This Topics in Current Physics (TCP) Volume 34 is concerned primarily with super- conductivity and magnetism, and the mutual interaction of these two phenomena in ternary rare earth compounds. It is the companion of TCP Volume 32 - Superconduc- tivity in Ternary Compounds: Structural, Electronic and Lattice Properties. The interplay between superconductivity and magnetism has intrigued theoreticians and experimentalists alike for more than two decades. V. L. Ginzburg first addressed the question of whether or not superconductivity and ferromagnetism could coexist in 1957, and B. T. Matthias and coworkers carried out the first experimental inves- tigations on this problem in 1959. The early experiments were made on systems that consisted of a superconducting element or compound into which small concentrations of rare earth impurities with partially-filled 4f electron shells had been intro- duced. These dilute impurity systems were chosen because the scattering of conduc- tion electrons by parama9. netic rare earth impurity ions usually has a strong de- structive "pa i r breaking" effect on superconducti vity, typi ca lly drivi ng the super- conducting transition temperature to zero at impurity concentrations of only a few atomic percent.Unfortunately, analysis of these early experiments was complicated by clustering and/or the formation of short range or "glassy" types of magnetic order so that definitive conclusions regarding the coexistence of superconductivity and magnetism could not be reached.
