Current-driven Magnetization Reversal and Spin Wave Excitations in Co/Cu/Co Pillars

Current-driven Magnetization Reversal and Spin Wave Excitations in Co/Cu/Co Pillars

(August 20, 1999) | J. A. Katine, F. J. Albert, R. A. Buhrman, E. B. Myers, D. C. Ralph
The authors investigate the effects of spin-polarized currents on the magnetization reversal and spin wave excitations in Co/Cu/Co pillars, which are thin film structures with two ferromagnetic Co layers of different thicknesses separated by a paramagnetic Cu spacer. Using these pillars, they examine the torque effects due to spin-polarized currents flowing perpendicular to the layers. According to spin-transfer theory, spin-polarized electrons flowing from the thinner to the thicker Co layer can switch the magnetic moments of the layers antiparallel, while a reversed electron flow causes switching to a parallel state. In low magnetic fields, this behavior is confirmed, but in larger fields, the current no longer fully reverses the magnetic moment but instead stimulates spin-wave excitations. The study uses GMR measurements to probe the relative orientation of the Co layers and confirms the predicted hysteretic jumps in resistance as a function of current, which are attributed to changes in the relative alignment of the magnetization. The critical currents required for switching are found to be consistent with spin-transfer theory, but the effect of the external field on these currents suggests a higher damping parameter $\alpha$ compared to previous point contact studies. The results indicate that the switching process is more complex than a simple uniform rotation, possibly involving multiple domains and domain wall motion. The findings provide insights into the behavior of isolated ferromagnetic particles and the role of spin-transfer effects in magnetic systems.The authors investigate the effects of spin-polarized currents on the magnetization reversal and spin wave excitations in Co/Cu/Co pillars, which are thin film structures with two ferromagnetic Co layers of different thicknesses separated by a paramagnetic Cu spacer. Using these pillars, they examine the torque effects due to spin-polarized currents flowing perpendicular to the layers. According to spin-transfer theory, spin-polarized electrons flowing from the thinner to the thicker Co layer can switch the magnetic moments of the layers antiparallel, while a reversed electron flow causes switching to a parallel state. In low magnetic fields, this behavior is confirmed, but in larger fields, the current no longer fully reverses the magnetic moment but instead stimulates spin-wave excitations. The study uses GMR measurements to probe the relative orientation of the Co layers and confirms the predicted hysteretic jumps in resistance as a function of current, which are attributed to changes in the relative alignment of the magnetization. The critical currents required for switching are found to be consistent with spin-transfer theory, but the effect of the external field on these currents suggests a higher damping parameter $\alpha$ compared to previous point contact studies. The results indicate that the switching process is more complex than a simple uniform rotation, possibly involving multiple domains and domain wall motion. The findings provide insights into the behavior of isolated ferromagnetic particles and the role of spin-transfer effects in magnetic systems.
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