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Relativistic kinematics of a magnetic soliton

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Science  18 Dec 2020:
Vol. 370, Issue 6523, pp. 1438-1442
DOI: 10.1126/science.aba5555

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Magnetic special relativity

Magnetic materials have domains, patches of ordered spins that are separated from one another by boundaries called domain walls. These boundaries can be driven by electrical currents, a phenomenon forming the basis of the so-called racetrack memories. The speed of the domain walls has steadily increased as experimenters have perfected these devices. Caretta et al. now show experimentally that this speed is fundamentally limited (see the Perspective by Daniels and Stiles). Just as no object can travel faster than the speed of light, the speed of the domain walls saturates to a constant determined only by the properties of the magnetic host material.

Science, this issue p. 1438; see also p. 1413

Abstract

A tenet of special relativity is that no particle can exceed the speed of light. In certain magnetic materials, the maximum magnon group velocity serves as an analogous relativistic limit for the speed of magnetic solitons. Here, we drive domain walls to this limit in a low-dissipation magnetic insulator using pure spin currents from the spin Hall effect. We achieve record current-driven velocities in excess of 4300 meters per second—within ~10% of the relativistic limit—and we observe key signatures of relativistic motion associated with Lorentz contraction, which leads to velocity saturation. The experimental results are well explained through analytical and atomistic modeling. These observations provide critical insight into the fundamental limits of the dynamics of magnetic solitons and establish a readily accessible experimental framework to study relativistic solitonic physics.

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