Skyrmions and multisublattice helical states in a frustrated chiral magnet

Abstract

We investigate the existence and stability of skyrmions in a frustrated chiral ferromagnet by considering the competition between ferromagnetic (FM) nearest-neighbour (NN) interaction (J1) and antiferromagnetic (AFM) next-nearest-neighbour (NNN) interaction (J2). Contrary to the general wisdom that long-range ferromagnetic order is not energy preferable under frustration, the skyrmion lattice not only exists but is even stable for a large field range when J2 ≤ J1 compared with frustration-free systems. We defend that the enlargement of stability window of skyrmions is a consequence of the reduced effective exchange interaction caused by the frustration. A multi-sublattice helical state is found below the skyrmion phase, which results from the competition between AFM coupling that favors a two-sublattice Néel state and the chiral interaction that prefers a helix. As a byproduct, the hysteresis loop of the frustrated chiral system shrinks as the magnetization goes to zero and then opens up again, known as wasp-waist hysteresis loop. The critical field that separates the narrow and wide part of the wasp-waist loop depends exponentially on the strength of NNN coupling. By measuring the critical field, it is possible to determine the strength of NNN coupling.

A theory on skyrmion size

“…For those systems with significant exchange interactions beyond nearest neighbor 36,37 , our model may not be directly applicable. However, if the extra exchange interaction (for instance, a nextnearest neighbor interaction) is small compared to A, its effect can be approximately considered by modifying A to an effective value A eff 38 . Our model is a classical one, in which quantum fluctuations and finite-size effects are not considered.…”

Static and dynamic properties of bimerons in a frustrated ferromagnetic monolayer

“…However, the dynamics of magnetic bimerons driven by different external forces as well as the static properties of different forms of bimerons still remain elusive, especially for the bimerons in frustrated magnetic systems. Some most recent studies have focused on the existence and manipulation of skyrmions in frustrated magnetic systems [48,[64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83]. Therefore, it is also imperative to study bimerons in frustrated magnetic systems, of which the physical properties are essential for designing future bimeron-based device applications.…”

Skyrmion Crystals and Phase Transitions in Magneto-Ferroelectric Superlattices: Dzyaloshinskii–Moriya Interaction in a Frustrated J1 − J2 Model

“…We see that, as the frustration becomes stronger, the ratio N 2 /N 1 tends towards 1. Let us discuss about some theoretical observations of skyrmions in frustrated magnets [31,[37][38][39][40][41][42][43]46]. Each of these works used a different model, so comparison is impossible.…”

“…Real magnetic materials have complicated interactions; there are large families of frustrated systems, such as the heavy lanthanides metals (holmium, terbium, and dysprosium) [34,35] and helical MnSi [36]. Other interesting properties of skyrmions in magnetically frustrated systems have also been investigated [31,[37][38][39][40][41][42][43][44][45][46]. Multiferroics and superlattices of multiferroics (for example, PZT/LSMO and BTO/LSMO) currently attract many research activities on these materials because of the coexistence of ferroelectric and magnetic orderings.…”