NEUTRON-SCATTERING STUDY OF ANTIFERROMAGNETIC ORDER IN THE MAGNETIC SUPERCONDUCTORS RNI(2)B(2)C

Neutron diffraction has been used to investigate the magnetic order an d crystallography of RNi(2)B(2)C. For R=Dy the magnetic order that dev elops at 10 K consists of ferromagnetic sheets of spins in the tetrago nal a-b plane which are stacked antiferromagnetically along the c-axis . An identical magnetic order is observed for R = Ho at low T, while f rom similar to 5 K to T-N approximate to 8 K each layer along the c-ax is is rotated by 163 degrees to form a spiral state, which strongly co uples the magnetic order to the superconductivity. Small a-axis peaks are also observed in a narrow temperature range, but the c-axis peaks are dominant. In ErNi2B2C (T-c=11 K) an antiferromagnetic a-axis spin density wave state develops below T-N=6.8 K, which readily coexists wi th the superconductivity indicating that weak coupling occurs between this type of magnetic state and the superconducting state. There is no evidence of magnetic order for either R=Ce or R=Y.