Magnetic and transport properties of pure and carbon-doped divalent RE hexaboride single crystals

Abstract

Single crystals of pure and carbon-substituted divalent hexaborides EuB6 and YbB6 have been prepared by the Al flux method. Lattice parameters, density measurements, and microprobe analysis show that the single crystals are very close to stoichiometry. EuB6 is ferromagnetic (TC=12.5 K and ϑp=15 K) whileYbB6 is diamagnetic. Resistivity and Hall effect measurements carried out over a wide temperature range show that both pure hexaborides are semiconductors with a small intrinsic gap. The carbon-substituted hexaborides RE B6−xCx (RE=Eu, Yb) have a metallic behavior due to the donation of electrons to the conduction band by carbon atoms. Metallic conduction, at low temperature, in the ’’pure’’ hexaborides is attributed to the formation of an impurity band containing more than 1018 electrons per cm3.