Long-distance propagation of short-wavelength spin waves

Abstract

Recent years have witnessed a rapidly growing interest in exploring the use of spin waves for information transmission and computation toward establishing a spin-wave-based technology that is not only significantly more energy efficient than the CMOS technology, but may also cause a major departure from the von-Neumann architecture by enabling memory-in-logic and logic-in-memory architectures. A major bottleneck of advancing this technology is the excitation of spin waves with short wavelengths, which is a must because the wavelength dictates device scalability. Here, we report the discovery of an approach for the excitation of nm-wavelength spin waves. The demonstration uses ferromagnetic nanowires grown on a 20-nm-thick Y₃Fe₅O₁₂ film strip. The propagation of spin waves with a wavelength down to 50 nm over a distance of 60,000 nm is measured. The measurements yield a spin-wave group velocity as high as 2600 m s^-1, which is faster than both domain wall and skyrmion motions.

Fig. 1

Different magnetic states in a nanopatterned magnetic heterostructure. a Sketch of an YIG (20 nm)/Ti (1 nm)/Co (25 nm) heterostructure with a coplanar waveguide (CPW) prepared on the top. The applied field H is parallel to the Co nanowires (along the y axis). b Color-coded reflection spectra S₁₁ measured on the device structure shown in a. The field is set to −3000 Oe to magnetize the Co nanowires and the YIG film to saturation first and then swept from −400 Oe to 400 Oe with a field step of 2.5 Oe. The spectra have several different regions corresponding to three different magnetic states: parallel state (P), antiparallel state (AP), and random state (R). c A line plot extracted from b at a field of 400 Oe. d Color-coded reflection spectra S₁₁ with a reversed field sweeping direction, as indicated. The field is set to 3000 Oe first and then swept from 400 Oe to −400 Oe with a field step of −2.5 Oe. e TEM image of the YIG/Ti/Co heterostructure. The horizontal scale bar is 2 nm long. f SEM surface image of the heterostructure. The Co nanowires are color-coded in red and the YIG film beneath the wires are in blue. The scale bar is 500 nm long

Fig. 2

Propagating short-wavelength spin waves and their group velocities. a Sketch of a YIG/Ti/Co heterostructure-based device for spin-wave propagation measurements. The external field H is in the plane and parallel to the Co nanowires. b Color-coded transmission spectra S₂₁ measured on a YIG/Ti/Co device with a nanowire period of 200 nm (device A1). The field is set to −3000 Oe first and then swept from 600 Oe to 1000 Oe with a field step of 2.5 Oe. c A line plot extracted as a cutoff from b at the field value of 800 Oe. Δf is extracted for the calculation of the group velocity. d Group velocities of different spin-wave modes. Black squares show the group velocities of CPW-excited spin waves in the 20-nm-thick plain YIG film extracted from the experimental data on a YIG/Ti/Co structure with a nanowire period of 180 nm (device A2). Yellow circles, blue triangles, and green diamonds show the PSWSW group velocities extracted from the experimental data with mode number n = 2, n = 4, and n = 6, respectively. The solid lines are the group velocities calculated based on the derivative of the spin-wave dispersion relation in the YIG film

Fig. 3

Dispersion relations for propagating short-wavelength spin waves. a Data points show the frequencies and wavenumbers of different spin-wave modes extracted from the experiments on device A1—YIG/Ti/Co with a = 200 nm (black squares), device A2—YIG/Ti/Co with a = 180 nm (purple dots), and device C1—YIG/Ti/CoFe with a = 200 nm (green diamonds). The red curve shows the dispersion relation of the DE spin wave in the YIG thin film, which is calculated using Eq. (2) for a field of 1000 Oe. The inset shows the transmission spectra S₂₁ for the n = 8 PSWSW mode detected in device C1. b Micromagnetic simulation results of the dispersion relation for the YIG/Ti/Co structure with a period of a = 200 nm. The simulation takes into account the interlayer dipolar interactions between the Co and the YIG, but not direct interlayer exchange coupling

Fig. 4

Reconfigurable short-wavelength spin waves. a Color-coded plot of transmission spectra S₂₁ measured in the P state. The field is set to 3000 Oe first and then swept from 200 Oe to 0 with a field step of −2.5 Oe. c Color-coded transmission spectra S₂₁ measured in the AP state. The field is set to −3000 Oe first and then swept from 0 to 200 Oe with a field step of 2.5 Oe. b Line spectra taken as cuts from the full spectra at an applied field of 100 Oe. These cuts are indicated by the dashed lines in a and c. d and e illustrate the excitation of n = 4 short-wavelength spin waves induced by the interlayer magnetic coupling in the P and AP configurations, respectively