In today’s fast-paced world, the significance of efficient radio-frequency (RF) signal processing cannot be overstated, as it’s the beating heart of modern communication and radar technology. Ensuring fast data transmission and pinpoint target detection is the crucial basis for smart infrastructure, IoT applications and autonomous driving. The global objective of M&MEMS is to create a novel spintronics-based platform with energy-efficient and tunable devices for microwave and RF technologies. With this platform, we aim to reduce energy consumption, achieve unprecedented agility and tunability, and reduce the size and ecological footprint of current RF and microwave technologies. This challenge is more relevant than ever, as demonstrated by the continuous roll-out of massive MIMO (multiple input multiple output) antennas and base stations for 5G and next generation communication standards.
To achieve this ambitious goal, M&MEMS combines several technologies that have been researched separately in the past. The centerpiece of M&MEMS' RF processing is magnonics, i.e. the use of spin waves (SWs) for data processing. The frequencies of the spin waves can be continuously tuned in a magnonic device using magnetic fields and thus cover a spectrum from a few 100 MHz up to 20 GHz and higher, depending on the material and magnetic field. The spin wave frequencies therefore have a perfect overlap with applications in the field of communication for 5G and 6G, satellite communication and GPS, radar and other RF-based detection methods. In M&MEMS, mainly frequencies in FR1 and FR3 are targeted, but also the low frequency part of FR2 can be principally reached if high enough bias magnetic fields or materials with high saturation magnetization are used. What makes them interesting for miniaturized applications is that the wavelength of spin waves is 4 to 5 orders of magnitude shorter than electromagnetic waves of the same frequency. This leads to µm-long wavelength of SWs in the GHz range, which enables a strong miniaturization of the RF components, as required for mobile systems. Possible applications include RF filters, phase shifters (PS), time-delay units (TDU), isolators and also non-linear systems such as signal limiters and auto-tune filters. In these systems, RF-to-SW transducers are used to excite coherent spin waves, which propagate inside a magnetic material and can be manipulated during their propagation by appropriate magnetic field landscapes.
Magnonic components have already been used to construct such RF systems in the past, but the tunability of the frequencies was usually achieved by a magnetic field generated by charge currents in coils (electromagnets). However, the charge currents in the electromagnets cause large Joule losses, which negate the original high energy efficiency of the magnonic device.
To reach the objective of increased energy efficiency, M&MEMS aims to develop demonstrators that use tunable magnetic fields without the use of charge currents. Initially, two approaches were employed to reach the objective of tunable and energy-efficient RF devices:
In the first approach, micro-structured permanent magnets are used, whose stray magnetic field represents the bias field for the magnonic device. These micromagnets are placed at µm distances from the magnonic components. The stray field acting on the magnonic component is changed by using MEMS (micro-electromechanical systems) to change the position of the permanent magnets relative to the magnonic element. A slightly modified approach is to use stationary permanent magnets and to use a mechanically movable flux concentrator made of a soft magnetic material to change the field at the magnonic component. These approaches are voltage-driven making permanent charge currents obsolete.
The second approach utilises the fact that spin waves can also be influenced by many types of so-called "effective magnetic fields", which originate from other energy terms that contribute to the total magnetic energy in addition to the Zeeman energy. An important contribution here is the magnetoelastic energy, which describes the interaction of elastic deformations of the solid with magnetism. The magnetoelastic energy can be used to generate magnetoelastic fields that depend on the mechanical strains. It is important for the energy efficiency of this approach that also mechanical stresses can be generated efficiently via the piezo-electric effect with the help of electric fields/voltages. This makes a constant charge current and the associated Joule losses obsolete. In the M&MEMS project, we use for instance acoustic waves (surface acoustic waves, SAW) to create dynamic strains in the magnetic material which translate into dynamic magneto-elastic fields.
Based upon these approaches, the final goal of M&MEMS is the fabrication of two demonstration devices which are exemplary for this novel hybrid platform. The first demonstrator is a tunable RF filter with high agility and the second is a multi-port phase shifter/time delay unit for directional RF antennas. These demonstrators are stand-alone devices with an advanced technological readiness level 5 (TRL 5) and they will serve as the cornerstones of a new technology platform for the application in 5G and beyond communication.
