Emergent electrodynamics of skyrmions in a chiral magnet

Abstract

When an electron moves in a smoothly varying non-collinear magnetic structure, its spin-orientation adapts constantly, thereby inducing forces that act on both the magnetic structure and the electron. These forces may be described by electric and magnetic fields of an emergent electrodynamics. The topologically quantized winding number of so-called skyrmions, i.e., certain magnetic whirls, discovered recently in chiral magnets are theoretically predicted to induce exactly one quantum of emergent magnetic flux per skyrmion. A moving skyrmion is therefore expected to induce an emergent electric field following Faraday's law of induction, which inherits this topological quantization. Here we report Hall effect measurements, which establish quantitatively the predicted emergent electrodynamics. This allows to obtain quantitative evidence of the depinning of skyrmions from impurities at ultra-low current densities of only 10^6 A/m^2 and their subsequent motion. The combination of exceptionally small current densities and simple transport measurements offers fundamental insights into the connection between emergent and real electrodynamics of skyrmions in chiral magnets, and promises to be important for applications in the long-term.Comment: 24 pages, supplementary information file include

Universal current-velocity relation of skyrmion motion in chiral magnets

“…31 and Supplementary of ref. 32. Using an approach proposed by Thiele 33 , the LLG equation can be mapped onto the translational mode in the continuum limit by assuming the rigidity of spin textures during the drift motion.…”

“…The first term in the left hand side of equation (3) is the Magnus force and the second one is the dissipative force. The third one is the phenomenological pinning force due to the impurities 31,32 :…”

“…We next discuss the results in the presence of impurities by taking into account F pin a0 in equations (3) and (4) 31,32 . Here we focus on the realistic value of a ¼ 0.04 as in the case of Fig.…”

A Centrosymmetric Hexagonal Magnet with Superstable Biskyrmion Magnetic Nanodomains in a Wide Temperature Range of 100–340 K

“…These topologically protected properties, together with their nanoscale dimensions (10 ~ 100 nm) and ultralow threshold for current-driven motion (10 5 ~ 10 6 Am -2 ) [17][18][19] , make the magnetic skyrmions be promising materials for technological applications in spincronic devices [20] .…”

SCES2013 Summary: Experiment

“…[48] Besides the scientific opportunities yet to be realized from the study of these topologically protected magnetic objects, their manipulation by an electric field opens new possibilities for designing spintronic devices that capitalize on their properties. [49,50,51] Though a topologically protected metallic surface state is now well-established in uncorrelated materials like Bi2Se3 [52,53], the possibility of such as state driven by electronic correlations [54,55] has led to a flurry of experiments on a proposed candidate SmB6. In particular, the saturation of resistivity in this Kondo insulator at temperatures below 5 K may be a signature of a protected metallic surface.…”