Observation of Anomalous Meissner Screening in Cu/Nb and Cu/Nb/Co Thin Films

Abstract

We have observed the spatial distribution of magnetic flux in Nb, Cu/Nb and Cu/Nb/Co thin films using muon-spin rotation. In an isolated 50 nm thick Nb film we find a weak flux expulsion (Meissner effect) which becomes significantly enhanced when adding an adjacent 40 nm layer of Cu. The added Cu layer exhibits a Meissner effect (due to induced superconducting pairs) and is at least as effective as the Nb to expel flux. These results are confirmed by theoretical calculations using the quasiclassical Green's function formalism. An unexpected further significant enhancement of the flux expulsion is observed when adding a thin (2.4 nm) ferromagnetic Co layer to the bottom side of the Nb. This observed cooperation between superconductivity and ferromagnetism, by an unknown mechanism, forms a key ingredient for developing superconducting spintronics.

Controlling the electromagnetic proximity effect by tuning the mixing between superconducting and ferromagnetic order

“…In the present work the new EM proximity theory is used successfully to model the effect of the interaction between S and F on the flux profile, demonstrating that the previously anomalous effect reported in [19] can be explained within this new theoretical picture. In order to further test the theory we additionally control the degree of coupling between S and F through the insertion of thin insulating barriers (I) as a means to investigate how EM proximity develops as a function of the contact between S and F. We utilize LE-μSR measurements on these resultant N/S/I/F thin film structures to directly probe the screening response of the proximitized system.…”

“…where d S is the Nb thickness and x = 0 corresponds to the top surface of the film [31]. While in principle an extension to the N/S bilayer case could be obtained by using the quasiclassical theory to calculate a numerical profile for the screening response, we find for our combination of sample material parameters the Cu becomes fully proximitized and screening develops across the full spatial extent of the bilayer in a nearly symmetric manner [19]. The resultant profile therefore becomes B NS (x) = B S (x) but where we now use the full thickness of the bilayer, d NS , in the model.…”

“…The presence of unconventional spin-triplet pairs has also been shown to be capable of producing a paramagnetic, as opposed to diamagnetic, Meissner response [12][13][14]. More recently, an anomalous flux lowering has been reported in N/S/F systems (with N a normal metal), over lengthscales far greater than any coherence length phenomena (e.g., inverse proximity [15][16][17]), which could not be understood within the existing quasiclassical theory [18,19]. This latter result, which at the time of its publication was unanticipated by theory, has led to the development of a new theory of electromagnetic (EM) proximity which describes how the interaction of the superconductivity with the vector potential of the F layer influences the screening response [20][21][22][23].…”

“…In order to measure directly any differences in screening behavior one is required to probe locally the magnetic flux profile across a sample. Low-energy muon-spin rotation (LE-μSR) provides a means to achieve this and has been used extensively to probe new physics within these S/F proximity coupled systems [14,18,19,24].…”

Magnetism induced by nonlocal spin-entangled electrons in a superconducting spin-valve

“…So the induced magnetic moment can not be measured in the experiment [42]. Very recently, an anomalous Meissner screening has been observed in normal-metal/superconductor (Cu/Nb) and normal-metal/superconductor/ferromagnet (Cu/Nb/Co) thin films [52]. Comparing with isolated superconductor (50 nm thick Nb film), the flux expulsion becomes significantly enhanced when adding an adjacent normal-metal (40 nm Cu layer).…”