Antiferromagnetic Materials Research Articles

Large Room Temperature Anomalous Transverse Thermoelectric Effect in Kagome Antiferromagnet YMn6 Sn6.

Kagome magnets possess several novel nontrivial topological features owing to the strong correlation between topology and magnetism that extends to their applications in the field of thermoelectricity. Conventional thermoelectric (TE) devices use the Seebeck effect to convert heat into electrical energy. In contrast, transverse thermoelectric devices based on the Nernst effect are attracting recent attention due to their unique transverse geometry, which uses a single material to eliminate the need for a multitude of electrical connections compared to conventional TE devices. Here, a large anomalous transverse thermoelectric effect of ≈2 µV K-1 at room temperature in a kagome antiferromagnet YMn6 Sn6 single crystal is obtained. The obtained value is larger than that of state-of-the-art canted antiferromagnetic (AFM) materials and comparable with ferromagnetic systems. The large anomalous Nernst effect (ANE) can be attributed to the net Berry curvature near the Fermi level. Furthermore, the ANE of the AFM YMn6 Sn6 exceeds the magnetization scaling relationship of conventional ferromagnets. The results clearly illustrate that AFM material YMn6 Sn6 is an ideal topological material for room-temperature transverse thermoelectric applications.

Multiferroic antiferromagnetic artificial synapse

Artificial intelligence frameworks utilizing unsupervised learning techniques can avoid the bottleneck of labeled training data required in supervised machine learning systems, but the programming time of these systems is inherently limited by their hardware implementations. Here, a finite-element model coupling micromagnetics and dynamic strain is used to investigate a multiferroic antiferromagnet as a high-speed artificial synapse in artificial intelligence applications. The stability of strain-induced intermediate antiferromagnetic magnetization states (non-uniform magnetization states between a uniform 0 or 1), along with the minimum time scale at which these states can be programmed is investigated. Results show that due to the antiferromagnetic material's magnetocrystalline anisotropy, two intermediate states (Néel vector 1/3z, 2/3x, and Néel vector 2/3z, 1/3x) between fully x and fully z Néel vector orientations can be successfully programmed using 375 με strain pulses, and that the time associated with this programming is limited to ∼0.3 ns by the material's antiferromagnetic resonance frequency.

On-Surface Design of a 2D Cobalt-Organic Network Preserving Large Orbital Magnetic Moment.

The design of antiferromagnetic nanomaterials preserving large orbital magnetic moments is important to protect their functionalities against magnetic perturbations. Here, we exploit an archetype H6HOTP species for conductive metal-organic frameworks to design a Co-HOTP one-atom-thick metal-organic architecture on a Au(111) surface. Our multidisciplinary scanning probe microscopy, X-ray absorption spectroscopy, X-ray linear dichroism, and X-ray magnetic circular dichroism study, combined with density functional theory simulations, reveals the formation of a unique network design based on threefold Co+2 coordination with deprotonated ligands, which displays a large orbital magnetic moment with an orbital to effective spin moment ratio of 0.8, an in-plane easy axis of magnetization, and large magnetic anisotropy. Our simulations suggest an antiferromagnetic ground state, which is compatible with the experimental findings. Such a Co-HOTP metal-organic network exemplifies how on-surface chemistry can enable the design of field-robust antiferromagnetic materials.

Surface plasmon-phonon-magnon polariton in a topological insulator-antiferromagnetic bilayer structure

We present a robust technique for computationally studying surface polariton modes in hybrid materials. We use a semiclassical model that allows us to understand the physics behind the interactions between collective excitations of the hybrid system and develop a scattering and transfer matrix method that imposes the proper boundary conditions to solve Maxwell's equations and derive a general equation describing the surface polariton in a heterostructure consisting of N constituent materials. We apply this method to a test structure composed of a topological insulator (TI) and an antiferromagnetic material (AFM) to study the resulting surface Dirac plasmon-phonon-magnon polariton (DPPMP). We find that interactions between the excitations of the two constituents result in the formation of hybridized modes and the emergence of avoided-crossing points in the dispersion relations for the DPPMP. For the specific case of a $\mathrm{Bi}_{2}\mathrm{Se}_{3}$ TI material combined with a 3D AFM such as NiO, $\mathrm{MnF}_{2}$, or $\mathrm{FeF}_{2}$, the polariton branch with low frequency below 2 THz redshifts upon increasing the thickness of TI thin film, which leads to an upper bound on the thickness of the TI layer that will allow an observable signature of strong coupling and the emergence of hybridized states. We also find that the strength of the coupling between the TI and the AFM, which is parametrized by the amplitude of the avoided-crossing splitting between the two polariton branches at the magnon resonance frequency, depends on the magnitude of the magnetic dipole and the linewidth of the magnon in the AFM material as well as on the Fermi energy of Dirac plasmon in the TI. Finally, we show that materials with extremely high quality, i.e., low scattering loss rate, are essential to achieve an experimentally observable strong coupling between a TI and 3D AFM material. The overall analysis identifies the material properties that are necessary to achieve experimentally observable strong coupling for the interaction between THz excitations in a TI/AFM heterostructure and can thereby guide experimental efforts.

Anisotropic magnetotransport in the layered antiferromagnet TaFe1.25Te3

The discovery of fascinating ways to control and manipulate antiferromagnetic materials have garnered considerable attention as an attractive platform to explore novel spintronic phenomena and functionalities. Layered antiferromagnets (AFMs) exhibiting interesting magnetic structures can serve as an attractive starting point to establish novel functionalities down to the two-dimensional limit. In this work, we explore the magnetoresistive properties of the spin-ladder AFM ${\mathrm{TaFe}}_{1.25}{\mathrm{Te}}_{3}$. Magnetization studies reveal an anisotropic magnetic behavior resulting in the stabilization of a spin-flop configuration for $H\ensuremath{\perp}$ (10-1) plane (i.e., out-of-plane direction). Angle-dependent longitudinal and transverse magnetoresistances show an unusual anharmonic behavior. A significant anisotropic enhancement of magnetoresistance when $H\ensuremath{\perp}$ (10-1) plane compared to $H||$ (10-1) directions has been observed. The present results deepen our understanding of the magnetoresistive properties of low-dimensional layered AFMs, and point towards the possibility of utilizing these novel material systems for antiferromagnetic spintronics.

Tuneable hysteresis loop and multifractal oscillations of magnetisation in weakly disordered antiferromagnetic–ferromagnetic bilayers

Coupled antiferromagnetic–ferromagnetic bilayers have been intensively investigated as low-dimensional memory materials. However, the connection between their architecture and emergent hysteresis loop phenomena remains elusive. Here, we revealed this relation through low-temperature simulations of the field-driven Ising spin reversal dynamics in heterostructures of coupled ferromagnetic and antiferromagnetic layers of varied thicknesses and a weak random-field disorder. The hysteresis loop exhibits the fractional-magnetisation plateaus, where their number, the height of the central loop, and the structure of side sub-loops strictly depend on the antiferromagnetic layer thickness. Meanwhile, the interlayer coupling chiefly determines the coercive field values, modified by the magnetic disorder, the thickness of the ferromagnetic layer and the system size, in agreement with the derived theoretical formula. The magnetisation fluctuations are modulated with peaks at the transitions between successive plateaus, reflecting the active groups of spins with different levels of (anti)ferromagnetic couplings arising at the interplay of antiferromagnetic sublattices and disorder. The cyclic trend drives the magnetisation fluctuations within limited time intervals; multifractal fluctuations occur on larger time scales. These findings shed new light on the tuneable hysteresis loop properties and the related magnetisation fluctuations in thin antiferromagnetic–ferromagnetic bilayers. The described hysteresis-loop phenomena are not limited to these model systems but should be present in different antiferromagnetic materials with complex morphology. • The AFM-FM bilayer architecture determines the emergent hysteresis-loop properties. • Simulations of the magnetisation-reversal processes reveal the hysteresis-loop shape and the coercive fields. • Magnetisation fluctuations on the hysteresis loop exhibit cycles and multifractality. • Cycles indcate distinct spin groups activated by the field, geometry and weak disorder.

The Zn1-xPbxCr2Se4-Single Crystals Obtained by Chemical Vapour Transport-Structure and Magnetic, Electrical, and Thermal Properties.

Monocrystalline chalcogenide spinels ZnCr2Se4 are antiferromagnetic and semiconductor materials. They can be used to dope or alloy with related semiconducting spinels. Therefore, their Pb-doped display is expected to have unique properties and new potential applications. This paper presents the results of dc and ac magnetic measurements, including the critical fields visible on the magnetisation isotherms, electrical conductivity, and specific heat of the ZnCr2S4:Pb single crystals. These studies showed that substituting the diamagnetic Pb ion with a large ion radius for the Zn one leads to strong short-range ferromagnetic interactions in the entire temperature range and spin fluctuations in the paramagnetic region at Hdc = 50 kOe.

Antiferromagnetic resonance in α-Fe2O3 up to its Néel temperature

Hematite (α-Fe2O3) is an antiferromagnetic material with a very low spin damping and high Néel temperature. The temperature dependence of the antiferromagnetic resonance in a bulk single crystal of hematite was characterized from room temperature up to the Néel temperature in the frequency range of 0.19–0.5 THz. From these data, the Néel temperature was estimated as 966 K.

Elementary excitations at anomalous interfaces: Dispersion relations from thermodynamics constraints

The dispersion relation, assumed as a power law of a given index, defines the degrees of freedom of elementary excitations. In an anomalous interface, where the degrees of freedom are in principle unspecified, the dispersion relation is determined by thermodynamic constraints. Magnons at the junction between a ferromagnetic material (index 2) and an antiferromagnetic material (index 1) are an explicit example. At high temperatures, the statistically averaged index is 4/3 and varies with temperature. There are large fluctuations at high temperatures and an expression for the lifetime of excitations for any spatial dimension (fractional or not) is proposed.

Dramatic effect of electrode type on tunnel junction based molecular spintronic devices

A new class of molecular spintronic devices can be fabricated by chemically bonding magnetic molecular channels to the electrodes of a prefabricated tunnel junction with exposed side edges. Prior experimental studies showed that the cyanide-bridged octametallic molecular cluster, [(pzTp)FeIII(CN)3]4[NiII(L)]4¬[O3SCF3]4 [(pzTp) = tetra(pyrazol-1-yl)borate; L = 1-S(acetyl)tris(pyrazolyl)decane] molecule impact depended on the type of metallic electrodes used in the tunnel junction testbed. Experimental magnetization and transport studies showed a dramatic difference in molecule response on tunnel junctions with different combinations of metallic electrodes. Transport via paramagnetic molecular channels on a tunnel junction involving paramagnetic and ferromagnetic metal electrodes was dramatically different than the suppressed current state observed on tunnel junctions involving two ferromagnetic electrodes. We conducted theoretical studies to understand the experimental data and explore a wide range of electrode materials on tunnel junction-based molecular spintronics devices (TJMSD). Here, we report a Monte Carlo simulation study that focuses on understanding the effect of electrodes on the magnetic and physical properties of TJMSD. A 3D Heisenberg model of cross-junction-shaped TJMSD was used for the simulation study. We studied the effects of ferromagnetic, paramagnetic, and antiferromagnetic electrode materials. This study provides insights for designing and understanding futuristic molecular spintronics devices.

Light-Controlled Ultrafast Magnetic State Transition in Antiferromagnetic-Ferromagnetic van der Waals Heterostructures.

Manipulating spin in antiferromagnetic (AFM) materials has great potential in AFM opto-spintronics. Laser pulses can induce a transient ferromagnetic (FM) state in AFM metallic systems but have never been proven in two-dimensional (2D) AFM semiconductors and related van der Waals (vdW) heterostructures. Herein, using 2D vdW heterostructures of FM MnS2 and AFM MXenes as prototypes, we investigated optically induced interlayer spin transfer dynamics based on real-time time-dependent density functional theory. We observed that laser pulses induce significant spin injection and interfacial atom-mediated spin transfer from MnS2 to Cr2CCl2. In particular, we first demonstrated the transient FM state in semiconducting AFM-FM heterostructures during photoexcited processes. The proximity magnetism breaks the magnetic symmetry of Cr2CCl2 in heterostructures. Our results provide a microscopic understanding of optically controlled interlayer spin dynamics in 2D magnetic heterostructures and open a new way to manipulate magnetic order in 2D materials for ultrafast opto-spintronics.

Giant exchange bias in antiferromagnetic Pr2CoFe0.5Mn0.5O6: a structural and magnetic properties study

Antiferromagnetic (AFM) materials with a colossal exchange bias (EB) effect find applications as high-density spintronic devices. We report structural (geometrical and electronic) and magnetic studies in the polycrystalline Pr2CoFe0.5Mn0.5O6 double perovskite system. The observed lack of training effect suggests the existence of robust EB. In addition, the detailed magnetic studies and Raman studies unravel the Griffith-like phase along with the spin-phonon coupling in the present system. The x-ray photoemission spectroscopy (XPS) analysis supports more than one valence state of B-site elements, which is accountable for the competition between ferromagnetic (FM) and AFM interactions in addition to the anti-site disorder in the system. The neutron measurement confirms the G-type AFM spin arrangement, accredited by the DFT calculation. The magnetic studies have correlated with the electronic structure, neutron study, and theoretical first principle calculations.

Spin Conductivity Based on Magnetic Toroidal Quadrupole Hidden in Antiferromagnets

We report our theoretical results on spin conductivity in antiferromagnets by focusing on the role of the magnetic toroidal quadrupole (MTQ) in electron systems. The MTQ is characterized as a time-reversal-odd rank-2 polar tensor degree of freedom in electrons, which is distinct from conventional rank-1 magnetic and magnetic toroidal dipoles. Based on a microscopic sd model analysis for a tetragonal system under both collinear and noncollinear antiferromagnetic orderings, we clarify that the MTQ becomes a source of an extrinsic spin conductivity even with neither a uniform magnetization nor spin–orbit coupling. We also list all the magnetic point groups to accommodate the MTQs as a primary order parameter as well as the candidate antiferromagnetic materials.

New Type of Anticommutative Dynamical Magnetoelectric Response.

Axion field induced topological magnetoelectric response has attracted lots of attention since it was first proposed by Qi etal. [Phys. Rev. B 78, 195424 (2008).PRBMDO1098-012110.1103/PhysRevB.78.195424]. Here we find a new type of anticommutative magnetoelectric response β^{ξ}(ω), which can induce a dynamical magnetoelectric current driven by a time-varying magnetic field. Unlike the Chern-Simons axion term, this magnetoelectric response term is gauge independent and nonquantized, and manifests in the systems breaking the symmetries of the time reversal, inversion, and mirror. In particular, we propose the antiferromagnetic material Mn_{2}Bi_{2}Te_{5} as a material candidate to observe dynamical magnetoelectric current, in which a large magnetoelectric response term β^{ξ}(ω) originates from band inversion.

SmI3:4f5 honeycomb magnet with spin-orbital entangled Γ7 Kramers doublet

We report magnetic properties of a 4f-honeycomb iodide SmI3 made up of edge-shared network of SmI6 octahedra. High temperature magnetic susceptibility indicates {\Gamma}7 Kramers doublet ground state of Sm3+ (4f5) ions stabilized by the spin-orbit coupling and octahedral crystal electric field, which interact with Sm-I-Sm bond angle nearly 90 degree. Magnetization measurements down to 0.1 K detected antiferromagnetic correlations and an anomaly in the magnetization curve before saturation without a sign of long-range order. Relevance between SmI3 and the antiferromagnetic Kitaev material proposed in the 4f-electron system is discussed.

Self-Supervised Graph Neural Networks for Accurate Prediction of Néel Temperature

Antiferromagnetic materials are exciting quantum materials with rich physics and great potential for applications. On the other hand, an accurate and efficient theoretical method is highly demanded for determining critical transition temperatures, Néel temperatures, of antiferromagnetic materials. The powerful graph neural networks (GNNs) that succeed in predicting material properties lose their advantage in predicting magnetic properties due to the small dataset of magnetic materials, while conventional machine learning models heavily depend on the quality of material descriptors. We propose a new strategy to extract high-level material representations by utilizing self-supervised training of GNNs on large-scale unlabeled datasets. According to the dimensional reduction analysis, we find that the learned knowledge about elements and magnetism transfers to the generated atomic vector representations. Compared with popular manually constructed descriptors and crystal graph convolutional neural networks, self-supervised material representations can help us to obtain a more accurate and efficient model for Néel temperatures, and the trained model can successfully predict high Néel temperature antiferromagnetic materials. Our self-supervised GNN may serve as a universal pre-training framework for various material properties.

Giant magnetization jumps in multiscale-distortion dual-antiferromagnetic system

Large magnetization jumps (MJs) can realize an avalanched flip of the spin structure from a low spin state (antiferromagnetic) to a high spin state (ferromagnetic) and has potential applications in spin devices. Here, we report giant MJs in dual-antiferromagnetic hematite-ilmenite (Fe2O3)0.1(FeTiO3)0.9 (HI-9) solid solution. The obtained intensity of MJs (the ratio of an abrupt change in magnetization to saturation magnetization) increases to 53.3%, which is about twice as much as previously reported values in HI-9. These unusually large MJs are achieved by intentionally introducing multiscale distortions with high-stress compression deformation. Both experiments and Monte Carlo simulations demonstrate that the increase in MJs' intensity originates from the tunable atomic-scale and nano-scale distortions induced by crystal strain energy during the deformation process. Our findings provide an approach to modulate metamagnetic transitions and may inspire fresh ideas for creating high-performance antiferromagnetic materials.

Variation between Antiferromagnetism and Ferrimagnetism in NiPS3 by Electron Doping

AbstractTo electrically control magnetic properties of material is promising toward spintronic applications, where the investigation of carrier doping effects on antiferromagnetic (AFM) materials remains challenging due to their zero net magnetization. In this work, the authors find electron doping dependent variation of magnetic orders of a 2D AFM insulator NiPS3, where doping concentration is tuned by intercalating various organic cations into the van der Waals gaps of NiPS3 without introduction of defects and impurity phases. The doped NiPS3 shows an AFM‐ferrimagnetic (FIM) transition at a doping level of 0.2–0.5 electrons/cell and a FIM‐AFM transition at a doping level of ≥0.6 electrons/cell. The authors propose that the found phenomenon is due to competition between Stoner exchange dominated inter‐chain ferromagnetic order and super‐exchange dominated AFM order at different doping level. The studies provide a viable way to exploit correlation between electronic structures and magnetic properties of 2D magnetic materials for realization of magnetoelectric effect.

Tilted spin current generated by the collinear antiferromagnet ruthenium dioxide

We report measurements demonstrating that when the Neel vector of the collinear antiferromagnet RuO2 is appropriately canted relative to the sample plane, the antiferromagnet generates a substantial out of plane damping-like torque. The measurements are in good accord with predictions that when an electric field, E is applied to the spin split band structure of RuO2 it can cause a strong transverse spin current even in the absence of spin-orbit coupling. This produces characteristic changes in all three components of the E induced torque vector as a function of the angle of E relative to the crystal axes, corresponding to a spin current with a well defined tilted spin orientation s approximately (but not exactly) parallel to the Neel vector, flowing perpendicular to both E and S. This angular dependence is the signature of an antiferromagnetic spin Hall effect with symmetries that are distinct from other mechanisms of spin-current generation reported in antiferromagnetic or ferromagnetic materials.

Abnormal Magnetoresistance Transport Properties of van der Waals Antiferromagnetic FeNbTe2

The emergence of two-dimensional (2D) van der Waals magnetic materials has attracted enormous attention due to their novel physical phenomena and potential application in the fields of spintronics and information storage technology. Here, we systematically study the magnetic and transport properties of a van der Waals antiferromagnetic material, FeNbTe2. The magnetic and magnetoresistance measurements verified its antiferromagnetic properties, spin glass state, and negative magnetoresistance effect at lower temperatures. In addition, the measurement results of transport also show the existence of angle-dependent anisotropic magnetoresistance in a wide temperature range and anisotropic magnetoresistance inversion in a certain temperature range.