Electrical and Magnetic Properties of Quasicrystal Approximants RCd6 (R: Rare Earth)

We measured the electrical resistivity, magnetic susceptibility, magnetization, and specific heat of the quasicrystal approximants RCd 6 (R: rare earth, Y--Lu) with a body-centered cubic (bcc) crystal structure. Single crystals were grown by the Cd-self flux method and annealing method. We confirmed that the structural order--disorder transition is realized at about 160 K when the lattice constant $a$ is larger than 15.481 $Å$ in YCd 6 , namely, for R = Pr, Nd, Sm, Gd, Tb, Dy, and Yb. At lower temperatures, RCd 6 compounds, except non-$4f$ reference compounds YCd 6 and LuCd 6 , and a divalent compound YbCd 6 , are found to order antiferromagnetically. We clarified that the structural order--disorder transition has a great influence on the magnetic ordering and transport properties. The Néel temperature of RCd 6 (R: Nd, Sm, Tb, and Dy) with the structural order--disorder transition is appreciably higher than the de Gennes scaling normalized by the Néel temperature of GdCd 6 , while the the Néel temperature of RCd 6 (R: Ho, Er, and Tm) without the structural transition approximately follows the de Gennes scaling. Moreover, the electrical resistivity of RCd 6 with the structural transition decreases monotonically below the Néel temperature, while it increases below the Néel temperature and a large residual resistivity remains at low temperatures in RCd 6 without the structural transition. The contribution of an Einstein oscillator to the phonon specific heat is also discussed in YCd 6 and LuCd 6 .