Electrical and magnetic properties of quasicrystal approximants RCd ₆ (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 160K when the lattice constant a is larger than 15.481 A 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 YCd6 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.