Absence of magnetic interactions in Ni-Nb ferromagnet-superconductor bilayers

TL;DR

This study investigates magnetic interactions in Ni-Nb ferromagnet-superconductor bilayers using neutron reflectometry and Josephson junctions, finding no detectable electromagnetic proximity effect at zero field, challenging existing theories.

Contribution

The paper provides experimental evidence showing the absence of EM proximity effects in Ni-Nb bilayers, setting an upper limit and suggesting the need for theoretical extensions.

Findings

No signals of EM proximity effect detected at zero field

Establishes an upper limit of ±0.27 mT for the effect

Results challenge current theoretical predictions

Abstract

Studies of ferromagnet-superconductor hybrid systems have uncovered magnetic interactions between the competing electronic orderings. The electromagnetic (EM) proximity effect predicts the formation of a spontaneous vector potential inside a superconductor placed in proximity to a ferromagnet. In this work, we use a Nb superconducting layer and Ni ferromagnetic layer to test for such magnetic interactions. We use the complementary, but independent, techniques of polarised neutron reflectometry and detection Josephson junctions to probe the magnetic response inside the superconducting layer at close to zero applied field. In this condition, Meissner screening is negligible, so our measurements examine only additional magnetic and screening contributions from proximity effects. We report the absence of any signals originating from EM proximity effect in zero applied field. Our observations indicate that either EM proximity effect is below the detection resolution of both of our experiments or may indicate a new phenomenon that requires extension of current theory. From our measurements, we estimate a limit of the size of the zero field EM proximity effect in our Ni-Nb samples to be $\pm0.27$ mT.