In 1930, Bloch1 proposed that when the magnetic moment in a magnetic system is disturbed, it deviates from equilibrium states. Such disturbances will propagate through the material in the form of waves, known as spin waves. Later in 1957, Brockhouse2 confirmed the existence of spin waves through neutron diffraction experiments, and the quantized units of spin waves are called magnons. Magnons, as the elementary excitations in magnetic systems, have several advantages: they propagate without generating Joule heat, exhibit wave characteristics, have intrinsic frequencies ranging from gigahertz to terahertz, and support a rich set of techniques for exciting, manipulating, and detecting their transport. Therefore, they are poised as a potential candidate to replace electrons as the carriers for the next-generation information technology. In recent years, due to advancements in research techniques for magnons, the study of magnons has attracted increasing attention. Thanks to the organization by the Editorial Board of Applied Physics Letters, we are honored to organize a special issue on the topic of Magnonics. Focusing on materials, physics, and devices related to magnonics,3 we accepted and published a total of 55 papers.
This special topic issue was proposed in Applied Physics Letters to draw more interest to this emerging field. It includes cutting-edge research in magnonics across various directions:
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Magnon and spin wave in ferromagnets, ferrimagnets, antiferromagnets, magnetic semiconductors, and organic materials.4–7
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Excitation/generation, modulation, and detection of magnon current.8–28
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Magnonic crystals and their magnon (spin wave) transport properties.29–31
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Interactions among magnon, phonon, photon, and other quasiparticles.32–45
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Magnon quantum effects in magnon heterostructures.46–49
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Magnon devices, including magnon valve, junction, transistor, logic, and circuits.50–57
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Magnonics in two-dimensional (2D) materials.58
In the research of magnon in different magnetic materials, for example, the properties of magnons in a newly discovered magnetic material, the altermagnets, have been studied.5 Sødequist and Olsen have proposed a high-throughput computational search method for altermagnetism in two-dimensional materials based on a calculated database of 2D materials. They have calculated and predicted the magnon properties in altermagnetic materials such as RuF4 and FeBr3. In addition, the magnons at the compensation point of ferrimagnetic materials4 and in rare earth ferrimagnets7 have also been studied theoretically and experimentally.
The most studied aspects are the excitation, manipulation, and detection techniques of magnons. For instance, in antiferromagnets, due to the absence of dipole interactions, magnetic moments can quickly recover to equilibrium after perturbation, resulting in higher intrinsic frequencies of magnons. Ge et al.13 theoretically demonstrated the excitation of terahertz magnons by moving antiferromagnetic domains under the energy gradient of magnetic anisotropy, combining Lorentz contraction from relativity. The broadening of domain width leads to a reduction in anisotropic energy, which is converted into the energy for exciting magnons. Additionally, terahertz magnons in two-dimensional antiferromagnetic materials exhibit rich phenomena of magnon–plasmon coupling and magnon–photon coupling.18 In terms of manipulating magnons, for example, microwave nonreciprocal transport is achieved using pump-induced magnon modes,8 and nonreciprocal magnon couplers can be realized with metal-coated yttrium iron garnet (YIG) strips.15 Introducing local vertical magnetic anisotropy nanofilm stacks in spin wave transmission channels20 allows for fine-tuning the propagation characteristics of spin waves, such as frequency-selective coherent spin wave transmission. Zhou et al. proposed spin torque nano-oscillators,23 which enable the control of spin wave packet excitation using electric fields. They used electrically tuned magnetic droplets to generate spin waves, adjusting the amplitude and period of the spin wave packet by varying the pulse width of the electric field.
Magnon crystals possess periodic magnetic structures, where the magnon spectrum significantly differs from that of bulk materials, exhibiting bandpass phenomena for specific frequencies of magnons. In this special topic, for example, Martyshkin et al. experimentally confirmed29 that yttrium iron garnet (YIG) thin films with periodic metallic stripe arrays on the surface exhibit bandpass characteristics in the propagation of magnetostatic surface waves.
Magnons and their coupling with other particles and quasiparticles also exhibit a wealth of physical phenomena. For instance, magnon–phonon coupling provides a method to manipulate the magnon system using external stress. Xiong38 achieved controllable flipping of magnon excitations based on magnetostrictive effect. Chen et al.39 measured the thermal conductivity of 2D MnPSe3 and found that due to magnon–phonon coupling, the measured peak thermal conductivity changes with increasing thickness, and the magnon–phonon scattering rate increases with thickness, leading to a significant suppression of thermal conductivity. Bogdanova et al.40 studied the effect of stress on antiferromagnetic resonance in α-Fe2O3, employing Brillouin light scattering spectroscopy to detect the excitation of quasi-ferromagnetic and antiferromagnetic resonance modes in bulk α-Fe2O3. The results indicate that α-Fe2O3 is a suitable material for strain-controlled magnon devices.
Magnons are bosons and, like photons, can exhibit the Bose–Einstein effect. Recent experiments have confirmed this conclusion. Schweizer et al. proposed that variations in magnetization and demagnetizing fields jointly affect the local increasement in the minimum frequency value of magnon dispersion in the Bose–Einstein condensate state, which leads to a magnon superfluid away from the hot region.48
The understanding of magnon properties in different magnetic materials and the advancement of experimental techniques have promoted the development of magnonic devices, providing many interesting potential applications in magnonics. For example, similar to the capacitors commonly used in electronics, Gunnink et al.50 proposed a magnon spin capacitor that can accumulate spin at the junction between two exchange-coupled ferromagnets. Additionally, Klima et al.54 proposed a zero-field spin wave turn, which was demonstrated through micromagnetic simulations and Brillouin light scattering microscopy experiments. They showed that dipole spin waves can propagate through a 90° turn without distortion, demonstrating the possibility of spin wave transport in curved geometric materials. Ustinov et al.56 utilized current-controlled magnonic reservoir to achieve physical reservoir computing.
Due to the influence of surface and interfacial effects, two-dimensional materials exhibit many novel properties that are distinct from their bulk counterparts. For example, Chen et al.58 studied magnons in the two-dimensional Weyl magnet Fe/W(110). They discovered that the chiral asymmetric magnon dispersion is related to the presence of Weyl fermions near the Fermi level and surface Fermi arcs. This provides an excellent example of introducing topology into the field of magnonics.
The development of electronics has laid the foundation for modern information technology and industry, greatly promoting the advancement of human technology and lifestyle. Electronics primarily relies on the charge degree of electrons, and further the spin degree of electrons is developed for information storage, transmission, and processing, i.e., spintronics. Analogously to electronics and spintronics, magnonics utilizes the quasiparticle magnon as the information carrier to avoid Joule heating. Additionally, magnons have an intrinsic frequency range from gigahertz to terahertz and possess a rich set of excitation, manipulation, and detection techniques, making them promising candidates for the next generation of information carriers and attracting significant research interest. This special topic issue on magnonics aims to introduce the cutting-edge progress in materials, physics, and devices related to magnonics, drawing more scholars into this emerging field.
We thank all authors who have contributed to this special topic issue on magnonics.