A magnetically-controlled superconducting switch Norman Birge, Michigan State University, DMR The interplay between superconductivity and ferromagnetism is a fascinating area of current research. Normally, these two states of solids “hate” each other, because superconductivity requires forming Cooper pairs of electrons with opposite spins, whereas the electron spins in a ferromagnet tend to line up. Strange things happen when superconducting (S) and ferromagnetic (F) metals are placed in contact. In an FSF sandwich, the temperature where the superconductor loses its electrical resistance depends on whether the magnetizations of the two F layers are parallel (P) or antiparallel (AP). If the critical temperature difference between the P and AP states could be made large enough, FSF sandwiches could be used as a magnetically-controlled superconducting switch. P state AP State F (Ni) S (Nb) F (Ni) P stateAP state

A magnetically-controlled superconducting switch Norman Birge, Michigan State University, DMR One of the goals of research in solid-state physics is to understand the properties of materials in all their possible states. Two common states of metals are ferromagnetism and superconductivity. The former has been known about for thousands of years, and the latter for nearly 100 years. Nevertheless, new understanding about how these materials interact with each other is being achieved even today. A few years ago, two groups of theorists predicted that a trilayer consisting of two ferromagnetic layers surrounding a superconducting layer could have properties that depend on the mutual orientations of the magnetizations of the two ferromagnets. These properties occur only when the middle superconducting layer is a few tens of nanometers thick – a level of control that is easily achievable using modern thin-film deposition techniques. Specifically, the critical temperature (the temperature where the superconductor loses all of its electrical resistance) should depend on whether the magnetizations of the ferromagnets are parallel or antiparallel. This effect was first observed using weak ferromagnets made from alloys of copper and nickel. We have shown that the effect can be observed using pure nickel, which is a strong ferromagnet. This is an important step, because understanding the interplay between strong ferromagnets and superconductors presents special challenges. In addition, there are some new theoretical predictions – so far untested in the laboratory – for some truly astounding behavior involving combinations of ferromagnetic and superconducting metals. Those new effects require that the electrons in the ferromagnetic metal retain their spin memory over long distances. That is much more likely to be the case in pure ferromagnets than in alloys.

A magnetically-controlled superconducting switch Norman Birge, Michigan State University, DMR Education: Graduate students working in experimental condensed matter physics learn the concepts and experimental tools that are crucial to the future development and applications of nanosciences. Former graduate students from this NSF-sponsored research program have gone on to careers in academia, industry, and government labs. Undergraduate students working in the lab get valuable research experience that helps them choose an appropriate career path. Current graduate students at MSU: Michael Crosser, Ion Moraru, Gassem Al- Zoubi Current & recent undergrads: Hamood Arham, Mark Stockett, Nick Moroz Broad scientific impact: Understanding the basic states of matter, such as magnetism and superconductivity, still plays a central role in condensed matter physics. New techniques for controlling materials on the nanoscale will lead to new technological applications.