Active Ferromagnetic Metasurface with Topologically Protected Spin Texture for Spectral Filters

Electromagnetic metasurfaces modulate a material's response to electromagnetic waves by specifically arranged elements with dimensions below the wavelength. They have opened new fields of research, including flat optics and nanophotonics on a chip. Ferromagnetic metasurfaces could become the building blocks for manipulation of both microwaves and spin waves (magnons). So far, the functionality of magnonic devices has been limited by high intrinsic damping of the materials employed, suppressing long‐distance spin‐wave propagation. Here ferromagnetic metasurfaces are reported, which are created from periodic arrays of either 15 nm thick Co20Fe60B20, Ni80Fe20 or Co nanodisks on ferrimagnetic yttrium iron garnet (YIG) hosting topologically protected vortex states. This device, a reconfigurable spectral filter, operates in the microwave regime near 0.9 GHz and manipulates long‐distance spin wave transmission in thin YIG. An efficiency of 98.5% is demonstrated, with the metasurface covering only 15% of the microwave antenna. This first dem-onstration of a ferromagnetic metasurface opens unprecedented possibilities for on-chip control of microwaves in low-damping ferrimagnetic insulators.

Magnonic crystals with reconfigurable magnetic defects for spin-based microwave electronics

Korbinian Baumgärtl

Collective spin excitations can propagate in magnetically ordered materials in the form of waves. These so-called spin waves (SWs) or magnons are promising for low-power beyond-CMOS information processing, which does not rely anymore on the lossy movement of electric charges. SWs in the few GHz frequency regime possess nanoscale wavelengths about five orders of magnitude smaller than electromagnetic waves of the same frequency. This property makes SWs ideally suited for application in microwave technology, essential for on-chip processing of wireless telecommunication signals. In this thesis, three crucial challenges relevant for the technological application of SWs are addressed: First, to functionalize SWs and exploit their small wavelengths, it is necessary to control them at the nanoscale. Here, periodically nanostructured materials, denoted magnonic crystals, are promising, as they allow to tailor the band structure of SWs. We report on SWs propagating in a prototypical one-dimensional magnonic crystal consisting of dipolarly coupled magnetic nanostripes. The remanent magnetization of individual stripes was designed to be bistable along the long axis. By magnetizing an individual stripe in opposite direction to the others, we created a magnetic defect. We measured by means of all-electrical spin wave spectroscopy and Brillouin light scattering microscopy phase and amplitude of SWs trespassing the defect. We found that SW phases and amplitudes were modified at the nanoscale, and phase shifts could be tuned by an applied bias magnetic field. Using micromagnetic simulations, we identified specific bias fields for which phase shifts of Pi are achieved without suppressing SW amplitudes. This result is highly relevant for the implementation of logic gates based on interference of phase-controlled SWs. We further measured propagation of short-waved SWs in an antiferromagnetically ordered one-dimensional magnonic crystal, where every second stripe was magnetized in opposite direction. We found a band gap closing at the Brillouin zone boundary when no magnetic bias field was applied. Our observations are promising for reprogrammable microwave filters capable of adjusting stop- and passband. Second, we address how long-waved electromagnetic waves can be coupled efficiently to nanoscale SWs. We demonstrate by space- and time-resolved scanning X-ray transmission measurements, that excited nanogratings allow to transfer their reciprocal lattice vector and multiple of it to an underlying magnetic thin film, in which nanoscale propagating SWs are launched. Additionally, we discovered a second method for short-waved SW generation based on magnetic microwave guides. This approach is easy to fabricate and relies on the adaption of the SW wavelength to a changing effective magnetic field. Efficient coupling of electromagnetic waves to nanoscale SWs promises an unprecedented miniaturization of microwave components. Third, we found that the magnetization direction of bistable nanomagnets can be switched by propagating SWs in an underlying magnetic thin film when a threshold amplitude is reached. This discovery is promising for the realization of a non-volatile magnonic memory, which stores SW amplitudes. A possible application are SW logic gates, which encode the outcome of a logic operation in the output SW amplitude. Magnonic memory would allow for storing these amplitudes directly, without requiring lossy conversion into the electrical domain.

EPFL2021

Magnons, worms and nanogratings in artificial magnetic quasicrystals

Sho Watanabe

Magnons (spin waves, SWs) are elementary spin excitations in magnetically ordered materials. They are the promising quanta for the transmission and processing of information. Magnons can be coupled to the electromagnetic waves utilized for the wireless communication technologies. A wavelength of magnons is orders of magnitude shorter than that of electromagnetic waves at the same frequency. Therefore, wave-based computing with magnons would be promising for the next generation information technology. For the optimized information processing, artificial magnetic media have the potential to offer advanced controls of SWs. A magnonic crystal, consisting of a periodically arranged nanomagnets, provides a tailored magnon band structure. Periodic magnonic grating couplers (MGCs) efficiently convert centimeter-scaled microwaves to sub-100 nm wavelength SWs. Artificial spin ices (ASIs) are expected to offer reprogrammable magnetization configurations. However, the studies on artificial magnetic materials based on the quasicrystalline and aperiodic arrangement are in their infancy. A quasicrystal, a long range ordered material with a lack of translational invariance, exhibits manifold rotational symmetry. The reciprocal vectors associated with the quasicrystal densely fill out all reciprocal space. These exotic properties would be advantageous for the control of SWs. During my PhD study, I explored SW properties in two dimensional (2D) artificial magnetic quasicrystals (AMQs) to understand the effect of aperiodicity on magnetic properties by means of broadband SW spectroscopy, inelastic light scattering spectroscopy, X-ray magnetic circular dichroism and micromagnetic simulation. By nanofabrication, AMQs made of different magnetic materials were created based on Penrose P2, P3 and Ammann tilings. First, we fabricated AMQs based on aperiodically arranged nanoholes etched into ferromagnetic thin films. Angular-dependent SW spectra exhibited tenfold rotational symmetry reflecting the lattice symmetry of the quasicrystalline nanohole arrangement. Intriguingly, worm-like nanochannels, each exhibiting different SW states, were generated in the AMQs. Our findings imply that the nanohole-based AMQ would become a new class of dense-wavelength division multiplexer. AMQs based on low damping Yttrium iron garnet (YIG) allowed for omnidirectional SW emission thanks to the unconventional rotational symmetry of the quasicrystal. Absorption spectra exhibited forbidden frequency gap openings and a corresponding modification of the magnon density of states, indicating the formation of a magnonic band structure. MGCs, prepared from aperiodically arranged ferromagnetic nanopillars on YIG thin films, allowed also for omnidirectional SW emission with a broad range of wave vectors. The constructive/destructive interference of SWs excited by two emitters allowed for a binary 1/0 output operation. ASIs composed of interconnected ferromagnetic nanobars were found to show non-stochastic switching relevant for reconfigurable functionalities. An ASI integrated on a YIG film showed MGC modes and SW channeling with the presence of an external field. The SW spectra at remnant state exhibited reprogrammable characteristics depending on the magnetic field history. Our study on AMQs is important for the fundamental understanding of quasicrystals as well as fabrication of future magnonic devices targeting at information processing by wave-based computing system on the nanoscale.

EPFL2021

Controlled Sub-Micrometer Hierarchical Textures Engineered in Polymeric Fibers and Microchannels via Thermal Drawing

Alba Carla De Luca, Stéphanie Lacour, Alexis Gérald Page, Yunpeng Qu, Fabien Sorin, Marco Volpi, Wei Yan

The controlled texturing of surfaces at the micro- and nanoscales is a powerful method for tailoring how materials interact with liquids, electromagnetic waves, or biological tissues. The increasing scientific and technological interest in advanced fibers and fabrics has triggered a strong motivation for leveraging the use of textures on fiber surfaces. Thus far however, fiber-processing techniques have exhibited an inherent limitation due to the smoothing out of surface textures by polymer reflow, restricting achievable feature sizes. In this article, a theoretical framework is established from which a strategy is developed to reduce the surface tension of the textured polymer, thus drastically slowing down thermal reflow. With this approach the fabrication of potentially kilometers-long polymer fibers with controlled hierarchical surface textures of unprecedented complexity and with feature sizes down to a few hundreds of nanometers is demonstrated, two orders of magnitude below current configurations. Using such fibers as molds, 3D microchannels are also fabricated with textured inner surfaces within soft polymers such as poly(dimethylsiloxane), at dimensions and a degree of simplicity impossible to reach with current techniques. This strategy for the texturing of high curvature surfaces opens novel opportunities in bioengineering, regenerative scaffolds, microfluidics, and smart textiles.

Wiley-Blackwell2017

Magnons, worms and nanogratings in artificial magnetic quasicrystals

Sho Watanabe

EPFL2021

Magnonic crystals with reconfigurable magnetic defects for spin-based microwave electronics

Korbinian Baumgärtl

EPFL2021