Weak ferromagnetism in single crystalline YbB6-delta

Abstract

We present a study of magnetization and transport properties of single crystals of YbB6 and YbB6 +/-delta (delta=0.3 as a nominal value). The YbB6-delta crystal revealed a weak ferromagnetism, manifested by hysteresis loop in M(H) at room temperature with a saturation magnetic moment of 1.5x10(-4) mu(B)/f.u. and the ferromagnetic transition temperature higher than T=300 K, whereas YbB6 and YbB6+delta crystals revealed diamagnetism and paramagnetism, respectively. It was found that the bulk magnetic property of YbB6 +/-delta crystals is intrinsically diamagnetic and that the magnetism, ferromagnetism, or paramagnetism is a surface effect. Based on x-ray photoelectron spectroscopy and electron probe microanalyzer measurements, we believed that the extraneous magnetic impurities, such as Fe and Ni, are not the origin of the ferromagnetism or paramagnetism. While we observed from electrical resistivity and Hall resistivity that the off stoichiometry in YbB6 +/-delta causes carrier doping effect, it is not clear at present what surface state creates the magnetic signals.