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 nonstandard 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.