Large quality factor of oscillations in magnetic field feedback oscillator
Dinesh Kumar1*, K. Konishi2, Nikhil Kumar3, S. Miwa2, A. Fukushima4, K. Yakushiji4, S. Yuasa4, H. Kubota4, A. Prabhakar3, C. V. Tomy5, Y. Suzuki2, A. Tulapurkar6
1Department of Physics, National Cheng Kung University, Tainan, Taiwan
2Graduate School of Enginnering Science, Osaka University, Osaka, Japan
3Department of Physics, Indian Institute of Technology Madras, Chennai, India
4Spintronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki, Japan
5Department of Physics, Indian Institute of Technology Bombay, Mumbai, India
6Department of Electrical Enginnering, Indian Institute of Technology Bombay, Mumbai, India
* presenting author:Dinesh Kumar,
Radio frequency (RF) voltage can be generated in a magnet resistive element, referred as spin torque oscillator (STO). In the early stage output power and Q factor (f/Δf ) of STO were very small (~nW and several). We improved both power and Q factor using magnetic tunnel junction (MTJ) based STOs. Recently, we have succeeded to stabilize further the frequency by developing a phase-lock loop (PLL) circuit specialized for STO. In another approach, we proposed a new design of spintronics oscillator, in which a part of the output RF current is fed back to the coplanar waveguide (CPW) locating just above an oscillation layer of the spintronics oscillator. The RF current passing through the CPW generates a RF magnetic field, which can stabilize the oscillation of the RF-field-feedback oscillator [5]. An advantage of the RF-field-feedback oscillator is simple circuit compared to the PLL controlled STO. In this study, we prepared elliptical samples consisting of CoFeB/MgO/CoFeB MTJ with dimensions of 300 nm × 500 nm. Without feedback we observed microwave emission of frequency of 2.5GHz with Q factor of 4 by applying a dc bias current (-2.7mA) and a dc magnetic field (58 Oe) at room temperature, where an emission power of 0.24 nW was obtained. Using the feedback, highly coherent oscillations were observed which resulted in a very large Q factor of 12800. The spin dynamics of the RF-field-feedback oscillator has also been validated by micromagnetic simulations. The observed results will be of great significance for designing MgO-based spintronics oscillators for ultra-sensitive magnetic field sensors and future wireless communication applications.

Keywords: Spintronics oscillator, Radio frequency magnetic field, Coplanar waveguide