Yongsheng Gui

Towards Low-Power High-Efficiency RF and Microwave Energy Harvesting

Since the very beginning of RF and microwave integrated techniques and energy harvesting, Schottky diodes are most often used in mixing and rectifying circuits. However, in specific μW power-harvesting applications, the Schottky diode technique fails to provide a satisfactory RF-dc conversion efficiency mainly because of its high zero-bias junction resistance. This paper examines the state-of-the-art low-power microwave-to-dc energy conversion techniques. A comprehensive picture of the state-of-the-art on this aspect is given graphically, which compares different technologies such as transistor, diode, and CMOS schemes. Subsequent to the highlighted limitations of current devices, this work introduces, for the first time, a nonlinear component for low-power rectification based on a recent discovery in spintronics, namely, the spindiode. Along with an analysis of the role of nonlinearity and zero bias resistance in the rectification process of the spindiode, it is shown how the spindiode could enhance the rectification efficiency even at a very low-power level and how this technique would shift the design paradigms of diode-based devices and circuits.

Universal method for separating spin pumping from spin rectification voltage of ferromagnetic resonance.

We develop a method for universally resolving the important issue of separating spin pumping from spin rectification signals in bilayer spintronics devices. This method is based on the characteristic distinction of spin pumping and spin rectification, as revealed in their different angular and field symmetries. It applies generally for analyzing charge voltages in bilayers induced by the ferromagnetic resonance (FMR), independent of FMR line shape. Hence, it solves the outstanding problem that device-specific microwave properties restrict the universal quantification of the spin Hall angle in bilayer devices via FMR experiments. Furthermore, it paves the way for directly measuring the nonlinear evolution of spin current generated by spin pumping. The spin Hall angle in a Py/Pt bilayer is thereby directly measured as 0.021±0.015 up to a large precession cone angle of about 20°.

Electrical detection of magnetization dynamics via spin rectification effects

The purpose of this article is to review the current status of a frontier in dynamic spintronics and contemporary magnetism, in which much progress has been made in the past decade, based on the creation of a variety of micro- and nano-structured devices that enable electrical detection of magnetization dynamics. The primary focus is on the physics of spin rectification effects, which are well suited for studying magnetization dynamics and spin transport in a variety of magnetic materials and spintronic devices. Intended to be intelligible to a broad audience, the paper begins with a pedagogical introduction, comparing the methods of electrical detection of charge and spin dynamics in semiconductors and magnetic materials respectively. After that it provides a comprehensive account of the theoretical study of both the angular dependence and line shape of electrically detected ferromagnetic resonance (FMR), which is summarized in a handbook formate easy to be used for analyzing experimental data. We then review and examine the similarity and differences of various spin rectification effects found in ferromagnetic films, magnetic bilayers and magnetic tunnel junctions, including a discussion of how to properly distinguish spin rectification from the spin pumping/inverse spin Hall effect generated voltage. After this we review the broad applications of rectification effects for studying spin waves, nonlinear dynamics, domain wall dynamics, spin current, and microwave imaging. We also discuss spin rectification in ferromagnetic semiconductors. The paper concludes with both historical and future perspectives, by summarizing and comparing three generations of FMR spectroscopy which have been developed for studying magnetization dynamics.

Spin rectification enabled by anomalous Hall effect

We report the observation of a transverse dc voltage which appears when a radio frequency (rf) current flows along the longitudinal direction of a ferromagnetic Hall device. This effect is fully explained through the spin rectification enabled by the anomalous Hall effect, which is nonlinear coupling between the dynamic magnetization and the rf current. The observed resonant feature and angular dependent line shape are related to the magnetization precession driven by a rf magnetic field. This suggests a method for detection of spin dynamic and rf magnetic field vector.

Electric detection of the thickness dependent damping in Co90Zr10 thin films

In this letter, we propose a dc electrical detection method for investigating the spin dynamics of ferromagnetic thin films. Based on anomalous Hall effect (AHE), the out-of-plane component of the dynamic magnetization can directly rectify the rf current into a time-independent Hall voltage at the ferromagnetic resonance. This method is applied for studying the damping mechanism in Co90Zr10 films. The thickness dependent zero-frequency linewidth and the effective Gilbert damping are related to the surface roughness and microstructure evolution. Compared with standard cavity ferromagnetic resonance, the AHE rectification is more suitable for studying the dynamic properties of local magnetic moment.

Microwave reflection imaging using a magnetic tunnel junction based spintronic microwave sensor

A far-field microwave imaging technique has been developed using a spintronic sensor based on a magnetic tunnel junction (MTJ). Such a sensor can directly rectify a microwave field into a dc voltage signal using the Seebeck effect. Thanks to the high conversion efficiency of the microwave rectification in MTJs, the microwave power sensitivity of the spintronic sensor is on the order of 1–10 mV/mW. This high sensitivity allows the sensor to directly measure the coherent spatial scattered microwave field distribution, which gives it the ability to non-destructively detect hidden objects down to a few wavelengths in size.

Spintronics-based devices for Microwave Power Harvesting

Since nearly the beginning, the Schottky diode rules in the development of RF/microwave mixing and rectifying circuits. However, in the specific μW power harvesting applications, the diodes fail to provide satisfying RF-to-DC conversion efficiency mainly due to their high zero-bias junction resistance. This work introduces for the first time a nonlinear component for power rectification based on a recent discovery in spintronics: the spindiode. Along with an analysis of the role of the nonlinearity and the zero bias resistance in the rectification process, it will be shown how the spindiode can provide 10 times more power than a Schottky diode.

On-chip artificial magnon-polariton device for voltage control of electromagnetically induced transparency

We demonstrate an on-chip device utilizing the concept of artificial cavity magnon-polariton (CMP) coupling between the microwave cavity mode and the dynamics of the artificial magnetism in a split ring resonator. This on-chip device allows the easy tuning of the artificial CMP gap by using a DC voltage signal, which enables tuneable electrodynamically induced transparency. The high tunability of the artificial magnon-polariton system not only enables the study of the characteristic phenomena associated with distinct coupling regimes, but also may open up avenues for designing novel microwave devices and ultra-sensitive sensors.

Angular dependence of ferromagnetic resonance measurements in exchange coupled Ni80Fe20/NiO bilayers

We investigate the angular dependence of the ferromagnetic resonance (FMR) frequency in a Ni80Fe20/NiO bilayer for varied external magnetic fields by means of broadband microwave absorption spectroscopy. The dispersion curves of the FMR frequencies exhibit an angular-dependent low-field behaviour not predicted by current ferromagnetic/antiferromagnetic interface models. Our experimental data can be modelled using a single domain approach assuming that NiO introduces anisotropies at the interface.

Electromagnetic Field Enhancement and Its Application in Spin Rectification

A subwavelength antenna, which has the capability to enhance both the microwave electric and magnetic fields, is proposed for use in spintronic devices. The geometric resonance of the microwave electric and magnetic fields in the antenna are determined by spintronic techniques, and are in remarkable agreement with measurements taken using a vector network analyzer and simulations based on the finite-difference time-domain method. Our simulations predict that the performance of a spin dynamo may be improved by three orders of magnitude if properly integrated with this antenna on-chip.