Real-space observation of a two-dimensional skyrmion crystal

Abstract

Crystal order is not restricted to the periodic atomic array, but can also be found in electronic systems such as the Wigner crystal or in the form of orbital order, stripe order and magnetic order. In the case of magnetic order, spins align parallel to each other in ferromagnets and antiparallel in antiferromagnets. In other, less conventional, cases, spins can sometimes form highly nontrivial structures called spin textures. Among them is the unusual, topologically stable skyrmion spin texture, in which the spins point in all the directions wrapping a sphere. The skyrmion configuration in a magnetic solid is anticipated to produce unconventional spin-electronic phenomena such as the topological Hall effect. The crystallization of skyrmions as driven by thermal fluctuations has recently been confirmed in a narrow region of the temperature/magnetic field (T-B) phase diagram in neutron scattering studies of the three-dimensional helical magnets MnSi (ref. 17) and Fe(1-x)Co(x)Si (ref. 22). Here we report real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe(0.5)Co(0.5)Si using Lorentz transmission electron microscopy. With a magnetic field of 50-70 mT applied normal to the film, we observe skyrmions in the form of a hexagonal arrangement of swirling spin textures, with a lattice spacing of 90 nm. The related T-B phase diagram is found to be in good agreement with Monte Carlo simulations. In this two-dimensional case, the skyrmion crystal seems very stable and appears over a wide range of the phase diagram, including near zero temperature. Such a controlled nanometre-scale spin topology in a thin film may be useful in observing unconventional magneto-transport effects.

Universal current-velocity relation of skyrmion motion in chiral magnets

“…The DM interaction naturally leads to a helical spin order (HL), which turns into a triangular skyrmion crystal (SkX) under an external magnetic field B (refs 11,12) as observed in a narrow region of the B-T phase diagram for bulk samples by neutronscattering experiments 13 . Enhanced stability of the SkX in twodimensional (2D) systems or in thin-plate samples was predicted theoretically 14,15 , and was indeed confirmed by Lorentz microscope experiments 16,17 . Figure 1 shows a phase diagram at T ¼ 0 as well as schematic pictures of spin configurations in the HL, SkX and ferromagnetic phases in the 2D system.…”

Confinement of chiral magnetic modulations in the precursor region of FeGe

“…Similar skyrmionic states have been observed in thin films of Fe 0.5 Co 0.5 Si [51], MnSi [52], and in the non-centrosymmetric cubic ferrimagnet Cu 2 OSeO 3 [52]. These breakthrough investigations are based on the direct observation of helical and skyrmionic states by Lorentz TEM.…”

Scanned probe imaging of nanoscale magnetism at cryogenic temperatures with a single-spin quantum sensor

“…Examples include vortices in high temperature superconductors 1 , ferromagnetism at the interface between oxide band insulators 2 , and skyrmion phases in helimagnets 3,4 . Many experimental tools, including real space imaging techniques such as magnetic force microscopy (MFM) 5 , scanning superconducting quantum interference devices (SQUIDs) 6 and Lorentz transmission electron microscopy (TEM) 7 , and reciprocal space techniques including neutron scattering 8 have been successfully utilized to study magnetism in these systems. However, each of these techniques has limitations that must be considered.…”