High-Power Coherent Microwave Emission from Magnetic Tunnel Junction Nano-oscillators with Perpendicular Anisotropy

Abstract

The excitation of the steady-state precessions of magnetization opens a new way for nanoscale microwave oscillators by exploiting the transfer of spin angular momentum from a spin-polarized current to a ferromagnet, referred to as spin-transfer nano-oscillators (STNOs). For STNOs to be practical, however, their relatively low output power and their relatively large line width must be improved. Here we demonstrate that microwave signals with maximum measured power of 0.28 μW and simultaneously narrow line width of 25 MHz can be generated from CoFeB-MgO-based magnetic tunnel junctions having an in-plane magnetized reference layer and a free layer with strong perpendicular anisotropy. Moreover, the generation efficiency is substantially higher than previously reported STNOs. The results will be of importance for the design of nanoscale alternatives to traditional silicon oscillators used in radio frequency integrated circuits.

Oscillation characteristics of zero-field spin transfer oscillators with field-like torque

“…Recently, various solutions in the zero magnetic field case have been suggested to produce the steady-state precession, namely, STO with a perpendicularly magnetized pinned layer, 19,20 magnetic vortex oscillators, 21,22 a tilted magnetization of the fixed layer respect to the film plane, 23 and a synthetic ferromagnetic free layer. 24 Recently, the discovery of interfacial perpendicular anisotropy (IPA) between the ferromagnetic electrodes and the tunnel barrier of MTJs [25][26][27] enabled the realization of a STO with a perpendicularly magnetized free layer (PMF) and an in-plane magnetized pinned layer, which may be referred to as PMF-STO. This structure was able to exhibit a reduced threshold current, a high power and a high Q factor.…”

Resonant Spin-Transfer-Torque Nano-Oscillators

“…Various studies have focussed on lowering I C by tailoring the magnetic properties of the ferromagnetic layer while preserving the higher MR. They have estimated that the maximum power delivered to a matched load is around 1µW, while the maximum achieved power in experiments is still around 0.3µW 11 . In this work, we thus propose to harvest the higher MR and the lower switching bias emerging from resonant spin filtering physics 12,13,16 to increase the microwave power and the conversion efficiency of STNOs.…”

Mutual synchronization of spin-torque oscillators consisting of perpendicularly magnetized free layers and in-plane magnetized pinned layers

“…The synchronization of spin-torque oscillators results in an enhancement of the emission power and an increase of the quality factor of the practical devices. In addition, new applications such as brain-inspired computing based on the synchronized spin-torque oscillators are proposed very recently [16][17][18][19].An attractive structure of spin-torque oscillator for practical applications is that consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer [20][21][22] because this type of spin-torque oscillator results in high emission power, narrow linewidth, and wide frequency tunability simultaneously. The oscillation properties of this type of spin-torque oscillator, such as the relation between the injected current and the oscillation frequency, as a single oscillator have been investigated both experimentally [22] and theoretically [23].…”

“…An attractive structure of spin-torque oscillator for practical applications is that consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer [20][21][22] because this type of spin-torque oscillator results in high emission power, narrow linewidth, and wide frequency tunability simultaneously. The oscillation properties of this type of spin-torque oscillator, such as the relation between the injected current and the oscillation frequency, as a single oscillator have been investigated both experimentally [22] and theoretically [23].…”