Antiferromagnetic Materials Research Articles

Spin-valley switch and conductance oscillations in antiferromagnetic transition metal dichalcogenides

Antiferromagnetic materials are regarded as the outstanding candidates for the next generation of spintronics applications thanks to the numerous interesting features. We theoretically study the spin and valley transport in transition metal dichalcogenides in the present of antiferromagnetic exchange field. It is found that the spin and valley dependent band gap can be controlled by the exchange field. The system could become a spin-valley half metal, where a certain spin-valley electron is metallic state and other electrons are insulating states. The normal/antiferromagnetic/normal junction could work as an effective spin-valley switch controlled by the gate voltage. In the normal/antiferromagnetic/normal/ferromagnetic/normal junction, quantum beats occur in the oscillations of total conductance. The beat phenomenon results from the interference of two different spin-valley dependent conductances with similar frequencies. In addition, the junction can also work as a magnetoresistance device.

Phonon-mediated strong coupling between a three-dimensional topological insulator and a two-dimensional antiferromagnetic material

We numerically study strong coupling between terahertz excitations in a hybrid material consisting of a three-dimensional (3D) topological insulator (TI) and a quasi-two-dimensional (2D) van der Waals antiferromagnet. We find that the interaction between a surface Dirac plasmon polariton in the 3D TI and a magnon polariton in the 2D antiferromagnet is mediated by the phonon coupling in the 3D TI material and can result in emergence of a new hybridized mode, namely, a surface Dirac plasmon-phonon-magnon polariton. We numerically study the dependence of the strong coupling on a variety of structural parameters of the 3D-TI/2D-antiferromagnetic (AFM) hybrid material. Our results reveal that the strength of the coupling depends primarily on the anisotropy constant of the 2D AFM material, as well as on its thickness, and reaches a maximum when the AFM layer is sufficiently thick to be considered a half-infinite slab. We show that the extremely large anisotropy constant reported for certain 2D van der Waals antiferromagnets results in a coupling strength that should be experimentally observable even in the presence of realistic scattering losses.

Field-free spin–orbit torque switching of an antiferromagnet with perpendicular Néel vector

The field-free spin–orbit torque induced 180° reorientation of magnetization is beneficial for the high performance magnetic memory. The antiferromagnetic material (AFM) can provide a higher operation speed than the ferromagnetic counterpart. In this paper, we propose a trilayer AFM/insulator/heavy metal structure as the AFM memory device. We show that the field-free switching of the AFM with a perpendicular Néel vector can be achieved by using two orthogonal currents, which provide a uniform damping-like torque and uniform field-like torque, respectively. The reversible switching can be obtained by reversing either current. A current density of 1.79 × 1011 A/m2 is sufficient to induce the switching. In addition, the two magnetic moments become noncollinear during switching. This enables an ultrafast switching within 40 ps. The device and switching mechanism proposed in this work offer a promising approach to deterministically switch the AFM with the perpendicular Néel vector. It can also stimulate the development of an ultrafast AFM-based MRAM.

Identification of phonon symmetry and spin-phonon coupling in van der Waals antiferromagnetic FePSe3

For engineering of van der Waals antiferromagnetic material FePSe3 and expanding its potential technological promise in spintronic fields, a detailed knowledge of the underlying physics of vibrational phonon modes and their coupling with the spin degree of freedom are undoubtedly needed. In this work, by angle-resolved polarized Raman spectroscopy in corroboration with first-principles calculation, the Raman modes of bulk FePSe3 have been rationally assigned, which clarify the ambiguity about symmetry identification of some Raman-active modes in previous studies. Moreover, through detailed temperature-dependent Raman scattering experiments, the abnormal shift of the frequency, linewidth, and integrated intensity across the Néel temperature have been identified for phonon modes associating with a non-magnetic [P2Se6]4- cluster. This can be well explained with the assistance of the spin-phonon coupling mechanism that involves the variation of the d electron transfer with lattice vibration in magnetic materials. Our results are helpful for uncovering the rich physics in FePSe3 and also for enriching the further understanding of magnetic van der Waals materials down to the 2D limit.

Ab initio prediction of anomalous Hall effect in antiferromagnetic CaCrO3

While the anomalous Hall effect takes place typically in ferromagnets with finite magnetization, large anomalous Hall conductivity in noncollinear antiferromagnetic systems has been recently observed and attracted much attention. In this paper, we predict the anomalous Hall effect in perovskite ${\mathrm{CaCrO}}_{3}$ as a representative of collinear antiferromagnetic materials. Our result shows that the $C$-type antiferromagnetic ordering generates the sizable anomalous Hall conductivity. Based on symmetry analyses, we show that the antiferromagnetic order parameter belongs to the same irreducible representation as the ferromagnetic order parameter in the nonsymmorphic space group, allowing the nonvanishing Berry curvatures in $k$ space. By performing first-principles density-functional theory calculations, we find that the Berry-curvature ``hot spots'' lie along the gapped nodal lines where spin-orbit coupling induces the spin splitting of Cr-$3d$ bands near the Fermi energy and enhances the anomalous Hall effect in ${\mathrm{CaCrO}}_{3}$.

ARPES studies of the ground state electronic properties of the van der Waals transition metal trichalcogenide CoPS[formula omitted

The electronic structure of a wide bandgap van der Waals antiferromagnetic material CoPS3 (transition metal phosphorus trichalcogenide) is studied using angle-resolved photoelectron spectroscopy at room temperature which is well above the TN transition temperature for this compound. The observed dispersion of the electronic states in the valence band is correctly described using spin-resolved band structure. This observation points to the importance of the local electron–electron correlations and fluctuations for the description of the electronic and magnetic properties of transition metal phosphorus trichalcogenides and draws further ideas for the studies of electronic structure of CoPS3 and other similar materials.

Out-of-plane weak ferromagnetism at room temperature in lattice-distortion non-collinear antiferromagnet of single-crystal Mn3Sn

Out-of-plane weak ferromagnetic (OWFM) spin arrangements with topological properties can realize a series of interesting physical properties. However, this spin structure tends to exist at low temperatures. The OWFM structure can also be induced at room temperature by hydrostatic pressure, whereas this isotropic approach tends to form helical AFM structures. We report the OWFM spin arrangement in single crystal Mn3Sn by an anisotropic strategy of high-stressconstrained compression deformation at room temperature. Both experimental and theoretical simulation results show that the alignment of the OWFM spin structure is due to the distortion of the atomic scale caused by the strain energy during deformation. The OWFM spin arrangement can significantly change the magnetic property of Mn3Sn. As a result, the remanent magnetization M r for the deformed sample (0.056 μ B/f.u.) is about eleven times that for the pre-deformed sample (0.005 μ B/f.u.), and the coercivity (H c) increases from 0 kOe (pre-deformed sample) to 6.02 kOe (deformed sample). Our findings provide a way to generate the OWFM spin structure at room temperature and may give fresh ideas for creating antiferromagnetic materials with excellent physical properties.

Acoustically assisted energy efficient field free spin orbit torque switching of out of plane nanomagnet

Deterministic spin orbit torque (SOT) magnetization switching of the perpendicular magnetic anisotropy structures requires an external magnetic field, which is unsuitable for on-chip applications. Various approaches are there to debar the external magnetic field requirement. In this work, we use static stress, which can generate a virtual magnetic field via magnetoelastic coupling. We show that this field can be used for deterministic magnetization switching. In our simulations, we use an antiferromagnetic material for generating the SOT and exchange bias field and avail the benefits of the field like torque (FLT). With the exclusion of thermal noise, this reduces the threshold current density from 114 to 36 MA/cm2, thus mitigating the energy dissipation by more than nine times. To study the effect of thermal noise, we simulate 500 trajectories and find out the switching probability. We perform simulations to study the effects of current density, stress, pulse width, and FLT on the switching probability in great details. Our simulation results show that we can get sub-ns switching with a probability of 0.99 requiring only 45.5 fJ of energy dissipation. This can be very attractive for artificial neural network kind of applications.

The effect of anisotropic field on magnon dispersion of antiferromagnetic metal fluoride materials

This article investigates the dynamics of spin waves in two sublattice antiferromagnetic (AFM) system spins’ interaction and magnon dispersion relation. The analysis is done starting with a standard model that includes a uniaxial magnetic anisotropy field. Quantum field theory is employed in formulating the problem, and double-time temperature-dependent Green function technique is used to obtain magnon dispersion, whereby random phase approximation is considered to decouple and diagonalize the higher order terms. At low temperatures and long wavelength approximation, dispersion of uniaxial symmetric AFM crystal lattice of metallic fluoride materials, such as MnF2, FeF2, and RbMnF3, is analyzed for illustration. Our findings indicate that as the anisotropic field increases, the magnon dispersion vs wave vector k gradually varies, attempts linearity from being curved, and attains a sinusoidal structure when extended further in the first Brillouin zone. Perhaps, these results are useful to understand the feature of magnon dispersion of stable similar AFM materials for practical application.

First principles study in two-dimensional antiferromagnetic Mn2Cl8 with strain-controllable and hydrogenation

With the rapid development of spintronics, two-dimensional antiferromagnetic materials have attracted much attention because of their unique physical properties. Here, the monolayer Mn2Cl8 is discovered to be an intrinsically antiferromagnetic semiconductor in current work. The results show that monolayer Mn2Cl8 and Mn2Cl4X4 (X = F, Br) are stable semiconductors with indirect bandgaps of 0.34eV, 0.95eV, and 0.55eV, respectively, and Mn2Cl8 has a Néel temperature (TN) of 245 K. In the systematic study of strain effects, TN changes significantly under strains from −4% to 4% when the antiferromagnetic ground state is not affected. And the compression strain can increase TN to 469 K due to the enhancement of antiferromagnetic coupling of the nearest adjacent magnetic atoms. Moreover, the bandgap and TN of monolayer Mn2Cl8 can be tuned by hydrogenation. This work finds that elemental substitution, strains, and hydrogen passivation is efficient routes to tune the electronic properties of monolayer antiferromagnetic semiconductor Mn2Cl8.

Emission of coherent THz magnons in an antiferromagnetic insulator triggered by ultrafast spin–phonon interactions

Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices owing to their ultrafast spin dynamics. Manipulating coherently their spin dynamics, however, remains a key challenge that is envisioned to be accomplished by spin-orbit torques or direct optical excitations. Here, we demonstrate the combined generation of broadband THz (incoherent) magnons and narrowband (coherent) magnons at 1 THz in low damping thin films of NiO/Pt. We evidence, experimentally and through modeling, two excitation processes of spin dynamics in NiO: an off-resonant instantaneous optical spin torque in (111) oriented films and a strain-wave-induced THz torque induced by ultrafast Pt excitation in (001) oriented films. Both phenomena lead to the emission of a THz signal through the inverse spin Hall effect in the adjacent heavy metal layer. We unravel the characteristic timescales of the two excitation processes found to be < 50 fs and > 300 fs, respectively, and thus open new routes towards the development of fast opto-spintronic devices based on antiferromagnetic materials.

Calculating spin-lattice interactions in ferro- and antiferromagnets: The role of symmetry, dimension, and frustration

Recently, the interplay between spin and lattice degrees of freedom has gained a lot of attention due to its importance for various fundamental phenomena as well as for spintronic and magnonic applications. Examples are ultrafast angular momentum transfer between the spin and lattice subsystems during ultrafast demagnetization, frustration driven by structural distortions in transition-metal oxides, or in acoustically driven spin-wave resonances. In this work, we provide a systematic analysis of spin-lattice interactions for ferro- and antiferromagnetic materials and focus on the role of lattice symmetries and dimensions, magnetic order, and the relevance of spin-lattice interactions for angular momentum transfer as well as magnetic frustration. For this purpose, we use a recently developed scheme, which allows an efficient calculation of spin-lattice interaction tensors from first principles. In addition to that, we provide a more accurate and self-consistent scheme to calculate ab initio spin-lattice interactions by using embedded clusters, which allows us to benchmark the performance of the scheme introduced previously.

Iron oxyhalides monolayers with excellent optical anisotropic properties and large anisotropic carrier mobility

Developing two-dimensional layered materials is highly desirable for nanoscale device applications. Antiferromagnetic materials have attracted considerable attention through their absence of production of parasitic stray fields, ultrafast dynamics, and the generation of large magnetotransport effects. Here, based on first-principles calculations, we demonstrate a series of promising two-dimensional semiconductors in the family of iron oxyhalides FeOX (X = F, Cl, Br, I) with antiferromagnetism, anisotropic optical properties and large carrier mobility. Combined with appropriate gaps, this renders FeOX monolayers with high absorption coefficient up to 3.90×105cm−1 in the visible region. In addition, due to their large band dispersion, FeOX monolayers are also found to harbor considerably high carrier mobilities, especially for FeOBr (6.84×103cm2V-1s-1), suggesting their enormous potential for optoelectronic fields. All of these discoveries make these FeOX monolayers compelling materials for next-generation nanoscale photovoltaic device.

Theory of optical generation and detection of propagating magnons in an antiferromagnet

We report a theory of optical generation and detection of the propagating spin waves in antiferromagnetic materials relevant for ultrafast pump-probe experiments. We derive and solve the equations of motion for antiferromagnetic spins in response to the light-induced effective magnetic field in the linear regime. Different forms of the excitation and the properties of the generated spin waves are analyzed. We theoretically show the selective detection of the spin waves by the magneto-optical Kerr effect. The developed formalism is readily applicable to inform future experiments on antiferromagnetic optomagnonics.

A DFT study of structural, electronic, optical, mechanical, thermoelectric, and magnetic properties of Pb-halide perovskites LiPbX3 (X = Cl, Br, and I) for photovoltaic applications

The structural, electronic, optical, mechanical, thermal, and magnetic properties of Pb-based cubic perovskites LiPbX3 (X = Cl, Br, and I) were evaluated by using the LDA + U technique within the CASTEP code. The calculated lattice constants of studied materials are found 5.8048 Å, 6.0707 Å and 6.3792 Å for LiPbCl3, LiPbBr3 and LiPbI3, respectively. The electronic properties show that all three materials are semiconductor in nature. The calculated energy band gaps are 1.801 eV, 2.337 eV and 1.943 eV for LiPbCl3, LiPbBr3 and LiPbI3, respectively. The optical properties of all three materials are discussed for the photon energy from 0 to 25 eV to explain interaction of light with the material. The mechanical properties reveal that all three materials possess a ductile behavior. The thermal properties revealed that the value of Debye temperature is 169 K, 128 K and 87 K for LiPbBr3, LiPbCl3 and LiPbI3, respectively. The magnetic properties suggest that the total magnetic dipole of studied materials is zero and thus all three materials behave as antiferromagnetic materials. The theoretical power conversion efficiency is 24.37%, 16.17% and 08.14% for LiPbI3, LiPbBr3 and LiPCl3, respectively. The electronic and optical properties reveal that all considered materials are suitable for photovoltaic and optoelectronics applications.

Towards a theory of surface orbital magnetization

The theory of bulk orbital magnetization has been formulated both in reciprocal space based on Berry curvature and related quantities, and in real space in terms of the spatial average of a quantum mechanical local marker. Here we consider a three-dimensional antiferromagnetic material having a vanishing bulk but a nonzero surface orbital magnetization. We ask whether the surface-normal component of the surface magnetization is well defined, and if so, how to compute it. As the physical observable corresponding to this quantity, we identify the macroscopic current running along a hinge shared by two facets. However, the hinge current only constrains the difference of the surface magnetizations on the adjoined facets, leaving a potential ambiguity. By performing a symmetry analysis, we find that only crystals exhibiting a pseudoscalar symmetry admit well-defined magnetizations at their surfaces at the classical level. We then explore the possibility of computing surface magnetization via a coarse-graining procedure applied to a quantum local marker. We show that multiple expressions for the local marker exist, and apply constraints to filter out potentially meaningful candidates. Using several tight-binding models as our theoretical test bed and several potential markers, we compute surface magnetizations for slab geometries and compare their predictions with explicit calculations of the macroscopic hinge currents of rod geometries. We find that only a particular form of the marker consistently predicts the correct hinge currents.

Tuning magnetism by electric field in MnPS3/Sc2CO2 van der Waals heterostructure

Combining a two-dimensional (2D) antiferromagnetic (AFM) material, MnPS3 and a 2D ferroelectric material, Sc2CO2, we propose 2D van der Waals (vdW) heterostructure multiferroics to realize strong magnetoelectric coupling, which is important for designing high-performance magnetoelectric devices. By using first-principles simulations, it is found that the transition from an AFM state to a ferromagnetic (FM) state of a MnPS3 layer could be realized by reversing the polarization direction of a Sc2CO2 layer. We further reveal that such strong magnetoelectric effects originate from the large inter-layer charge transfer due to the competitive interaction between the difference of the interface work functions between MnPS3 and Sc2CO2 and the strong electronegativity of the O atom interface in the Sc2CO2 layer. Our results suggest a feasible scheme for constructing 2D vdW heterostructure multiferroics with very strong inter-layer magnetoelectric coupling effect.

Electronic structure and layer-dependent magnetic order of a new high-mobility layered antiferromagnet KMnBi

Room-temperature two-dimensional antiferromagnetic (AFM) materials are highly desirable for various device applications. In this letter, we report the low-energy electronic structure of KMnBi measured by angle-resolved photoemission spectroscopy, which confirms an AFM ground state with the valence band maximum located at −100 meV below the Fermi level and small hole effective masses associated with the sharp band dispersion. Using complementary Raman, atomic force microscope and electric transport measurement, we systematically study the evolution of electric transport characteristics of micro-mechanically exfoliated KMnBi with varied flake thicknesses, which all consistently reveal the existence of a probable AFM ground state down to the quintuple-layer regime. The AFM phase transition temperature ranges from 220 K to 275 K, depending on the thickness. Our results suggest that with proper device encapsulation, multilayer KMnBi is indeed a promising 2D AFM platform for testing various theoretical proposals for device applications.

Controlling Magnetism with Light in a Zero Orbital Angular Momentum Antiferromagnet.

Antiferromagnetic materials feature intrinsic ultrafast spin dynamics, making them ideal candidates for future magnonic devices operating at THz frequencies. A major focus of current research is the investigation of optical methods for the efficient generation of coherent magnons in antiferromagnetic insulators. In magnetic lattices endowed with orbital angular momentum, spin-orbit coupling enables spin dynamics through the resonant excitation of low-energy electric dipoles such as phonons and orbital resonances which interact with spins. However, in magnetic systems with zero orbital angular momentum, microscopic pathways for the resonant and low-energy optical excitation of coherent spin dynamics are lacking. Here, we consider experimentally the relative merits of electronic and vibrational excitations for the optical control of zero orbital angular momentum magnets, focusing on a limit case: the antiferromagnet manganese phosphorous trisulfide (MnPS_{3}), constituted by orbital singlet Mn^{2+} ions. We study the correlation of spins with two types of excitations within its band gap: a bound electron orbital excitation from the singlet orbital ground state of Mn^{2+} into an orbital triplet state, which causes coherent spin precession, and a vibrational excitation of the crystal field that causes thermal spin disorder. Our findings cast orbital transitions as key targets for magnetic control in insulators constituted by magnetic centers of zero orbital angular momentum.

Generalized form of the magnetic anisotropy field in micromagnetic and atomistic spin models

We present a general approach to the derivation of the effective anisotropy field which determines the dynamical behaviour of magnetic spins according to the Landau-Lifshitz-Gilbert equation. The approach is based on the gradient in spherical polar coordinates with the final results being expressed in Cartesian coordinates as usually applied in atomistic and micromagnetic model calculations. The approach is generally valid for all orders of anisotropies including higher order combinations of azimuthal and rotational anisotropies often found in functional magnetic materials such as permanent magnets and an emerging class of antiferromagnetic materials with applications in spintronics. Anisotropies are represented in terms of spherical harmonics which have the important property of rational temperature scaling. Effective field vectors are given for anisotropies up to sixth order, presenting a unified framework for implementing higher order magnetic anisotropies in numerical simulations.