Anomalous Meissner Screening Probed using Low Energy Muon Spin Spectroscopy

In mesoscopic systems the Meissner response of a superconducting film can be very different from bulk. For normal metal (N) superconducting (S) bilayers screening can be enhanced depending on material properties.\footnote{W. Belzig, \textbf{PRB} 53, 5727} It has been shown that if N is replaced by an inhomogeneous ferromagnet (F) an inverse Meissner response is possible\footnote{M. Alidoust, \textbf{PRB} 89, 054508}. Low Energy Muon Spin Spectroscopy has recently been used to observe this effect\footnote{A. Di Bernadino, \textbf{PRX} 5, 041021}. We present low energy muon spin spectroscopy measurements on bi and trilayer systems where an enhancement of 300 \% is observed in the Meissner response when compared with an S layer. Using the quasiclassical approach we compare these results with calculations. Whilst for the N/S system theory and experiment are consistent, a large discrepancy results when adding an F layer to form an N/S/F trilayer. Rather than finding a reduction in the Meissner response we observe a 500 \% increase in flux expulsion. Moreover, this F layer enhancement can be tuned by modifying the interface. These observations contradict current SF proximity theory and are valuable for understanding more complex S, F, N systems in which there is much current interest.