The 2024 Magnonics Roadmap.

Magnonicsis a research field that has gained an increasing interest in both the fundamental and applied sciences in recent years. This field aims to explore and functionalize collective spin excitations in magnetically ordered materials for modern information technologies, sensing applications, and advanced computational schemes. Spin waves, also known as magnons, carry spin angular momenta that allow for the transmission, storage, and processing of information without moving charges. In integrated circuits, magnons enable on-chip data processing at ultrahigh frequencies without the Joule heating, which currently limits clock frequencies in conventional data processors to a few GHz. Recent developments in the field indicate that functional magnonic building blocks for in-memory computation, neural networks, and Ising machines are within reach. At the same time, the miniaturization of magnonic circuits advances continuously as the synergy of materials science, electrical engineering, and nanotechnology allows for novel on-chip excitation and detection schemes. Such circuits can already enable magnon wavelengths of 50 nm at microwave frequencies in a 5G frequency band. Research into non-charge-based technologies is urgently needed in view of the rapid growth of machine learning and artificial intelligence applications, which consume substantial energy when implemented on conventional data processing units. In its first part, the 2024 Magnonics Roadmap provides an update on the recent developments and achievements in the field of nano-magnonics while defining its future avenues and challenges. In its second part, the Roadmap addresses the rapidly growing research endeavors on hybrid structures and magnonics-enabled quantum engineering. We anticipate that these directions will continue to attract researchers to the field and, in addition to showcasing intriguing science, will enable unprecedented functionalities that enhance the efficiency of alternative information technologies and computational schemes.

Manipulating ultrafast magnetization dynamics of ferromagnets using the odd-even layer dependence of two-dimensional transition metal di-chalcogenides.

Two-dimensional transition metal dichalcogenides (TMDs) have drawn immense interest due to their strong spin-orbit coupling and unique layer number dependence in response to spin-valley coupling. This leads to the possibility of controlling the spin degree of freedom of the ferromagnet (FM) in thin film heterostructures and may prove to be of interest for next-generation spin-based devices. Here, we experimentally demonstrate the odd-even layer dependence of WS2 nanolayers by measurements of the ultrafast magnetization dynamics in WS2/Co3FeB thin film heterostructures by using time-resolved Kerr magnetometry. The fluence (photon energy per unit area) dependent magnetic damping (α) reveals the existence of broken symmetry and the dominance of inter- and intraband scattering for odd and even layers of WS2, respectively. The higher demagnetization time, τm, in 3 and 5 layers of WS2 is indicative of the interaction between spin-orbit and spin-valley coupling due to the broken symmetry. The lower τm in even layers as compared to the bare FM layer suggests the presence of a spin transport. By correlating τm and α, we pinpointed the dominant mechanisms of ultrafast demagnetization. The mechanism changes from spin transport to spin-flip scattering for even layers of WS2 with increasing fluence. A fundamental understanding of the two-dimensional material and its odd-even layer dependence at ultrashort timescales provides valuable information for designing next-generation spin-based devices.

Investigation of ultrafast demagnetization and Gilbert damping and their correlation in different ferromagnetic thin films grown under identical conditions.

Following the demonstration of laser-induced ultrafast demagnetization in ferromagnetic nickel, several theoretical and phenomenological propositions have sought to uncover its underlying physics. In this work we revisit the three temperature model (3TM) and the microscopic three temperature model (M3TM) to perform a comparative analysis of ultrafast demagnetization in 20 nm thick cobalt, nickel and permalloy thin films measured using an all-optical pump-probe technique. In addition to the ultrafast dynamics at the femtosecond timescales, the nanosecond magnetization precession and damping are recorded at various pump excitation fluences revealing a fluence-dependent enhancement in both the demagnetization times and the damping factors. We confirm that the Curie temperature to magnetic moment ratio of a given system acts as a figure of merit for the demagnetization time, while the demagnetization times and damping factors show an apparent sensitivity to the density of states at the Fermi level for a given system. Further, from numerical simulations of the ultrafast demagnetization based on both the 3TM and the M3TM, we extract the reservoir coupling parameters that best reproduce the experimental data and estimate the value of the spin flip scattering probability for each system. We discuss how the fluence-dependence of inter-reservoir coupling parameters so extracted may reflect a role played by nonthermal electrons in the magnetization dynamics at low laser fluences.

Bias field orientation driven reconfigurable magnonics and magnon-magnon coupling in triangular shaped Ni80Fe20nanodot arrays.

Reconfigurable magnonics have attracted intense interest due to their myriad advantages including energy efficiency, easy tunability and miniaturization of on-chip data communication and processing devices. Here, we demonstrate efficient reconfigurability of spin-wave (SW) dynamics as well as SW avoided crossing by varying bias magnetic field orientation in triangular shaped Ni80Fe20nanodot arrays. In particular, for a range of in-plane angles of bias field, we achieve mutual coherence between two lower frequency modes leading to a drastic modification in the ferromagnetic resonance frequency. Significant modification in magnetic stray field distribution is observed at the avoided crossing regime due to anisotropic dipolar interaction between two neighbouring dots. Furthermore, using micromagnetic simulations we demonstrate that the hybrid SW modes propagate longer through an array as opposed to the non-interacting modes present in this system, indicating the possibility of coherent energy transfer of hybrid magnon modes. This result paves the way for the development of integrated on-chip magnonic devices operating in the gigahertz frequency regime.

Ferromagnetic Ni1-xVxO1-y Nano-Clusters for NO Detection at Room Temperature: A Case of Magnetic Field-Induced Chemiresistive Sensing.

Surface modulation of functional nanostructures is an efficient way of improving gas sensing properties in chemiresistive materials. However, synthesis methods employed so far in achieving desired performances are cumbersome and energy intensive. Moreover, nano-engineering-induced magnetic properties of these materials which are expected to enhance sensing responses have not been utilized until now in improving their interaction with target gases. In particular for gasses with paramagnetic nature such as NO or NO2, the inherent magnetic property of the chemiresistor might assist in enabling superior sensing performance. In this work, vanadium-doped NiO nano-clusters with ferromagnetic behavior at room temperature have been synthesized by a simple and effective combination of soft chemical routes and employed in efficient and selective detection of paramagnetic NO gas. While NiO is typically anti-ferromagnetic, the nanoscale engineering of NiO- and V-doped NiO samples have been found to tune the inherent anti-ferromagnetic behavior into room-temperature ferromagnetism. Surface modification in terms of formation of nano-clusters led to an increased Brunauer-Emmett-Teller surface area of ∼120 m2/g. The sample Ni0.636V0.364O has been observed to exhibit a selective and high response of ∼98% to 1 ppm NO at room temperature with fast response (14 s) and recovery (95 s). The improved sensing response of this sample compared to other doped NiO variants could be explained in terms of lower remnant magnetic moment of the sample accompanied with higher excess negative charge at the surface. The sensing response of this sample was increased by 30% in the presence of an external magnetic field of 280 gauss, highlighting the importance of magnetic ordering in chemiresistive gas sensing between the magnetic sensor material and target analyte. This material stands as a potential gas sensor with excellent NO detection properties.

Anomalous Infrared Absorbance of S═O: A Perturbation Study of α-C-H/D.

Solvatochromic shifts of S═O vibrational probes describe the strength of the surrounding electric fields and the hydrogen bonding status. Herein, we demonstrated how the solvents alter the infrared (IR) spectra of the S═O vibrating mode. The experimental measurement of the involvement of α-H/D isotopic interactions with different solvents and their effects on the IR absorbance spectra of the vibrational probe provides detailed knowledge of the microsolvation environment despite the complexity of overlapping bands in the spectra. Herein, we discover how the solvents interact differently with DMSO and DMSO-d6, while being electronically and structurally the same. Interestingly, the IR spectrum of the S═O mode remains unaltered during α-isotopic replacement in the presence of aprotic solvents (acetone, acetonitrile, and dichloromethane), but in strongly coordinating polar solvents (D2O), it is altered remarkably. There is a lack of quantitative information about the influence of the α-H atom or α-isotopic substitution on the vibrational probe in the literature. Our experiments provide a detailed molecular understanding of the structure of DMSO in DMSO-solvent binary mixtures. As DMSO plays an important role in virtually all subdisciplines of chemistry and biology, we believe that our work will be of interest to a large diversity of studies in these fields.

On-Off Infrared Absorption of the S═O Vibrational Probe of Dimethyl Sulfoxide.

Dimethyl sulfoxide (DMSO), a polar solvent molecule, is used in a wide range of therapeutic and pharmacological applications. Different intermolecular interactions, such as dimerization and hydrogen bonding with water, are crucial to understanding the role of DMSO in applications. Herein, we study DMSO in various solvation environments to decipher the environment-dependent dimerization and hydrogen-bonding propensity. We use a combination of infrared spectroscopy, quantum mechanical calculations, and molecular dynamics simulations to reach our conclusions. Although DMSO can exist in a dynamic equilibrium between monomers and dimers, our results show that the relative intensity of the S═O stretch and the CH3 rocking modes is a spectroscopic indicator of the extent of DMSO dimerization in solution. The dimerization (self-association) is seen to be maximum in neat DMSO. When dissolved in different solvents, the dimerization propensity decreases with increasing solvent polarity. In the presence of a protic solvent, such as water, DMSO forms a hydrogen bond with the solvent molecules, thereby reducing the extent of dimerization. Further, we estimate the hydrogen-bond occupancy of DMSO. Our results show that DMSO predominantly exists as doubly hydrogen-bonded in water.

Magnetic Configuration Driven Femtosecond Spin Dynamics in Synthetic Antiferromagnets.

Ultrafast demagnetization in diverse materials has sparked immense research activities due to its captivating richness and contested underlying mechanisms. Among these, the two most celebrated mechanisms have been the spin-flip scattering (SFS) and spin transport (ST) of optically excited carriers. In this work, we have investigated femtosecond laser-induced ultrafast demagnetization in perpendicular magnetic anisotropy-based synthetic antiferromagnets (p-SAFs) where [Co/Pt]n-1/Co multilayer blocks are separated by Ru or Ir spacers. Our investigation conclusively shows that the ST of optically excited carriers can have a significant contribution to the ultrafast demagnetization in addition to SFS processes. Moreover, we have also achieved an active control over the individual mechanisms by specially designing the SAF samples and altering the external magnetic field and excitation fluence. Our study provides a vital understanding of the underlying mechanism of ultrafast demagnetization in synthetic antiferromagnets, which will be crucial in future research and applications of antiferromagnetic spintronics.

Strain and crystallite size controlled ordering of Heusler nanoparticles having high heating rate for magneto-thermal application.

Heusler compound nanoparticles with good structural ordering need to be investigated as a potential material class for magneto-thermal applications requiring heat generation in presence of an oscillating magnetic field. Here, we report an important finding of a structural parameter related to the product of the strain and the coherent crystallite size, that can be used to efficiently control the structural ordering and the magnetic property of the Heulser compound nanoparticles. The optimization of this product parameter is found to enhance both the structural ordering and magnetic transition temperature in Co2FeSn Heusler nanoparticles. Furthermore, using magnetic hyperthermia measurements we demonstrate the possibility of heat generation using Heusler compound nanoparticles comparable to that of conventional magnetic nanoparticles. This shall lead to the development of Heulser compounds for similar applications.

Spin Wave Electromagnetic Nano-Antenna Enabled by Tripartite Phonon-Magnon-Photon Coupling.

Tripartite coupling between phonons, magnons, and photons in a periodic array of elliptical magnetostrictive nanomagnets delineated on a piezoelectric substrate to form a 2D two-phase multiferroic crystal is investigated. Surface acoustic waves (SAW) (phonons) of 5-35 GHz frequency launched into the substrate cause the magnetizations of the nanomagnets to precess at the frequency of the wave, giving rise to confined spin-wave modes (magnons) within the nanomagnets. The spin waves, in turn, radiate electromagnetic waves (photons) into the surrounding space at the SAW frequency. Here, the phonons couple into magnons, which then couple into photons. This tripartite phonon-magnon-photon coupling is thus exploited to implement an extreme sub-wavelength electromagnetic antenna whose measured radiation efficiency and antenna gain exceed the approximate theoretical limits for traditional antennas of the same dimensions by more than two orders of magnitude at some frequencies. Micro-magnetic simulations are in excellent agreement with experimental observations and provide insight into the spin-wave modes that couple into radiating electromagnetic modes to implement the antenna.

Temperature-Dependent Dielectric Relaxation in Ionic Acetamide Deep Eutectics: Partial Viscosity Decoupling and Explanations from the Simulated Single-Particle Reorientation Dynamics and Hydrogen-Bond Fluctuations.

We report here temperature-dependent (293 ≤ T (K) ≤ 336) dielectric relaxation (DR) measurements of (acetamide + LiBr/NO3-/ClO4-) deep eutectic solvents (DESs) in the frequency window of 0.2 ≤ ν (GHz) ≤ 50 and explore, via molecular dynamics simulations, the relative roles for the collective single-particle reorientational relaxations and the H-bond dynamics of acetamide in the measured DR response. In addition, DR measurements of neat molten acetamide were performed. Recorded DR spectra of these DESs require multi-Debye fits and produce well-separated DR time scales that are spread over several picoseconds to ∼1 ns. Simulations suggest DR time scales derive contributions from both the collective reorientational (Cl(t)) relaxation and structural H-bond (CHB(t)) dynamics of acetamide. A good correlation between the measured and simulated activation energies further reveals a strong connection between the measured DR and the simulated Cl(t) and CHB(t). Average DR times exhibit a strong fractional viscosity dependence, suggesting substantial microheterogeneity in these media. Simulations of Cl(t) and CHB(t) reveal strong stretched exponential relaxations with a stretching exponent, 0.4 ≤ ß ≤ 0.7. The ratio between the average reorientational correlation times of first and second ranks, ⟨τ⟩l=1/⟨τ⟩l=2, deviates appreciably from Debye's l(l+1) law for homogeneous media. Importantly, a pronounced translation-rotation decoupling between the simulated reorientation and center-of-mass diffusion times was observed.

Applications of nanomagnets as dynamical systems: I.

When magnets are fashioned into nanoscale elements, they exhibit a wide variety of phenomena replete with rich physics and the lure of tantalizing applications. In this topical review, we discuss some of these phenomena, especially those that have come to light recently, and highlight their potential applications. We emphasize what drives a phenomenon, what undergirds the dynamics of the system that exhibits the phenomenon, how the dynamics can be manipulated, and what specific features can be harnessed for technological advances. For the sake of balance, we point out both advantages and shortcomings of nanomagnet based devices and systems predicated on the phenomena we discuss. Where possible, we chart out paths for future investigations that can shed new light on an intriguing phenomenon and/or facilitate both traditional and non-traditional applications.

Applications of nanomagnets as dynamical systems: II.

In Part I of this topical review, we discussed dynamical phenomena in nanomagnets, focusing primarily on magnetization reversal with an eye to digital applications. In this part, we address mostly wave-like phenomena in nanomagnets, with emphasis on spin waves in myriad nanomagnetic systems and methods of controlling magnetization dynamics in nanomagnet arrays which may have analog applications. We conclude with a discussion of some interesting spintronic phenomena that undergird the rich physics exhibited by nanomagnet assemblies.

Femtosecond laser-induced spin dynamics in single-layer graphene/CoFeB thin films.

Graphene/ferromagnet hybrid heterostructures are important building blocks of spintronics due to the unique ability of graphene to transport spin current over unprecedented distances and possible increase in its spin-orbit coupling due to proximity and hybridization. Here, we present magnetization dynamics over a femtosecond to nanosecond timescale by employing an all-optical time-resolved magneto-optical Kerr effect technique in single-layer graphene (SLG)/CoFeB thin films with varying CoFeB thickness and compared them with reference CoFeB thin films without an SLG underlayer. Gilbert damping variation with CoFeB thickness is modelled to extract spin-mixing conductance for the SLG/CoFeB interface and isolate the two-magnon scattering contribution from spin pumping. In SLG/CoFeB, we have established an inverse relationship between ultrafast demagnetization time (τm) and the Gilbert damping parameter (α) induced by interfacial spin accumulation and pure spin-current transport via a spin pumping mechanism. This systematic study of ultrafast demagnetization in SLG/CoFeB heterostructures and its connection with magnetic damping can help to design graphene-based ultrahigh-speed spintronic devices.

Comparison of Spin-Wave Modes in Connected and Disconnected Artificial Spin Ice Nanostructures Using Brillouin Light Scattering Spectroscopy.

Artificial spin ice systems have seen burgeoning interest due to their intriguing physics and potential applications in reprogrammable memory, logic, and magnonics. Integration of artificial spin ice with functional magnonics is a relatively recent research direction, with a host of promising results. As the field progresses, direct in-depth comparisons of distinct artificial spin systems are crucial to advancing the field. While studies have investigated the effects of different lattice geometries, little comparison exists between systems comprising continuously connected nanostructures, where spin-waves propagate via dipole-exchange interaction, and systems with nanobars disconnected at vertices, where spin-wave propagation occurs via stray dipolar field. Gaining understanding of how these very different coupling methods affect both spin-wave dynamics and magnetic reversal is key for the field to progress and provides crucial system-design information including for future systems containing combinations of connected and disconnected elements. Here, we study the magnonic response of two kagome spin ices via Brillouin light scattering, a continuously connected system and a disconnected system with vertex gaps. We observe distinct high-frequency dynamics and magnetization reversal regimes between the systems, with key distinctions in spin-wave localization and mode quantization, microstate trajectory during reversal and internal field profiles. These observations are pertinent for the fundamental understanding of artificial spin systems and broader design and engineering of reconfigurable functional magnonic crystals.

Observation of magnon-magnon coupling with high cooperativity in Ni80Fe20cross-shaped nanoring array.

We report here an experimental observation of dynamic dipolar coupling induced magnon-magnon coupling and spin wave (SW) mode splitting in Ni80Fe20cross-shaped nanoring array. Remarkably, we observe an anticrossing feature with minimum frequency gap of 0.96 GHz and the corresponding high cooperativity value of 2.25. Interestingly, splitting of the highest frequency SW mode occurs due to the anisotropic dipolar interactions between the cross nanorings. Furthermore, using micromagnetic simulations we demonstrate that the coupled SW modes propagate longer as opposed to other modes present in this system. Our work paves the way towards integrated hybrid systems-based quantum magnonics and on-chip coherent information transfer.

Resonant amplification of intrinsic magnon modes and generation of new extrinsic modes in a two-dimensional array of interacting multiferroic nanomagnets by surface acoustic waves.

Using time-resolved magneto-optical Kerr effect (TR-MOKE) microscopy, we demonstrate surface-acoustic-wave (SAW) induced resonant amplification of intrinsic spin-wave (SW) modes, as well as generation of new extrinsic or driven modes at the SAW frequency, in a densely packed two-dimensional array of elliptical Co nanomagnets fabricated on a piezoelectric LiNbO3 substrate. This system can efficiently serve as a magnonic crystal (MC), where the intrinsic shape anisotropy and the strong inter-element magnetostatic interaction trigger the incoherent precession of the nanomagnets' magnetization in the absence of any bias magnetic field, giving rise to the 'intrinsic' SW modes. The magnetoelastic coupling leads to a rich variety of SW phenomena when the SAW is launched along the major axis of the nanomagnets, such as 4-7 times amplification of intrinsic modes (at 3, 4, 7 and 10 GHz) when the applied SAW frequencies are resonant with these frequencies, and the generation of new extrinsic modes at non-resonant SAW frequencies. However, when the SAW is launched along the minor axis, a dominant driven mode appears at the applied SAW frequency. This reveals that the magnetoelastic coupling between SW and SAW is anisotropic in nature. Micromagnetic simulation results are in qualitative agreement with the experimental observations and elucidate the underlying dynamics. Our findings lay the groundwork for bias-field free magnonics, where the SW behavior is efficiently tuned by SAWs. It has important applications in the design of energy efficient on-chip microwave devices, SW logic, and extreme sub-wavelength ultra-miniaturized microwave antennas for embedded applications.

Observation of Coherent Spin Waves in a Three-Dimensional Artificial Spin Ice Structure.

Harnessing high-frequency spin dynamics in three-dimensional (3D) nanostructures may lead to paradigm-shifting, next-generation devices including high density spintronics and neuromorphic systems. Despite remarkable progress in fabrication, the measurement and interpretation of spin dynamics in complex 3D structures remain exceptionally challenging. Here, we take a first step and measure coherent spin waves within a 3D artificial spin ice (ASI) structure using Brillouin light scattering. The 3D-ASI was fabricated by using a combination of two-photon lithography and thermal evaporation. Two spin-wave modes were observed in the experiment whose frequencies showed nearly monotonic variation with the applied field strength. Numerical simulations qualitatively reproduced the observed modes. The simulated mode profiles revealed the collective nature of the modes extending throughout the complex network of nanowires while showing spatial quantization with varying mode quantization numbers. The study shows a well-defined means to explore high-frequency spin dynamics in complex 3D spintronic and magnonic structures.

Structural Phase-Dependent Giant Interfacial Spin Transparency in W/CoFeB Thin-Film Heterostructures.

Pure spin current has transformed the research field of conventional spintronics due to its various advantages, including energy efficiency. An efficient mechanism for generation of pure spin current is spin pumping, and high effective spin-mixing conductance (Geff) and interfacial spin transparency (T) are essential for its higher efficiency. By employing the time-resolved magneto-optical Kerr effect technique, we report here a giant value of T in substrate/W (t)/Co20Fe60B20 (d)/SiO2 (2 nm) thin-film heterostructures in the beta-tungsten (ß-W) phase. We extract the spin diffusion length of W and spin-mixing conductance of the W/CoFeB interface from the variation of damping as a function of W and CoFeB thickness. This leads to a value of T = 0.81 ± 0.03 for the ß-W/CoFeB interface. A stark variation of Geff and T with the thickness of the W layer is obtained in accordance with the structural phase transition and resistivity variation of W with its thickness. Effects such as spin memory loss and two-magnon scattering are found to have minor contributions to damping modulation in comparison to the spin pumping effect which is reconfirmed from the unchanged damping constant with the variation of Cu spacer layer thickness inserted between W and CoFeB. The giant interfacial spin transparency and its strong dependence on crystal structures of W will be important for future spin-orbitronic devices based on pure spin current.

The 2021 Magnonics Roadmap.

Magnonics is a budding research field in nanomagnetism and nanoscience that addresses the use of spin waves (magnons) to transmit, store, and process information. The rapid advancements of this field during last one decade in terms of upsurge in research papers, review articles, citations, proposals of devices as well as introduction of new sub-topics prompted us to present the first roadmap on magnonics. This is a collection of 22 sections written by leading experts in this field who review and discuss the current status besides presenting their vision of future perspectives. Today, the principal challenges in applied magnonics are the excitation of sub-100 nm wavelength magnons, their manipulation on the nanoscale and the creation of sub-micrometre devices using low-Gilbert damping magnetic materials and its interconnections to standard electronics. To this end, magnonics offers lower energy consumption, easier integrability and compatibility with CMOS structure, reprogrammability, shorter wavelength, smaller device features, anisotropic properties, negative group velocity, non-reciprocity and efficient tunability by various external stimuli to name a few. Hence, despite being a young research field, magnonics has come a long way since its early inception. This roadmap asserts a milestone for future emerging research directions in magnonics, and hopefully, it will inspire a series of exciting new articles on the same topic in the coming years.