Tuning RF Acoustic Waves via Magnetism

Abstract

Acoustically-driven ferromagnetic resonance (ADFMR) (1,2) is a powerful technique used to transmit RF surface acoustic waves (SAWs) and tune their transmission using a thin magnetic film and a static magnetic field HDC. We explore tunability of ADFMR devices for their potential in agile RF communications applications. Two examples are nonreciprocal ADFMR devices, and devices with curved interdigital transducers (IDTs), which focus SAWs with enhanced intensity in the magnetic film.Nonreciprocity of RF transmission has an important role in critical components like isolators and circulators. We demonstrate that a thin synthetic antiferromagnet using FeGaB (3) can selectively absorb waves travelling in one direction with 48.4 dB isolation- performance that can compete with state-of-the-art commercial devices despite much smaller package size and power draw.Another tuning knob which can have device impacts is nonlinearity. With curved interdigitated transducers (IDTs), we focus SAW energy to a small, intense region near the center of the magnet. We then study the dependence of the magnetoacoustic interaction on input power. Reaching moderate input powers of 10 dBm or less allows access to a nonlinear regime previously unavailable in these devices.Using a scanning acoustic-wave interferometer, we can observe the change in amplitude of the acoustic waves as they travel across the device surface. The power dependence is studied in depth, and an applied magnetic field modifies the amplitude wherever the magnet is present. Using these tools we gain new insight into how the magnetoelastic coupling works in ADFMR, which guides our understanding in the pursuit of new tunable RF communications device types.  Operation of a magnetoacoustic isolator. With external magnetic field applied in one direction, the device transmits RF unidirectionally with 48.4 dB isolation. Reversing the external field switches the direction of RF transmission.  A) Image of a focused IDT ADFMR device. Curved IDTs transmit and receive SAWs, which travel across a 20-nm-thick Ni film on a y-cut LiNbO3 substrate. B) ADFMR pattern showing output with 860-MHz 27-dBm input as a function of HDC angle and magnitude. C) SAW amplitude measured by interferometry.