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Spin-Transfer Induced Dynamic Modes in Single-Crystalline Fe/Ag/Fe Nanopillars

We performed measurements and simulations of spin-transfer torque (STT)-induced magnetization dynamics in nanopillars containing a thin, circular, single-crystalline Fe nanomagnet with four-fold in-plane magnetocrystalline anisotropy as a free layer. The magnetocrystalline anisotropy inherent to bcc-Fe allows for a consecutive switching of the magnetization by 90° between parallel and antiparallel alignment to the fixed layer magnetization. Additionally, the anisotropy gives rise to steady-state precession of the magnetization at low or even zero applied magnetic fields as well as at large fields exceeding the coercive field. While the low-field mode is governed by the interplay between the STT and the anisotropy, the high-field dynamics result from the STT acting against the externally applied magnetic field.


Figure: Simulated STT-induced switching of a macrospin in the presence of cubic magnetocrystalline anisotropy and demagnetizing field. The free layer Mfree switches under the influence of a persistent DC current first from parallel (+x-direction) to a 90°-orientation (+y-direction) with respect to the fixed layer Mfixed and then from the 90°-orientation to the antiparallel alignment (−x-direction).  (a,b) Trajectories of the two switching events. (c,d) Representation of the STT (blue arrows) and damping torque (red arrows) viewed along (b) the initial, parallel and (c) the 90°-orientation of the macrospin. Only a fraction of the trajectory in the immediate vicinity of the switching event (a) is shown in (c) and (d).

R. Lehndoff, D. E. Bürgler, A. Kákay, R. Hertel, and C. M. Schneider
Spin-Transfer Induced Dynamic Modes in Single-Crystalline Fe-Ag-Fe Nanopillars
IEEE Trans. Magn. 44, 1951 (2008).