Many useful electronic devices are made from ferromagnetic films, such as read and write heads for computer hard drives. Useful devices are made from superconducting films, too, including medical systems that detect magnetic fields from heartbeats and brain activity. Recent advances in the sophistication of experimental apparatus for making thin-film structures and for measuring them, and advances in theory, have opened a new avenue for possible devices that incorporate superconducting and ferromagnetic films in close proximity. The really exciting aspect of the science, and an important mechanism for new device functionalities, comes from the persistence of superconductivity inside the ferromagnet when the two materials are in such intimate contact such that electrons can diffuse freely from superconductor to ferromagnet and back. This project will study superconducting electrons in the ferromagnet directly via magnetic field screening measurements in large and nano-scale devices, and indirectly via electron tunneling measurements. The project will involve graduate and undergraduate students in fabrication of SC/FM devices as well as sophisticated measurements.
Superconductivity and ferromagnetism are antithetical, the former demanding that electrons form spin-zero pairs and the latter pressing for parallel electron spins. Recently it has been appreciated that sample fabrication and measurement techniques have become sufficiently sophisticated that the mutual interactions of superconductors in intimate contact with ferromagnets can be reproducibly created and studied. A surprising result of theory is that superconductivity can persist into the ferromagnet. This project will explore the effective superfluid density associated with supercurrents in the FM layer of various SC/FM bilayer and trilayer structures. The superfluid density will be measured directly via magnetic screening measurements involving the mutual inductance of coils on opposite sides of a SC/FM bilayer or trilayer, and via transport measurements on nanoscale patterned wires, to see the effects of reduced dimensionality. It will also be studied via tunneling measurements into the FM film of a SC/FM bilayer that is carrying a supercurrent. The project will involve graduate and undergraduate students in fabrication of SC/FM devices as well as sophisticated measurements.
This is an experimental study of thin layers of different metals grown one on top of the other, and therefore in intimate contact. One layer is a ferromagnet. The other is a superconductor, a material which knits electrons together into quantum entities called "Cooper pairs". These pairs regularly hit the interface and occasionally hop into the ferromagnet. The goal is to learn how easily the pairs cross the F/S interface and, once they get across, how do they evolve with time inside the ferromagnet. We know they eventually decay, but the question is how long does it take. We find that pairs must strike the interface as many as 10 times in order to cross, even though the interface is grown to be as "clean" as possible. More interestingly, we find that the Cooper pairs live about 10 times longer in the ferromagnet that had been expected and found in earlier studies. Both of these results contradict previous studies. This result is generally important because a great deal of effort goes into understanding the quantum ground states of electrons in complex materials. In this regard, the competition between superconductivity and ferromagnetism is one of the central problems currently under attack. Also, superconducting devices involving S/F structures are envisioned and even prototyped. A correct understanding of the microscopics will be essential to develop useful technology. In our study, we probed the super/ferro bilayers and super/ferro/super trilayers by passing a magnetic field through the sample. A "drive" coil on one side of the sample produced a small magnetic field at 50 kHz while a "pick-up" coil on the other side measured how much magnetic field made it through the sample. The stronger superconductivity is in the sample, the greater the reduction in field. When the sample goes superconducting at its transition temperature, Tc, screening of the magnetic field grows rapidly to a typical value of about 100 at low temperature. The magnitude and temperature dependence of the screening factor tell us about what is happening in the superconductor and the ferromagnet. A significant part of the project was to turn abstruce mathematical theory for F/S structures into basic physical concepts to simplify interpretation of data. We were able to reduce the number of physical parameters needed from a dozen or so to just a few. As a side project, we explored the physics of the experiment when the magnetic field pushed through the bi (tri) layer sample was so large that superconductivity was broken down. We discovered how to extract an important microscopic parameter, the superconducting coherence length from such measurements.
