The effect of spreading resistance on the magnetoresistance of current-perpendicular-to-plane spin valves with patterned spacer layers

There has been intense research effort to increase the magnetoresistance (MR) of current-perpendicular-to-plane (CPP) spin valve, so as to maintain the high rates of areal density growth of magnetic media platters of the hard disk towards 1 Tbit/in² and beyond. It has been proposed recently that MR can be increased with the insertion of ferromagnetic or half-metalic layers into the basic CPP spin-valve multilayer structure. In this article, we suggest that enhancement of MR can also be achieved by patterning the spacer layer (Cu) of the spin valve trilayer to form cylindrical nanopillar structures. The patterning of the Cu spacer leads to current confinement within the small cylinders, and significant spreading resistances R_sp at the interfaces between the spacer and ferromagnetic layers, due to the abrupt change of cross sectional area at the ends of the cylinders. The MR of the spin-valve may be enhanced if R in the FM layers can offset the increase in the spacer resistance due to current confinement. The extent of R_sp is dependent on a number of factors, i.e. size, cross-sectional shape and packing density of the cylinders. Thus, in our computations we consider both a single-cylinder and multi-cylinder spacer layers. The spreading resistance is determined numerically via a finite-element Poisson solver (Ansys), while the spin-dependent transport through the device is modeled via the spin drift-diffusion equation. In our model for MR, we have assumed no spin flip scattering across the cross-sectional areas, constant total current j, and zero spin accumulation at both device terminals.