Nanoscale Nonreciprocal Spin-Wave-Based Devices for Microwave Signal Processing

Recently discovered physics phenomena in magnetic heterostructures such as giant interfacial Dzyaloshnskii-Moriya interaction, spin-flexoelectric effect and voltage-controlled magnetic anisotropy create opportunities for the development of new types of energy-efficient ultra-compact nonreciprocal devices for microwave signal processing. Such devices are based on control of spin waves and magneto-elastic waves in magnetic nano- and microstructures by electric field and anisotropic exchange interaction. In this talk, recent advances in the development and characterization of such devices will be presented. It will be shown that surface acoustic waves (SAWs) propagating in a piezoelectric substrate (LiNbO3) covered with a thin ferromagnetic/heavy metal bilayer (Ni/Pt) exhibit nonreciprocity, i.e., the frequencies of these waves are nondegenerate with respect to the inversion of the SAW propagation direction. The simultaneous action of the magneto-elastic interaction in the ferromagnetic layer and the interfacial Dzyaloshinskii-Moriya interaction at the ferromagnet/heavy metal interface results in the openings of magneto-elastic band gaps in the SAW spectrum, and the frequency position of these band gaps is different for opposite SAW propagation directions. The band-gap widths and the frequency separation between them can be controlled by a proper selection of the magnetization angle and the thickness of the ferromagnetic layer. The degree of isolation between SAWs propagating in opposite directions in such a system can exceed the direct SAW propagation losses by more than one order of magnitude. Parametric excitation by voltage-controlled magnetic anisotropy (VCMA) is an energy-efficient method of generating short wavelength, high group velocity spin waves (SWs) for use as information carriers in novel nonvolatile magnetic logic devices. Parametric excitation of SWs in nanoscale magnetic devices by microwave voltages as low as 0.15V is demonstrated. A spin wave field effect transistor (SW-FET), which can turn on and off microwave-frequency SW propagation in a ferromagnetic nanowire with a gate voltage not exceeding 2V via VCMA effect will be presented. Operation of a nanoscale SW-FET based on a Ta/CoFeB/MgO multilayer nanowire is demonstrated. The device consists of a ferromagnetic nanowire channel magnetized out of the sample plane for SW propagation and a voltage gate on top of the wire. Propagating SWs are excited on one side of the gate via application of a localized microwave spin Hall torque at the SW resonance frequency. SW propagation to the other side of the gate is detected via anomalous Hall effect (AHE). Utilizing VCMA, a DC voltage applied to the gate adjusts perpendicular magnetic anisotropy under the gate and transforms the spatially uniform magnetic energy landscape into a potential well or barrier for SW propagation depending on the gate voltage polarity. VCMA-induced magnetic barrier can efficiently suppress SW propagation under the top gate. In addition, electric field applied to the gate can induce nonreciprocity of the SW propagation via spin-flexoelectric effect. Operation of ultra-compact nonreciprocal microwave devices such as circulators and directional couplers based on nonreciprocal SAW and SW will be discussed. This research was supported by the NSF EFRI NewLAW program under award EFMA-1641989.