The transfer of spin angular momentum to a nanomagnet from a spin polarized current provides an efficient means of controlling the magnetization direction in nanomagnets. A unique consequence of this spin torque is that the spontaneous oscillations of the magnetization can be induced by applying a combination of a dc bias current and a magnetic field. Here we experimentally demonstrate a different effect, which can drive a nanomagnet into spontaneous oscillations without the need of external spin torque injection. For the demonstration of this effect, we use a nano-pillar of magnetic tunnel junction (MTJ) powered by a dc current and connected to a coplanar waveguide (CPW) lying above the free layer of the MTJ. Any fluctuation of the free layer magnetization is converted into oscillating voltage via the tunneling magneto-resistance effect and is fed back into the MTJ by the CPW through inductive coupling. As a result of this feedback, the magnetization of the free layer can be driven into a continual precession. The combination of MTJ and CPW behaves similar to a laser system and outputs a stable rf power with quality factor exceeding 10,000.
@article{arxiv.1608.03708,
title = {Coherent microwave generation by spintronic feedback oscillator},
author = {Dinesh Kumar and K. Konishi and Nikhil Kumar and S. Miwa and A. Fukushima and K. Yakushiji and S. Yuasa and H. Kubota and C. V. Tomy and A. Prabhakar and Y. Suzuki and A. Tulapurkar},
journal= {arXiv preprint arXiv:1608.03708},
year = {2018}
}