Asymmetric spin-transfer torque in single-crystalline $\mathrm{Fe}∕\mathrm{Ag}∕\mathrm{Fe}$ nanopillars

We investigate current-perpendicular-plane giant magnetoresistance (CPP-GMR) and current-induced magnetization switching in single-crystalline $\mathrm{Fe}∕\mathrm{Ag}∕\mathrm{Fe}$ nanopillars of $70\mathrm{nm}$ 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 behavior, 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 $\mathrm{Fe}∕\mathrm{Ag}\left(001\right)$ interfaces [M. D. Stiles and D. R. Penn, Phys. Rev. B 61, 3200 (2000)]. 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 1

Angular variations of (a) the GMR signal and (b) the spin-transfer torque for three different $\Lambda$ $\left(\chi \right)$ values according to Eqs. (1, 2, 3). The dotted line shows the symmetric behavior as found experimentally in Ref. 12. The solid line is what we got from our GMR data and the broken line is obtained from our ratio of switching current densities ${\rho }_{c2}∕{\rho }_{c1}$ (see Table ). The experiment clearly finds a pronounced deviation from the symmetric behavior, which stems from the enhanced spin accumulation at the $\mathrm{Fe}∕\mathrm{Ag}\left(100\right)$ interfaces.

Figure 2

CPP-GMR data (black) measured at $5\mathrm{K}$ with the magnetic field being applied along (a) an easy or (b) hard axis, respectively, of the single-crystalline Fe layers. Gray lines are least and circles are second least energy solutions of Stoner-Wohlfarth fits. Equation (1) is used to calculate the GMR signal. The fit parameters are ${\chi }_{\mathit{GMR}}=1.6$ $({\Lambda }_{\mathit{GMR}}=1.6)$, $K_1^{\mathit{\text{free}}}=63\mathrm{kJ}∕{\mathrm{m}}^3$, and $K_1^{\mathit{\text{fixed}}}=98\mathrm{kJ}∕{\mathrm{m}}^3$. The edges of the dashed squares indicate the magnetic easy axis of the Fe layers and the thick (thin) arrow indicates the magnetization direction of the fixed (free) layer.

Figure 3

Two-step current-induced switching of the free-layer magnetization at $5\mathrm{K}$. A weak field of $5.6\mathrm{mT}$ is applied along a HA. $I_{c1}$ and $I_{c2}$ denote the critical currents for the switching from parallel to perpendicular alignment and from perpendicular to antiparallel alignment, respectively.