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Spin wave nonreciprocity for logic device applications
Mahdi Jamali, Jae Hyun Kwon, Soo-Man Seo, Kyung-Jin Lee, Hyunsoo Yang
TL;DR
The paper addresses the fragility of phase-based spin-wave logic by proposing a complete system based on amplitude and nonreciprocal spin-wave behavior. Experiments show that magnetostatic surface-spin-wave nonreciprocity can be tuned by bias magnetic field, while the logic system is proposed to implement Boolean functions and operate in GHz ranges.
Problem
Phase-based spin-wave logic relies on a delicate phase that can be disrupted, motivating an alternative based on spin-wave amplitude.
Method
The paper proposes a complete logic system using spin-wave amplitude and nonreciprocal behavior excited by microstrip antennas.
Results
The nonreciprocity of magnetostatic surface spin waves can be tuned by bias magnetic field, and the proposed logic system can implement all Boolean functions.
Takeaways & Limitations
The device concept uses tunable nonreciprocal amplitude behavior for spin-wave logic and can operate in GHz frequency ranges.
Takeaways & Limitations
The logic design assumes a ferromagnetic structure with an easy axis in the y-direction and corresponding anisotropy-field conditions.
Abstract
from arXiv · showhide
The utilization of spin waves as eigenmodes of the magnetization dynamics for information processing and communication has been widely explored recently due to its high operational speed with low power consumption and possible applications for quantum computations. Previous proposals of spin wave Mach-Zehnder devices were based on the spin wave phase, a delicate entity which can be easily disrupted. Here, we propose a complete logic system based on the spin wave amplitude utilizing the nonreciprocal spin wave behavior excited by microstrip antennas. The experimental data reveal that the nonreciprocity of magnetostatic surface spin wave can be tuned by the bias magnetic field. Furthermore, engineering of the device structure could result in a high nonreciprocity factor for spin wave logic applications.
3 KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul
Spin waves are promising information-processing candidates because they combine high-frequency operation, substantial group velocity, and guided propagation. This section motivates amplitude-based logic by contrasting it with fragile phase-based designs and introduces tunable nonreciprocity in magnetostatic surface spin waves.
- Spin waves operate at gigahertz frequencies, have group velocities of a few tens of µm/ns, and can be guided along ferromagnetic materials.
- Phase-based spin-wave logic requires a controlled phase shifter, complicating device structures, particularly at submicron dimensions.
- The spin-wave phase is continuous, sensitive to imperfections, magnetic inhomogeneities, and device dimensions, reducing reliability and bit density.
- Damon-Eshbach surface spin waves exhibit amplitude nonreciprocity for opposite wavevectors, arising from asymmetric out-of-plane excitation-field distributions.
- The nonreciprocity value can be tuned with the external bias magnetic field, and device engineering may increase the nonreciprocity ratio.
- The proposed complete logic device uses nonreciprocal spin-wave amplitude rather than phase for its operations.
Results
Experiments and simulations show that surface spin-wave nonreciprocity depends on bias magnetic field and changes amplitude with magnetization or propagation direction. The resulting amplitude asymmetry is used to propose complete one- and two-input logic gates for Boolean functions.
- Nonreciprocal propagation: Surface spin waves show clear nonreciprocity for positive and negative magnetic fields, equivalent to opposite wavevector signs.
- Frequency-domain measurements: Two spin-wave frequencies arise from different wavevectors, with the higher-frequency peak much smaller than the lower-frequency peak.A narrow 3 µm stripline supports different wavevectors, whereas a wide 10 µm stripline produces only one frequency.
- Frequency-domain measurements: The measured spin-wave spectrum follows a quadratic-like frequency dependence on magnetic field and yields k±Δk =0.57±0.33 µm-1.The fitted wavevector agrees with previous inductive-method reports.
- Field dependence: Nonreciprocity increases with bias magnetic field, and simulations match experimental results for both sinusoidal and impulse excitations.Time-resolved measurements show increased nonreciprocity from 60 to 200 Oe, consistent with frequency-domain data.
- Amplitude-based logic: Spin-wave packet amplitude changes when magnetization or propagation direction is reversed, supporting amplitude-based surface-spin-wave logic circuits.
- Logic gates: The proposed complementary outputs implement NOT and PASS gates, while one- and two-input gates together can realize any Boolean function.The output signal is a spin-wave packet that can be reshaped and fed into other gates.
Discussion
The proposed amplitude-based spin-wave logic uses tunable nonreciprocity and can operate at high speed with low power while supporting scalable device structures.
- A triggering signal can synchronize different logic gates as a clock, with timing determined by spin-wave and wave-shaping delays.
- The surface spin wave has a group velocity of a few tens of μm/ns, while the push-detector circuit can operate in the GHz range.
- The logic gate can operate in the GHz frequency range, and its performance can be enhanced at low dimensions.
- The device demonstrates scalability through designed logic gates with ferromagnetic-structure widths of 10 and 100 nm.
- Spin-wave propagation involves no charge transfer, supporting an energy-efficient logic scheme with triggering pulses below 80 ps and driving currents below 1 mA.
- The logic retains some non-volatility because the ferromagnetic magnetization direction is preserved in the ±y directions when power is off.
- Experiments show that the nonreciprocity factor varies with and can be tuned by the bias magnetic field in both frequency and time domains.
- The proposed amplitude-based logic system is complete and can implement all Boolean functions, operating in GHz ranges with very low power dissipation.
Methods
The devices were fabricated from patterned permalloy films with insulated asymmetric coplanar waveguides and characterized in frequency and time domains.
- A 20 nm permalloy film was deposited on Si/SiO2 substrates, patterned by electron-beam lithography, and transferred by Ar ion milling.
- The patterned ferromagnetic structures ranged from 1 to 400 μm in width, with multiple closely spaced wires used below 11 μm.
- A 50 nm SiO2 layer electrically isolated the ferromagnetic structure from subsequently fabricated waveguides.
- Asymmetric coplanar waveguides were fabricated from Cr (5 nm)/Ag (150 nm)/Pt (5 nm).
- Frequency-domain measurements used a vector network analyzer with 5 dBm excitation and cable-and-connector calibration.
- Time-domain measurements used a wide-bandwidth sampling oscilloscope and pulses generated at 1.6 V after 29 dB amplification.
Additional information
The additional information covers device geometry, measurement conditions, spin-wave spectra, nonreciprocity, and one- and two-input logic structures.
- The device uses a 20 nm permalloy film separated from striplines by 50 nm of SiO2.
- The asymmetric coplanar waveguide comprises Cr (5 nm)/Ag (150 nm)/Pt (5 nm), with signal width S = 3 µm and ground width W = 9 μm.
- A bias field is applied along y, and spin waves propagating along x are detected inductively by the antenna.
- The measured spectra and simulations characterize surface-spin-wave frequency and nonreciprocity across magnetic fields.
- Time-domain surface-spin-wave measurements used ±60, ±135, and ±200 Oe bias fields with a 1.6 V, approximately 80 ps excitation pulse.
- The logic structures include one-input complementary outputs implementing NOT and PASS behavior, plus a two-input gate with corresponding truth tables and Boolean expressions.