Fundamental mechanisms that could be used for new logic and data storage applications are explored in a study by Geoffrey Beach and colleagues, published in Nature Physics.
Magnonics is a branch of magnetism in which information is transported through spin waves — that is, ripples in the magnetization (the field is named after spin-wave quanta, known as magnons). Together with domain walls, which are magnetization twists separating magnetic domains with opposite polarization, spin waves could bring about high-speed, low-power storage and logic applications. However, generating spin waves with sufficiently high amplitudes and detecting them is no easy task.
The researchers provide new insight on the relationship between domain walls and spin waves, demonstrating how spin waves can be generated and detected using domain walls. “We showed that by driving two domain walls together, the magnetization can unwind rapidly and violently, releasing a burst of spin waves,” explains Beach. “Furthermore, we showed that the spin momentum carried by the spin waves can be used to depin and push magnetic domain walls, an effect that had been predicted but never observed.” The researchers combined micromagnetic simulations with experiments performed on ferromagnetic nanowires, in which domain walls can be created and annihilated by injecting current pulses; notches were patterned on the nanowires to act as pinning sites for domain walls. When driving the domain walls, the spin waves exerted a surprisingly strong force, equivalent to that of a magnetic field of 1 mT.
“We showed that by driving two domain walls together, the magnetization can unwind rapidly and violently, releasing a burst of spin waves”
Domain walls and spin waves hold potential for magnonics in specific ways: domain walls can be used to encode information in non-volatile data storage, but they have low propagation velocities; spin waves move quickly but are difficult to generate and detect. “Our experiments showed for the first time that the advantages of domain walls and spin waves can be combined owing to the very strong interactions between them,” remarks Beach. “Domain walls can be used to locally store energy, which can then be released on demand as a spin-wave pulse; spin waves can then be detected using their interactions with domain walls.” This opens the possibility of designing memory and logic devices that combine the most attractive attributes of domain walls and spin waves into a hybrid architecture.
Spin waves attenuate as they propagate; thus, they can carry momentum over only limited distances. This study used a material that has a relatively large damping; thus, the distance covered by the spin waves was limited to a few micrometres. “We plan to study these effects in materials with much lower damping, so that spin waves can propagate over much greater distances,” says Beach. “In such materials, simulations indicate that we can use spin waves to drive domain walls over considerable distances, rather than simply aiding their depinning.” Along with the use of materials with very low damping, the use of amplification schemes — for example, based on current-induced spin-transfer torque — will help in overcoming spin-wave damping, opening the way for the realization of more complex magnonic devices.
Woo, S., Delaney, T. & Beach, G. S. D. Magnetic domain wall depinning assisted by spin wave bursts. Nat. Phys. http://dx.doi.org/10.1038/nphys4022 (2017)
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Pacchioni, G. Magnetic materials: Riding the spin wave. Nat Rev Mater 2, 17007 (2017). https://doi.org/10.1038/natrevmats.2017.7