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Microscopic evidence for long-range magnetic ordering in the $$Gamma$$$$_{8}$$ ground quartet systems Sm$$Tr$$$$_{2}$$Al$$_{20}$$ ($$Tr$$: Ti, V, Cr)

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

Recently, $$RT_2$$Al$$_{20}$$ compounds ($$R$$: rare earth, $$T$$: transition metal) have attracted much attention due to interesting physical properties closely related to their caged-structure. Each $$R$$ ion is confined in a Frank-Kasper cage consisting of 16 Al atoms and the large coordination number leads to strong $$c$$-$$f$$ hybridization. In addition, the Frank-Kasper cage allows various multipole degrees of freedom for the $$f$$-electrons at low $$T$$ because of a highly symmetric crystalline-electric-field (CEF). Among the $$RT_2$$Al$$_{20}$$ series, Pr$$T_2$$Al$$_{20}$$ ($$T$$: Ti, V) and Sm$$T_2$$Al$$_{20}$$ ($$T$$: Ti, V, Cr) exhibit both these features. The $$Gamma_3$$ ground doublet systems Pr$$T_2$$Al$$_{20}$$ ($$T$$: Ti, V) show a phase transition at $$T_O$$ = 2.0K (Ti) and 0.6K (V). The order parameter is believed to be an electric quadrupole active in the $$Gamma_3$$ subspace. A possibility of the quadrupole Kondo effect has been discussed based on characteristic $$T$$-dependence in specific heat, resistivity, and magnetic susceptibility. The $$Gamma_8$$ ground quartet systems Sm$$T_2$$Al$$_{20}$$ ($$T$$: Ti, V, Cr) also exhibit a phase transition at $$T_O$$ = 6.4K (Ti), 2.3K (V), and 1.8K (Cr). A nuclear specific heat study in SmTi$$_2$$Al$$_{20}$$ 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 PrTi$$_2$$Al$$_{20}$$ and Sm$$T_2$$Al$$_{20}$$ ($$T$$: Ti, V, Cr) using muon spin relaxation. In PrTi$$_2$$Al$$_{20}$$, 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 $$T_O$$ in Sm$$T_2$$Al$$_{20}$$. This is consistent with the primary magnetic dipole ordering. The frequency distribution suggests that the magnetic structure is possibly different between the Sm$$T_2$$Al$$_{20}$$ compounds.



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