Studying spin-dependent scattering in magnetic multilayers by means of perpendicular (CPP) magnetoresistance measurements

Giant magnetoresistance (GMR) in multilayers composed of a ferromagnetic (F) and a non-magnetic (N) metal is usually assumed to result primarily from spin-orientation-dependent scattering in the bulk F metal and at the FN interfaces, which are described by a bulk spin-asymmetry parameter β and an interface parameter γ. Measurements of the specific resistance, ARt the sample cross-sectional area A times resistance Rt, with the current perpendicular to the layer planes (CPP), can give direct access to both parameters, as well as to the important interface specific resistance, ARFN. We describe how these measurements are performed at low temperatures, present the equations used to derive these parameters and discuss the conditions under which these equations are valid—the most important being that the spin diffusion lengths lsf in the N and F metals must be much longer than the layer thicknesses and the mean free paths for momentum transfer. We test the equations against low temperature data on sputtered CoAg, CoAgSn, CoCu and PyCu (PyNi84Fe16) multilayers, and list derived values of β, γ and ARFN for CoAg, CoCu and PyCu. As a further test, we examine how well the parameters found for the CoCu and PyCu multilayers predict the values of ARt for CoCuPyCu multilayers in the states where neighboring Co and Py layer magnetizations are either parallel (P) or anti-parallel (AP) to each other. Lastly, Valet and Fert recently generalized these equations to shorter values of lsf, and we show how their results can be used to derive lsf for AgMn and AgPt alloys.