Two-dimensional Magnetic Systems Research Articles

Stabilization and helicity control of hybrid magnetic skyrmion

The hybrid skyrmion, a type of magnetic skyrmion with intermediate helicity between Bloch and Néel skyrmion, has gained more attraction. It is tolerant toward the skyrmion Hall effect and a potential candidate for quantum bits. We investigated the stabilization and helicity control of the hybrid skyrmion in a two-dimensional magnetic system using an analytical model and micromagnetic simulation. We look at the interplaying factors of the bulk (Db ) and interfacial (Di ) Dzyaloshinskii–Moriya interactions (DMI) along with the dipolar interaction. We show that the hybrid skyrmion can stabilize through the interplay between interfacial DMI and either bulk DMI or dipolar interaction. We can also control the helicity of the hybrid skyrmion by tuning the ratio of Di/Db when there is no dipolar interaction, or simply by adjusting the Di when the Db is absent. Our results suggest that hybrid skyrmions can exist within 0<|Di|<0.4 mJ m−2 for Co-based magnetic systems.

Interlayer magnetic interactions and ferroelectricity in π/3-twisted CrX2 (X = Se, Te) bilayers

Recently, two-dimensional (2D) bilayer magnetic systems have been widely studied. Their interlayer magnetic interactions play a vital role in the magnetic properties. In this paper, we theoretically studied the interlayer magnetic interactions, magnetic states, and ferroelectricity of π/3-twisted CrX2 (X = Se, Te) bilayers (π/3-CrX2). Our study reveals that the lateral shift could switch the magnetic state of the π/3-CrSe2 between interlayer ferromagnetic and antiferromagnetic, while just tuning the strength of the interlayer antiferromagnetic interactions in π/3-CrTe2. Furthermore, the lateral shift can alter the off-plane electric polarization in both π/3-CrSe2 and π/3-CrTe2. These results show that stacking is an effective way to tune both the magnetic and ferroelectric properties of 1T-CrX2 bilayers, making the 1T-CrX2 bilayers hold promise for 2D spintronic devices.

Crystal structure of KMnPO4F with short- and long-range order inside the layered magnetic system.

Potassium manganese fluoride phosphate, KMnPO4F, has been obtained through mild hydrothermal synthesis and characterized by scanning electron microscopy, microprobe analysis and X-ray diffraction. The compound possesses an orthorhombic symmetry and chiral space group P212121 with a = 4.7884(2), b = 9.0172(4), c = 9.5801(4) Å, and Z = 4. Its crystal structure is composed of Mn3+O4F square pyramids sharing vertices with PO4 tetrahedra. This anionic framework is neutralized by K+ cations. As the temperature decreases, a short-range correlation state (Tmax ∼ 35 K) of KMnPO4F is formed, followed by the establishment of antiferromagnetic (AFM) long-range order at TN = 25 K. The latter is marked by sharp λ-type anomalies in both Fisher's specific heat d(χ‖T)/dT and heat capacity Cp. Pulsed magnetic field measurements on the single crystals identify the a axis as the easy magnetic axis and reveal a spin-flop transition at μ0Hspin-flop = 19 T. Density functional theory indicates that in variance with the three-dimensional network of KMnPO4F, it is a two-dimensional Ising magnetic system represented by buckled layers of integer spins S = 2 of Mn3+ ions. The strongest AFM exchange interaction, J1 ∼ -13 K, couples Mn3+ ions into linear chains running along the a axis. The chains themselves are ferromagnetically connected (J3 ∼ -4 K) within the ab plane. The interplane AFM exchange interaction (J2 ∼ -1 K) is weak and frustrated.

Terahertz Laser Pulse Boosts Interlayer Spin Transfer in Two-Dimensional van der Waals Magnetic Heterostructures.

Light-induced ultrafast dynamics in two-dimensional (2D) magnetic systems demonstrate substantial advancements in spintronics. Here, using the real-time time-dependent density functional theory (rt-TDDFT), we applied laser pulses with various frequencies, from terahertz (THz) to optical pulse, to systematically study the interlayer spin transfer dynamics in 2D van der Waals nonmagnetic-ferromagnetic heterostructures, including graphene-Fe3GeTe2 (Gr/FGT) and silicene-Fe3GeTe2 (Si/FGT). Our results demonstrate that low-frequency THz pulses are particularly effective in facilitating interlayer spin injection from the ferromagnetic FGT layers to the Si or Gr layers. On the contrary, high-frequency optical pulses exhibit a minimal influence on this process. Such an effect is attributed to the low-frequency THz pulses inducing in-phase oscillations of the electron charge density around atomic centers, leading to a highly efficient interlayer spin transfer. Our results provide a new insight into ultrafast THz radiation control intralayer spin transfer and magnetic proximity dynamics in the 2D limit.

Low-temperature magnetic ordering in Co core/CoO shell nanoparticles on the copper surface

Cobalt/cobalt oxide nanoparticles electrochemically deposited onto a copper surface revealed an unusual behavior of the coercivity and exchange bias at liquid helium temperature [V.G. Bayev et al., Appl. Surf. Sci. 440 (2018) 1252–1260], pointing out the necessity of careful consideration of magnetic properties at these temperatures. Here, it is shown that the approach based on the random anisotropy model and analysis of law of the approach to magnetic saturation adopted for the two-dimensional (2D) magnetic system allows obtaining the correlation function of the magnetic anisotropy axes in real space, as well as the exchange, random and coherent anisotropy magnetic fields. The long-range correlation in the magnetic anisotropy axes and the oscillating character of the correlation function is the characteristic feature, suggesting the important role of the interplay between the exchange bias and random anisotropy on the one hand and indirect exchange coupling via the conducting electrons of copper and magnetic dipole interaction, on the other.

Fingerprints of super spin-glass state in two-dimensional nanoscopic system

Novel two-dimensional magnetic assembly was designed, manufactured, and its static and dynamic magnetic response was investigated. The assembly contains Fe3O4 nanoparticles of nominal size 7 nm which were deposited on plasma-treated polypropylene substrate using a grafting technique. The nanoparticles create agglomerates with size distribution ranging nominally from 10 nm to 110 nm. The temperature dependence of zero-field and field-cooled susceptibility is consistent with glassy behavior. The relative shift of maximum in alternating susceptibility Γ ≈ 0.075 and zυ = 10 obtained from critical slowing down analysis also support the formation of the collective states akin to super spin-glass. However, the absence of collapse in dynamic scaling of alternating susceptibility data using predictions for three-dimensional spin-glass with Gaussian distribution of magnetic coupling and two-dimensional bimodal spin-glass suggests that the studied system is not representative of the used limiting models. The investigation of memory effects revealed the coexistence of relaxation phenomena of collective degrees of freedom and individual nanoparticles. The obtained results suggest that the studied assembly is appropriate for the investigation of super spin-glass state in two-dimensional magnetic system with dominant dipolar interaction.

Coexistence of magnetic and phononic second-order topological phases in two-dimensional NiZrCl6

Second-order topological phases (SOTPs) in two-dimensional (2D) magnetic and phononic systems are rarely reported. In this Letter, using first-principles calculations, we propose that the NiZrCl6 monolayer with space group P312 (No. 149) is a 2D ferromagnetic material with rich SOTPs: (i) magnetic SOTPs can be found in the band structures of both spin channels in NiZrCl6. NiZrCl6 hosts topologically protected corner states that have a quantized fractional charge (e/3) and are spin-polarized and pinned at the corners of the sample in real space. The SOTP nature in the NiZrCl6 monolayer is resistant to the spin–orbit coupling effect. (ii) Phononic SOTPs can be found in the phonon curves of NiZrCl6. The corner vibrational modes appear inside the frequency gap around 7.98 THz of the NiZrCl6 monolayer, and the secondary topological index can verify the nontrivial phase. The proposed 2D NiZrCl6 material can be a starting point for exploring higher-order topological phases in 2D magnetic and phononic systems.

A fast magnetic vector characterization method for quasi two-dimensional systems and heterostructures

The use of magnetic vector tomography/laminography has opened a 3D experimental window to access the magnetization at the nanoscale. These methods exploit the dependence of the magnetic contrast in transmission to recover its 3D configuration. However, hundreds of different angular projections are required leading to large measurement times. Here we present a fast method to dramatically reduce the experiment time specific for quasi two-dimensional magnetic systems. The algorithm uses the Beer-Lambert equation in the framework of X-ray transmission microscopy to obtain the 3D magnetic configuration of the sample. It has been demonstrated in permalloy microstructures, reconstructing the magnetization vector field with a reduced number of angular projections obtaining quantitative results. The throughput of the methodology is × 10–× 100 times faster than conventional magnetic vector tomography, making this characterization method of general interest for the community.

Chirality selective magnon-phonon hybridization and magnon-induced chiral phonons in a layered zigzag antiferromagnet

Two-dimensional (2D) magnetic systems possess versatile magnetic order and can host tunable magnons carrying spin angular momenta. Recent advances show angular momentum can also be carried by lattice vibrations in the form of chiral phonons. However, the interplay between magnons and chiral phonons as well as the details of chiral phonon formation in a magnetic system are yet to be explored. Here, we report the observation of magnon-induced chiral phonons and chirality selective magnon-phonon hybridization in a layered zigzag antiferromagnet (AFM) FePSe3. With a combination of magneto-infrared and magneto-Raman spectroscopy, we observe chiral magnon polarons (chiMP), the new hybridized quasiparticles, at zero magnetic field. The hybridization gap reaches 0.25 meV and survives down to the quadrilayer limit. Via first principle calculations, we uncover a coherent coupling between AFM magnons and chiral phonons with parallel angular momenta, which arises from the underlying phonon and space group symmetries. This coupling lifts the chiral phonon degeneracy and gives rise to an unusual Raman circular polarization of the chiMP branches. The observation of coherent chiral spin-lattice excitations at zero magnetic field paves the way for angular momentum-based hybrid phononic and magnonic devices.

Antiferromagnetism-driven two-dimensional topological nodal-point superconductivity

Magnet/superconductor hybrids (MSHs) hold the promise to host emergent topological superconducting phases. Both one-dimensional (1D) and two-dimensional (2D) magnetic systems in proximity to s-wave superconductors have shown evidence of gapped topological superconductivity with zero-energy end states and chiral edge modes. Recently, it was proposed that the bulk transition-metal dichalcogenide 4Hb-TaS2 is a gapless topological nodal-point superconductor (TNPSC). However, there has been no experimental realization of a TNPSC in a MSH system yet. Here we present the discovery of TNPSC in antiferromagnetic (AFM) monolayers on top of an s-wave superconductor. Our calculations show that the topological phase is driven by the AFM order, resulting in the emergence of a gapless time-reversal invariant topological superconducting state. Using low-temperature scanning tunneling microscopy we observe a low-energy edge mode, which separates the topological phase from the trivial one, at the boundaries of antiferromagnetic islands. As predicted by the calculations, we find that the relative spectral weight of the edge mode depends on the edge’s atomic configuration. Our results establish the combination of antiferromagnetism and superconductivity as a novel route to design 2D topological quantum phases.

High-Mobility Magnetic Two-Dimensional Electron Gas in Engineered Oxide Interfaces

The engineered interfaces of complex oxides have abundant physical properties and provide a powerful platform for the exploration of fundamental physics and emergent phenomena. In particular, research on the two-dimensional magnetic systems with high mobility remains a long-standing challenge for the discovery of quantum phase and spintronic applications. Here, we introduce a few atomic layers of the delta doping layer at LaAlO3/SrTiO3 interfaces through elaborately controllable epitaxial growth of SrRuO3. After inserting a SrRuO3 buffer layer, the interfaces exhibit a well-defined anomalous Hall effect up to 100 K and their mobility is enhanced by 3 orders of magnitude at low temperatures. More intriguingly, a large unsaturated positive magnetoresistance is created at interfaces. Combining with the density functional theory calculation, we attribute our findings to the electron transfer at interfaces and the magnetic moment of Ru4+ 4d bands. The results pave a way for further research of two-dimensional ferromagnetism and quantum transport in all-oxide systems.

The effect of defects on magnetic droplet nucleation

Defects and impurities strongly affect the timing and the character of the (re)ordering or disordering transitions of thermodynamic systems captured in metastable states. In this paper we analyse the case of two-dimensional magnetic systems. We adapt the classical JMAK theory to account for the effects of defects on the free energy barriers, the critical droplet area and the associated metastable time. The resulting predictions are successfully tested against the Monte-Carlo simulations performed by adopting Glauber dynamics, to obtain reliable time-dependent results during the out-of-equilibrium transformations. We also focus on finite-size effects, and study how the spinodal line (separating the single-droplet from the multi-droplet regime) depends on the system size, the defect fraction, and the external field.

Tunable large spin Nernst effect in a two-dimensional magnetic bilayer

We theoretically investigate topological spin transport of the magnon polarons in a bilayer magnet with two-dimensional square lattices. Our theory is motivated by recent reports on the van der Waals magnets which show the reversible electrical switching of the interlayer magnetic order between antiferromagnetic and ferromagnetic orders. The magnetoelastic interaction opens band gaps and allows the interband transition between different excitation states. In the layered antiferromagnet, due to the interband transition between the magnon-polaron states, the spin Berry curvature which allows the topological spin transport occurs even if the time-reversal symmetry is preserved. We find that the spin Berry curvature in the layered antiferromagnet is very large due to the small energy spacing between two magnonlike states. As a result, the spin Nernst conductivity shows a sudden increase (or decrease) at the phase transition point between ferromagnetic and antiferromagnetic phases. Our results suggest that the ubiquity of tunable topological spin transports in two-dimensional magnetic systems.

Approaching the Topological Low-Energy Physics of the F Model in a Two-Dimensional Magnetic Lattice.

We demonstrate that the physics of the F model can be approached very closely in a two-dimensional artificial magnetic system. Faraday lines spanning across the lattice and carrying a net polarization, together with chiral Faraday loops characterized by a zero magnetic susceptibility, are imaged in real space using magnetic force microscopy. Our measurements reveal the proliferation of Faraday lines and Faraday loops as the system is brought from low- to high-energy magnetic configurations. They also reveal a link between the Faraday loop density and icelike spin-spin correlations in the magnetic structure factor. Key for this Letter, the density of topological defects remains small, on the order of 1% or less, and negligible compared to the density of Faraday loops. This is made possible by replacing the spin degree of freedom used in conventional lattices of interacting nanomagnets by a micromagnetic knob, which can be finely tuned to adjust the vertex energy directly, rather than modifying the two-body interactions.

Low-dimensional Magnetism in Compounds with Different Dimensions of Magnetic Interaction

The results of a comparison of the exchange interaction mechanisms in low dimensional magnetic systems are presented. It has been shown that ZnO crystal may be used as a semiconductor non-magnetic matrix for the formation of quasi-one-dimensional and quasi-zero-dimensional magnetic systems by introducing impurity atoms of Cr, Mn, Fe, Co and Ni. Structural parameters, electronic and magnetic properties were calculated at the atomic level in the framework of quantum mechanical simulation. The exchange interaction integrals were calculated at the microscopic level using the Heisenberg model. The exchange interaction mechanisms were determined on the obtained dependences of the exchange interaction integral on the structural and electronic properties, as well as on the features of the low-dimensional magnetic systems partial density of electronic states. The results of studying the exchange interaction mechanisms in two-dimensional magnetic systems formed in materials of the MAX3 (M= Cr, Fe, A = Ge, Si, X= S, Se, Te) group are summarized. The established mechanisms made it possible to compare the conditions for the formation of a ferromagnetic order in systems with different dimensions of magnetic interaction. The ferromagnetic order in all the structures under study is formed due to the indirect superexchange interaction between orbitals of different symmetry. Strategies aimed at enhancing the superexchange interactions between orbitals of different symmetry or attenuating the contributions of the exchange interaction between orbitals of the same symmetry contribute to the formation of stable hightemperature ferromagnetism.

Optimization of physical quantities in the autoencoder latent space

We propose a strategy for optimizing physical quantities based on exploring in the latent space of a variational autoencoder (VAE). We train a VAE model using various spin configurations formed on a two-dimensional chiral magnetic system. Three optimization algorithms are used to explore the latent space of the trained VAE. The first algorithm, the single-code modification algorithm, is designed for improving the local energetic stability of spin configurations to generate physically plausible spin states. The other two algorithms, the genetic algorithm and the stochastic algorithm, aim to optimize the global physical quantities, such as topological index, magnetization, energy, and directional correlation. The advantage of our method is that various optimization algorithms can be applied in the latent space containing the abstracted representation constructed by the trained VAE model. Our method based on latent space exploration is utilized for efficient physical quantity optimization.

Two-dimensional ferromagnetism detected by proximity-coupled quantum Hall effect of graphene

The recent discovery of a two-dimensional van der Waals magnet has paved the way for an enhanced understanding of two-dimensional magnetic systems. The development of appropriate heterostructures in this emerging class of materials is required as the next step towards applications. Here, we report on the electrical transport in monolayer graphene coupled with the two-dimensional ferromagnet Cr2Ge2Te6 (CGT). Graphene that forms an interface with CGT is electron-doped owing to charge transfer. The temperature-dependent resistance of graphene/CGT undergoes a nontrivial sudden change near the Curie temperature (Tc) of CGT. Apart from this, the behavior of various transport parameters also differs before and after Tc. Moreover, the contribution of the magnetization of CGT to the enhanced magnetic flux density leads to the critical evolution of the quantum Hall state. These results imply that graphene in the graphene/CGT hybrid structure can be utilized to electrically monitor the magnetic phase transition of the adjacent CGT layer.

Coexistence of multiple Weyl fermions and quantum anomalous Hall effect in 2D half-metallic Cr2NT2

Topological state in two-dimensional (2D) magnetic system has attracted widespread attention due to its potential application in spintronics and quantum computing fields. As a novel topological states of matter, Weyl semimetals with multiple topological phases have been studied. However, realizing multiple topological phases in 2D magnetic system and studying their entanglement still face great challenges. In this work, using first-principles calculations, we predict that 2D monolayer Cr2NT2 (T = -OH, -F, =O) has multiple Weyl fermions (WFs) and large spin-down gap. Ferromagnetic ground states of monolayer Cr2NT2meet the dynamic, thermal and mechanical stabilities. Without spin-orbit coupling (SOC), these WPs are fully spin-polarized. When considering SOC, apart from WP3 in Cr2NO2, all WPs are opened the gap, and the chiral edge states are observed in Cr2NT2. In particular, there is a quantum anomalous Hall effect in Cr2NF2. Further, a biaxial strain in the a-b plane leads to the emergence of rich topological phases without SOC. In addtion, an effective four-band tight-binding model is constructed to clarify the origin of WFs in the non-trivial band topology. Finally, we calculate the Berry curvature and abnormal Hall conductivity of Cr2NT2, and find that they are a non-zero energy points around the Fermi level, indicating that monolayer Cr2NT2 are promising candidates for spintronic applications. Therefore, it is necessary to explore 2D magnetic materials with multiple types of WFs and QAH phases.

The impacts of molecular adsorption on antiferromagnetic MnPS3 monolayers: enhanced magnetic anisotropy and intralayer Dzyaloshinskii–Moriya interaction

In two-dimensional (2D) magnetic systems, significant magnetic anisotropy is required to protect magnetic ordering against thermal fluctuation.

Induced ferromagnetism on late transition metals adsorbed on Antimony Arsenide monolayer from First-Principles

We investigate the structural, electronic and magnetic properties of transition metal (including Mn, Fe, Co, Ni, Cu, Rh and Pd) adsorbed on SbAs monolayer by using density functional theory. We found that an induced magnetic moment is observed for Mn, Fe, Co and Rh transition metal atoms, while in the case of Cu and Ni it is suppressed by local distortions. Our calculations show that Mn, Fe, Co and Rh can induce total spin magnetic moment of 4.86μB, 3.51μB, 1.99μB and 0.90μB, respectively. The origin of magnetism lays is their d orbital and depends on its filling level. In particular, the total magnetic moment decreases with increasing the number of d electrons. Among the studied transition metals, Mn seems to be the most promising candidate for the introduction of ferromagnetic ordering into SbAs, as it showed a high magnetic moment close to 5μB. Overall, the analysis of the electronic and magnetic properties indicates that transition metal doped SbAs are promising two-dimensional magnetic systems.