INTERTWINED SYMMETRY OF THE MAGNETIC MODULATION AND THE FLUX-LINE-LATTICE IN THE SUPERCONDUCTING STATE OF TMNI2B2C

Materials that can in principle exhibit both superconductivity and fer romagnetism are caught in a dilemma: both states represent long-range order, but are in general mutually exclusive. When the material favour s a ground state with a large magnetic moment, as is the case for Er4R h4B (ref. 1), superconductivity is destroyed. For superconductivity to persist, the magnetic structure would need to adopt an antiferromagne tic modulation of short enough wavelength to ensure a small net moment on the length scale of the superconducting coherence length. The inte rmetallic-borocarbide superconductors(2-4) RNi2B2C (where R is a rare- earth element) have shed new light on this balance between magnetism a nd superconductivity. The response of these materials in the supercond ucting state to a magnetic field is dominated by the formation of a fl ux-line lattice-a regular array of quantized magnetic vortices whose s ymmetry and degree of order are easily modified and thus can be expect ed to interact with an underlying magnetic modulation. In TmNi2B2C, su perconductivity and antiferromagnetic modulated ordering coexist below 1.5 K (refs 5-7). Here we present the results of a small-angle neutro n-scattering study of this compound which show that the structure of t he magnetic modulation and the symmetry of the flux-line lattice are i ntimately coupled, resulting in a complex phase diagram.