NMR studies of YbB₆

We report results of DC-magnetization and ¹¹B-NMR measurements on single crystalline YbB₆. The magnetization data at temperatures between 4 and 300 K reveal weak ferromagnetic order with a T _C>300 K. It involves very small ordered moments, of the order of 0.002 µ_B/f.u., representing only a small fraction of the effective paramagnetic moment per formula unit that is indicated by the magnetic susceptibility. The latter can be accounted for by assuming that 2% of all the Yb atoms adopt the Yb⁺³ configuration. Since almost all the Yb ions adopt the divalent configuration one expects YbB₆ to be a poor metal.