A-type Antiferromagnetic Research Articles

Design of spintronic devices based on adjustable half-metallicity induced by electric field in A-type antiferromagnetic bilayer NiI2

Exploring the attainment of half-metallic behavior in two-dimensional (2D) materials through external perturbations is a popular area of current research. In this work, we demonstrate, using first-principles calculations, that bilayer NiI2 (bi-NiI2) is an A-type antiferromagnetic (AFM) semiconductor with an indirect bandgap of 0.86 eV, with the most stable configuration being the AB stacking mode. Upon the application of a vertical electric field, the material transforms from its original semiconducting state into a half-metallic state. Moreover, the spin polarization reverses its orientation whenever the direction of the electric field is altered. This intriguing behavior has inspired us to design a spintronic device based on the A-type AFM bi-NiI2. By employing nonequilibrium Green's function (NEGF) combined with density functional theory (DFT) calculations, we find that the device achieves ON/OFF switching by applying vertical electric fields in parallel or anti-parallel configurations in the two leads. The device displays 100 % spin polarization in the parallel configuration (PC) scenario, driven by bias voltage or temperature differences. Utilizing either the parallel or antiparallel configuration (APC) for ON/OFF switching enables the device to exhibit tunneling magnetoresistance (TMR) of up to 1.45 × 1010 % due to bias voltage and up to 1011 % thermal TMR arising from temperature differences between the leads. These findings highlight the potential of NiI2 and A-type AFM bilayers in the design of spintronic devices.

Electronic and Transport Engineering of A-Type Antiferromagnets with Ferroelectric Sandwich Structure: Toward Multistate Nonvolatile Memory Applications.

Achieving higher-order multistates with mutual interstate switching at the nanoscale is essential for high-density storage devices; yet, it remains a significant challenge. Here, we demonstrate that integrating A-type antiferromagnetic semiconductors sandwiched between ferroelectric layers is an effective strategy to achieve high-performance multistate data storage. Taking the Sc2CO2/VSi2P4 bilayer (bi-VSi2P4)/Sc2CO2 van der Waals multiferroic heterostructure as an example, our first-principles calculations show that by switching the polarization direction of the upper and bottom ferroelectric Sc2CO2 layers, antiferromagnetic bi-VSi2P4 can exhibit four distinct states with different band structures. The intriguing band structure engineering stems from the polarization-field-induced band shift and interface charge transfer. Accordingly, the proposed Sc2CO2/bi-VSi2P4/Sc2CO2-based multiferroic device can achieve four different resistance states, accompanied by fully spin-polarized currents and giant tunneling electroresistance ratios. Our results propose a viable strategy for realizing nonvolatile electrical control of antiferromagnets at the nanoscale and provide insights into the development of advanced memories.

Tunable Magnetism in Atomically Thin Itinerant Antiferromagnet with Room-Temperature Ferromagnetic Order.

Addressing the need for modulated spin configurations is crucial, as they serve as the foundational building blocks for next-generation spintronics, particularly in atomically thin structures and at room temperature. In this work, we realize intrinsic ferromagnetism in monolayer flakes and tunable ferro-/antiferromagnetism in (Fe0.56Co0.44)5GeTe2 antiferromagnets. Remarkably, the ferromagnetic ordering (≥1 L) and antiferromagnetic ordering (≥4 L) remain discernible up to room temperature. The TC (∼310 K) of the monolayer flakes sets a record high for known exfoliated monolayer van der Waals magnets. Within the framework of A-type antiferromagnetism, a notable odd-even layer-number effect at elevated temperatures (T = 150 K) is observed. Of particular interest is the strong ferromagnetic order in even-layer flakes at low temperatures. The intricate interplay among magnetic field strength, layer number, and temperature gives rise to a diverse array of phenomena, holding promise not only for new physics but also for practical applications.

The underlying mechanism of the strain-induced reversible magnetic state transition in MXene nanosheets

MXenes have emerged as one of the most promising members among the two-dimensional (2D) material family owing to their unique physical-chemical properties and outstanding application prospects. Here, we investigated the unique magnetic properties of Ti2C MXenes monolayer by means of the first-principles calculations combining with the Heisenberg spin Hamiltonian approach. A strain-induced reversible magnetic transition for the Ti2C monolayer could be found in our study. When the strain is applied to the Ti2C monolayer, the magnetic configuration of the Ti2C monolayer transforms from the A-type antiferromagnetic (A-AFM) phase to the ferromagnetic (FM) phase with the biaxial tensile strain larger than 6%. Furthermore, the transition from the A-AFM phase to the FM phase was found to be closely related with the change of the Ti and C atomic magnetic moments. The direct magnetic coupling between Ti and C atoms with the increased Ti and C magnetic moments is able to stabilize the structure due to the weakened superexchange interaction between Ti and C atoms under large tensile strain. This is the underlying driving force for the transition from the A-AFM phase to the FM phase. This work provides a promising pathway to better understand the existence of magnetic reversal behavior in the Ti2C MXenes monolayer and opens the door to future magnetic applications of MXenes.

Competing itinerant and local spin interactions in kagome metal FeGe

The combination of a geometrically frustrated lattice, and similar energy scales between degrees of freedom endows two-dimensional Kagome metals with a rich array of quantum phases and renders them ideal for studying strong electron correlations and band topology. The Kagome metal, FeGe is a noted example of this, exhibiting A-type collinear antiferromagnetic (AFM) order at TN ≈ 400 K, then establishes a charge density wave (CDW) phase coupled with AFM ordered moment below TCDW ≈ 110 K, and finally forms a c-axis double cone AFM structure around TCanting ≈ 60 K. Here we use neutron scattering to demonstrate the presence of gapless incommensurate spin excitations associated with the double cone AFM structure of FeGe at temperatures well above TCanting and TCDW that merge into gapped commensurate spin waves from the A-type AFM order. Commensurate spin waves follow the Bose factor and fit the Heisenberg Hamiltonian, while the incommensurate spin excitations, emerging below TN where AFM order is commensurate, start to deviate from the Bose factor around TCDW, and peaks at TCanting. This is consistent with a critical scattering of a second order magnetic phase transition with decreasing temperature. By comparing these results with density functional theory calculations, we conclude that the incommensurate magnetic structure arises from the nested Fermi surfaces of itinerant electrons and the formation of a spin density wave order.

Negative magnetoresistance in the antiferromagnetic semimetal V1/3TaS2

Intercalated transition metal dichalcogenides (TMDCs) attract much attention due to their rich properties and potential applications. In this article, we grew successfully high-quality V1/3TaS2 crystals by a vapor transport method. We measured the magnetization, longitudinal resistivity ρxx (T, H), Hall resistivity ρxy (T, H), as well as performed calculations of the electronic band structure. It was found that V1/3TaS2 is an A-type antiferromagnet with the Neel temperature T N = 6.20 K, and exhibits a negative magnetoresistance (MR) near T N. Both band structure calculations and Hall resistivity measurements demonstrated it is a magnetic semimetal.

Charge Transfer and Asymmetric Coupling of MoSe2 Valleys to the Magnetic Order of CrSBr.

van der Waals heterostructures composed of two-dimensional (2D) transition metal dichalcogenides and vdW magnetic materials offer an intriguing platform to functionalize valley and excitonic properties in nonmagnetic TMDs. Here, we report magneto photoluminescence (PL) investigations of monolayer (ML) MoSe2 on the layered A-type antiferromagnetic (AFM) semiconductor CrSBr under different magnetic field orientations. Our results reveal a clear influence of the CrSBr magnetic order on the optical properties of MoSe2, such as an anomalous linear-polarization dependence, changes of the exciton/trion energies, a magnetic-field dependence of the PL intensities, and a valley g-factor with signatures of an asymmetric magnetic proximity interaction. Furthermore, first-principles calculations suggest that MoSe2/CrSBr forms a broken-gap (type-III) band alignment, facilitating charge transfer processes. The work establishes that antiferromagnetic-nonmagnetic interfaces can be used to control the valley and excitonic properties of TMDs, relevant for the development of opto-spintronics devices.

Fe Site Order and Magnetic Properties of Fe1/4NbS2.

Transition-metal dichalcogenides (TMDs) have long been attractive to researchers for their diverse properties and high degree of tunability. Most recently, interest in magnetically intercalated TMDs has resurged due to their potential applications in spintronic devices. While certain compositions featuring the absence of inversion symmetry such as Fe1/3NbS2 and Cr1/3NbS2 have garnered the most attention, the diverse compositional space afforded through the host matrix composition as well as intercalant identity and concentration is large and remains relatively underexplored. Here, we report the magnetic ground state of Fe1/4NbS2 that was determined from low-temperature neutron powder diffraction as an A-type antiferromagnet. Despite the presence of overall inversion symmetry, the pristine compound manifests spin polarization induced by the antiferromagnetic order at generic k points, based on density functional theory band-structure calculations. Furthermore, by combining synchrotron diffraction, pair distribution function, and magnetic susceptibility measurements, we find that the magnetic properties of Fe1/4NbS2 are sensitive to the Fe site order, which can be tuned via electrochemical lithiation and thermal history.

Interplay of magnetism and band topology in Eu[formula omitted]Ca[formula omitted]Mg[formula omitted]Bi[formula omitted] (x=0, 0.5, 0.67) from first principles study

Recent discovery of the time reversal symmetry breaking magnetic Weyl semimetals (WSMs) has created a huge surge of activities in the field of quantum topological materials. In this work, we have studied systematically various magnetic orders, electronic structure and the interplay between the magnetic order and band topology in one such material, EuMg2Bi2 (EMB) and its Ca doped variant using first principle methods within the framework of density functional theory (DFT). Our thorough investigation reveals the existence of various magnetic order driven topological phases (for example, topological insulator in the A-type antiferromagnetic (A-AFM) phase with Eu moments aligned along the crystallographic a or b direction, Dirac semimetal in the A-AFM phase when Eu moments are parallel to c direction and Weyl semimetal in the ferromagnetic (FM) phase with Eu moments pointing along the c direction) in this material. These phases are found to be energetically very close and hence are expected to be tunable from one to the other by an external handle such as magnetic field or chemical substitution. Most importantly, we observe the existence of a single pair of Weyl points (WPs) connecting valence and conduction band very close to the Fermi level (FL) along Γ-A direction in the FM state of EuMg2Bi2 with Eu moments aligned along the c direction making it an ideal Weyl semimetal in its FM state exactly similar to EuCd2As2. On doping 50% and 67% Ca at Eu sites, we observe the single pair of WPs to move even closer to the FL which is highly desirable for application purposes. Further, we observe that the separation between the WPs in the pair decreases in doped compounds compared to that in the parent compound which has direct consequence on anomalous Hall conductivity (AHC). Our first principles calculation of AHC shows high peak values exactly at these WPs and the peak height decreases when we dope the system with Ca. Therefore, Ca doping can be a good external handle to tune AHC in this system.

Flatbands in frustrated lattice X3MnN3 (X = Ca, Sr, Ba): A first-principles study

Frustrated lattices with dispersionless band structures and fully localized states are an exciting platform for exploring many-body physics. In this work, we identify X3MnN3 (X= Ca, Sr, Ba) as a frustrated lattice and investigate its physical properties in the ferromagnetic (FM) and antiferromagnetic (AFM) states based on first-principles calculations. Our results show that all three materials in FM and A-type AFM configurations have flatbands with band touching in the kz∼0 and kz∼π planes with bandwidths less than 0.2 eV. Intriguingly, the flatband is tuned to the Fermi level when X3MnN3 is transformed into the FM state. Furthermore, we find the two-dimensional dice model hidden in X3MnN3 by treating the coupled Mn and N atoms as a basic site, revealing that the compact localized state is generated by destructive interferences between the hopping amplitudes, and the dice models stack the system staggered along the c-axis. Our work provides new candidate materials for exploring strong correlation physics and reveals the mechanism of their localized state.

Spin density wave and antiferromagnetic transition in EuFe2As2: a high field transport and heat capacity study

Abstract Here we reported structural, electrical, magnetic, and thermal transport properties of ternary pnictide bulk iron based compound EuFe2As2 (Eu-122). This compound (Eu-122) crystallized in ThCr2Si2-type tetragonal phase structure with space group I4/mmm at ambient temperature. A promising divalent state (Eu2+) of Eu-ions was observed in the studied EuFe2As2. Magnetic ordering of Eu2+ ions takes place at very low temperature at around T N = 20 K in EuFe2As2. The ferromagnetic interactions between Fe–Fe ions were established at higher temperature which was revealed from magnetic susceptibility measurements with negative value of the paramagnetic Curie temperature. Both magnetic phase transitions (20 K and 190 K) were clearly established the intrinsic magnetic nature revealed by both electrical transport and specific heat measurement. However the phase transition at low temperature corresponds to the magnetic ordering of Eu2+ ions while the high transition temperature is due to the itinerant moment of Fe. EuFe2As2 is the only compound among various parent compounds of iron pnictide superconductor’s family, in which both spin density wave (SDW) of Fe and A-type antiferromagnetic (AFM) ordering of the localized Eu2+ magnetic moments take place simultaneously. We observed here that the localized character of Eu anti-ferromagnetism dominated via RKKY interactions, despite the largely itinerant nature of Fe magnetic interactions. The resistivity with applied magnetic field revealed that the AFM ordering temperature of Eu2+ ions suppress with applied magnetic field. Also resistivity under hydrostatic pressure measurements shows the T SDW (Fe) transition of the Fe moments shifts towards the lower temperatures while AFM ordering of Eu2+ decreases with pressure and the same is completely disappears at 2 GPa.

Electric-Field Control of Spin Polarization above Room Temperature in Single-Layer A-Type Antiferromagnetic Semiconductor

Two-dimensional (2D) antiferromagnets have drawn great interest for absence of stray fields in antiferromagnetic (AFM) spintronics. However, it remains challenging to manipulate their spin polarization above room temperature for practical applications. Herein, a general strategy is reported to realize the control of spin polarization above room temperature in 2D A-type AFM semiconductors by external electric field based on first-principles calculations, exemplified by transition metal monohalide MnCl and carbide MXenes Cr2CX2 (X = F, Cl, OH). It shows that 100% spin polarization can be induced around Fermi level with spin splitting gap related to the spatial distribution of spin density in real space. Meanwhile, the Neél temperature of 2D MnCl and Cr2CF2 remains above room temperature under external electric field up to 0.6 V/Å. This study exhibits the potential for application of 2D AFM semiconductors in electric-field-controlled spintronics.

High-pressure evolution of the magnetic order in LaMnO3

A relationship between evolution of the long-range magnetic order, crystal structure, and lattice distortions in $\mathrm{LaMn}{\mathrm{O}}_{3}$ was studied by a combination of neutron diffraction and Raman spectroscopy at high pressures up to 39 and 50 GPa, respectively, covering the temperature range 5 to 290 K. The Raman spectra reveal a gradual structural phase transformation evolving in the pressure range of 4 to 17 GPa, caused by a modification of the Jahn-Teller (JT) lattice distortions from the static cooperative character to the local one. A presence of residual JT-distorted regions associated with the initial phase is detected up to 32 GPa, where the insulator-metal transition occurs. At higher pressure, the local JT distortions also vanish completely at further compression up to 50 GPa. In the neutron diffraction data, a strong suppression of the A-type antiferromagnetic (AFM) phase is observed over the pressure region of the phase transformation. This is accompanied by a noticeable reduction in magnitude of the ${Q}_{2}$ and ${Q}_{3}$ JT local modes. The effective ordered magnetic manganese moment is reduced about twice at pressures up to 14 GPa. At higher pressures up to 30 GPa, residual regions of the A-type AFM phase coexist with the magnetically disordered phase. In the range 30 to 39 GPa, i.e., during the pressure-induced insulator-metal transition, these regions disappear and, finally, the magnetically disordered metallic phase becomes the only ground state. The possible models of the insulator-metal transition in $\mathrm{LaMn}{\mathrm{O}}_{3}$ are analyzed.

Electrical tuning of robust layered antiferromagnetism in MXene monolayer

A-type antiferromagnetism, with an in-plane ferromagnetic order and the interlayer antiferromagnetic coupling, owns inborn advantages for electrical manipulations but is naturally rare in real materials except in those artificial antiferromagnetic heterostructures. Here, a robust layered antiferromagnetism with a high Néel temperature is predicted in a MXene Cr2CCl2 monolayer, which provides an ideal platform as a magnetoelectric field effect transistor. Based on first-principles calculations, we demonstrate that an electric field can induce the band splitting between spin-up and spin-down channels. Although no net magnetization is generated, the inversion symmetry between the lower Cr layer and the upper Cr layer is broken via electronic cloud distortions. Moreover, this electric field can be replaced by a proximate ferroelectric layer for non-volatility. The magneto-optic Kerr effect can be used to detect this magnetoelectricity, even if it is a collinear antiferromagnet with zero magnetization.

Giant Tunneling Magnetoresistance in Spin-Filter Magnetic Tunnel Junctions Based on van der Waals A-Type Antiferromagnet CrSBr

Two-dimensional van der Waals magnetic materials have demonstrated great potential for new-generation high-performance and versatile spintronic devices. Among them, magnetic tunnel junctions (MTJs) based on A-type antiferromagnets, such as CrI3, possess record-high tunneling magnetoresistance (TMR) because of the spin filter effect of each insulating unit ferromagnetic layer. However, the relatively low working temperature and the instability of the chromium halides hinder applications of this system. Using a different technical scheme, we fabricated the MTJs based on an air-stable A-type antiferromagnet, CrSBr, and observed a giant TMR of up to 47000% at 5 K. Meanwhile, because of a relatively high Néel temperature of CrSBr, a sizable TMR of about 50% was observed at 130 K, which makes a big step towards spintronic devices at room temperature. Our results reveal the potential of realizing magnetic information storage in CrSBr-based spin-filter MTJs.

Theoretical investigation of the magnetic and optical properties in a transition metal-doped GaTeCl monolayer.

We investigate the structural, magnetic, electronic and optical properties of a transition metal-doped GaTeCl monolayer, denoted as M@GaTeCl (M = V, Cr, Mn, Fe and Co), by using first-principles calculations. It is found that the magnetic ground state can be regulated by different M elements. In the meantime, the electronic structure is different with the doping of different M metal atoms, and thus the optical absorption changes correspondingly. The electronic calculations of M@GaTeCl suggest that V@GaTeCl, Cr@GaTeCl, Mn@GaTeCl and Fe@GaTeCl are semiconductors and the magnetic ground states are G-type antiferromagnetic (AFM), C-type AFM, A-type AFM and C-type AFM order, respectively, while Co@GaTeCl is a metal and the ground state is ferromagnetic (FM) order. The different magnetic ground states are discussed with the Heisenberg model. The rough estimation of the ferroelectric polarization value of M@GaTeCl suggests that M@GaTeCl still exhibits multiferroicity. The electronic structure is explained by the projected density of states, band structure and decomposed charge of the valence band maximum (VBM) and conduction band minimum (CBM). Simultaneously, the absorption coefficient calculations indicate that M@GaTeCl absorption shows anisotropic properties, as the same as in a pure GaTeCl monolayer, there exists enhanced visible light absorption in these M@GaTeCl monolayers relative to the pure GaTeCl one, which can be interpreted by the anisotropic structure and by the peculiar electronic structure. Thus, we found that the magnetic ground state, the electronic structure, and the absorption coefficient of M@GaTeCl can be tuned by doping different transition metal M atoms, and the ferroelectricity is still retained, which makes M@GaTeCl a potential multifunctional material in spintronics and optics.

Layer-polarized anomalous Hall effects in valleytronic van der Waals bilayers.

The layer-polarized anomalous Hall effect (LP-AHE), derived from the coupling between the Berry curvature and the layer degree of freedom, is of importance for both fundamental physics and device applications. Nonetheless, the current research paradigm is rooted in topological systems, rendering such a phenomenon rather scarce. Here, through model analysis, we propose an alternative, but general, mechanism for realizing the LP-AHE in valleytronic van der Waals bilayers by interlayer sliding. The interaction between out-of-plane ferroelectricity and A-type antiferromagnetism gives rise to the layer-locked Berry curvature and thus the long-sought LP-AHE in bilayer systems. The LP-AHE can be strongly coupled with sliding ferroelectricity, rendering it ferroelectrically controllable and reversible. The mechanism is demonstrated in a series of real valleytronic materials, including bilayer VSi2P4, VSi2N4, FeCl2, RuBr2 and VClBr. The new mechanism and phenomena provide a significant new direction to realize the LP-AHE and explore its application in electronics.

Electric-Field-Tunable Spin Polarization and Carrier-Transport Anisotropy in an A-Type Antiferromagnetic van der Waals Bilayer

Two-dimensional (2D) magnetic semiconductor materials with the electric-field-tunable spin polarization and carrier-transport anisotropy are of great significance for the fundamental physics and device application. Here we propose a general strategy to tune the spin polarization and anisotropic effective mass in a 2D A-type antiferromagnetic (AFM) bilayer system. We take the A-type AFM bilayer $\mathrm{CrSBr}$ (with the intralayer ferromagnetic and interlayer AFM coupling) as an example to confirm this design principle. Firstly, a vertical electric field can lift the spin degeneracy in the A-type AFM bilayer $\mathrm{CrSBr}$. By flipping the direction of electric field, the spin polarization direction can be reversed. Secondly, in the A-type AFM bilayer $\mathrm{CrSBr}$ with an interlayer twist angle of 90\ifmmode^\circ\else\textdegree\fi{}, both the spin direction and the carrier-transport anisotropy can be tuned simultaneously by applying a vertical electric field due to the in-plane anisotropic carrier effective mass in each $\mathrm{CrSBr}$ monolayer. This opens an opportunity for controlling the spin polarization as well as carrier-transport anisotropy in the 2D magnetic van der Waals layered materials by interlayer twist and electric field, and provides an idea for utilizing A-type AFM semiconductors to design spintronic devices.

Interplay of electronic structure, magnetism, strain, and defects in carbide MXenes

The two-dimensional (2D) magnetic and semiconducting materials, including carbide MXenes, are in high demand for magneto-electronic devices and applications. Here, we report a first-principles study of electronic structure and magnetism of a Ti2C MXene, its derivatives Ti2CT2 (T = -F, -O, -OH, -H), and the effect of single vacancy defects (25% C, 12.5% O, and 12.5% and 5.5% Ti) and transition metal (V, Cr, Mn, and Co) dopings. The MAX phase Ti2AlC is a non-magnetic (NM) metal which upon removal of Al layer forms a pristine Ti2C MXene with A-type antiferromagnetic (AFM) semiconductor having reliable transition temperature (TN=41K) predicted by the Heisenberg model. All the functionalized MXenes are stable as NM metal except for Ti2CO2 which remains semiconducting with NM ground state. The significant effect of spin–orbit coupling (SOC) is evident in the band structure forming Dirac-like cones and band inversions near the Fermi level. Depending upon the type of vacancy defects Ti2CO2 is NM metal and ferromagnetic (FM) metal. The transition metal (TM) doped MXenes fulfill FM Stoner criterion. The V- and Co-doped MXenes are metals, whereas Cr- and Mn-doped are half-metal and semiconductor.

Coexisting Ferromagnetic–Antiferromagnetic Phases and Manipulation in a Magnetic Topological Insulator MnBi4Te7

Magnetic topological insulators (MTIs) have received considerable attention owing to the demonstration of various quantum phenomena, such as the quantum anomalous Hall effect and topological magnetoelectric effect. The intrinsic superlattice-like layered MTIs MnBi2Te4/(Bi2Te3)n have been extensively investigated mainly through transport measurements; however, a direct investigation of their superlattice-sensitive magnetic behaviors is relatively rare. In this paper, we report a microscopic real-space investigation of the magnetic phase behaviors in MnBi4Te7 using cryogenic magnetic force microscopy. The intrinsic robust A-type antiferromagnetic (AFM), surface spin-flip (SSF) + AFM, ferromagnetic (FM) + SSF + AFM, and forced FM phases are sequentially visualized via the increased external magnetic field, consistent with the magnetic behavior in the M–H curve. The temperature-dependent magnetic phase evolution behaviors are further investigated to obtain a complete H–T phase diagram of MnBi4Te7. Tentative local phase manipulation via the stray field of the magnetic tip is demonstrated by transforming the AFM into the FM phase in the surface layers of MnBi4Te7. Our study provides key real-space ingredients for understanding the complicated magnetic, electronic, and topological properties of such intrinsic MTIs and suggests new directions for manipulating spin textures and locally controlling their exotic properties.