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Shaped angular dependence of the spin-transfer torque and microwave generation without magnetic field


The generation of oscillations in the microwave frequency range is one of the most important applications expected from spintronics devices exploiting the spin-transfer phenomenon, which is the reorientation of the magnetization of a ferromagnetic domain by spin-polarized current. Here we report transport and microwave power measurements on specially designed nanopillars, for which a non-standard angular dependence of the spin-transfer torque is predicted by theoretical models. We observe a new kind of current-induced dynamics that is characterized by large angle precessions in the absence of any applied field. This is also predicted by simulations including a ‘wavy’ angular dependence of the torque. This type of nanopillar, which is able to generate microwave oscillations in zero applied magnetic field, could represent an interesting method for the implementation of spin-transfer oscillators. We also emphasize the theoretical implications of our results on the angular dependence of the torque.

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Figure 1: Angular dependence of the STT for a standard and a ‘wavy’ angular dependence.
Figure 2: Transport measurements on nanopillars with standard or ‘wavy’ angular dependence of the STT.
Figure 3: Microwave power spectra for the Co(8 nm)/Cu(10 nm)/Py(8 nm) nanopillar of Fig. 2b,c.
Figure 4: Experimental and simulated spin-transfer-induced high-frequency dynamics for a Co(8 nm)/Cu(10 nm)/Py(8 nm) nanopillar.


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The authors thank M. Gmitra for the calculations of Fig. 1b on the basis of the model of ref. 16. We would also like to acknowledge H. Hurdequint for FMR measurements, L. Vila for assistance in fabrication, O. Copie and B. Marcilhac for assistance in transport and frequency measurements and M. R. Pufall for discussions. This work was partly supported by the French National Agency of Research, ANR, through the PNANO programme (MAGICO PNANO-05-044-02) and the EU through the Marie Curie Training Network SPINSWITCH (MRTN-CT-2006-035327). J.B. acknowledges support by funds from the Polish Ministry of Science and Higher Education as a research project (2006–2009).

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Boulle, O., Cros, V., Grollier, J. et al. Shaped angular dependence of the spin-transfer torque and microwave generation without magnetic field. Nature Phys 3, 492–497 (2007).

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