Exotic skyrmion crystals in chiral magnets with compass anisotropy

Abstract

The compass-type anisotropy appears naturally in diverse physical contexts with strong spin-orbit coupling (SOC) such as transition metal oxides and cold atomic gases etc, and it has been receiving substantial attention. Motivated by recent studies and particularly recent experimental observations on helimagnet MnGe, we investigate the critical roles of this compass-type anisotropy in modulating various spin textures of chiral magnets with strong SOC, by Monte Carlo simulations based on a classical Heisenberg spin model with Dzyaloshinsky-Moriya interaction and compass anisotropy. A phase diagram with emergent spin orders in the space of compass anisotropy and out-of-plane magnetic field is presented. In this phase diagram, we propose that a hybrid super-crystal structure consisting of alternating half-skyrmion and half-anti-skyrmion is the possible zero-field ground state of MnGe. The simulated evolution of the spin structure driven by magnetic field is in good accordance with experimental observations on MnGe. Therefore, this Heisenberg spin model successfully captures the main physics responsible for the magnetic structures in MnGe, and the present work may also be instructive to research on the magnetic states in other systems with strong SOC.

Real-space observation of ferroelectrically induced magnetic spin crystal in SrRuO3

“…We perform here in-depth studies of the magnetic domain pattern in the field range where the topological Hall peak occurs. We reveal by MFM measurements emergent periodic chiral domains well correlated with the abnormal peak in the Hall response usually assigned to THE 23 , 24 . We show theoretically and with atomic-resolution scanning transmission electron microscopy (STEM) that the system exhibits broken surface inversion symmetry, itself causing a large iDMI.…”

“…For the double-q cycloidal magnetic spin texture in this investigation, a non-zero χ has already been theoretically established 23 , 24 ; however, a non-zero χ can be directly identified from observation of the spin texture in Fig. 3i .…”

“…Whilst this spin structure seemingly approximates a square skyrmion phase 23 , 24 , precise and careful analysis shows otherwise. The topological nature of a skyrmion means that the local spin configuration has to be directly mapped onto a unit sphere of magnetisation.…”

“…Here, the spin texture has been normalised with (defined in Eq. ( 2 )), which was introduced to ensure all magnetic moments are the same length 23 , 24 . Figure 3h, i shows the simulation of the magnetisation of the cycloidal phase and the anisotropic double-q structure, respectively.…”

“…Double-q vector magnetic phases have been subject to frequent investigation, both theoretically 23 , 24 and experimentally 25 – 27 , 29 , 30 ; however, any identification of such regions has been through reciprocal space techniques such as neutron diffraction, and associated with ac-magnetic susceptibility measured phase diagrams. The results obtained from MFM in this study closely agree with this magnetic spin texture, with this report being the first to display real-space images of such spin crystals.…”