RARE-EARTH MAGNETIC-ORDERING IN EXCHANGE-COUPLED SUPERCONDUCTORS

Magnetic neutron diffraction measurements are reviewed for two familie s of superconductors where exchange interactions dominate the energeti cs of the magnetic system. One system is cuprates that contain Pr, whi ch exhibit magnetic rare earth ordering temperatures that are an order -of-magnitude higher than for the other rare earths, and where f-elect ron hybridization effects destroy any chance for superconductivity. Th e second family of materials is the quaternary RNi2B2C borocarbides. T hese materials have comparable magnetic and superconducting transition temperatures, and exhibit a rich variety of commensurate and incommen surate magnetic structures that coexist and compete with the supercond ucting state.