Interdependence of magnetism and superconductivity in the borocarbide TmNi2B2C

We have discovered anew antiferromagnetic phase in TmNi2B2C by neutron diff raction. The ordering vector is Q(A) = (0.48,0,0) and the phase appears sho ve a critical in-plans magnetic field of 0.9 T. The field was applied in or der to test the assumption that the zero-field magnetic structure at Q(F) = (0.094, 0.094, 0) would change into a c-axis ferromagnet if superconductiv ity were destroyed. We present theoretical calculations which show that two effects are important a suppression of the ferromagnetic component of the RKKY exchange interaction in the superconducting phase and a reduction of t he superconducting condensation energy due to the periodic modulation of th e moments at Q(A).