Magnetization reversal and field annealing effects in perpendicular exchange-biased Co∕Pt multilayers and spin valves with perpendicular magnetization

Abstract

A study on the Co layer thickness dependence of the magnetization reversal process and magnetoresistance of perpendicular exchange-biased Co∕Pt multilayers and spin valves with perpendicular magnetization is presented. The hysteresis of [20ÅPt∕tÅCo]3∕100Å IrMn∕20Å Pt multilayers with t⩾5Å are found to be asymmetric after deposition. This asymmetry reflects a lateral variation in the perpendicular exchange-bias direction due to the growth of IrMn onto multidomain Co∕Pt multilayers. Magnetic annealing in a perpendicular field removes the switching asymmetry and leads to a maximum exchange-bias field of 16.3mT for t=7Å. Perpendicular spin valves with an optimized structure of 50 Ta∕20Å Pt∕tÅ Co∕30Å Cu∕tÅ Co∕20Å Pt∕6Å Co∕3Å Pt∕100 IrMn∕20Å Pt are found to exhibit good switching behavior but limited magnetoresistance for small Co layer thickness. The magnetoresistance of these spin valves increases with t up to a maximum of 3.9% for t=10Å, beyond which it decreases due to simultaneous magnetization reversal in the ferromagnetic layers. Magnetic field annealing of these top-pinned structures reduces the exchange-bias field and the magnetoresistance.