Microscopic evidence for long-range magnetic ordering in the ground quartet systems SmAl (: Ti, V, Cr)

Ito, Takashi   ; Higemoto, Wataru  ; Ninomiya, Kazuhiko; Sakai, Akito*; Nakatsuji, Satoru*

Recently, Al compounds (: rare earth, : transition metal) have attracted much attention due to interesting physical properties closely related to their caged-structure. Each ion is confined in a Frank-Kasper cage consisting of 16 Al atoms and the large coordination number leads to strong - hybridization. In addition, the Frank-Kasper cage allows various multipole degrees of freedom for the -electrons at low because of a highly symmetric crystalline-electric-field (CEF). Among the Al series, PrAl (: Ti, V) and SmAl (: Ti, V, Cr) exhibit both these features. The ground doublet systems PrAl (: Ti, V) show a phase transition at = 2.0K (Ti) and 0.6K (V). The order parameter is believed to be an electric quadrupole active in the subspace. A possibility of the quadrupole Kondo effect has been discussed based on characteristic -dependence in specific heat, resistivity, and magnetic susceptibility. The ground quartet systems SmAl (: Ti, V, Cr) also exhibit a phase transition at = 6.4K (Ti), 2.3K (V), and 1.8K (Cr). A nuclear specific heat study in SmTiAl suggests that the primary order parameter is a magnetic dipole in spite of field-insensitive nature of the phase boundary. A possible importance of a secondarily ordered octupole moment was discussed. We here present a microscopic study of PrTiAl and SmAl (: Ti, V, Cr) using muon spin relaxation. In PrTiAl, no additional local field was observed in the ordered phase. This strongly supports the quadrupole ordering scenario. On the other hand, an appearance of a coherent local field was observed below in SmAl. This is consistent with the primary magnetic dipole ordering. The frequency distribution suggests that the magnetic structure is possibly different between the SmAl compounds.