Filming the formation and fluctuation of skyrmion domains by cryo-Lorentz transmission electron microscopy

Abstract

Magnetic skyrmions are promising candidates as information carriers in logic or storage devices thanks to their robustness, guaranteed by the topological protection, and their nanometric size. Currently, little is known about the influence of parameters such as disorder, defects, or external stimuli on the long-range spatial distribution and temporal evolution of the skyrmion lattice. Here, using a large (7.3 × 7.3 μm 2 ) single-crystal nanoslice (150 nm thick) of Cu 2 OSeO 3 , we image up to 70,000 skyrmions by means of cryo-Lorentz transmission electron microscopy as a function of the applied magnetic field. The emergence of the skyrmion lattice from the helimagnetic phase is monitored, revealing the existence of a glassy skyrmion phase at the phase transition field, where patches of an octagonally distorted skyrmion lattice are also discovered. In the skyrmion phase, dislocations are shown to cause the emergence and switching between domains with different lattice orientations, and the temporal fluctuation of these domains is filmed. These results demonstrate the importance of direct-space and real-time imaging of skyrmion domains for addressing both their long-range topology and stability.skyrmions | Lorentz transmission electron microscopy | skyrmion dynamics | magnetic materials | strongly correlated systems I n a noncentrosymmetric chiral lattice, the competition between the symmetric ferromagnetic exchange, the antisymmetric Dzyaloshinskii-Moriya interaction, and an applied magnetic field can stabilize a highly ordered spin texture, presenting as a hexagonal lattice of spin vortices called skyrmions (1-4).Magnetic skyrmions have been experimentally detected in materials having the B20 crystal structure such as MnSi (5), Fe 1−x Co x Si (6, 7), FeGe (8), and Cu 2 OSeO 3 (9) and, recently, also on systems like GaV 4 S 8 (10) and beta-Mn-type alloys (11). Small-angle neutron scattering studies of bulk solids evidenced the formation of a hexagonal skyrmion lattice confined in a very narrow region of temperature and magnetic field (T-B) in the phase diagram (5, 6). In thin films and thinly cut slices of the same compounds, instead, skyrmions can be stabilized over a wider T-B range as revealed by experiments using cryo-Lorentz transmission electron microscopy (LTEM) (12, 13). Furthermore, it was proposed and recently observed that skyrmions can also exist as isolated objects before the formation of the ordered skyrmion lattice (14, 15). A recent resonant X-ray diffraction experiment also suggested the formation of two skyrmion sublattices giving rise to regular superstructures (16).In a 2D landscape, long-range ordering can be significantly altered by the presence of defects and disorder. Indeed, the competition between order and disorder within the context of lattice formation continues to be an issue of fundamental importance.Condensed matter systems are well known to provide important test beds for exploring theories of structural order in solids and glasses. An archetypal and conceptually relevant example ...

Resonant elastic x-ray scattering from the skyrmion lattice inCu2OSeO3

“…It is worth mentioning that the helical pitch λ h is equal to the helixto-helix distance a h for Cu 2 OSeO 3 , as well as other B20 metallic helimagnets. This is well-established by both SANS [27,29] and LTEM [4,30] studies. Above a certain magnetic field H c1 , the conical spiral state becomes the lowest energy solution.…”

Relaxation dynamics of modulated magnetic phases in the skyrmion host GaV4S8 : An ac magnetic susceptibility study

“…1 are not sharp but extend over relatively wide regions of magnetic fields. These regions are presumably characterized by the coexistence of the magnetic phases [34,37], as has been observed in various skyrmion host compounds in real space via magnetic force microscopy [46] and Lorentz-TEM imaging [4,6,10,47]. In the case of GaV 4 S 8 , the lamellar structure of the ferroelastic domains with a typical thickness in the micrometer range [27] may introduce significant disorder in the vicinity of domain boundaries and may also enhance demagnetizing fields in the sample.…”

“…The excitation of these defects over pinning barriers might lead to the observed frequency dependence of the susceptibility [38]. Moreover, the emerging SkL, coexisting with the helical states is prone to develop lattice defects such as irregular coordination or edge dislocations, eventually relaxing to the equilibrium single domain SkL state in the pure SkL phase [4,47]. The ac-field-induced relaxation of the defects in the SkL may also be responsible for the slow dynamics seen in the ac susceptibility measurements.…”

Imaging and manipulation of skyrmion lattice domains in Cu2OSeO3

“…38 Moreover, the close-packed ordering of the skyrmion lattice can have defects at the domain boundaries, e.g., "5-7 defects" where five or seven skyrmions are surrounding a reference skyrmion, instead of six. 37 In this case, a line defect may appear, forming the domain boundary. Depending on its configuration, the neighboring domains may change their orientations and assume an arbitrary angle, depending on the width of the defect-containing boundary.…”