OBSERVATION OF A PAIR-BREAKING FIELD AT THE NI SITE IN NONSUPERCONDUCTING RENI2B2C

Fe-57 Mossbauer spectra of RENi(2)B(2)C (RE = Tb, Dy, Ho, Er) doped wi th 1 at. % Fe-57 show a magnetic hyperfine field B-hf at the Fe-57 nuc leus in TbNi2B2C for T < T-N similar to 15 K and in HoNi2B2C for 5.5 > T > 4.2 K. No B-hf is observed in superconducting DyNi2B2C and ErNi2B 2C. Since TbNi2B2C is not superconducting at all and superconductivity disappears in HoNi2B2C for 6 > T > 4.7 K (reentrant state), we have t o conclude that the observed B-hf at the Fe-57 nucleus, resulting from the noncollinear antiferromagnetic spin structure of the RE moments, acts as a pair-breaking field at the Ni site.