Effect of magnetic flux distribution on GMR in Ag/Co multilayers

Abstract

Since the discovery of giant magnetoresistance (GMR) in multilayers (MLs) various applications of this effect are studied. One practical feature of ML GMR sensing elements is the magnetic flux distribution in the direction perpendicular to the surface of ML in the current-in-plane geometry. It results from the fact that top and bottom ferromagnetic layers concentrate more magnetic flux than central layers of the stack. This effect is studied in this work in evaporated Ag/Co MLs deposited onto Si substrates. The thickness of Co was 1.2–1.4 nm, i.e. Co layers were continuous. Ag layers were 5.4–5.8 nm thick. Number of Ag/Co periods was only ≤5 to expose the relative influence of top and bottom parts of ML. To make the ML structure symmetrical, an additional Co layer was deposited as first on the substrate. Ag/Co MLs were analyzed by X-ray diffraction (XRD) and X-ray reflectivity (XRR). The magnetic flux distribution was simulated. GMR measurements were performed in dynamic conditions in the external ac field (50 Hz) with increasing swing. By simulations of XRR spectra the effective roughness of ML interfaces 0.4–0.8 nm was obtained. The GMR signal at RT increased with number of periods and the external field ≤16 kA/m up to approximately 2%. The influence of distribution of the magnetic flux density on GMR was inferred from the behavior of the positions of maxima of GMR on the magnetic field axis vs. number of periods. It can be explained by the statistical distribution of coercivity of different domains in Co layers.