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 and crystallography of RNi(2)B(2)C. For R=Dy the magnetic order that develops at 10 K consists of ferromagnetic sheets of spins in the tetragonal a-b plane which are stacked antiferromagnetically along the c-axis. An identical magnetic order is observed for R = Ho at low T, while from similar to 5 K to T(N) approximate to 8 K each layer along the c-axis is rotated by 163 degrees to form a spiral state, which strongly couples 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 ErNi(2)B(2)C (T(c)=11 K) an antiferromagnetic a-axis spin density wave state develops below T(N)=6.8 K, which readily coexists with 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.