Excitations of incoherent spin-waves due to spin-transfer torque

The possibility of exciting microwave oscillations in a nanomagnet by a spin-polarized current, as predicted by Slonczewski¹ and Berger ², has recently been demonstrated³. This observation opens important prospects of applications in radiofrequency components. However, some unresolved inconsistencies are found when interpreting the magnetization dynamics within the coherent spin-torque model^4-6. In some cases, the telegraph noise caused by spin-currents could not be quantitatively described by that model. This has led to controversy about the need for an effective magnetic temperature model^7-11. Here we interpret the experimental results of Kiselev et al.³ using micromagnetic simulations. We point out the key role played by incoherent spin-wave excitation due to spin-transfer torque. The incoherence is caused by spatial inhomogeneities in local fields generating distributions of local precession frequencies. We observe telegraph noise with gigahertz frequencies at zero temperature. This is a consequence of the chaotic dynamics and is associated with transitions between attraction wells in phase space.