Spontaneous skyrmionic lattice from anisotropic symmetric exchange in a Ni-halide monolayer

Abstract

Topological spin structures, such as magnetic skyrmions, hold great promises for data storage applications, thanks to their inherent stability. In most cases, skyrmions are stabilized by magnetic fields in non-centrosymmetric systems displaying the chiral Dzyaloshinskii-Moriya exchange interaction, while spontaneous skyrmion lattices have been reported in centrosymmetric itinerant magnets with long-range interactions. Here, a spontaneous anti-biskyrmion lattice with unique topology and chirality is predicted in the monolayer of a semiconducting and centrosymmetric metal halide, NiI2. Our first-principles and Monte Carlo simulations reveal that the anisotropies of the short-range symmetric exchange, when combined with magnetic frustration, can lead to an emergent chiral interaction that is responsible for the predicted topological spin structures. The proposed mechanism finds a prototypical manifestation in two-dimensional magnets, thus broadening the class of materials that can host spontaneous skyrmionic states.

Interplay between Single-Ion and Two-Ion Anisotropies in Frustrated 2D Semiconductors and Tuning of Magnetic Structures Topology

“…A rich phase diagram is uncovered by means of Monte Carlo simulations, where both SIA and applied out-of-plane magnetic field trigger various transitions between different kinds of magnetic configurations, with the topological charge ranging from high- Q to zero values. The previously identified [ 40 ] spontaneous high- Q skyrmion lattice is found to extend over a significant range of SIA, confirming the robustness of the TIA-based mechanism. On the one hand, the in-plane component of all noncoplanar emerging triple- spin states is found to be always fixed by the two-ion anisotropy, confirming its primary role in determining the rotational direction of in-plane magnetization over the SIA.…”

“…In Figure 3 a,d, we show the high- Q topological lattices labelled C- and D- phases, respectively, which occupy the region of the phase diagram for zero and low magnetic field; in particular, the C-phase is the spontaneous topological phase reported in [ 40 ], and stabilized by the exchange magnetic couplings given in Table 1 .…”

“…The starting point of our analysis is the classical spin Hamiltonian (given in the following Equation ( 1 )) describing the magnetic properties of the semiconducting and centrosymmetric NiI monolayer [ 40 ]. NiI belongs to the family of transition-metal-based ( M ) van der Waals materials [ 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 ], comprising layers where magnetic cations form a 2D triangular lattice, thus displaying geometrical frustration [ 51 , 52 ].…”

“…NiI belongs to the family of transition-metal-based ( M ) van der Waals materials [ 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 ], comprising layers where magnetic cations form a 2D triangular lattice, thus displaying geometrical frustration [ 51 , 52 ]. Our previous first-principles based investigation [ 40 ] on NiI monolayer revealed that its magnetic properties are ruled by: strong magnetic frustration arising from competing nearest-neighbor ( ) ferromagnetic (FM) and third-nearest neighbor ( ) anti-ferromagnetic (AFM) exchange interactions; very weak easy-plane single-ion anisotropy against highly anisotropic symmetric exchange, both driven by spin-orbit coupling (SOC). …”

“…So far, the spontaneous stabilization of skyrmionic lattices has been proposed only in itinerant magnets, characterized by long-range exchange interactions mediated by conduction electrons [ 1 , 37 , 38 , 39 ]. Only recently, a frustrated semiconducting 2D-magnet, NiI monolayer, was predicted to show the spontaneous formation of a stable high- Q skyrmionic lattice ( ) [ 40 ], with well defined topology and chirality determined by the anisotropic part of the short-range symmetric exchange, in the absence of DM and Zeeman interactions. In particular, the authors pointed out that the anisotropic symmetric exchange, also referred to as two-ion anisotropy or bond-dependent exchange anisotropy [ 41 ], may act as an emergent chiral interaction, thus adding frustration in the relative orientation of spins, when displaying noncoplanar principal axes, and determining the topology of the localized spin texture.…”