Antiferromagnetic Materials Research Articles

Zeeman splitting in doped antiferromagnetic semiconductors

Zeeman splitting of conduction electrons in doped antiferromagnetic materials in the presence of a strong spin–orbit interaction is considered. Using symmetry arguments the Hamiltonian and the spectrum of conduction electrons near some time-reversal invariant momenta were constructed. It was shown, that in the vicinity of some time-reversal invariant momenta the external magnetic field perpendicular to the Neel axis does not lead to the Zeeman splitting. The Kramers degeneracy is preserved in that case. The absence of the Zeeman splitting in antiferromagnets leads to the absence of spin zeros in quantum oscillation experiments. Recent experimental confirmation of the predicted effects in organic antiferromagnet and High-Tc Superconductors is discussed.

Machine learning-accelerated discovery of novel 2D ferromagnetic materials with strong magnetization

Two-dimensional ferromagnetic (2DFM) semiconductors (metals, half-metals, and so on) are important materials for next-generation nano-electronic and nano-spintronic devices. However, these kinds of materials remain scarce, and “trial and error” experiments and calculations are time-consuming and expensive. In the present work, to obtain optimal 2DFM materials with strong magnetization, we established a machine learning (ML) framework to search the 2D material space containing over 2417 samples, and identified 615 compounds whose magnetic orders was then determined via high-through-put first-principles calculations. Using ML algorithms, we trained two classification models and a regression model. The interpretability of the regression model was evaluated through Shapley additive explanations (SHAP) analysis. Unexpectedly, we found that Cr2NF2 is a potential antiferromagnetic ferroelectric 2D multiferroic material. More importantly, 60 novel 2DFM candidates were predicted, and among them, 13 candidates have magnetic moments of > 7 μB. Os2Cl8, Fe3GeSe2, and Mn4N3S2 were predicted to be novel 2DFM semiconductors, metals, and half-metals, respectively. Our ML approach can accelerate the prediction of 2DFM materials with strong magnetization and reduce the computation time by more than one order of magnitude.

Spin Hall Magnetoresistance in Pt/BiFeO3 Bilayer

Multiferroic materials are general antiferromagnets with negligibly small net magnetization, which strongly limits their magnetoelectric applications in spintronics. Spin Hall magnetoresistance (SMR) is sensitive to the orientation of the Néel vector, which can be applied for the detection of antiferromagnetic states. Here, we apply SMR on the unique room-temperature antiferromagnetic multiferroic material BiFeO3 (BFO). The angular dependence of SMR in a bilayer of epitaxial BFO (001) and heavy metal Pt is studied. By rotating the sample under a magnetic field of 80 kOe in the film plane, the resistance shows the maximum when the field is perpendicular to the current while it shows the minimum when the field is along the current. This can be well explained by the SMR in the bilayer of heavy metal/antiferromagnet with the relative orientation between the Néel vector and current direction. In contrast, the angular dependence of the resistance of Pt directly deposited on a SrTiO3 (001) substrate shows a 90° shift with the magnetic field rotating in the film plane, which originates from the Hanle magnetoresistance of Pt. The obtained spin mixing conductance at the Pt/BFO interface clearly confirms the efficient spin transmission. Our results provide a possible solution for applications of antiferromagnetic multiferroic materials in spintronics.

Single spin magnetometry and relaxometry applied to antiferromagnetic materials

Despite the considerable interest for antiferromagnets that appeared with the perspective of using them for spintronics, their experimental study, including the imaging of antiferromagnetic textures, remains a challenge. To address this issue, quantum sensors, and, in particular, the nitrogen-vacancy (NV) defects in diamond have become a widespread technical solution. We review here the recent applications of single NV centers to study a large variety of antiferromagnetic materials, from quantitative imaging of antiferromagnetic domains and non-collinear states, to the detection of spin waves confined in antiferromagnetic textures and the non-perturbative measurement of spin transport properties. We conclude with recent developments improving further the magnetic sensitivity of scanning NV microscopy, opening the way to detailed investigations of the internal texture of antiferromagnetic objects.

Visualization of antiferromagnetic domains by nonreciprocal directional dichroism and related optical responses

Nonreciprocal directional dichroism (NDD) is a phenomenon in which the optical absorption is changed by reversing the direction of light propagation or the sign of the magnetic order parameters. While the NDD has mostly been observed in materials with macroscopic magnetization, recent experiments have shown that the NDD can be induced by a specific antiferromagnetic (AFM) spin structure that breaks both space-inversion and time-reversal symmetries. This opens the possibility of visualizing the spatial distribution of AFM domains via the NDD effect. This article reviews the basic features of the NDD, a brief history of the NDD in AFM materials, and recent achievements in visualizing AFM domains via the NDD and related optical responses, and finally provides a perspective on applications of this method for future AFM spintronics research.

Intrinsic Néel Antiferromagnetic Multimeronic Spin Textures in Ultrathin Films.

Topological antiferromagnetism is a vibrant and captivating research field, generating considerable enthusiasm with the aim of identifying topologically protected magnetic states of key importance in the hybrid realm of topology, magnetism, and spintronics. While topological antiferromagnetic (AFM) solitons bear various advantages with respect to their ferromagnetic cousins, their observation is scarce. Utilizing first-principles simulations, here we predict new chiral particles in the realm of AFM topological magnetism, exchange-frustrated multimeronic spin textures hosted by a Néel magnetic state, arising universally in single Mn layers directly grown on an Ir(111) surface or interfaced with Pd-based films. These nanoscale topological structures are intrinsic; i.e. they form in a single AFM material, can carry distinct topological charges, and can combine in various multimeronic sequences with enhanced stability against external magnetic fields. We envision the frustrated Néel AFM multimerons as exciting highly sought after AFM solitons having the potential to be utilized in novel spintronic devices hinging on nonsynthetic AFM quantum materials, further advancing the frontiers of nanotechnology and nanophysics.

Direct Imaging of Antiferromagnet‐Ferromagnet Phase Transition in van der Waals Antiferromagnet CrSBr

AbstractThe advent of van der Waals (vdW) ferromagnetic (FM) and antiferromagnetic (AFM) materials offers unprecedented opportunities for spintronics and magneto‐optic devices. Combining magnetic Kerr microscopy and density functional theory calculations, the AFM‐FM transition is investigated and a surprising abnormal magneto‐optic anisotropy in vdW CrSBr associated with different magnetic phases (FM, AFM, or paramagnetic state) is discovered. This unique magneto‐optic property leads to different anisotropic optical reflectivity from different magnetic states, permitting direct imaging of the AFM Néel vector orientation and the dynamic process of the AFM‐FM transition within a magnetic field. Using Kerr microscopy, not only the domain nucleation and propagation process is imaged but also the intermediate spin‐flop state in the AFM‐FM transition is identified. The unique magneto‐optic property and clear identification of the dynamics process of the AFM‐FM phase transition in CrSBr demonstrate the promise of vdW magnetic materials for future spintronic technology.

Strain-mediated voltage controlled magnetic anisotropy based switching for magnetic memory applications

Reliability and packing density concerns are the two major shortcomings of spin transfer torque and spin orbit torque based magnetic memory, respectively. Voltage controlled magnetic anisotropy (VCMA) becomes energy efficient and fast, showing transcendence for the writing mechanism in the magnetic tunnel junction. Deterministic switching cannot be achieved by VCMA alone in the out of plane nanomagnet. It requires an external in-plane magnetic field, but the use of an external field is inconvenient for on-chip applications. We exploit stress and exchange bias provided by an antiferromagnetic material to mitigate the external magnetic field requisite. We perform macro-spin simulations using the Landau–Lifshitz–Gilbert equation at room temperature. We use the VCMA effect cum stress effect to investigate field free switching performance, and this improves the write error rate (WER) to 5×10−5 against WER of 0.1 with the VCMA effect alone. We studied the effects of applied voltage (amplitude and pulse width), exchange bias field, and VCMA coefficient on the switching performance in detail. This proposed two-terminal device can be helpful in achieving high cell density to implement nonvolatile magnetic memory.

Electric field controlled perpendicular exchange bias in Ta/Pt/Co/IrMn/Pt heterostructure

Perpendicular exchange bias (PEB) is an essential effect of antiferromagnetic materials. It has the advantages over the in-plane exchange bias in application of magnetic random access memory (MRAM) technology, particularly in terms of higher packing density. The influence of the strain effect on PEB of Ta/Pt/Co/Ir20Mn80/Pt multilayers deposited on the piezoelectric substrate is studied in this work. It is found that the exchange bias field (Hex) can be regularly regulated by the strain effect introduced by the piezoelectric substrate. It indicates that the uncompensated spins at the interface of Ir20Mn80/Co can be reoriented under the strain effect, resulting in the variation of Hex. Furthermore, we propose a MRAM device structure based on the strain-mediated PEB, which enables field-free magnetization switching driven by the pure strain. Our study demonstrates the controllability of PEB and its application in spintronics, providing a method for the development of antiferromagnetic material-based next generation MRAM.

Realization of High Magnetization in Artificially Designed Ni/NiO Layers through Exchange Coupling.

High-magnetization materials play crucial roles in various applications. However, the past few decades have witnessed a stagnation in the discovery of new materials with high magnetization. In this work, Ni/NiO nanocomposites are fabricated by depositing Ni and NiO thin layers alternately, followed by annealing at specific temperatures. Both the as-deposited samples and those annealed at 373 K exhibit low magnetization. However, the samples annealed at 473 K exhibit a significantly enhanced saturation magnetization exceeding 607 emu cm-3 at room temperature, surpassing that of pure Ni (480 emu cm-3). Material characterizations indicate that the composite comprises NiO nanoclusters of size 1-2nm embedded in the Ni matrix. This nanoclustered NiO is primarily responsible for the high magnetization, as confirmed by density functional theory calculations. The calculations also indicate that the NiO clusters are ferromagnetically coupled with Ni, resulting in enhanced magnetization. This work demonstrates a new route toward developing artificial high-magnetization materials using the high magnetic moments of nanoclustered antiferromagnetic materials.

Valence band electronic structure of the van der Waals antiferromagnet FePS3

Antiferromagnetic van der Waals materials have gained a lot of interest in recent years. They can be exfoliated down to the two-dimensional (2D) limit while potentially preserving intriguing properties of antiferromagnets, such as insensitivity to external magnetic fields and ultrafast spin dynamics in the THz range. The investigation of the electronic band structure of these materials is crucial to understand their behavior and thus to identify paths for future applications. Here, we investigate the valence band structure of one of the most studied 2D antiferromagnets –iron phosphorus trisulfide (FePS3)– using angle-resolved photoemission spectroscopy (ARPES) and compare our results with first-principles calculations based on Hubbard-corrected density functional theory (DFT+U). This allows us to identify the bands originating respectively from the Fe 3d, the S 3p, and the P 3p orbitals and to describe their dispersion throughout the whole Brillouin zone. Our results represent an important step towards an accurate theoretical description of the electronic properties of transition metal phosphorus trisulfides, which is a pre-requisite for understanding the behavior of antiferromagnetic materials at the 2D limit.

Anomalous Hall effect of van der Waals metallic antiferromagnetic V5Se8

Two-dimensional (2D) magnetic materials have developed rapidly in recent years, which exhibited a lot of novel and interesting properties, such as the topological Hall effect, magnetic skyrmions and charge density waves. Most hitherto known 2D antiferromagnetic materials are insulating, while 2D antiferromagnets with intrinsic metallicity are rarely reported. Hindered by such shortage, the electrical properties of the metallic 2D antiferromagnetic materials remain largely unexplored. Here we report high-quality synthesis and comprehensive characterization of a van der Waals antiferromagnetic metal, V5Se8, and observation of an anomalous Hall effect in magneto-transport measurements. The temperature dependence of longitudinal-resistance reveals a metallic behavior in the as-grown V5Se8 flakes. The transversal resistance measurements reveal a non-zero Hall resistance with two antisymmetric peaks, which is attributed to a non-zero Berry curvature. Our work demonstrates the novel property of 2D metallic antiferromagnetic material V5Se8 for the application of 2D spintronics.

Strongly correlated antiferromagnetic vanadates

Antiferromagnetic (AFM) materials have zero net magnetic moments due to the alignment of adjacent magnetic moments antiparallel to each other, which have drawn extensive interest in the field of next generation spintronics, thanks to the merits of negligible stray fields, fast spin dynamics, and robustness against magnetic disturbances. In the past decades, the strongly correlated vanadates of VO2, V2O3, and RVO3 have extensively been studied because of noteworthy metal–insulator transitions. Nevertheless, the magnetism (especially antiferromagnetism) in the vanadates has attracted less attention. This Perspective provides a brief overview of the strongly correlated AFM vanadates. First, an introduction to the basic concept of antiferromagnetism and vanadates is presented. Then, the antiferromagnetism in vanadates containing V4+ (VO2, NaV2O5, and Sr2VO4), V3+ (V2O3, RVO3, AV2O4, and VOCl), and V2+ (VO) with different spin moments is presented. Finally, a summary and outlook with respect to the antiferromagnetism in vanadates are included.

Current‐Driven Switching of Néel Vector of an Antiferromagnetic Insulator Thin Film

AbstractManipulation of antiferromagnetic (AFM) materials as active elements provides a crucial combination of electrical, thermal, and magnetic properties for spintronics. This study shows how the spin current generated in heavy metal is induced by spin‐orbit torque into an adjacent AFM insulator. The bulk unpinned spins of the AFM layer drive a spin current that is transmitted to the top ferromagnet. This mechanism allows the electrical control of the exchange bias, coercive field, and blocking temperature of the system. Further support is provided by a model calculation that quantitatively describes the effect of the spin current injection into the AFM.

Engineering buffer layers to improve temperature resilience of magnetic tunnel junction sensors

Improving the thermal resilience of magnetic tunnel junctions (MTJs) broadens their applicability as sensing devices and is necessary to ensure their operation under harsh environments. In this work, we are address the impact of temperature on the degradation of the magnetic reference in field sensor stacks based on MgO-MTJs. Our study starts by simple MnIr/CoFe bilayers to gather enough insights into the role of critical morphological and magnetic parameters and their impact in the temperature dependent behavior. The exchange bias coupling field (H ex), coercive field (H c), and blocking temperature (T b) distribution are tuned, combining tailored growth conditions of the antiferromagnet and different buffer layer materials and stackings. This is achieved by a unique combination of ion beam deposition and magnetron sputtering, without vaccum break. Then, the work then extends beyond bilayers into more complex state-of-the-art MgO MTJ stacks as those employed in commercial sensing applications. We systematically address their characteristic fields, such as the width of the antiferromagnetic coupling plateau ΔH, and study their dependence on temperature. Although, [Ta/CuN] buffers showed higher key performance indications (e.g. H ex) at room temperature in both bilayers and MTJs, [Ta/Ru] buffers showed an overall wider ΔH up to 200 °C, more suitable to push high temperature operations. This result highlights the importance of properly design a suitable buffer layer system and addressing the complete MTJ behavior as function of temperature, to deliver the best stacking design with highest resilience to high temperature environments.

Magnetic super-structure and active surface role in the onset of magnetic excitons revealed in TbCu2 nanoparticles

Antiferromagnetic materials are receiving renewed interest on behalf of their potential for information technologies. Recent reports have also revealed how the physics governing such magnetic arrangements and their excitations become more complex compared to traditional ferromagnetic materials, especially at the nanoscale. Here, we address two main issues that are of prime interest to their technological transfer. First, using small-angle neutron scattering, we show the existence of a magnetic helix-like super-structure in a polycrystalline TbCu2 alloy, preserved at both bulk and nanoparticle ensembles of 8 nm. Second, using inelastic neutron scattering, we elucidate the magnetic excitons and the crystalline electric field energy level schemes of TbCu2 in bulk and nanoparticle ensembles. This allows to understad the effect of the surface broken symmetry on the quantum energy levels at the nanoscale, so as the key role of interfacial effects on the propagation of magnetic excitations. Our research provides insights for the realization of magnetic moment dynamics models based on complex nanometric super-structures, and for nanoparticles to be integrated in spintronics and information technology applications.

Intrinsic Nonlinear Hall Detection of the Néel Vector for Two-Dimensional Antiferromagnetic Spintronics.

The respective unique merit of antiferromagnets and two-dimensional (2D) materials in spintronic applications inspires us to exploit 2D antiferromagnetic spintronics. However, the detection of the Néel vector in 2D antiferromagnets remains a great challenge because the measured signals usually decrease significantly in the 2D limit. Here we propose that the Néel vector of 2D antiferromagnets can be efficiently detected by the intrinsic nonlinear Hall (INH) effect which exhibits unexpected significant signals. As a specific example, we show that the INH conductivity of the monolayer manganese chalcogenides MnX (X=S, Se, Te) can reach the order of nm·mA/V^{2}, which is orders of magnitude larger than experimental values of paradigmatic antiferromagnetic spintronic materials. The INH effect can be accurately controlled by shifting the chemical potential around the band edge, which is experimentally feasible via electric gating or charge doping. Moreover, we explicitly demonstrate its 2π-periodic dependence on the Néel vector orientation based on an effective k·p model. Our findings enable flexible design schemes and promising material platforms for spintronic memory device applications based on 2D antiferromagnets.

Thermo-Magnetostrictive Effect for Driving Antiferromagnetic Two-Dimensional Material Resonators.

Magnetostrictive coupling has recently attracted interest as a sensitive method for studying magnetism in two-dimensional (2D) materials by mechanical means. However, its application in high-frequency magnetic actuators and transducers requires rapid modulation of the magnetic order, which is difficult to achieve with external magnets, especially when dealing with antiferromagnets. Here, we optothermally modulate the magnetization in antiferromagnetic 2D material membranes of metal phosphor trisulfides (MPS3), to induce a large high-frequency magnetostrictive driving force. From the analysis of the temperature-dependent resonance amplitude, we provide evidence that the force is due to a thermo-magnetostrictive effect, which significantly increases near the Neél temperature, due to the strong temperature dependence of the magnetization. By studying its angle dependence, we find the effect is observed to follow anisotropic magnetostriction of the crystal lattice. The results show that the thermo-magnetostrictive effect results in a strongly enhanced thermal expansion force near the critical temperature of magnetostrictive 2D materials, which can enable more efficient actuation of nano-magnetomechanical devices and can also provide a route for studying the high-frequency coupling among magnetic, mechanical, and thermodynamic degrees of freedom down to the 2D limit.

Proximity effects of 2D antiferromagnets on superconductivity in exfoliated niobium disulfide

The conventional theory of superconductivity holds that Cooper pairs form due to electron–phonon coupling; however, this description may not be adequate to describe certain unconventional superconductors such as cuprates and iron chalcogenides. In these unconventional superconductors, it has been proposed that spin fluctuations may be responsible for the formation of Cooper pairs. In this study, we explore spin interactions in the transition metal, dichalcogenide niobium disulfide, induced through proximity effects by fabricating antiferromagnet/NbS2 heterostructures. We tested three different 2D antiferromagnetic materials, each with different spin textures: anganese phosphorus trisulfide, manganese phosphorus triselenide, and chromium trichloride. Our results showed a substantial reduction in the critical temperature in the case of NbS2/MnPSe3. We hypothesize that this could be due to spin fluctuations in MnPSe3 inducing proximity effects in NbS2.

Giant spin-splitting and tunable spin-momentum locked transport in room temperature collinear antiferromagnetic semimetallic CrO monolayer

In this work, we present theoretical predictions of a two-dimensional collinear antiferromagnetic semimetal, CrO, which exhibits a giant spin-split band structure, spin-momentum locked transport properties, and a high Néel temperature. Specifically, CrO features two pairs of spin-polarized anisotropic Weyl points at the Fermi level. By manipulating the position of these Weyl points with strain, we demonstrate that four different antiferromagnetic spintronic states with zero net magnetic moments can be achieved, including semimetals with two spin-polarized transport channels, half-semimetals, semiconductors with two spin-polarized transport channels, and half-semiconductors. The strain-induced semiconducting state also preserves the ultra-high carrier mobility of Weyl points, and the bandgap can be easily tuned. These findings offer a good avenue in spintronics without net magnetic moment or strong spin–orbit coupling and could lead to the development of antiferromagnetic materials for spintronic applications.