Heavy electron YbNi

We report low temperature 170Yb Mössbauer spectroscopy and magnetization measurements on two tetragonal non-superconducting quaternary borocarbides, YbNi2B2C and YbNiBC, and present LIII edge X-ray absorption data in YbNiBC. Both compounds show evidence of 4f-conduction band hybridisation. In the heavy electron YbNi2B2C, measurements down to 0.023 K show that only a small hyperfine interaction is present and we estimate an upper limit of 0.05-0.1 $\mu_{\rm B}$ for any Yb moment. In YbNiBC, magnetic ordering is observed below 4.2 K, in agreement with previous specific heat measurements. The single ion easy magnetization axis lies along the ${\bf c}$ axis but the ordered Yb moments lie in the basal (a, b) plane. Their (extrapolated) saturated value is unusually low (0.35 $\mu_{\rm B}$). The Yb-Yb exchange interaction is very large and highly anisotropic with the dominant component lying in the basal plane. We also report 166Er Mössbauer and specific heat data in the ferromagnet ErNiBC