Coherent microwave generation by spintronic feedback oscillator.

Dinesh Kumar,S Miwa,S Yuasa,H Kubota,A Fukushima,A Tulapurkar,A Prabhakar,K Yakushiji,Nikhil Kumar,C V Tomy,K Konishi,Y Suzuki

doi:10.1038/srep30747

Abstract

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 any need of spin torque. 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.

Highlights

The easy axis of the free layer is taken to be along the x-axis; in-plane hard axis is along the y-axis and the out-of-plane hard axis is along the z-axis
The co-planar waveguide (CPW) is oriented in such a way that the current passing through it creates a magnetic field along the x-axis
The radio frequency (RF) port of the bias-T is connected to the CPW through a power splitter and an amplifier to amplify the feedback signal. (A phase shifter can be inserted in the feedback path.) The CPW lies right above the free layer, and is electrically insulated from the magnetic tunnel junction (MTJ) by a thick (100 nm) SiO2 layer

Summary

Methods

Multilayers with the stacking structure of bottom contact (50)/Ta(3)/Ru(5)/IrMn(7)/CoFe(3)/Ru(0.8)/CoFeB(3)/ CoFe(0.4)/MgO(0.9)/CoFeB(3)/Ta(5)/Ru(5)/top contact (45) (thickness in nanometers) were fabricated. The sample was post-annealed at 300 °C for 2 hours in an in-plane field of 6 kOe. CoFe(3)/Ru(0.8)/CoFeB(3) is the synthetic antiferromagnetic (SAF) polarizing layer. CoFe and CoFeB are coupled antiferromagnetically through Ru. Top CoFeB layer acts as a free layer. The microwave emission spectra were measured with a spectrum analyzer. In order to obtain the correct RF emitted output power from the MTJ, we have subtracted the background data from the raw output power data. Each spectrum is obtained by averaging 100 spectral scans. The power output has been corrected for the inclusion of the power splitter in the system and the impedance mismatch. All measurements were carried out at room temperature. The micromagnetic simulations have been performed with the MuMax[3] program[23]

Author Contributions

Findings

Additional Information