Magnetic Transitions in B₁₂ Icosahedral Cluster Compounds REB₅₀ (RE=Tb, Dy, Ho, Er)

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REB₅₀ (RE=Tb, Dy, Ho, Er) compounds were synthesized and magnetic properties were measured. A measurement of the magnetic susceptibility of TbB₅₀ at high field confirmed the antiferromagnetic transition at 17.5 K. Antiferromagnetic transitions were also observed in the other three compounds with transition temperatures T_N ranging from 7.5 K to 4.6 K. These compounds are the first found type of higher boride compounds in which a magnetic transition occurs. The variation of the transition temperatures for the rare earth series indicated that the shortness of the B₁₂ icosahedral bonding length is not the only important factor of the magnetic transition. The resistivity of a REB₅₀ type isostructural compound, a TbB₄₁Si_1.2 crystal showed a metallic-type temperature dependence at low temperatures. Electrical and magnetic properties of the REB₅₀ type compounds differ largely from other higher boride compounds such as REB₂₅ and REB₆₆.