Neutron scattering study on UTGa and NpTGa

UTGa and NpTGaの中性子散乱による研究

目時 直人   

Metoki, Naoto

UTGa及びNPtGaの中性子散乱による研究結果を報告する。これらの化合物の磁気構造のバラエティー及び、二段の磁気転移から、これらの遍歴的な多体5電子系において、軌道の自由度が重要な役割を示していることが明らかになった。局在モデルとの比較から、NpNiGa, NpRhGa, NpNiGaにおいて、四極子秩序と反強磁性秩序が共存していると考えられる。それを確かめるため、現在放射光を用いた共鳴X線の実験が進行中である。

The results of our recent neutron scattering studies on actinide 115 compounds are reviewed. The itinerant many body f electron system and cross over to the localized character are realized only in actinide system. Actinide 115 family is very interesting, because we can grow high quality single crystals of many iso-structural compounds with different number and character of f electrons and valence d electrons. Variety of the ground state have been reported, e.g. paramagnetic, itinerant magnetism, and heavy fermion superconductivity in PuCoGa. Very recently we have intensively studied the magnetic structure in UTGa and NpTGa, These compounds exhibit a variety of the magnetic structure with A-, C-, G-type antiferromagnetic and ferromagnetic ordering and canted structure. NpFeGa exhibits an unusual magnetic anisotropy with the direction of the magnetic moment inclined about 25 degrees from the tetragonal basal plane. On the other hand, the localized compounds RRhIn (R: Tb, Ho, Dy, Nd) shows the antiferromagnetic structure with =(1/2 0 1/2). The magnetic moment is parallel to the c-axis. An identical magnetic structure in rare earth compounds is a consequence of the localized nature of 4 electrons. We found that NpFeGa, NpNiGa, and NpRhGa exhibit 5 electronic transition between the low- and high-moment states associated with a change in the direction of the magnetic moment. The diverse magnetic ordering and the 5 electronic transition suggest that the orbital degree-of-freedom of itinerant 5f electrons may play important role for the physical properties in many body electron systems. Actually a localized model predicts the magnetic structure and the phase diagram with magnetic and quarupolar order parameters. A resonant X-ray scattering study is in progress, if the predicted antiferroquadrupole order coexists with the canting antiferromagnetic dipole order in NpNiGa.