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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)

$$Gamma_8$$基底Sm$$Tr$$$$_{2}$$Al$$_{20}$$($$Tr$$: Ti, V, Cr)における長距離磁気秩序の微視的証拠

伊藤 孝; 髭本 亘; 二宮 和彦; 酒井 明人*; 中辻 知*

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

f電子系化合物$$RT_2$$Al$$_{20}$$ ($$R$$: Pr, Sm, $$T$$: Ti, V, Cr)は特異な籠状構造に起因する興味深い物性を示す。$$R$$イオンは16個ものAl原子からなる「籠」に内包されており、$$R$$サイトには対称性の高い結晶場が生じる。これにより多極子自由度が活性な結晶場基底が形成され、低温において多極子秩序が生じる。一方、多数のAl原子が$$R$$イオンに配位することにより、顕著な$$cf$$混成が生じる。$$cf$$混成の度合いは$$T$$元素の置換によってチューン可能であることから、多極子秩序に関する量子臨界性の系統的研究が可能な系として注目を集めている。本研究では、上述の議論の前提となっている多極子秩序を微視的な観点から検証するために、PrTi$$_2$$Al$$_{20}$$及びSm$$T_2$$Al$$_{20}$$($$T$$: Ti, V, Cr)に対して$$mu$$SR測定を行った。その結果、PrTi$$_2$$Al$$_{20}$$及びSm$$T_2$$Al$$_{20}$$($$T$$: Ti, V, Cr)の秩序変数が、それぞれ電気四極子と磁気多極子であることが明らかになった。

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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