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Spin waves control the magnetization around them

8 January 2020

The quasiparticles essential for proposed magnonic devices exert a spin-transfer torque analogous to that of electrons.

As electronics get faster and smaller with elements more densely packed, Joule heating from electron motion and the resulting collisions becomes more prominent. Heat limits the performance of electrical devices and requires built-in methods to cool systems down. A device that transports information without moving electrons would avoid such power dissipation, and so-called magnonic devices would do just that.

In magnonics, spin waves embody and transfer information in the form of spin: The collective precessions of electron spins (blue and red arrows in the figure) transport angular momentum while the electrons stay put. The spin-wave quasiparticle, known as a magnon, flows and carries a spin angular momentum in the same way electrons do. But a magnon is trickier to direct and measure than an electron is. Now two groups have taken a step toward making magnons more manageable. Their experiments have investigated how magnon spin waves both control and are controlled by their magnetic environment.

Diagram of the effect
Credit: Adapted from J. Han et al., Science 366, 1121 (2019)

Spin waves arise from propagating disturbances to the aligned electron spins in magnetic materials. Luqiao Liu and his colleagues at MIT explored how spin waves interact with the interface between different magnetic domains. They used a microwave antenna to induce a spin wave in a cobalt/nickel multilayer film with an up magnetic region (blue in figure) and a down magnetic region (pink). As the spin wave moved from the up to the down domain, or vice versa, its amplitude shrank, and its phase shifted by 175° consistently across different devices.

At the domain wall, the magnon angular momentum also changes by 2ℏ from spin down to spin up, and there’s no reflection. Angular momentum is conserved, so that of the magnon is transferred to the domain wall, and the torque from that spin transfer drives the domain wall in the opposite direction of the spin wave.

When Liu and his group mapped the magnetization, they saw the domain wall shift a few micrometers after the spin wave passed through. Other groups have observed a magnetic domain wall shift due to the electron spin-transfer torque, but Liu and his group are the first to see the behavior from magnon torque.

In a companion paper published in the same issue, Hyunsoo Yang of the National University of Singapore and colleagues demonstrated that magnon spin-transfer torque also flips the magnetization of an entire magnet. They created a spin wave in a nickel oxide antiferromagnetic insulator through the spin Hall effect, which converts an adjacent electrical spin current into a spin wave. (For more on the spin Hall effect, see Physics Today, May 2010, page 13.)

When the spin wave flowed into a nickel–iron alloy magnet, it flipped the magnetization of the 6-nm-thick layer from up to down or from down to up. In the future, spin waves may be a way to encode magnetic bits in a device without Joule heating.

Spin-wave control of magnetization offers many potential applications. For example, in an all-magnon device, spin waves could modulate the transmission of subsequent spin waves by moving the domain wall. Or spin waves can be detected through changes in the magnetization rather than through small changes in the electrical resistance. But first, the process of creating spin waves in a material needs to become more efficient. (J. Han et al., Science 366, 1121, 2019; Y. Wang et al., Science 366, 1125, 2019.)

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