Probing hybrid metallic sandwiches with nonlocal four-terminal electrical measurements

Despite its history of more than a century, the four-probe technique has remained a cornerstone of electrical measurements in thin conductive layers. Its traditional on-sample configuration consists of four electrical contacts arranged along a straight line to measure local sheet resistances. In multilayers, the results of such measurements cannot be interpreted straightforwardly due to their significant dependence on the properties of individual films and contact resistances between them. To address this challenge, we propose a through-sample nonlocal four-terminal method based on the Landauer-Büttiker scattering approach, which has been tested on hybrid all-metallic sandwiches composed of two 80 nm thick NbN films and a 50 nm thick core made of three archetypal ferromagnets, Co, Ni, or NiCu alloy. Results obtained for the trilayers are compared with the corresponding data for single NbN films 160 nm thick. At temperatures above the critical temperature of NbN, we have found negative values of nonlocal resistances which are explained using an equivalent circuit model with six resistances connecting the four probes. The key advantage of the proposed methodology lies in its simple design enabling the detection of subtle physical effects in transversely heterogeneous devices that might otherwise go unnoticed.