Fast Pulse Measurements and Temperature Variation of Enhanced Magnetic Damping of Spin-Transfer Excitation

Recently, light terbium (Tb) doping in thin films of permalloy (Py) has been shown to increase the damping parameter $\alpha $ by several orders of magnitude [1]. To directly study the effect of increased $\alpha $ on spin-transfer systems, we have fabricated 0.004 um$^{2}$ Py/Cu/Py nanopillar spin valves with 0 and 2{\%} Tb in the free layer, and measured critical currents across a range of temperatures from 4.2 K to 295 K. We find that the critical currents for reversibly switching the free layer, generally expected to be proportional to $\alpha $, are several times larger on average in the 2{\%} Tb samples than in pure Py samples, and increase linearly with decreasing temperature. We will also discuss FMR data, and data for switching with fast pulses from 1 to 100 nsec at both 150 K and room temperature, along with matching simulations that allows us to extract $\alpha $ and other spin-torque parameters [2]. These results suggest one approach for controllably reducing the negative impact of spin-torque effects on nanoscale spin valve and read head sensors, and achieving a deeper understanding of these spin-torque devices. [1] W. Bailey, P. Kabos, F. Mancoff, and S. E. Russek, IEEE Trans. Magn. 37, 1749 (2001). [2] P. M. Braganca, et al. Appl. Phys. Lett. 87, 112507 (2005).