(P31)
NMR study of neptunium-based filled skutterudite compound NpFe4 P12
Y. Tokunaga1 , D. Aoki2 , H. Sakai1 , T. Fujimoto1 , S. Kambe1 , Y. Homma2 , T. D. Matsuda1 ,
S. Ikeda1 , Y. Haga1 , E. Yamamoto1 , A. Nakamura1 , Y. Shiokawa2 , Y. Ōnuki3
1
ASRC, JAEA, 2-4 Shirane, Tokai, Naka, Ibaraki 319-1195
IMR Tohoku University, Oarai, Ibaraki, 311-1313
3
Graduate School of Science, Osaka University, Toyonaka, Osaka, 560-0043
2
In the family of filled skutterudites, the only Np-based compound successfully crystallized
to date is NpFe4 P12 . There is no other filled skutterudite including transuranium elements
reported up to now. NpFe4 P12 is isostructural with UFe4 P12 , except for the replacement of U
by Np. The lattice constant is 7.7702 Å, which is the smallest value of all the filled skutterudite
compounds. Recently, D.Aoki et al. have performed electrical resistivity, magnetic susceptibility, magnetization, and specific heat measurements using a single crystal [1]. Magnetization
measurements indicate the occurrence of ferromagnetic ordering with a Curie temperature
TC = 23 K. The magnetic moment is directed along the 100 direction with the saturated moment of 1.35 μB /Np. The large resistivity value indicates that NpFe4 P12 might be a semimetal,
where the 5f electrons are localized with a 5f 3 configuration, indicating Np4+ .
In this study, we report microscopic magnetic properties of NpFe4 P12 obtained via 31 PNMR measurements. The field-orientation dependence of the hyperfine interaction has been
measured using a single crystal ( Fig.1 ), and the results compared with the field-orientation
dependence obtained previously in UFe4 P12 [2]. We have also measured the field and temperature dependence of the nuclear spin-lattice relaxation rate (1/T1 ) in several fields between 1.9
and 7.8 T. The results are discussed on the basis of a localized picture for the neptunium 5f
electrons.
[110]
80
[111]
60
40
θ (°)
20
[001]
80.15
80.20
80.25
80.30
Frequency (MHz)
80.35
0
80.40
Figure 1: The angular dependence of the 31 P NMR spectrum in a single crystal taken at 100 K
and 4.7 T. θ indicates the angle of tilt for the field as it rotates from [ 001 ] to [ 110 ] through
the [ 111 ] directions of the NpFe4 P12 crystal.
[1] D. Aoki et al. Journal of the Physical Society of Japan 75 (2006), 073703
[2] Y. Tokunaga et al. Physical Review B 71 (2005), 045124