INTERPLAY BETWEEN SUPERCONDUCTIVITY AND MAGNETISM

The f-electrons in rare earth and actinide intermetallic compounds are responsible for a rich variety of novel superconducting and magnetic states that have been discovered during the past two decades. In this paper, we briefly review these remarkable superconducting and magnetic states, how they are believed to originate, and the f-electron materials in which they are found. Examples of phenomena that are produced by superconducting-magnetic interactions in certain ternary and quaternary rare earth compounds include the coexistence of superconductivity and antiferromagnetism, the destruction of superconductivity by the onset of ferromagnetism at a second critical temperature T-c2 below the superconducting critical temperature T-c1, accompanied by a new sinusoidally modulated magnetic state that coexists with superconductivity in a narrow temperature interval above T-c2, and the induction of superconductivity by the application of high magnetic fields. Multiple superconducting phases have been discovered in several U ''heavy fermion'' compounds, f-electron materials in which the electrons have enormous effective masses as high as several hundred times the mass of the free electron. The multiple superconducting phases are apparently the result of the interaction of an unconventional superconducting state with a coexisting weakly antiferromagnetic state. This unconventional superconducting state is presumed to involve electrons that are paired in states with angular momentum greater than zero via antiferromagnetic spin fluctuations, and is similar to the superconducting state that is believed by many (but not all!) researchers to be responsible for high-T-c, superconductivity in the cuprates.