Antiferromagnetic Films Research Articles

Magneto-ionic modulation of the interlayer exchange interaction in synthetic antiferromagnets

The electric-field control of magnetism is a highly promising and potentially effective approach for realizing energy-efficient applications. Recent interest has focused on the magneto-ionic effect in synthetic antiferromagnets, driven by its potential to enable high-density data storage devices with ultra-low power consumption. However, the underlying mechanism responsible for the magneto-ionic effect on the interlayer exchange coupling remains elusive. In our work, we find that the modulation of the interlayer exchange coupling is highly sensitive to the thickness of the ferromagnetic layer. We have identified that the changes in the interlayer exchange coupling induced by the gate voltage can be associated with the magneto-ionic effects on the top ferromagnetic layer of the synthetic antiferromagnet. The direct contact between the high ion mobility oxide and the top ferromagnetic layer plays a crucial role in facilitating these effects, largely modifying the anisotropy of the layers. Our findings highlight the important role of magneto-ionic control over the properties of the top ferromagnetic layer in governing the observed modifications in the interlayer exchange coupling. This study provides crucial insight into the intricate interplay between stack structure and magneto-ionic effect on magnetic properties in synthetic antiferromagnetic thin film systems.

Tuning magnetocrystalline anisotropy by Au ion induced defects in NiO thin films

Intercalations and doping effectively induce room-temperature ferromagnetism in antiferromagnetic NiO thin films. This report investigates the defect-controlled ferromagnetic properties of Au ion implanted highly crystalline NiO thin films at different ion fluences. Low energy heavy ion irradiation leads to preferential sputtering of Ni and O atoms that create various kinds of vacancies and interstitial defect states within the matrix. These defect states are responsible for bandgap narrowing and changing the crystallite size and show the defect luminescence in the NiO thin films. It has been observed that the intrinsic Ni and O defects with the non-stoichiometric ratio of these atoms during crystal growth, resulting in uncompensated spins, induce the ferromagnetic properties in the pristine sample and are further modulated with ion fluences. The presence of defects gives rise to energy variations due to the modification of the magnetic environment of Ni ions in the lattice and tune the blocking temperature and magnetocrystalline anisotropy. The magnetocrystalline anisotropy constant (K) reaches its maximum at 70 erg/cm3 with ion fluence and decreases at the highest fluence because of the suppression of exchange interaction by a large number of defect centers. The modulation of anisotropy in NiO thin films is intricately linked to crystallite size, bandgap narrowing, blocking temperature, and the magnetization mechanism, which is a direct result of defect concentrations in the material.

Microwave Field-Induced Changes in Raman Modes and Magnetic Force Images of Antiferromagnetic NiO Films

Effective control of domain walls or magnetic textures in antiferromagnets promises to enable robust, fast, and nonvolatile memories. The lack of net magnetic moment in antiferromagnets implies the need for creative ways to achieve such a manipulation. We conducted a study to investigate changes in magnetic force microscopy (MFM) imaging and in the magnon-related mode in Raman spectroscopy of virgin NiO films under a microwave pump. After MFM and Raman studies were conducted, a combined action of broadband microwave (0.01–20 GHz, power scanned from −20 to 5 dBm) and magnetic field (up to 3 kOe) were applied to virgin epitaxial (111) NiO and (100) NiO films grown on (0001) Al2O3 and (100) MgO substrates, following which the MFM and Raman studies were repeated. We observed a suppression of the magnon-related Raman mode subsequent to the microwave exposure. Based on MFM imaging, this effect appeared to be caused by the suppression of large antiferromagnetic domain walls due to the possible excitation of antiferromagnetic spin oscillations localized within the antiferromagnetic domain walls.

Reconfigurable Skyrmion-Based Logic Gates: Versatile Design and Full-Scale Implementation.

Herein, we investigate the behavior of skyrmions within a racetrack design incorporating voltage-controlled magnetic anisotropy (VCMA) gates. Our analysis encompassed multiple forces, including spin currents and anisotropy gradients induced by bias voltages. As a result, the efficient control of skyrmion dynamics was achieved across various VCMA gate configurations. Building upon these findings, we propose an efficient approach to reconfigurable skyrmion logic (RSL) in a thin antiferromagnetic (AFM) film through a versatile design. Our RSL harnesses the selective integration of VCMA, spin-polarized currents, and skyrmion-skyrmion (sky-sky) interactions to implement multiple logic gates, including AND, OR, XOR, NOT, NAND, XNOR, and NOR. The design brings a significant advantage with its simplified fabrication process, making the implementation of the RSL practical and accessible for various applications. Furthermore, the RSL enables seamless dynamic switching between logic gates, thereby enhancing its multifunctionality. Additionally, the strategic incorporation of sky-sky interactions and skyrmion-edge repulsion prominently facilitates the realization of complex gates, such as NAND, XNOR, and NOR gates, that typically require intricate design efforts. Hence, this streamlined integration of RSL, coupled with its adaptability to changing computational needs, underscores its potential as a practical solution for implementing high-functionality skyrmion-based logic gates.

Effect of spin transfer torque on antiferromagnetic film: computer simulations

Effect of spin transfer torque on antiferromagnetic film: computer simulations

Skyrmion-deriven topological spin and charge Hall effects in diffusive antiferromagnetic thin films

We investigate topological Hall effects in a metallic antiferromagnetic (AFM) thin film and/or at the interface of an AFM insulator–normal metal bilayer with a single skyrmion in the diffusive regime. To determine the spin- and charge Hall currents, we employed a Boltzmann kinetic equation with both spin-dependent and spin-flip scatterings. The interaction between conduction electrons and static skyrmions is included in the Boltzmann equation via the corresponding emergent magnetic field arising from the skyrmion texture. We compute intrinsic and extrinsic contributions to the topological spin Hall effect and spin accumulation, induced by an AFM skyrmion. We show that although the spin Hall current vanishes rapidly outside the skyrmion, the spin accumulation can be finite at the edges far from the skyrmion, provided the spin diffusion length is longer than the skyrmion radius. In addition, We show that in the presence of a spin-dependent relaxation time, the topological charge Hall effect is finite and we determine the corresponding Hall voltage. Our results may help to explore antiferromagnetic skyrmions by electrical means in real materials.

Investigation of the Extraordinary Phase Transition in Antiferromagnetic Thin Films: Computer Simulation

Investigation of the Extraordinary Phase Transition in Antiferromagnetic Thin Films: Computer Simulation

STUDY OF EXTRAORDINARY PHASE TRANSITION IN THIN ANTI-FERROMAGNETIC FILMS: COMPUTER SIMULATION

In the article, an extraordinary phase transition in thin antiferromagnetic films was investigated by computer modeling. The simulation uses the Ising model and Metropolis algorithm. Cubic crystal lattice epitaxial films containing multiple monoatomic layers are studied in the paper. The difference in the value for the exchange integrals in the bulk and on the surface of the film is a condition for the appearance of surface and extraordinary phase transitions. The film shall contain at least eight monoatomic layers for appearance of surface and extraordinary phase transitions. The extraordinary phase transition is investigated at different film thicknesses. Magnetic susceptibility demonstrates a logarithmic dependence on Neel temperature near the phase transition point. The dependence of the critical exponent of magnetic susceptibility on film thickness is calculated for the logarithmic phase.

Magnetic Field-Induced Spin Nematic Phase Up to Room Temperature in Epitaxial Antiferromagnetic FeTe Thin Films Grown by Molecular Beam Epitaxy.

Electronic nematicity, where strong correlations drive electrons to align in a way that lowers the crystal symmetry, is ubiquitous among unconventional superconductors. Understanding the interplay of such a nematic state with other electronic phases underpins the complex behavior of these materials and the potential for tuning their properties through external stimuli. Here, we report magnetic field-induced spin nematicity in a model system tetragonal FeTe, the parent compound of iron chalcogenide superconductors, which exhibits a bicollinear antiferromagnetic order. The studies were conducted on epitaxial FeTe thin films grown on SrTiO3(001) substrates by molecular beam epitaxy, where the bicollinear antiferromagnetic order was confirmed by in situ atomic resolution scanning tunneling microscopy imaging. A 2-fold anisotropy is observed in in-plane angle-dependent magnetoresistance measurements, indicative of magnetic field-induced nematicity. Such 2-fold anisotropy persists up to 300 K, well-above the bicollinear antiferromagnetic ordering temperature of 75 K, indicating a magnetic field-induced spin nematic phase up to room temperature in the antiferromagnet FeTe.

Spin-mediated shear oscillators in a van der Waals antiferromagnet.

Understanding how microscopic spin configuration gives rise to exotic properties at the macroscopic length scale has long been pursued in magnetic materials1-5. One seminal example is the Einstein-de Haas effect in ferromagnets1,6,7, in which angular momentum of spins can be converted into mechanical rotation of an entire object. However, for antiferromagnets without net magnetic moment, how spin ordering couples to macroscopic movement remains elusive. Here we observed a seesaw-like rotation of reciprocal lattice peaks of an antiferromagnetic nanolayer film, whose gigahertz structural resonance exhibits more than an order-of-magnitude amplification after cooling below the Néel temperature. Using a suite of ultrafast diffraction and microscopy techniques, we directly visualize this spin-driven rotation in reciprocal space at the nanoscale. This motion corresponds to interlayer shear in real space, in which individual micro-patches of the film behave as coherent oscillators that are phase-locked and shear along the same in-plane axis. Using time-resolved optical polarimetry, we further show that the enhanced mechanical response strongly correlates with ultrafast demagnetization, which releases elastic energy stored in local strain gradients to drive the oscillators. Our work not only offers the first microscopic view of spin-mediated mechanical motion of an antiferromagnet but it also identifies a new route towards realizing high-frequency resonators8,9 up to the millimetre band, sothe capability of controlling magnetic states on the ultrafast timescale10-13 can be readily transferred to engineering the mechanical properties of nanodevices.

The anomalous Hall effect controlled by residual epitaxial strain in antiferromagnetic Weyl semimetal Mn3Sn thin films grown by molecular beam epitaxy

The large anomalous Hall effect (AHE) in antiferromagnetic(AFM) Weyl semimetal Mn3Sn attracts intensive attentions in spintronics. Here, we report the structural property of high quality Mn3Sn thin film on insulator substrate MgO(110) by molecular beam epitaxy (MBE), and AHE in control of residual mismatch strain between Mn3Sn film and substrate. We are able to grow strain-free Mn3Sn(10 1¯ 0) films or alternatively strained Mn3Sn(11 2¯ 0) films via a three-step process. The strain-free Mn3Sn film has large anomalous Hall conductivity up to 30 Ω-1cm−1 at room temperature, which is comparable to bulk Mn3Sn. In contrast, AHE is switched off in strained Mn3Sn film due to piezomagnetic effect under a uniaxial compress strain of ∼2.0%. These findings provide a deeper understanding on AFM spintronic applications.

Composite Spin Hall Conductivity from Non-Collinear Antiferromagnetic Order.

Non-collinear antiferromagnets (AFMs) are an exciting new platform for studying intrinsic spin Hall effects (SHEs), phenomena that arise from the materials' band structure, Berry phase curvature, and linear response to an external electric field. In contrast to conventional SHE materials, symmetry analysis of non-collinear antiferromagnets does not forbid non-zero longitudinal and out-of-plane spin currents with polarization and predicts an anisotropy with current orientation to the magnetic lattice. Here, multi-component out-of-plane spin Hall conductivities are reported in L12 -ordered antiferromagnetic PtMn3 thin films that are uniquely generated in the non-collinear state. The maximum spin torque efficiencies (ξ = JS /Je ≈ 0.3) are significantly larger than in Pt (ξ ≈ 0.1). Additionally, the spin Hall conductivities in the non-collinear state exhibit the predicted orientation-dependent anisotropy, opening the possibility for new devices with selectable spin polarization. This work demonstrates symmetry control through the magnetic lattice as a pathway to tailored functionality in magnetoelectronic systems.

Terahertz Spin Current Pulses in Antiferromagnetic Oxide: The Role of Vacancy‐Induced Ferromagnetism

Antiferromagnetic oxides have attracted increasing attention for their outstanding peculiarities in spintronics. Crystal lattice defects that are present in antiferromagnetic oxides can influence their physical properties, such as vacancy‐induced ferromagnetism. Meanwhile, the generation and manipulation of ultrafast spin currents of antiferromagnetic insulators are highly desired. Although the generation and detection of terahertz spin current pulses in antiferromagnetic oxides have been realized, the effect of vacancy‐induced ferromagnetism on spin current in antiferromagnetic oxides is not yet known. Herein, the role of vacancy‐induced ferromagnetism on the terahertz spin current in antiferromagnetic nickel oxide thin films is reported. Structural and magnetic characterizations reveal that nickel vacancies effectively break the strong antiferromagnetic exchange coupling, giving rise to the coexistence of antiferromagnetism and ferromagnetism in NiO thin films. Notably, the enhancement of terahertz radiation associated with the photo‐induced ultrafast spin current of NiO thin film with the strongest ferromagnetism is the most significant. Besides, the nonlinear susceptibility tensor parameters related to the antiferromagnetic property of NiO thin films also change distinctly. The findings indicate that the defects of antiferromagnetic materials play a decisive role in the application of antiferromagnetic spintronics in the terahertz field.

Tuning crystal orientation and chiral spin order in Mn3Ge by annealing process and ion implantation

The non-collinear antiferromagnetic Weyl semimetal Mn3X (X = Ga, Ge, Sn) system has attracted a lot of attentions owing to its robust anomalous Hall effect (AHE), large spin Hall angle and small net magnetization at room temperature. The high spin-charge interconversion efficiency makes it a super candidate in topological antiferromagnetic spintronic devices, which could facilitate ultra-fast operation of high-density devices with low energy consumption. In this work, we have realized to obtain different chiral spin structures in Heusler alloy Mn3Ge thin films, which originate from different crystalline orientations. The high-quality (0002)- and (200)-oriented single phase hexagonal Mn3Ge films are achieved by controllable growth, annealing process and ion implantation. The various magnetic properties and AHE behaviors are observed along a and c crystal axes, equivalent to magnetic field in and out of the inverse triangular spin plane. The observation demonstrates the manipulation of crystal structure accompanied with chiral spin order in a non-collinear antiferromagnetic Mn3Ge film, which is induced by energy conversion and defect introduction. The in situ thermal treatment induces crystal phase rotation up to 90° and robust AHE modulation, which is significantly important and highly desirable for flexible spin memory device applications.

Antiferromagnetic half-skyrmions electrically generated and controlled at room temperature.

Topologically protected magnetic textures are promising candidates for information carriers in future memory devices, as they can be efficiently propelled at very high velocities using current-induced spin torques. These textures-nanoscale whirls in the magnetic order-include skyrmions, half-skyrmions (merons) and their antiparticles. Antiferromagnets have been shown to host versions of these textures that have high potential for terahertz dynamics, deflection-free motion and improved size scaling due to the absence of stray field. Here we show that topological spin textures, merons and antimerons, can be generated at room temperature and reversibly moved using electrical pulses in thin-film CuMnAs, a semimetallic antiferromagnet that is a testbed system for spintronic applications. The merons and antimerons are localized on 180° domain walls, and move in the direction of the current pulses. The electrical generation and manipulation of antiferromagnetic merons is a crucial step towards realizing the full potential of antiferromagnetic thin films as active components in high-density, high-speed magnetic memory devices.

Switchable Anomalous Hall Effects in Polar-Stacked 2D Antiferromagnet MnBi2Te4.

van der Waals (vdW) assembly of two-dimensional (2D) materials allows polar layer stacking to realize novel properties switchable by the induced electric polarization. Here, based on symmetry analyses and density-functional calculations, we explore the emergence of the anomalous Hall effect (AHE) in antiferromagnetic MnBi2Te4 films assembled by polar layer stacking. We demonstrate that breaking P̂T̂ symmetry in an MnBi2Te4 bilayer produces a magnetoelectric effect and a spontaneous AHE switchable by electric polarization. We find that reversible polarization at one of the interfaces in a three-layer MnBi2Te4 film drives a metal-insulator transition, as well as switching between the AHE and quantum AHE (QAHE). Finally, we predict that engineering interlayer polarization in a three-layer MnBi2Te4 film allows converting MnBi2Te4 from a trivial insulator to a Chern insulator. Overall, our work emphasizes the topological properties in 2D vdW antiferromagnets induced by polar layer stacking, which do not exist in a bulk material.

Atomic Displacements Enabling the Observation of the Anomalous Hall Effect in a Non-Collinear Antiferromagnet.

Antiferromagnets with non-collinear spin structures display various properties that make them attractive for spintronic devices. Some of the most interesting examples are an anomalous Hall effect despite negligible magnetization and a spin Hall effect with unusual spin polarization directions. However, these effects can only be observed when the sample is set predominantly into a single antiferromagnetic domain state. This can only be achieved when the compensated spin structure is perturbed and displays weak moments due to spin canting that allows for external domain control. In thin films of cubic non-collinear antiferromagnets, this imbalance waspreviously assumed to require tetragonal distortions induced by substrate strain. Here weshow that in Mn3 SnN and Mn3 GaN, spin canting is due to structural symmetry lowering induced by large displacements of the magnetic manganese atoms away from high symmetry positions. These displacements remain hidden in X-ray diffraction when only probing the lattice metric and require measurement of a large set of scattering vectors to resolve the local atomic positions. In Mn3 SnN, the induced net moments enable the observation of the anomalous Hall effect with an unusual temperature dependence, which weconjecture results from a bulk-like temperature-dependent coherent spin rotation within the Kagomeplane. This article is protected by copyright. All rights reserved.

Room temperature positive exchange bias in CoFeB/D019-Mn3Ge noncollinear antiferromagnetic thin films

Polycrystalline and single crystal antiferromagnetic D019-Mn3Ge thin films were grown by magnetron sputtering on GaN (0001) and Pt (111) templates at different substrate temperatures. Later, a CoFeB layer was grown on top of the D019-Mn3Ge films at room temperature. Single crystal D019-Mn3Ge thin films, with high surface roughness, induced an increase of the coercivity of the CoFeB ultrathin films, meanwhile polycrystalline D019-Mn3Ge thin films, with uniaxial texture and low surface roughness, induced positive exchange bias at room temperature by cooling the sample under a magnetic field of 2 T. No exchange bias was observed with lower cooling magnetic fields. The non-zero magnetic moment of uncompensated D019-Mn3Ge grains of the textured films couple antiferromagnetically leading to positive exchange bias.

Antiferromagnetic domain formation and spin frustration induced by adjacent paired screw dislocations in 10 monolayer-thick Cr(001) films

We investigated the growth and surface morphology of 10 monolayer (ML)-thick Cr(001) films on clean Au(001) surfaces. High quality epitaxial Cr(001) films with large atomically flat terraces and distinct surface states were successfully fabricated through growth at 300 K and subsequent post-annealing at 520 K. At 300 K, spin-polarized scanning tunneling microscopy images of both the topological and magnetic structures of this Cr film were obtained. The magnetic images exhibited the following features: (1) The layered antiferromagnetic (AF) order appeared in adjacent terraces and one ML-depth shallow hole in the terraces; (2) significant spin frustrations induced by adjacent paired screw dislocations caused the AF domain formation with 90 degrees quantum axis rotation and a large spin frustration area, not always limited in the vicinity of screw dislocations. The feature (2) was qualitatively reproduced by the micromagnetic simulation. These findings may be essential for the further development of spin-electronics utilizing thin AF films.

Perpendicular magnetic anisotropy induced by antiferromagnetic Mn-Pd alloy films: Dual effects of exchange and spin-orbit coupling

Ferromagnetic (FM) films with perpendicular magnetic anisotropy (PMA) are crucial building blocks of state-of-the-art perpendicular magnetic devices. This study investigated the effects of triggering PMA in FM thin films by applying ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}$-based antiferromagnetic (AFM) thin films with strong AFM exchange coupling from Mn and high spin-orbit coupling from Pd atoms. Our results reveal that ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}$ films can trigger PMA in Co/Ni films with weak in-plane magnetic anisotropy at room temperature through either ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}$-facilitated interfacial perpendicular crystalline anisotropy (spin-orbit coupling, $x \ensuremath{\ge}10%$) or incorporation of them with antiferromagnet-induced exchange coupling ($x \ensuremath{\le}10%$). In Co/Fe films with strong in-plane magnetic anisotropy, Mn-rich ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}$ films ($x \ensuremath{\le}10%$) can induce stable PMA at low temperatures through concurrently enhanced spin-orbit coupling and antiferromagnet-induced exchange coupling across the AFM--FM interface. Our research clarifies the dual effects of exchange and spin-orbit coupling in ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}$ films, which trigger the PMA of adjacent FM films. This indicates that ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}$ films are promising candidates for achieving controllable PMA.