Effects of engineered Cu spacer on the interlayer coupling and giant magnetoresistance behavior in Pd/[Pd/Co]2/Cu/[Co/Pd]4 pseudo-spin-valves with perpendicular anisotropy

Abstract

Effects of perpendicular interlayer coupling formed between two perpendicularly magnetized ferromagnetic multilayers separated by engineered Cu spacer on the giant magnetoresistance (GMR) behavior were investigated in the Pd (3 nm)/[Pd (1.2 nm)/Co (0.6 nm)]2/Cu (x nm)/[Co (0.3 nm)/Pd (0.6 nm)]4/Pd (3 nm) pseudo-spin-valves. It was clearly observed that an increase in Cu spacer thickness from 1.6 to 4.9 nm, decreased the perpendicular interlayer coupling field through the Cu spacer and correspondingly decreased the magnetoresistance ratio. It was found that this is due to the degradation of the perpendicular anisotropy caused by strain relaxation in the soft [Co/Pd] multilayers. Further experimental and theoretical analyses verified that the physical nature of the perpendicular interlayer coupling field is directly relevant to the topological and Ruderman–Kittel–Kasuya–Yosida (RKKY) type oscillatory coupling created in the [Co/Pd]2/Cu/[Co/Pd]4 multilayers. Unlike the spin valves with in-plane anisotropy, it was clearly verified that the contribution of topological coupling field to the perpendicular interlayer coupling field is negligibly small. In contrast, the oscillatory RKKY coupling field depending on the magnetization angle between the soft and hard [Co/Pd] multilayers dominantly determined the physical characteristics of perpendicular interlayer coupling field. Furthermore, even at a larger surface roughness of greater than 0.25 nm for tCu=6–16 nm, the slight oscillations in the GMR ratio corresponded well with the oscillations in the interlayer coupling field, although the GMR ratio showed an overall decreasing trend possibly due to shunting through the thicker Cu spacer. This indicates that the perpendicular interlayer coupling rather than the topological coupling is dominant in determining the GMR behavior. Based on the experimental and theoretical results, a physical model enabling the explanation of GMR behavior in [Co/Pd] based pseudo-spin-valves with perpendicular anisotropy is proposed: a perpendicular magnetostatic field, induced in between the soft and hard [Co/Pd] multilayers through Cu spacer by perpendicular anisotropy, is directly relevant to the perpendicular interlayer coupling field and dominantly controls the GMR characteristics. In addition, this model proposes that the GMR ratio is proportional to the sine of the angle between the soft and hard layer magnetizations from the perpendicular direction during magnetic reversal of the soft layer by an applied magnetic field. However this model is only applicable once the magnetization of the soft layer is slightly tilted away from the perpendicular direction by a critical angle (θC1=5–10°). Similar perpendicular interlayer coupling characteristics were also observed for the Cu spacers engineered by different input sputtering powers.