Heavy electron YbNi2B2C and giant exchange YbNiBC: Yb-170 Mossbauer spectroscopy and magnetization studies

We report low temperature Yb-170 Mossbauer spectroscopy and magnetization m easurements on two tetragonal non-superconducting quaternary borocarbides, YbNi2B2C and YbNiBC, and present L-III 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 smal l hyperfine interaction is present and we estimate an upper limit of 0.05-0 .1 mu(B) for any Yb moment. In YbNiBC, magnetic ordering is observed below 4.2 K; in agreement with previous specific heat measurements. The single io n easy magnetization axis lies along the c axis but the ordered Yb moments lie in the basal (a, b) plane. Their (extrapolated) saturated Value is unus ually low (0.35 mu(B)). The Yb-Yb exchange interaction is very large and hi ghly anisotropic with the dominant component lying in the basal plane. We a lso report Er-166 Mossbauer and specific heat data in the ferromagnet ErNiB C.