The family of monolayer two-dimensional (2D) materials hosts a wide range of interesting phenomena, including superconductivity, charge density waves, topological states and ferromagnetism, but direct evidence for antiferromagnetism in the monolayer has been lacking. Nevertheless, antiferromagnets have attracted enormous interest recently in spintronics due to the absence of stray fields and their terahertz resonant frequency. Despite the great advantages of antiferromagnetic spintronics, controlling and detecting Néel vectors have been limited in bulk materials. In this work, we developed a sensitive second harmonic generation (SHG) microscope and detected long-range Néel antiferromagnetic (AFM) order and Néel vector switching down to the monolayer in MnPSe$_3$. Temperature-dependent SHG measurement in repetitive thermal cooling surprisingly collapses into two curves, which correspond to the switching of an Ising type Néel vector reversed by the time-reversal operation, instead of a six-state clock ground state expected from the threefold rotation symmetry in the structure. We imaged the spatial distribution of the Néel vectors across samples and rotated them by an arbitrary angle irrespective of the lattice in the sample plane by applying strain. By studying both a Landau theory and a microscopic model that couples strain to nearest-neighbor exchange, we conclude that the phase transition of the XY model in the presence of strain falls into the Ising universality class instead of the XY one, which could explain the extreme strain tunability. Finally, we found that the 180° AFM domain walls are highly mobile down to the monolayer after thermal cycles, paving the way for future control of the antiferromagnetic domains by strain or external fields on demand for ultra-compact 2D AFM terahertz spintronics.
The electronic structures and magnetic properties of strained monolayer MnPSe3 are investigated systematically by first-principles calculations. It is found that the magnetic ground state (GS) of monolayer MnPSe3 can be significantly affected by biaxial strain engineering, while the semiconducting characteristics are well preserved. Owing to the sensitivity of the magnetic coupling towards the structural deformation, a biaxial tensile strain about 13% can lead to an antiferromagnetic-ferromagnetic (AFM-FM) transition. The underlying physical mechanism of strain-dependent magnetic stability is …
Converting light into steady currents and spin-currents in two-dimensional (2D) platform is essential for future energy harvesting and spintronics. We show that the giant and modulable bulk photovoltaic effects (BPVEs) can be achieved in air-stable 2D antiferromagnet (AFM) monolayer MnPSe3, with nonlinear photoconductance > 4000 nm$\cdotμ$A/V2 and photo-spin-conductance > 2000 (nm$\cdotμ$A/V2 $\hbar$/2e) in the visible spectrum. The propagation and the spin-polarizations of photocurrents can be switched via simply rotating the N$é$el vector. We unveil that the PT-symmetry, mirror symmetries, a…
Anomalous valley Hall (AVH) effect is a fundamental transport phenomenon in the field of condensed-matter physics. Usually, the research on AVH effect is mainly focused on 2D lattices with ferromagnetic order. Here, by means of model analysis, we present a general design principle for realizing AVH effect in antiferromagnetic monolayers, which involves the introduction of nonequilibrium potentials to break of PT symmetry. Using first-principles calculations, we further demonstrate this design principle by stacking antiferromagnetic monolayer MnPSe3 on ferroelectric monolayer Sc2CO2 and achieve…
Antiferromagnetic skyrmion crystal (AF-SkX), a regular array of antiferromagnetic skyrmions, is a fundamental phenomenon in the field of condensed-matter physics. So far, only very few proposals have been made to realize the AF-SkX, and most based on three-dimensional (3D) materials. Herein, using first-principles calculations and Monte-Carlo simulations, we report the identification of AF-SkX in two-dimensional lattice of Janus monolayer CrSi2N2As2. Arising from the broken inversion symmetry and strong spin-orbit coupling, large Dzyaloshinskii-Moriya interaction is obtained in Janus monolayer…
Two-dimensional (2D) multiferroic materials with controllable magnetism have promising prospects in miniaturized quantum device applications, such as high-density data storage and spintronic devices. Here, using first-principles calculations, we propose a coexistence of antiferromagnetism and ferroelasticity in multiferroic $MnF_{4}$ monolayer. The $MnF_{4}$ monolayer is found to be an intrinsic wide-gap semiconductor with large spin polarization ~3 $μ_{B}$/Mn, in which the antiferromagnetic order originates from the cooperation and competition of the direct exchange and super exchange. $MnF_{…
Based on density functional theory (DFT), we performed first-principle studies on the electronic structure, magnetic state and optical properties of two-dimensional (2D) transition-metal phosphorous trichalcogenide MnPX$_3$ (X=S and Se). The calculated interlayer cleavage energies of MnPX$_3$ monolayers indicate the energetic possibility to be exfoliated from bulk phase, with good dynamical stability confirmed by the absence of the imaginary contributions in the phonon spectra. MnPX$_3$ monolayers are both Néel antiferromagnetic (AFM) semiconductors with direct band gaps falling into the visib…
The chiral spin textures of a two-dimensional (2D) triangular system, where both antiferromagnetic (AF) Heisenberg exchange and chiral Dzyaloshinsky-Moriya interactions co-exist, are investigated numerically with an optimized quantum Monte Carlo method based on mean-field theory. We find that: helical, skyrmionic and vortical AF crystals can be formed when an external magnetic field is applied perpendicular to the 2D monolayer; the sizes of these skyrmions and vortices change abruptly at several critical points of the external magnetic field; each of these AF crystals can be decomposed into th…
Topological nodal line semimetals, hosting one-dimensional Fermi lines with symmetry protection, has become a hot topic in topological quantum matter. Due to the breaking of time reversal symmetry in magnetic system, nodal lines require protection by additional symmetries. Here, we report the discovery of antiferromagnetic type-I and type-II nodal lines coexist in the monolayer CrAs$_{2}$ based on a systematic first-principles calculation. Remarkably, the type-I nodal line in CrAs$_{2}$ form a concentric loop centered around the $Γ$ point is filling-enforced by nonsymmorphic analogue symmetry …
Spin-Flop Ordering from Frustrated Ferro- and Antiferromagnetic Interactions: A Combined Theoretical and Experimental Study of a $\mathrm{Mn}/\mathrm{Fe}(100)$ Monolayer
Lifting the valley degeneracy is an efficient way to achieve valley polarization for further valleytronics operations. In this work, we demonstrate that a large valley splitting can be obtained in monolayer transition metal dichalcogenides by magnetic proximity coupling to an insulating antiferromagnetic substrate. As an example, we perform first-principles calculations to investigate the electronic structures of monolayer WS$_2$ on the MnO(111) surface. Our calculation results suggest that a large valley splitting of 214 meV, which corresponds to a Zeeman magnetic field of 1516 T, is induced …
The topological states of matters arising from the nontrivial magnetic configuration provide a better understanding of physical properties and functionalities of solid materials. Such studies benefit from the active control of spin orientation in any solid, which is yet known to rarely take place in the two-dimensional (2D) limit. Here we demonstrate by the first-principles calculations that spin-orientation dependent topological states can appear in the geometrically frustrated monolayer antiferromagnet. Different topological states including quantum anomalous Hall (QAH) effect and time-rever…
Intrinsic antiferromagnetism in van der Waals (vdW) monolayer (ML) crystals enriches the understanding regarding two-dimensional (2D) magnetic orders and holds special virtues over ferromagnetism in spintronic applications. However, the studies on intrinsic antiferromagnetism are sparse, owing to the lack of net magnetisation. In this study, by combining spin-polarised scanning tunnelling microscopy and first-principles calculations, we investigate the magnetism of vdW ML CrTe2, which has been successfully grown through molecular beam epitaxy. Surprisingly, we observe a stable antiferromagneti…
By means of the first-principles calculations and magnetic topological quantum chemistry, we demonstrate that the low energy physics in the checkerboard antiferromagnetic (AFM) monolayer FeSe, very close to an AFM topological insulator that hosts robust edge states, can be well captured by a double-degenerate fragile topologically flat band just below the Fermi level. The Wilson loop calculations identify that such fragile topology is protected by the $S_{4z}$ symmetry, which gives rise to an AFM higher-order topological insulator that support the bound state with fractional charge $e/2$ at th…
Dirac point in two-dimensional (2D) materials has been a fascinating subject of research. Recently, it has been theoretically predicted that Dirac point may also be stabilized in 2D magnetic systems. However, it remains a challenge to identify concrete 2D materials which host such magnetic Dirac point. Here, based on first-principles calculations and theoretical analysis, we propose a stable 2D material, the monolayers TaCoTe$_2$, as an antiferromagnetic (AFM) 2D Dirac material. We show that it has an AFM ground state with an out-of-plane Néel vector. It hosts a pair of 2D AFM Dirac points on …
Two-dimensional ferromagnetic materials with strong ferromagnetism and high Curie temperature are significantly desired for the applications of nanoscale devices. Here, based on first-principles computations, we report hydrogenated/oxygenated Mn2C monolayer is a such material with strong room-temperature ferromagnetism. The bare Mn2C monolayer is an antiferromagnetic metal with the local magnetic moment of Mn ~ 3μB. However, the antiferromagnetic coupling of Mn atoms can transform into the ferromagnetic order under hydrogenation/oxygenation. Especially, the magnetic moments in hydrogenated/oxy…
The effects of strain and hydrogenation on the electronic and magnetic properties of monolayers of Ta based dichalcogenides (TaX2; X = S, Se, Te) are investigated using density-functional theo-ry. We predict a complex scenario of strain-dependent magnetic phase transitions involving par-amagnetic, ferromagnetic, and modulated antiferromagnetic states. Covering one of the two chalcogenide surfaces with hydrogen switches the antiferromagnetic/nonmagnetic TaX2 mono-layers to a semiconductor. Our research opens new pathways towards the manipulation of mag-netic properties for future optoelectronic…
Magnetization and magnetoresistivity studies of Fe/V (001) superlattices are reported. The first giant magnetoresistance peak with respect to the vanadium and iron layer thicknesses is investigated. The interlayer antiferromagnetic coupling strength is found to show a peak at a vanadium layer thickness of 13 atomic monolayers ($\approx$ 20 Å) with a full width at half maximum of about 2 monolayers. The antiferromagnetic coupling shows a maximum at an iron layer thickness of about 6 monolayers ($\approx$ 9 Å) for series of superlattices with vanadium thicknesses around 13 monolayers. The magnit…
We report the current-driven dynamics of frustrated skyrmions in an antiferromagnetically exchange coupled bilayer system, where the bilayer skyrmion consists of two monolayer skyrmions with opposite skyrmion numbers $Q$. We show that the in-plane current-driven bilayer skyrmion moves in a straight path, while the out-of-plane current-driven bilayer skyrmion moves in a circular path. It is found that the in-plane current-driven mobility of a bilayer skyrmion is much better than the monolayer one at a large ratio of $β/α$, where $α$ and $β$ denote the damping parameter and non-adiabatic spin tr…
Two-dimensional (2D) magnetoelectric multiferroics are promising multifunctional materials for miniaturized logic and memory devices. Herein, we explore the effectiveness of strain-engineering for tuning the properties of a recently predicted 2D antiferromagnetic-ferroelectric, VOCl2 monolayer. Interestingly, we find that magnetic-ordering and electric polarization can be tuned independently using uniaxial tensile strain along different in-plane lattice vectors. A 4% tensile strain along lattice vector b induces a transition from an antiferromagnetic (AFM) ground state with an out-of-plane mag…
Two-dimensional intrinsic antiferromagnetic semiconductors are expected to stand out in the spintronic field. The present work finds the monolayer T'-MoTeI is intrinsically an antiferromagnetic semiconductor by using first-principles calculation. Firstly, the dimerized distortion of the Mo atoms causes T'-MoTeI to have dynamic stability, which is different from the small imaginary frequency in the phonon spectrum of T-MoTeI. Secondly, T'-MoTeI is an indirect-bandgap semiconductor with 1.35 eV. Finally, in the systematic study of strain effects, there are significant changes in the electronic s…
We present first-principles results on the structural, electronic, and magnetic properties of a new family of two-dimensional antiferromagnetic (AFM) manganese chalcogenides, namely monolayer MnX and Janus XMnY (X, Y= S, Se, Te), among which monolayer MnSe was recently synthesized in experiments [\href{https://pubs.acs.org/doi/abs/10.1021/acsnano.1c05532}{ACS Nano 15 (8),13794 (2021)}]. By carrying out calculations of the phonon dispersion and \textit{ab-initio} molecular dynamics simulations, we first confirmed that these systems, characterized by an unconventional strongly coupled bilayer at…
Designing two-dimensional (2D) materials with magnetic and topological properties has continuously attracted intense interest in fundamental science and potential applications. Here, on the basis of first-principles calculations, we predict the coexistence of antiferromagnetism and Dirac nodal loop (NL) in the monolayer MnB, where the band crossing points are very close to the Fermi Level. Remarkably, a moderate strain can induce an antiferromagnetic to ferromagnetic phase transition, driving the monolayer MnB to a ferromagnetic metal with Weyl NLs. Such a type of topological quantum phase tra…
Van der Waals (vdW) heterostructures have attracted great interest because of their rich material combinations.The discovery of two-dimensional magnets has provided a new platform for magnetic vdW heterointerfaces; however, research on magnetic vdW heterointerfaces has been limited to those with ferromagnetic surfaces. Here we report a magnetic vdW heterointerface using layered intralayer-antiferromagnetic MPSe3 (M=Mn, Fe) and monolayer transition metal dichalcogenides (TMDs). We found an anomalous upshift of the excitonic peak in monolayer TMDs below the antiferromagnetic transition temperatu…
Two-dimensional transition metal dichalcogenides have attracted great attention recently. Motivated by a recent study of crystalline bulk VTe$_2$, we theoretically investigated the spin-charge-lattice interplay in monolayer VTe$_2$. To understand the controversial experimental reports on several different charge density wave ground states, we paid special attention to the 'hidden' role of antiferromagnetism as its direct experimental detection may be challenging. Our first-principles calculations show that the 4$\times$1 charge density wave and the corresponding lattice deformation are accompa…
The van der Waals magnets CrX$_3$ (X = I, Br, and Cl) exhibit highly tunable magnetic properties and are promising candidates for developing novel two-dimensional (2D) magnetic devices such as magnetic tunnel junctions and spin tunneling transistors. Previous studies of CrCl$_3$ have mainly focused on mechanically exfoliated samples. Controlled synthesis of high quality atomically thin flakes is critical for their technological implementation but has not been achieved to date. Here, we report the growth of large CrCl$_3$ flakes with well-defined facets down to monolayer thickness (~0.6 nm) via…
