S. Kambe, D. Aoki, H. Sakai, E.
Yamamoto, A. Nakamura, Y. Shiokawa, R.E.Walstedt, and H.
Yasuoka, Phys. Rev. Lett. 94, 137209 (2005).
[3] Y. Tokunaga, D. Aoki, Y. Homma, S. Kambe, H .Sakai, S. Ikeda,
T. Fujimoto, R.E. Walstedt, H. Yasuoka, E. Yamamoto, A. Nakamura,and Y. Shiokawa, Phys. Rev. Lett. 97, 257601 (2006).
Contact toYo Tokunaga (Advanced Science Research Center, Japan Atomic
Energy Agency (JAEA))
e-mail: tokunaga.yo@jaea.go.jp
Yoshiya Homma (Radiochemistry of Metals Division)e-mail: yhomma@imr.tohoku.ac.jp

Fig.2. Field-angle dependence of the internal field at O sites in
NpO2. The solid and dotted lines are the results of model
calculations with and without the contribution from the field-induced
magnetic octupolar ordering, respectively [3].

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