Current-perpendicular and current-parallel giant magnetoresistances in Co/Ag multilayers

We report results of measurements of the perpendicular [current perpendicular to the layer planes (CPP)] and parallel [current in the layer planes (CIP)] magnetoresistances (MRs) of Co/Ag multilayers having Co and Ag thicknesses t(Co) and t(Ag) ranging from 1.5 to 60 nm. (1) We find the variations of the CPP- and CIP-MRs with t(Ag) and t(Co) to be qualitatively similar to each other, suggesting that the parameters determining both may also be similar. (2) In accord with predictions that CPP-MR>CIP-MR, we find their ratio pi=CPP-MR/CIP-MR to range from about 3 to at least 6. (3) We analyze current perpendicular to the layer plane (CPP) specific resistances, Ar-t, of Co/Ag multilayers having a wide range of Co and Ag thicknesses t(Co) and t(Ag) in terms of a two-current, series resistor model based upon the assumption that the spin-diffusion lengths in the Co and Ag are much longer than t(Co) and t(Ag), respectively. We show first that a six parameter fit gives rather good agreement with four data sets, with the three parameters that can be separately checked agreeing within experimental uncertainties with independent measurements of the same quantities. We then examine the ability of these fits to predict the behavior of additional sets of data. Here we find some agreements and some disagreements. Possible reasons for the disagreements are examined.