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Low-energy neutrons are essential for the analysis and characterization of materials and magnetic structures. However, both continuous (reactor-based) and pulsed (spallation-based) sources of such neutrons suffer from low fluence. Steering and lensing devices could improve this situation dramatically, so increasing spatial resolution, detectable sample volume limits and even perhaps opening the way for the construction of a neutron microscope. Neutron optics have to date exploited either Bragg diffraction1,2, such as bent crystals, or reflection, as in mirror3 guides or a Kumakhov lens4,5. Refractive optics remain an attractive alternative as they would permit full use of the beam cross-section, allow a compact and linear installation and, because of similarity to conventional optics, enable the use of commercial design and simulation tools. These advantages notwithstanding, single-element refractive optics have previously been considered impractical as they are too weakly focusing, too absorptive and too dispersive. Inspired by the recent demonstration6 of a compound refractive lens (CRL) for high-energy X-rays, we have designed, built and tested a prototype CRL for 9–20-A neutrons by using readily available optical components: our CRL has gains greater than 15 and focal lengths of 1–6 m, well matched to small-angle neutron scattering.

MAGNETISM and superconductivity are manifestations of two different ordered states into which metals can condense at low temperatures. In general these states are mutually exclusive1; they do not coexist at the same place in a sample. The study of the interplay between these properties has recently been revitalized by the discovery2,3 of a class of compounds with formula RNi2B2C (where R is a rare-earth element) which are both antiferromagnetic and superconducting at sufficiently low temperature4. It has been suggested5 that magnetic and superconducting order can coexist in these materials on an atomic scale. Here we use small-angle neutron scattering to study the structure of the superconducting vortex lattice in ErNi2B2C. Our results show that the development of magnetic order causes the vortex lines to disorder and rotate away from the direction of the applied magnetic field. This coupling of superconductivity and magnetism provides clear evidence for microscopic coexistence of magnetic and superconducting order, and indicates that magnetic superconductors may exhibit a range of unusual phenomena not observed in conventional superconductors.