Size-Independent Drive of One-Dimensional Skyrmion Motion Using Exchange Energy Control

We theoretically study the motion of skyrmions under an exchange-energy gradient. Our results show excellent agreement between the analytically predicted speeds and the results of the micromagnetic simulations. Furthermore, the one-dimensional motion under the gradient of the exchange energy is independent of the skyrmion size, in contrast to the skyrmion motion driven by the magnetic-field gradient and the magnetic-anisotropy gradient studied previously, whose velocities exhibit significant dependence on the size and shape of skyrmions. This property of motion under the exchange-energy gradient enables the synchronized manipulation of skyrmions indispensable to racetrack memory and synchronized logic circuits. To confine the skyrmion in tracks, we also determined the height of the exchange energy wall, which is revealed to be less effective than using the anisotropy wall.