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Asymmetric Spin-transfer Torque

We investigated current-perpendicular-plane giant magnetoresistance (CPP-GMR) and current-induced magnetization switching in single-crystalline Fe/Ag/Fe nanopillars of 70 nm diameter. The interplay between the in-plane, fourfold magnetocrystalline anisotropy of the Fe(001) layers and the spin-transfer torque (STT) gives rise to a two-step switching behaviour, which allows an investigation of the angular dependences of CPP-GMR and spin-transfer torque. Both behave asymmetrically with respect to the perpendicular alignment of the two Fe layer magnetizations as theoretically predicted, due to strong spin accumulation at the Fe/Ag(001) interfaces. The asymmetry parameter determined from the STT data quantitatively agrees with calculated spin-dependent interface resistances, whereas CPP-GMR yields a smaller degree of asymmetry.


Figure: Two-step current-induced switching of the free layer magnetization at 5 K. Ic1 and Ic2 denote the critical currents for the switching from parallel to perpendicular and from perpendicular to antiparallel alignment, respectively. The perpendicular state is stabilized by the cubic anisotropy of bcc-Fe. Inset: Schematic sample structure.

R. Lehndoff, D. E. Bürgler, A. Kákay, R. Hertel, and C. M. Schneider
Asymmetric spin-transfer torque in single-crystalline Fe/Ag/Fe nanopillars
Phys. Rev. B 76, 214420 (2007)