Nanomagnonics

Spin dynamics in bulk materials was discovered in the late 1940s [1]. For decades, it was exploited in discrete microwave components—such as band pass filters and oscillators—in e.g. radar technology. In the recent few years, the technological perspectives have changed dramatically. The versatility of spin dynamics and, particularly, spin waves (magnons) increased tremendously once that nanotechnology entered the field. Nowadays on-chip nanodevices are discussed [2] and undergo benchmarking in the framework of beyondCMOS strategies [3, 4]. Such strategies are urgently needed to overcome the possible end of Moore’s law [5]. This rapid development was not foreseen even in 2009 when the first international workshop on ‘magnonics’ took place in Dresden, Germany [6]. At that time, magnonic crystals [7, 8], i.e. the magnetic analog of photonic crystals [9], were in the focus. These artificial crystals promised an unprecedented control of spin waves when transmitted through magnetic materials. The workshop substantiated that interest and expectations in the field were indeed high. It was decided to publish a cluster of review articles on magnonics [10] that defined its current state and scope at that time. Our review [11] argued that particularly nanotechnology would put a new spin on magnonics and its technological perspectives. It is timely to revisit this statement. Spin waves are collective spin excitations in magnetic materials such as ferro-, ferriand antiferromagnets [12]. Correlated matter with even more complex spin structures (e.g. spin helices and skyrmion lattices [13]) supports spin waves as well. As information carriers they transport angular momentum but no electrical charge. Thereby they allow for transmitting and processing information without Joule heating. At the same time, spin-wave dispersion relations and propagation characteristics are susceptible to various external parameters [12, 14] that promise low power consumption for excitation and control. Spin-wave frequencies range from GHz to THz depending on the material and type of magnetic order [12]. Note that modern information technology and the steadily growing internet of things—not apparent in 2009—make extensive use of wireless communication at microwave frequencies around 1 GHz. At this frequency, the free-space electromagnetic wave exhibits a wavelength of about 30 cm. If coupled to a spin wave, the wavelength is decreased by orders of magnitude, and miniaturized microwave devices are possible. The long coherence length of spin waves outperforms the one of electrons by orders of magnitude. This way, coherent processing of data in cellular networks becomes possible, helping to optimize computationally demanding tasks [15]. In recent years, relevant device concepts such as the microwave-to-magnon transducer [16], spin-wave multiplexer [17], and magnon transistor [18] were prototyped and explored on microand macroscopic samples. In parallel, nanotechnology of magnetic materials advanced considerably. The reconfigurable magnonic crystal [11] was adapted to route spin waves around corners in a power-saving approach [19]. Individual domain walls were functionalized as nanoscale magnonic conduits that allowed for a rewritable nanocircuitry [20]. In more general terms, this advancement contributed to graded-index magnonics [21] and the emerging field of domain wall nanoelectronics [22]. Anisotropic Dzyaloshinskii–Moriya interaction has further enriched the toolbox to tailor spin-wave conduits on the nanoscale [23]. For reconfigurable magnonics, GHz dynamics of skyrmion lattices [13, 24] and artificial spin ice [25] offer visionary possibilities [26, 27]. Breakthroughs were presented also for interfacing magnonics with electronics and conventional microwave circuits. In [28], a dc charge current was applied to a multilayer D Grundler