Antiferromagnetic Materials Research Articles

Multifunctional antiferromagnetic materials with giant piezomagnetism and noncollinear spin current

We propose a new type of spin-valley locking (SVL), named C-paired SVL, in antiferromagnetic systems, which directly connects the spin/valley space with the real space, and hence enables both static and dynamical controls of spin and valley to realize a multifunctional antiferromagnetic material. The new emergent quantum degree of freedom in the C-paired SVL is comprised of spin-polarized valleys related by a crystal symmetry instead of the time-reversal symmetry. Thus, both spin and valley can be accessed by simply breaking the corresponding crystal symmetry. Typically, one can use a strain field to induce a large net valley polarization/magnetization and use a charge current to generate a large noncollinear spin current. We predict the realization of the C-paired SVL in monolayer V2Se2O, which indeed exhibits giant piezomagnetism and can generate a large transverse spin current. Our findings provide unprecedented opportunities to integrate various controls of spin and valley with nonvolatile information storage in a single material, which is highly desirable for versatile fundamental research and device applications.

Light-induced static magnetization: Nonlinear Edelstein effect

We theoretically and computationally demonstrate that static magnetization can be generated under light illumination via nonlinear Edelstein effect (NLEE). NLEE is applicable to semiconductors under both linearly and circularly polarized light, and there are no constraints from either spatial inversion or time-reversal symmetry. Remarkably, magnetization can be induced under linearly polarized light in nonmagnetic materials. With ab initio calculations, we reveal several prominent features of NLEE. We find that the orbital contributions can be significantly greater than the spin contributions. And magnetization with various orderings, including anti-ferromagnetic, ferromagnetic, etc., are all realizable with NLEE, which may facilitate many applications, such as unveiling hidden physical effects, creating a spatially varying magnetization, or manipulating the magnetization of anti-ferromagnetic materials. The relationship between NLEE and other magneto-optic effects, including the inverse Faraday effect and inverse Cotton-Mouton effect, is also discussed.

Quadratic magneto-optical effect in hexagonal non-collinear antiferromagnets

Quadratic magneto-optical (MO) effects can be utilized to investigate the spin order in antiferromagnetic (AFM) materials; however, the previously reported studies were all limited in antiferromagnets with collinear AFM order. Here, we develop a phenomenological theory to investigate the quadratic MO effects in hexagonal non-collinear AFM crystals with triangular spin structures. Based on the permittivity tensor up to the second-order in magnetization, we derive the formula to describe the quadratic MO responses and perform numerical calculations to obtain the MO rotation angle for different AFM spin configurations and sample orientations. For the sample with its spin plane lying perpendicular to the surface plane, we have revealed the emergence of quadratic MO response, which exhibits a strong dependence on the light incident angle. The MO rotation angle shows an approximately sinusoidal variation with a periodicity of 180° when the sample is rotated around its surface normal. The size of the MO response and its deviation from the sinusoidal form are analyzed for different values of the second-order permittivity tensor elements. This study provides important insights into the non-collinear AFM spin-induced quadratic MO effect, which may be used as a guidance for optical detections of the non-collinear AFM order, and, in particular, the ultrafast spin dynamics using the optical pump–probe technique.

Logical devices based on the antiferromagnetic-antimeron in a ferromagnet nanodot with gain

The antiferromagnetic spintronics is attracting intensive attention due to the recent progress of antiferromagnetism. Especially, the topological magnetic structures, discovered in the antiferromagnetic materials, have opened an alternative path for energy efficient information processing, e.g., constructing logical devices. Herein, we theoretically extend the building block of spintronics devices by utilizing the antiferromagnetic-antimeron, which can be formed in ferromagnet nanodots with gain. A rotational mode of the antiferromagnetic-antimeron is observed under the injection of spin polarized current that originates from the excitation of spin waves propagating along the radial normal of the nanodots. The frequency of rotation (f) can be analytically calculated, expressed in the form of f∝kJα, where k is a proportionality coefficient, and J and α are the current density and Gilbert damping, respectively. The value of f is proportional to the current density (J), but inversely proportional to α. Finally, the logical gates based on the antiferromagnetic-antimeron are proposed for Boolean logic operation of OR, XOR, XNOR, and AND. Utilizing the unique dynamic behaviors of the antiferromagnet-antimeron under the injection of current sheds light on the development of the building block of spintronics devices.

Machine Learning Model for High-Throughput Screening of Perovskite Manganites with the Highest Néel Temperature

Neel temperature (TN), the antiferromagnetic to paramagnetic phase transition temperature, is also one of the important magnetic properties of antiferromagnets. Antiferromagnets could be used to write and read information below TN in electronic memory devices. Perovskite manganites are an important antiferromagnetic material with superior magnetic properties. However, it is challengeable to seek novel perovskite manganites with TN higher than room temperature by trial-and-error method. In this work, the maximum correlation minimum redundancy (mRMR) integrated machine learning approaches were used to screen the optimal subset of features, which included chemical compositions and atomic parameters of perovskite manganites. The machine learning model called support vector regression (SVR) was constructed to predict the TN of perovskite manganites. The correlation coefficient (R) between experimental TN and predicted TN reached as high as 0.87 for the training set in leave-one-out cross-validation (LOOCV) and 0.86 for the independent testing set, respectively. The high-throughput screening of new perovskites with higher TN temperature was then carried out by using our online computation platform for materials data mining (OCPMDM). The TN of designed perovskite manganites (Sr0.7Pm0.3MnO3) was predicted to be 307.5K, increasing by 6% compared to the maximum TN of Sr0.9Ce0.1MnO3 (290K) reported. Our machine learning model can be accessible for public on web server: http://materials-data-mining.com/lkl/lkl_model .

Interfacial exchange coupling-modulated magnetism in the insulating heterostructure of CoOx/yttrium iron garnet

The combination of yttrium iron garnet (Y3Fe2(FeO4)3, YIG) and ferromagnetic materials or antiferromagnetic materials has attracted much attention because of its potential in applications such as spin pumping. In this study, a high-temperature and oxygen-poor environment was used to deposit a CoOx top layer on YIG for the observation of magnetic modulation. The temperature- and saturation-field-dependent bias shift of the YIG hysteresis loop was observed. The complex modulation of the magnetic behavior originated from the magnetic coupling of the antiferromagnetic CoO and an additional interfacial coupling of the ferromagnetic CoOx. Depth-profiling X-ray photoemission spectroscopy revealed the nonuniform stoichiometric distribution in the 20-nm CoOx layer, which confirmed the origin of the various magnetic coupling effects on the bottom YIG layer. The comparative ferromagnetic resonance measurements also demonstrated the considerable magnetic coupling effect on the dynamic magnetism of the YIG.

The magnetic, electronic, optical, and structural properties of the AB2O4 (A = Mn, Fe, co; B = Al, Ga, In) spinels: Ab initio study

The effect of cation composition on the magnetic, electronic, optical, and structural properties of the spinel oxides AB2O4 (A = Fe, Mn, Co; B = Al, Ga, In) were studied within DFT-GGA + U approximation. The spinels were considered both in the normal and inverse structure. FeB2O4 (B = Al, Ga, In) spinels have an inverse structure, whereas AB2O4 (A = Mn, Co; B = Al, Ga, In) prefer a normal structure. We find that the studied spinels are antiferromagnetic materials with the composition-dependent bandgap. The bandgap width is determined by the minimum of the conductive s-band formed by B-site cations states and can be increased by the applied pressure. The microscopic mechanisms of the relationship between composition, structural and electronic properties are analyzed. The ability to manipulate the structural, electronic, and optical properties is attributed to the different s-orbital energies and sizes of the B-site cations.

Spin photogalvanic effect in two-dimensional collinear antiferromagnets

Recent discovered two-dimensional (2D) antiferromagnetic (AFM) van der Waals quantum materials have attracted increasing interest due to the emergent exotic physical phenomena. The spintronic properties utilizing the intrinsic AFM state in 2D antiferromagnets, however, have been rarely found. Here we show that the spin photogalvanic effect (SPGE), which has been predicted in three-dimensional (3D) antiferromagnets, can intrinsically emerge in 2D antiferromagnets for promising spintronic applications. Based on the symmetry analysis of possible AFM orders in the honeycomb lattice, we conclude suitable 2D AFM candidate materials for realizing the SPGE. We choose two experimentally synthesized 2D collinear AFM materials, monolayer MnPS3, and bilayer CrCl3, as representative materials to perform first-principles calculations, and find that they support sizable SPGE. The SPGE in collinear 2D AFM materials can be utilized to generate pure spin current in a contactless and ultra-fast way.

Role of NiO in the nonlocal spin transport through thin NiO films on Y3Fe5O12

In spin transport experiments with spin currents propagating through antiferromagnetic (AFM) material, the antiferromagnet is treated as a mainly passive spin conductor not generating nor adding any spin current to the system. The spin current transmissivity of the AFM NiO is affected by magnetic fluctuations, peaking at the N\'eel temperature and decreasing by lowering the temperature. In order to study the role of the AFM in local and nonlocal spin transport experiments, we send spin currents through NiO of various thickness placed on Y$_3$Fe$_5$O$_{12}$. The spin currents are injected either electrically or by thermal gradients and measured at a wide range of temperatures and magnetic field strengths. The transmissive role is reflected in the sign change of the local electrically injected signals and the decrease in signal strength of all other signals by lowering the temperature. The thermally generated signals, however, show an additional upturn below 100$\,$K which are unaffected by an increased NiO thickness. A change in the thermal conductivity could affect these signals. The temperature and magnetic field dependence is similar as for bulk NiO, indicating that NiO itself contributes to thermally induced spin currents.

Temperature dependence of the effective Gilbert damping constant of FeRh thin films

Antiferromagnetic (AFM) materials have attracted attention for device applications due to the absence of the stray field and high-frequency response. To integrate AFM materials into magnetic devices, the understanding of the interfacial effect between AFM and ferromagnetic (FM) materials is required. In particular, magnetization dynamics and magnetic damping are critical phenomena to be elucidated since they govern magnetization switching, spin-wave propagation, etc. Although a conventional method for studying the interfacial effects is stacking materials, the approach may cause unfavorable factors. To get insight into the dynamic properties at the AFM and FM interfaces, we have focused on B2-ordered FeRh, showing the first-order phase transition from the AFM to FM states, since the coexistence of AFM and FM domains occurs during transitions, which is an ideal platform for studying interfacial effects. For this study, we have studied ferromagnetic resonance (FMR) of FeRh thin films during the AFM–FM phase transition as a function of temperature. From the FMR measurements, we characterize the temperature dependence of the effective Gilbert damping constant αeff. We find that αeff decreases with increasing temperature, indicating that the temperature variation of the effective Gilbert damping constant originates from the exchange interaction between the AFM and FM domains in the film and/or AFM domains as a spin sink.

Hidden Magnetic Structures Revealed

Researchers capture the previously hidden twists of spins in the domain wall of a “proper” antiferromagnetic material, chromium oxide.

Atomistic origin of the athermal training effect in granular IrMn/CoFe bilayers

Anti-ferromagnetic materials have the possibility to offer ultra fast, high data density spintronic devices. A significant challenge is the reliable detection of the state of the antiferromagnet, which can be achieved using exchange bias. Here we develop an atomistic spin model of the athermal training effect, a well known phenomenon in exchange biased systems where the bias is significantly reduced after the first hysteresis cycle. We find that the setting process in granular thin films relies on the presence of interfacial mixing between the ferromagnetic and antiferromagnetic layers. We systematically investigate the effect of the intermixing and find that the exchange bias, switching field and coercivity all increase with increased intermixing. The interfacial spin state is highly frustrated leading to a systematic decrease in interfacial ordering of the ferromagnet. This metastable spin structure of initially irreversible spins leads to a large effective exchange coupling and thus large increase in the switching field. After the first hysteresis cycle these metastable spins drop into a reversible ground state that is repeatable for all subsequent hysteresis cycles, demonstrating that the effect is truly athermal. Our simulations provide new insights into the role of interface mixing and the importance of metastable spin structures in exchange biased systems which could help with the design an optimisation of antiferromagnetic spintronic devices.

Atomistic simulations of the magnetic properties of IrxMn1−x alloys

Iridium manganese (IrMn) is arguably the most important antiferromagnetic material for device applications due to its metallic nature, high N\'eel temperature, and exceptionally high magnetocrystalline anisotropy. Despite its importance, its magnetic properties are poorly understood due to its intrinsic complexity and the interplay between structural and magnetic properties. Here we present a unifying atomistic model of ${\mathrm{Ir}}_{x}{\mathrm{Mn}}_{(1\ensuremath{-}x)}$ alloys which reproduces the key experimental facts of the material, while providing unprecedented understanding of the compositional and structural origins of its magnetic ground state and thermodynamic properties. We find that the N\'eel temperature is strongly dependent on the nature of the ground-state magnetic order which varies with $x$ from a triangular to tetrahedral spin structure, leading to different levels of geometric spin frustration. The N\'eel temperature increases linearly with manganese concentration for the disordered phase, while the ordered phases show a peak for ${\mathrm{Ir}}_{50}{\mathrm{Mn}}_{50}$ followed by a decrease due to increased spin frustration. The ground-state tetrahedral spin structure of the disordered phase is composition independent for manganese concentrations in the 50--95% range, while the degree of spin order varies strongly in the same range. For low manganese concentrations, we find antiferromagnetic spin-glass and ferromagnetic ground-state spin structures. The magnetic anisotropy energy exhibits a complex dependence on the lattice symmetry, presenting easy-plane, cubic, and unconventional symmetries for the principal phases, and a similarly complex variation of magnitude. The complexity of behavior represents a dual blessing and a curse in that the properties of a particular sample depend strongly on the degree of order and composition, while also providing a large state space to engineer an antiferromagnet with optimal symmetry, magnetic anisotropy, and thermal stability. Such effects are important for the future development of nanoscale sensor devices and antiferromagnetic spintronics.

Structural and magnetic properties of two-dimensional layered BiFeO3 from first principles

Using first-principles calculations, we predict a unique $\mathrm{BiFe}{\mathrm{O}}_{3}$ (BFO) phase, which is coined $I$-BFO, with layered characteristics based on the ilmenite structure. The interactions between interlayer bismuth atoms make it easy to exfoliate monolayer BFO with a configuration in which Bi ions are above and below two-dimensional (2D) iron-oxygen octahedra sharing common edges. The calculated phonon spectra reveal the dynamic stability of both bulk and monolayer forms of $I$-BFO. Combining total energy computation and crystal structure search, we corroborate that the $I$-BFO monolayer maintains the lowest-energy ground state, as compared to other 2D BFO exfoliations. We also discover that monolayer $I$-BFO is a robust antiferromagnetic material with a N\'eel temperature (${T}_{N}$) of 4 K in the fully relaxed case. Such ${T}_{N}$ can be prominently improved by strain, for example, ${T}_{N}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at 6% tension. Furthermore, our electronic structure calculations with PBE and hybrid HSE functionals show the $I$-BFO monolayer to be an indirect-gap semiconductor. Finally, we indicate that other ternary $AB{\mathrm{O}}_{3}$ compounds also have the potential to produce layered materials from the ilmenite structure. This work thus provides guidance to explore unconventional layered materials, and further broadens the application of BFO and other promising ilmenite oxides in two dimensions.

Magnetic properties and anomalous Hall effect of Mn3Sn thin films controlled by defects and ferroelectric 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 substrate

Noncollinear antiferromagnetic Mn3Sn films have received much attention due to their potential applications in antiferromagnetic spintronic devices. In this work, single-phase polycrystalline antiferromagnetic Mn3Sn thin films were successfully prepared by magnetron sputtering. The defects in the thin films were regulated by adjusting the sputtering power. The relationship among the films structure, the anomalous Hall effect (AHE) and the defects was investigated. High defect concentration in the Mn3Sn films led to large room temperature ferromagnetic moments. The maximum saturation magnetization reached up to ~ 16 kA·m−1 (36 mμB/Mn), which was much larger than the values reported in literatures. The coercive field of 38 mT was obtained in a high-quality Mn3Sn film, which effectively reduced the flipping magnetic field. Moreover, the anomalous Hall resistance and coercive field of the Mn3Sn films prepared on the ferroelectric substrates were manipulated through an applied electric field, indicating that the piezoelectric stress has a great influence on the nonzero Berry curvature of the triangular spin structure in the antiferromagnetic materials. These results will promote the potential application of Mn3Sn in high-density and low-power antiferromagnetic spintronic devices.

Giant spin torque efficiency in single-crystalline antiferromagnet Mn2Au films

Antiferromagnetic (AFM) materials have attracted wide attention in spin-orbit torque (SOT)-based spintronic due to its abundant spin-dependent properties and unique advantage of immunity against external field perturbations. To act as the charge-to-spin conversion source in energy-saving spintronic devices, it is of great importance for the AFM material to possess a large spin torque efficiency (ξDL). In this work, using the spin torque ferromagnetic resonance (ST-FMR) technique and a Mn2Au/NiFe(Py) bilayer system, we systemically study the ξDL of AFM Mn2Au films with different crystal structures. Compared with polycrystalline Mn2Au with effective ξDL < 0.051, we show a much larger ξDL of ~0.333 in single-crystal Mn2Au, which arises from the large spin Hall conductivity instead of electrical resistivity. Moreover, with a further contribution of interfacial effects, the effective ξDL of single-crystalline Mn2Au/Py system increases to 0.731, which is more than two times larger than the value of ∼0.22 reported for the Mn2Au/CoFeB system. By utilizing the large ξDL of Mn2Au in a perpendicularly magnetized MnGa/Mn2Au system, energy-efficient deterministic magnetization switching with a current density at ~106 A cm−2 is achieved. Our results reveal a significant potential of Mn2Au as an efficient SOT source and shed light on its application in future AFM material-based SOT integration technology.

Imaging non-collinear antiferromagnetic textures via single spin relaxometry

Antiferromagnetic materials are promising platforms for next-generation spintronics owing to their fast dynamics and high robustness against parasitic magnetic fields. However, nanoscale imaging of the magnetic order in such materials with zero net magnetization remains a major experimental challenge. Here we show that non-collinear antiferromagnetic spin textures can be imaged by probing the magnetic noise they locally produce via thermal populations of magnons. To this end, we perform nanoscale, all-optical relaxometry with a scanning quantum sensor based on a single nitrogen-vacancy (NV) defect in diamond. Magnetic noise is detected through an increase of the spin relaxation rate of the NV defect, which results in an overall reduction of its photoluminescence signal under continuous laser illumination. As a proof-of-concept, the efficiency of the method is demonstrated by imaging various spin textures in synthetic antiferromagnets, including domain walls, spin spirals and antiferromagnetic skyrmions. This imaging procedure could be extended to a large class of intrinsic antiferromagnets and opens up new opportunities for studying the physics of localized spin wave modes for magnonics.

Anisotropic coercivity and the effects of interlayer exchange coupling in CoFeB/FeRh bilayers

In an amorphous CoFeB layer, coercivity becomes anisotropic with fourfold symmetry when the CoFeB layer exchange couples to an FeRh layer. The angular dependence of coercivity of the CoFeB layer coincides with the in-plane easy-axis direction of the FeRh layer and experiences a ${45}^{\ensuremath{\circ}}$ shift with the occurrence of a metamagnetic phase transition of the FeRh layer from antiferromagnetism at room temperature to ferromagnetism at 400 K. The intriguing phenomena are well reproduced by our unbiased Monte Carlo simulation. The interfacial exchange and anisotropy energies, as well as the interfacial magnetization in the CoFeB/FeRh bilayer, are disentangled to demonstrate the strong dependence of the imprinting of anisotropy in the CoFeB layer on the interfacial exchange coupling. The evolution of the easy-axis direction of the induced anisotropy arises from the reconstruction of the interfacial exchange energy profile accompanied with the change of the magnetic state of FeRh, which governs the magnetization reversal of the CoFeB layer at both branches. Moreover, the imprinting is further applicable for the uniaxial magnetocrystalline anisotropy. This work not only presents the possibility of directly duplicating anisotropy between dissimilar materials, but it also provides a powerful tool to probe the hidden magnetic structures and/or the properties of materials that have weak magnetism, such as antiferromagnetic materials.

Magnetic and electronic structures of antiferromagnetic topological material candidate EuMg2Bi2

EuMg2Bi2 has been investigated to understand the electronic and magnetic behaviors as an antiferromagnetic (AFM) topological semimetal candidate. High-quality single crystals of EuMg2Bi2 were grown via a Bi flux and, subsequently, characterized to be consistent with the previously reported bulk magnetic and resistivity properties. A ferromagnetic interaction is indicated by the positive Curie–Weiss temperature obtained through fitting the bulk magnetic susceptibility data. The bulk resistivity measurements reveal an interesting electronic behavior that is potentially influenced by a competing antiferromagnetic and ferromagnetic interaction in and out of the ab plane. From the resulting refinement of the neutron diffraction data, EuMg2Bi2 was found to exhibit an A-type magnetic structure with Eu2+ moments ferromagnetically aligned in the plane and antiferromagnetically stacked between neighbor ferromagnetic Eu layers. The power law fitting magnetic ordering parameter below TN ∼ 8 K agrees with the 2D Heisenberg model, indicating a weak interlayer antiferromagnetic interaction. Considering the magnetic structure determined by neutron diffraction, the surface state calculation suggests that EuMg2Bi2 is an AFM topological insulator candidate. Linearly dispersed Dirac surface states were also observed in our angle-resolved photoemission spectroscopy measurements, consistent with the calculation.

Antiferromagnetic materials pave the path for quantum entanglement

The application of quantum laws ensures faithful, secure and high-speed information processing and computation. Quantum entanglement is a strange superposition of states by which one can have the result of state of both particles (for bipartite system) simultaneously. There are number of quantifiers to measure the amount of quantum entanglement as required for many applications of quantum entanglement. Entanglement entropy, concurrence and the negativity in entanglement spectrum are commonly used quantifiers. The weirdness of quantum entanglement can only be met in the uncanny properties of materials. The alignment of electron’s spin in antiferromagnetic materials may result in quantum entanglement by displaying geometrical and quantum frustrations. Kitaev model and Ising models are those in which electron’s spins get frustrated due to overchoice. The bonding between spins does not tend to minimize even at extremely low temperature resulting a degenerate ground state. Hamiltonian for nearest-neighbor one-dimensional spin- ½ particle for N particles is used to show the boundary conditions to attain zero total magnetization. To prevent stabilization of low energy states against decay, material should be exposed to frustration. Entanglement Entropy of the weakly frustrated spins is discussed for ferromagnetic and antiferromagnetic materials and found that it has nearly same value for both types of material.