Effects of interfacial roughness on the GMR of Ta/Co/Ta multilayers studied by neutron reflectometer

Abstract

To study the effects of the roughness of magnetic nano-multilayer films on the giant magnetoresistance (GMR) performance, we used magnetron sputtering to prepare Ta/Co/Ta films with different Co thicknesses on a silicon substrate. The surface and interfacial roughness of the multilayer films were investigated using atomic force microscopy (AFM) and neutron reflectometry. The AFM results with a range of 10 μm indicated that the surface undulation increased with the increment of the Co thickness and the root-mean-square (RMS) roughness increased from 0.48 to 2.26 nm. The neutron reflectometry results proved that the thicker the Co layer, the rougher the interface in the multilayer film, with an increase from 1 to 2.7 nm. The four probe method was used to study the GMR properties of the films. It indicated that the GMR effect was enhanced and the GMR value shifted from − 0.04% to − 1.13% with increasing Co thickness from 6.5 to 20 nm. The enhancement in the GMR effect originated from the thicker Co layer, which led to increasing interfacial roughness, thereby enhancing the spin-dependent scattering of the interface and increasing the absolute value of the GMR. Nevertheless, when the Co thickness increased to 42 nm, the GMR absolute value decreased from 1.13 to 0.8% and the GMR effect was destroyed. This behavior originated from the increased shunt of the ferromagnetic layer and the rougher Ta capping layer increasing the spin-independent scattering center, thereby eventually undermining the GMR effect. Diffusion between the multilayer films was observed using a polarized neutron reflection experiment, further illustrating that the GMR effect could be adjusted by the interfacial roughness.