Towards graded-index magnonics: Steering spin waves in magnonic networks

Abstract

Magnonics explores precessional excitations of ordered spins in magnetic materials-so-called spin wavesand their use as information and signal carriers within networks of magnonic waveguides. Here, we demonstrate that the nonuniformity of the internal magnetic field and magnetization inherent to magnetic structures creates a medium of graded refractive index for propagating magnetostatic waves and can be used to steer their propagation. The character of the nonuniformity can be tuned and potentially programmed using the applied magnetic field, which opens exciting prospects for the field of graded-index magnonics. Over the past decade, magnonics (the study of spin waves-precessional excitations of ordered spins in magnetic materials [1]) has emerged as one of the most rapidly growing research fields in magnetism [2,3]. Moreover, recent advances in the understanding of fundamental properties of spin waves in magnetic micro-and nanostructures have highlighted magnonics as a potential rival of or complement to semiconductor technology in the field of data communication and processing [4]. The push for miniaturization renders ferromagnetic transition metals and their alloys to be materials of choice for the fabrication of spin-wave devices [5,6]. However, loss reduction, the shortening of the wavelength of studied spin waves, and the associated miniaturization of the implemented magnonic concepts and devices remain major challenges in both experimental research and technological development in magnonics [2,3].In this Rapid Communication, we explore an approach to meet these challenges that is based on the concept of gradedindex (or gradient-index) optics [7]. As applied to spin waves, this concept is based on the following basic ideas. First, the propagation of spin waves is controlled using subwavelength, often continuously varying, magnetic nonuniformities [8,9]. This should minimize scaling of the device size with the magnonic wavelength, in contrast to, e.g., magnonic crystal based approaches [3], and thereby ease the associated patterning resolution requirements. Indeed, nonuniform effective magnetic field and magnetization configurations have been shown to confine [10,11] and channel [12][13][14][15] spin waves, to continuously modify their character [16][17][18], and to enable their coupling to essentially uniform free space microwaves [19,20]. Here, we go further by exploiting in addition the anisotropic dispersion inherent to spin waves dominated by the dynamic magneto-dipole field-so-called magnetostatic spin waves [1]. The symmetry axis of the anisotropic magnetostatic dispersion coincides with the direction of the magnetization [21][22][23]. This anisotropic dispersion leads to the formation of nondiffracting caustic spin-wave beams * Corresponding author: v.v.kruglyak@exeter.ac.uk [24][25][26][27][28][29][30] and to anomalous spin-wave reflection, refraction, and diffraction [31][32][33][34][35]. Here, we explore these ideas in networks of magnonic waveguides [6,8,[12][13][14][15][16][17][18][19][20]24,...

Electric-field control of spin-wave power flow and caustics in thin magnetic films

“…Furthermore, recent experimental studies showed an efficient spin-wave electrical tuning in thin magnetic films, such as YIG (Y 3 Fe 5 O 12 ) [4]. At present, interest to such phenomena is very active due to their potential applications in magnon spintronics and magnonics [7][8][9], where an electrically controlled phase shifter for spin waves could become an essential component of spin-wave devices. Nonreciprocity and unidirectionality of spin-wave propagation would be also valuable for this purpose [10].…”

Generation of Propagating Spin Waves From Edges of Magnetic Nanostructures Pumped by Uniform Microwave Magnetic Field

“…This mechanism is capable of exciting both magneto-dipole and dipole-exchange spin waves, depending on the frequency of excitation and / or the film thickness. Furthermore, the uniformity of the exciting magnetic field implies that the excitation could also accompany such common measurement techniques of magnonics as cavity and waveguide based ferromagnetic resonance spectroscopy [28], time-resolved scanning Kerr microscopy [1] and microwave assisted Brillouin Light Scattering imaging [14], [20].…”

“…HE PATTERNING of magnetic nanostructures, in order to steer [1]- [3], modulate [4], [5], or otherwise modify [6], [7] the properties of propagating spin waves, has led to the successful design of many elements of perceived magnonic computing architecture [8]- [12]. Indeed, it has been predicted [13] that this progress could eventually lead to the use of spin waves as information carriers within future data processing and communication technologies.…”

Field-Controlled Phase-Rectified Magnonic Multiplexer

“…Electrically measured spin-wave transmission characteristics of straight YIG stripes of 470 μm width and DB-05 7.5 μm thickness magnetized by a field of 1160 Oe revealed that the propagation of spin waves was most efficient at frequencies between 5.16 and 5.45 GHz [18]. BLS was used to image spin waves propagating at the frequency of 5.26 GHz through the multiplexer magnetized by the field of the same value applied at 7° to the leg length.…”