Antiferromagnetic Materials Research Articles

Supramolecular Rotor Assembly for the Design of a Hybrid Ferroelectric‐Antiferromagnetic Multiferroic Semiconductor

Ferroelectric (FE)‐antiferromagnetic (AFM) multiferroic materials have sparked growing interest due to their huge possibilities in energy‐saving, photoelectric devices, nonvolatile storage, and switches. However, realizing FE‐AFM properties in a hybrid molecular material is difficult because ferroelectric and magnetic orders are commonly mutually exclusive. Here, we report an FE‐AFM multiferroic semiconductor [NH4(18‐crown‐6)]2[Mn(SCN)4] (NCMS) by supramolecular assembly approach via molecular rotor synthon [NH4(18‐crown‐6)] and inorganic magnetic module [Mn(SCN)4]. Interestingly, NCMS shows good ferroelectricity with a spontaneous polarization (Ps) of 5.94 μC cm−2 higher than most crown‐ether‐based ferroelectrics. Especially, the realization of antiferromagnetism is for the first time in the crown ether hybrid perovskite ferroic systems. Additionally, semiconductor NCMS displays an X‐ray radiation detection response with a large photo/dark current on‐off ratio (197). Our study not only gives a deep insight into understanding multiferroic properties but also provides a novel and efficient approach to realizing high‐performance hybrid multiferroic materials.

Supramolecular Rotor Assembly for the Design of a Hybrid Ferroelectric-Antiferromagnetic Multiferroic Semiconductor.

Ferroelectric (FE)-antiferromagnetic (AFM) multiferroic materials have sparked growing interest due to their huge possibilities in energy-saving, photoelectric devices, nonvolatile storage, and switches. However, realizing FE-AFM properties in a hybrid molecular material is difficult because ferroelectric and magnetic orders are commonly mutually exclusive. Here, we report an FE-AFM multiferroic semiconductor [NH4(18-crown-6)]2[Mn(SCN)4] (NCMS) by supramolecular assembly approach via molecular rotor synthon [NH4(18-crown-6)] and inorganic magnetic module [Mn(SCN)4]. Interestingly, NCMS shows good ferroelectricity with a spontaneous polarization (Ps) of 5.94 μC cm-2 higher than most crown-ether-based ferroelectrics. Especially, the realization of antiferromagnetism is for the first time in the crown ether hybrid perovskite ferroic systems. Additionally, semiconductor NCMS displays an X-ray radiation detection response with a large photo/dark current on-off ratio (197). Our study not only gives a deep insight into understanding multiferroic properties but also provides a novel and efficient approach to realizing high-performance hybrid multiferroic materials.

Engineering exchange bias at the interface of self-polarized ultrathin ferroelectric BaTiO3 and ferromagnetic La0.67Sr0.33MnO3

We investigate the emergence and optimization of conventional exchange bias (EB) in ultrathin (<10 nm) ferroelectric (FE) BaTiO3 (BTO)/ferromagnetic (FM) La0.67Sr0.33MnO3 (LSMO) epitaxial bilayers without an antiferromagnetic (AFM) material. The EB originates from the electronic orbital reconstruction at the FE-FM interface due to the ferroelectric polarization. We achieve maximum EB of approximately 42 Oe with single-domain polarization in nine-unit-cell-thick BTO, setting the BTO thickness above the critical threshold for ferroelectricity yet below the thickness of strain relaxation and multidomain breakdown. Furthermore, the LSMO layer needs to be thick enough to sustain both the FM layer and polarization-induced AFM spin configuration at the LSMO/BTO interface, yet as thin as possible to enable the EB loop shift. The temperature, training, field, and thickness dependence of the EB confirm that the LSMO/BTO interface exhibits conventional EB despite its unconventional origin. Using x-ray magnetic circular dichroism, scanning transmission electron microscopy, and density-functional-theory calculations, we confirm that the macroscopic EB effect originates from the interfacial AFM spin configuration in LSMO driven by FE-induced -orbital modifications in interfacial Mn ions. Thus, we engineer strong interfacial EB coupling in artificial multiferroics without a conventional AFM material by controlling FE polarization, highlighting the potential for advanced spintronic applications. Published by the American Physical Society 2024

Magnetic proximity-induced anomalous Hall effect in 2D CrOCl/Pt heterostructure

Two-dimensional (2D) van der Waals antiferromagnetic (AFM) materials boast exceptional properties for spintronics, including high spin-wave speeds and negligible stray fields. Their layer-by-layer assembly into heterostructures enables the exploration of next-generation spintronic devices. However, most 2D AFM materials are semiconductors or insulators. Thus, magneto-transport, a key segment of spintronics, is difficult to obtain especially at low temperatures. Herein, we report the observation of anomalous Hall effect (AHE) in 2D CrOCl/Pt bilayer heterostructure. Magneto-transport measurements supported by density functional theory calculations reveal that the appearance of AHE is generated by spin polarization in Pt due to the magnetic proximity effect. In addition, it is demonstrated that the magnetic easy-axis changes from the z-axis to the xy-plane at the interface of the heterostructure. Our work sheds light on the magneto-transport properties of 2D CrOCl and its potential in emerging spintronic devices.

Single-molecule Magnets (SMM) spin channels connecting FeMn antiferromagnet and NiFe ferromagnetic electrodes of a tunnel junction

The integration of single-molecule magnets (SMMs) into magnetic tunnel junctions (MTJs) offers significant potential for advancing molecular spintronics, particularly for next-generation memory devices, quantum computing, and energy storage technologies such as solar cells. In this study, we present the first demonstration of SMM-induced spin-dependent properties in an antiferromagnet-based MTJ molecular spintronic device (MTJMSD). We engineered cross-junction-shaped devices comprising FeMn/AlOx/NiFe MTJs. The AlOx barrier thickness where the exposed junction edges meet was comparable to the SMM length, facilitating the incorporation of SMM molecules as spin channels for spin-dependent transport. The SMM channels enabled long-range magnetic moment ordering around molecular junctions, which were precisely engineered via fabrication processes. The SMM, composed of a [Mn6(μ3-O)2(H2N-sao)6(6-atha)2(EtOH)6] (H2N-saoH = salicylamidoxime, 6-atha = 6-acetylthiohexanoate) complex, featured thioester groups at the ends that upon hydrolysis they form bonds with the magnetic electrodes. SMM-treated junctions demonstrated a significant current enhancement, reaching up to 7 μA at an input voltage of 60 mV. Furthermore, SMM-doped junctions exhibited current stabilization in the μA range at lower temperatures, whereas the bare electrodes showed current suppression to the picoampere range. Magnetization measurements conducted at 55 K and 300 K on pillar-shaped devices revealed a reduction in magnetic moment at low temperatures. Additionally, Kelvin probe atomic force microscopy (KPAFM) measurements confirmed that SMM integration transformed the electronic properties over long ranges.These findings are attributed to the spin channels formed between magnetic metal electrodes, which enhance spin polarization at each magnetic electrode. Our research highlights the potential of using antiferromagnetic materials, characterized by minimal stray fields and zero net magnetization, to transform MTJMSD devices.

Quantum spin systems: Toroidal classification and geometric duality

We demonstrate a toroidal classification for quantum spin systems, revealing an intrinsic geometric duality within this structure. Through our classification and duality, we reveal that various bipartite quantum features in magnon systems can manifest equivalently in both bipartite ferromagnetic and antiferromagnetic materials, based upon the availability of relevant Hamiltonian parameters. Additionally, the results highlight the antiferromagnetic regime as an ultrafast dual counterpart to the ferromagnetic regime, both exhibiting identical capabilities for quantum spintronics and technological applications. Concrete illustrations are provided, demonstrating how splitting and squeezing types of two-mode magnon quantum correlations can be realized across ferro- and antiferromagnetic regimes. Published by the American Physical Society 2024

Controllable syntheses and magnetic and photoelectric properties modulations of CuFeS2 nanoplates

As an antiferromagnetic material, chalcopyrite CuFeS2 has both magnetic and semiconductor properties, which brings great prospects for the application of spintronics devices. Here, chalcopyrite CuFeS2 nanoplates were successfully synthesized through a template mediated method. The CuFeS2 exhibits a tetragonal phase while largely maintaining the hexagonal morphology of the CuS template, with particle sizes ranging between 90 and 120 nm. Specifically, the magnetic properties of CuFeS2 can be tuned by varying the Fe content. The strong room-temperature ferromagnetism is attributed to the coexistence of the ferromagnetic Fe7S8 and the antiferromagnetic CuFeS2, as well as to the unsaturated spin states of Fe on the surfaces of the nanoplates. Furthermore, the photoresponses of CuFeS2 nanoplates were confirmed by I-V measurements under dark and light illumination. The stable photoresponse capability is attributed to its semiconductor properties. This novel room-temperature ferromagnetism and photoelectric effect promotes the development of CuFeS2 nanoplates, and has great significance for the synthetic pathways of other ternary magnetic compounds.

Electric polarization evolution equation for antiferromagnetic multiferroics with the polarization proportional to the scalar product of the spins

The spin current model of electric polarization in multiferroics is justified via the quantum hydrodynamic method and the mean-field part of the spin-orbit interaction. The spin current model is applied to derive the electric polarization proportional to the scalar product of the spins of the nearby ions, which appears to be caused by the Dzylaoshinskii-Moriya interaction. The symmetric tensor spin structure of the polarization is discussed as well. We start our derivations for the ferromagnetic multiferroic materials and present further generalizations for the antiferromagnetic multiferroic materials. We rederive the operator of the electric dipole moment, which provides the macroscopic polarization obtained via the spin current model. Finally, we use the quantum average of the found electric dipole moment operator to derive the polarization evolution equation for the antiferromagnetic multiferroic materials. The possibility of spiral spin structures is analyzed.

PT Symmetry Breaking Induced Anomalous Valley Hall Effect in 2D Antiferromagnetic Semiconductor.

Realizing the anomalous valley Hall (AVH) effect in two-dimensional (2D) materials is of crucial importance for information processing and recording technology. While the research in this field mainly focuses on ferromagnetic systems, little is known about antiferromagnetic systems. Here, using k·p model analysis, we report a novel mechanism of realizing the AVH effect in 2D antiferromagnetic materials. This physics is related to the PT symmetry breaking induced by intrinsic staggered sublattice potential, which is introduced by asymmetric magnetic ions located at different sublattices. With reversal of the magnetic orientation on different sublattices, the AVH effect can be reversed. Based on first-principles calculations, we further demonstrate this mechanism in an antiferromagnetic monolayer of NiRuCl6. Intriguingly, due to the d orbital mismatch near the Fermi level, monolayer NiRuCl6 simultaneously owns zero net magnetization and large spin splitting and valley polarizations, which facilitates the observation of the AVH effect. Our findings greatly enrich the research on valley physics in antiferromagnetic systems.

Towards MnN as a replacement for IrMn

There is an urgent need to identify new antiferromagnetic materials that offer a replacement for IrMn alloys for room temperature and above applications. This is driven by the scarcity and high cost of Ir. Recently, MnN with {002} texture grown on a Ta seed layer has been proposed as a cost-effective alternative. However, two key issues need to be addressed before this material can be considered a realistic alternative to IrMn: thick layers of approximately 30 nm are required due to its relatively low magnetocrystalline anisotropy and nitrogen diffusion into the Ta layer at relatively low temperatures results in poor temperature performance. In this work, we show a potential pathway to overcome these issues. By using a W rather than a Ta seed layer, N diffusion is minimized, if not eliminated, at temperatures exceeding 300∘C\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$300\\,^{\\circ }\\hbox {C}$$\\end{document}. Furthermore, preferential {111} growth is achieved and a significantly enhanced anisotropy, 2.6·106erg/cm3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2.6 \\cdot 10^{6}\\,\\hbox {erg}/\\hbox {cm}^{3}$$\\end{document}, has been measured. This value is almost identical to that measured for 3D randomly oriented IrMn.

Antiferro skyrmion crystals consisting of skyrmions and anti-skyrmions on a bilayer square lattice

An antiferro-type skyrmion crystal consisting of skyrmions with a negative topological charge and antiskyrmions with a positive topological charge manifests itself in its peculiar topological magnetism. We theoretically investigate the emergence of such an antiferro-type skyrmion crystal, where the topological charge exists in each sublattice but vanishes in the whole system. By performing the simulated annealing for an effective spin model with competing exchange interactions and staggered Dzyaloshinskii-Moriya interaction in momentum space on a bilayer square lattice, we find that the synergy between the antiferromagnetic interlayer exchange interaction and high-harmonic wave-vector interaction in addition to the staggered Dzyaloshinskii-Moriya interaction results in the antiferro-type skyrmion crystal in an external magnetic field. We show that the antiferro-type skyrmion crystal turns into the ferro-type skyrmion crystal with the same topological charge for both layers when the magnitude of the external magnetic field is varied. We also demonstrate that the competing interactions between ordering wave-vector channels and their high-harmonic wave-vector channels are essential in stabilizing the antiferro-type skyrmion crystal. Our results provide important ingredients to realize the antiferro-type skyrmion crystal by the external magnetic field, which can be utilized for low-power and high-speed spintronic devices based on antiferromagnetic materials.

Low Temperature Plasma-Assisted Double Anodic Dissolution: A New Approach for the Synthesis of GdFeO3 Perovskite Nanoparticles.

Orthorhombic perovskite GdFeO3 nanostructures are promising materials with multiferroic properties. In this study, a new low-temperature plasma-assisted approach is developed via dual anodic dissolution of solid metallic precursors for the preparation of perovskite GdFeO3 nanoparticles (NPs) that can be collected both as colloids as well as deposited as a thin film on a substrate. Two solid metallic foils of Gd and Fe are used as precursors, adding to the simplicity and sustainability of the method. The formation of the orthorhombic perovskite GdFeO3 phase is supported by high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman measurements, while a uniform elemental distribution of Gd, Fe, and O is confirmed by energy dispersive X-ray spectroscopy, proving the successful preparation of ternary compound NPs. The magnetic properties of the NPs show zero remnant magnetization typical of antiferromagnetic materials, and saturation at high fields that can be caused by spin interaction between Gd and Fe magnetic sublattices. The formation mechanism of ternary compound NPs in this novel plasma-assisted method is also discussed. This method is also modified to demonstrate the direct one-step deposition of thin films, opening up opportunities for their future applications in the fabrication of magnetic memory devices and gas sensors.

Spin chain orientation and magneto-optical coupling in twisted NiPS3 homostructures

Magnetic two-dimensional (2D) materials have garnered significant attention due to their unique electronic, magnetic, and optical properties and their potential applications in next-generation electronic and optoelectronic devices. However, the magneto-optical effects of oligolayer antiferromagnetic materials remain inadequately understood. Here, we investigate the magnetic properties of few-layer nickel phosphorus trisulfide (NiPS3) and its twisted heterostructures, emphasizing the observation of optical phenomena at low temperatures (1.65 K). By stacking few-layer NiPS3 to fabricate twisted homostructures, we probe their magnetic characteristics using photoluminescence (PL) spectroscopy. Our results reveal that sharp exciton peaks emerge at low temperatures and that the spin chain orientation in oligolayer NiPS3 can be discerned through the polarization dependence of exciton PL intensity. Notably, fewer-layered NiPS3 exhibits a significant magneto-optical effect under an applied magnetic field, allowing the modulation of the polarization angle of its exciton PL spectrum. Additionally, the polarization-dependent Raman spectrum of NiPS3 shows substantial changes under the influence of a magnetic field. These findings underscore the potential of few-layer NiPS3 for future magneto-optical device applications.

Ultrahigh Néel Temperature Antiferromagnetism and Ultrafast Laser-Controlled Demagnetization in a Dirac Nodal Line MoB3 Monolayer.

Two-dimensional (2D) antiferromagnetic (AFM) materials boasting a high Néel temperature (TN), high carrier mobility, and fast spin response under an external field are in great demand for efficient spintronics. Herein, we theoretically present the MoB3 monolayer as an ideal 2D platform for AFM spintronics. The AFM MoB3 monolayer features a symmetry-protected, 4-fold degenerate Dirac nodal line (DNL) at the Fermi level. It demonstrates a high magnetic anisotropy energy of 865 μeV/Mo and an ultrahigh TN of 1050 K, one of the highest recorded for 2D AFMs. Importantly, we reveal the ultrafast demagnetization of AFM MoB3 under laser irradiation, which induces a rapid transition from a DNL semimetallic state to a metallic state on the time scale of hundreds of femtoseconds. This work presents an effective method for designing advanced spintronics using 2D high-temperature DNL semimetals and opens up a new idea for ultrafast modulation of magnetization in topological semimetals.

High temperature ferrimagnetic semiconductors by spin-dependent doping in high temperature antiferromagnets

To realize room temperature ferromagnetic (FM) semiconductors is still a challenge in spintronics. Many antiferromagnetic (AFM) insulators and semiconductors with high Neel temperature TN are obtained in experiments, such as LaFeO3, BiFeO3, etc. High concentrations of magnetic impurities can be doped into these AFM materials, but AFM state with very tiny net magnetic moments was obtained in experiments because the magnetic impurities were equally doped into the spin up and down sublattices of the AFM materials. Here, we propose that the effective magnetic field provided by a FM substrate could guarantee the spin-dependent doping in AFM materials, where the doped magnetic impurities prefer one sublattice of spins, and the ferrimagnetic (FIM) materials are obtained. To demonstrate this proposal, we study the Mn-doped AFM insulator LaFeO3 with FM substrate of Fe metal by the density functional theory (DFT) calculations. It is shown that the doped magnetic Mn impurities prefer to occupy one sublattice of the AFM insulator and introduce large magnetic moments in La(Fe, Mn)O3. For the AFM insulator LaFeO3 with high TN = 740 K, several FIM semiconductors with high Curie temperature TC > 300 K and the band gap less than 2 eV are obtained by DFT calculations when 1/8 or 1/4 Fe atoms in LaFeO3 are replaced by the other 3d, 4d transition metal elements. The large magneto-optical Kerr effect (MOKE) is obtained in these LaFeO3-based FIM semiconductors. In addition, the FIM semiconductors with high TC are also obtained by spin-dependent doping in some other AFM materials with high TN, including BiFeO3, SrTcO3, CaTcO3, etc. Our theoretical results propose a way to obtain high TC FIM semiconductors by spin-dependent doping in high TN AFM insulators and semiconductors.

Non-relativistic torque and Edelstein effect in non-collinear magnets

The Edelstein effect is the origin of the spin-orbit torque: a current-induced torque that is used for the electrical control of ferromagnetic and antiferromagnetic materials. This effect originates from the relativistic spin-orbit coupling, which necessitates utilizing materials with heavy elements. Here, we show that in magnetic materials with non-collinear magnetic order, the Edelstein effect and, consequently, a current-induced torque can exist even in the absence of the spin-orbit coupling. Using group symmetry analysis, model calculations, and realistic simulations on selected compounds, we identify large classes of non-collinear magnet candidates and demonstrate that the current-driven torque is of similar magnitude as the celebrated spin-orbit torque in conventional transition metal structures. We also show that this torque can exist in an insulating material, which could allow for highly efficient electrical control of magnetic order.

Room-temperature spontaneous perpendicular exchange bias in IrMn/[Co/Pt]3 multilayers

Abstract Perpendicular exchange bias (PEB) is highly desirable for the development of advanced nanoscale spintronics devices. The attainment of conventional PEB typically involves a field-cooling process through the Néel temperature of antiferromagnetic materials. In this study, we demonstrated the realization of spontaneous PEB (SPEB) in IrMn/[Co/Pt]3 multilayers utilizing isothermal crystallization of IrMn at room temperature. And the SPEB generated isothermally at IrMn/Co interface does not destroy the perpendicular magnetic anisotropy of the multilayers. The magnetic domains of the multilayers captured by Kerr microscopy after different magnetization time also indicate the generation of SPEB. The magnitude of SPEB can be controllable by varying the isothermal magnetization time and the annealing temperature of IrMn. The relationship between magnetization waiting time and SPEB reveals that even slight isothermal crystallization can generate substantial SPEB. Our results provide an alternative approach to isothermally generate PEB in IrMn/[Co/Pt]3 multilayers at room temperature.

Exchange bias and topological Hall effect of Fe and Co intercalated NbS2 single crystals

In recent years, the topological Hall effect generated by noncoplanar magnetic structures has been studied extensively. Some two-dimensional chalcogenides with 3d transition metal intercalations also exhibit similar properties. In this study, we investigated the crystalline structure and the magnetic and transport properties of Fe and Co codoped NbS2 single crystals. The structural results indicate disorder between the Fe and Co atoms within the interlayers of NbS2. Magnetic measurements showed that the vertical exchange bias effect appeared below the Néel temperature (TN), with a maximum value of approximately 2.5 mμB/f.u. at 2 K, which can be ascribed to the appearance of a spin-glass state. Additionally, accompanying the antiferromagnetic phase transition, nontrivial electrical transport properties were observed. Below TN, the magnetoresistance became positive and exhibited an initial increase, followed by a decrease with decreasing temperature. The Hall resistivity shows a hump-like curve near TN, indicating a topological Hall effect. This behavior is related to fluctuations in the spin chirality, which affects the scattering of charge carriers. Based on the correlation between the magnetoresistance and Hall resistivity responses, it is believed that the magnetic moments of the doped Fe and Co atoms in a single crystal compete with each other near the Néel temperature, resulting in complex magnetic structures such as noncoplanar and noncollinear arrangements. Such a two-dimensional antiferromagnetic material system provides an ideal platform for studying unconventional transport mechanisms.

Unraveling the Magnetic Properties of NiO Nanoparticles: From Synthesis to Nanostructure

NiO nanoparticles have garnered significant interest due to their diverse applications and unique properties, which differ markedly from their bulk counterparts. NiO nanoparticles are p-type semiconductors with a wide bandgap, high discharge capacity, and high carrier density, making them ideal for use in batteries, sensors, and catalysts. Their ability to generate reactive oxygen species also imparts disinfectant and antibiotic properties. Additionally, the higher Néel temperature of NiO compared with other antiferromagnetic materials makes it suitable for high-temperature applications in spintronic devices and industrial settings. This review focuses on the critical role of structure and composition in determining the magnetic properties of NiO nanoparticles. It examines how finite-size surface effects, morphology, crystallinity, and nickel distribution influence these properties. Fundamental physical properties and characterization techniques are discussed first. Various synthesis methods and their impact on NiO nanoparticle properties are then explored. Their magnetic phenomenology is examined in detail, highlighting the effects of finite size, particle composition and surface, and crystal quality. The review concludes with a summary of key insights and future research directions for optimizing NiO nanoparticles in technological applications.

Observation of the multiple magnetic phases in double perovskite Pr1.8La0.2CoFeO6

The structural and magnetic properties of the double perovskite Pr1.8La0.2CoFeO6 have been investigated. The X-ray diffraction study shows that the system acquires room-temperature orthorhombic phase with the Pnma space group. The X-ray photoemission spectroscopy (XPS) measurement confirmed that the B-site ions are present in mixed valence states i.e. Co has been found in Co2+ and Co3+ valance states whereas Fe has been found in Fe3+/Fe4+. Magnetic measurements of the system confirm the existence of several fascinating magnetic behaviors, such as long-range canted antiferromagnetism withTN ∼ 271 K, giant exchange bias, and re-entrant cluster glass phase (TG ∼ 33 K). Field-dependent magnetization shows a large spontaneous exchange bias; HSEB∼ 1.8 kOe and giant conventional exchange bias, HCEB∼ 2.4 kOe at 5 K. Antiferromagnetic materials with large exchange bias can be utilized in high-density spintronic devices. Temperature-dependent Raman study demonstrates that the observed phonon mode exhibits anomalous behavior close to the magnetic transition temperature. Analysis of anomalous softening of phonon mode below TN, clarifies the presence of significant spin-phonon coupling while the magnetostriction effect does not play any significant role in the observed phonon anomaly.