Antiferromagnetic Materials Research Articles

Dynamics of domain-wall motion driven by spin-orbit torque in antiferromagnets

Ultrafast dynamics of antiferromagnetic materials is an appealing feature for novel spintronic devices. Several experiments have shown that both, the static states and the dynamical behavior of the antiferromagnetic order, are strictly related to stabilization of domains and domain wall (DW) motion. Hence for a quantitative understanding of statics and dynamics of multidomain states in antiferromagnetic materials a full micromagnetic framework is necessary. Here, we use this model to study the antiferromagnetic DW motion driven by the spin-orbit torque. The main result is the derivation of analytical expressions for the DW width and velocity that exhibit a very good agreement with the numerical simulations in a wide range of parameters. We also find that a mechanism limiting the maximum applicable current in an antiferromagnetic racetrack memory is the continuous nucleation of the domains from the edge, which is qualitatively different from what is observed in ferromagnetic racetracks.

Permanent magnet design assisted by antiferromagnet-ferromagnet interface coupling: A Monte Carlo study

In the pursuit of rare-earth-free permanent magnets, materials with large saturation magnetization (Ms) stand out. However, large Ms results in low anisotropy field, thus additional source of coercivity is needed to make useful magnets for a broad range of applications. In this paper, we investigate the use of interface coupling to an antiferromagnet (AFM) as a means to increase coercivity for permanent magnet design. Results on the effects of Néel temperature, AFM anisotropy, interface coupling strength, AFM volume ratio, as well as temperature dependence are obtained using Monte Carlo (MC) simulations. And before that, we clarify some practices in MC simulations of magnetic hysteresis. The origin of coercivity enhancement is investigated by examining the magnetization reversal process, which is found to agree with well-established theory of exchange bias. Finally, discussions and design suggestions are made on the AFM materials, the geometry, and experimental realization.

Spin accumulation dynamics in spin valves in the terahertz regime

The notion of spin accumulation and spin relaxation in the diffusive regime was introduced by Valet and Fert in 1993 to describe the current-perpendicular-to-plane (CPP) diffusive transport in metallic magnetic multilayers. This theory has been quite successful in explaining the giant magnetoresistance of magnetic multilayers in CPP geometry in the frequency range from DC to a few gigahertz. In this paper, we investigate the dynamic aspect of spin accumulation from the theoretical point of view when reaching the terahertz (THz) frequency range. The characteristic relaxation time of electron elastic scattering is typically in the femtosecond range. However, since spin accumulation results from a diffusion process involving a very large number of individual scattering events, the characteristic time of spin accumulation variation when the current and/or the magnetic configuration are varied can be significantly longer than that of the input signal, eventually reaching the picosecond range. In spintronic devices operating in the THz range such as those based on ferrimagnetic or antiferromagnetic materials, the spin accumulation amplitude and, correlatively, the device magnetoresistance can therefore depend on the actual device operating frequency. We investigate this question by extending the Valet and Fert theory in the time domain.

Magnetic structure and uniaxial negative thermal expansion in antiferromagnetic CrSb.

Negative thermal expansion (NTE) has been found in a growing number of ferromagnetic and ferrimagnetic materials; however, it remains a challenge to discover antiferromagnetic (AFM) NTE materials. Here, we report the uniaxial NTE properties of AFM intermetallic CrSb systematically, and reveal its uniaxial NTE mechanism for the first time. The present AFM intermetallic CrSb shows uniaxial NTE at high temperature and over a broad temperature window (αa = -6.55 × 10-6 K-1, 360-600 K). The direct experimental evidence of neutron powder diffraction reveals that NTE is induced by the AFM ordering of the Cr atom. The present study demonstrates that due to the transition from an AFM ordered structure to a paramagnetic disordered configuration, the negative contribution to the thermal expansion from the magnetovolume effect overwhelms the positive contribution from anharmonic phonon vibration. This study is of interest to find antiferromagnetic NTE materials.

Spin reorientation transition and spin dynamics study of perovskite orthoferrite TmFeO3 detected by electron paramagnetic resonance.

The temperature-dependent spin-reorientation transition (SRT) and spin interaction mechanism of bulk TmFeO3 were studied by the electron paramagnetic resonance (EPR) method. The combined experimental results of magnetic curves and EPR spectra confirmed that there is an antiferromagnetic transition at 85 K with a reentering ferromagnetic state due to the spin-reorientation behavior. In the high-temperature region of T > 90 K, there are three distinct resonance peaks in the EPR spectrum, which indicates the presence of multiple magnetic phases (canted antiferromagnetic, weak ferromagnetic, and paramagnetic phases). In the low-temperature region (T < 85 K), the temperature dependence of the EPR linewidth, effective g-factor, and intensity can be used to infer a strong spin-lattice correlation. Different magnetic interactions such as Fe3+-Fe3+, Fe3+-Tm3+, and Tm3+-Tm3+ lead to a paramagnetic-canted antiferromagnetic phase at T > 85 K, with SRT between 85-65 K and ferromagnetic interaction at the lower temperature, respectively. Above 90 K, we find that the spin relaxation mechanism is determined by the mixture of spin-spin and spin-lattice interactions. Below 85 K, the transverse relaxation rate increases with the decrease in temperature, which is consistent with the weakening of the fluctuating internal field in this temperature region. This EPR detection provides a new method to clarify the strong spin coupling in antiferromagnetic materials.

2D tetragonal transition-metal phosphides: an ideal platform to screen metal shrouded crystals for multifunctional applications.

Two-dimensional (2D) metal shrouded crystals, a new kind of conceptional material, have attracted remarkable attention due to their unique properties. Here, we propose a novel class of 2D metal shrouded materials, tetragonal transition-metal phosphides (TM2Ps), which show peculiar features of coexistence of in-plane TM-P covalent bonds and TM-TM interlayer metallic bonds. From a combination of high throughput searching and first-principles calculations, Fe2P, Co2P, Ni2P, Ru2P, and Pd2P monolayer sheets stand out because they simultaneously have high thermal, dynamical, and mechanical stability. All these five TM2P materials are metals, especially Pd2P, which can be a promising catalyst for the hydrogen evolution reaction with a very low overpotential. Moreover, these 2D TM2Ps show good ductility since they can withstand a tensile strain of up to 45%. Even in the large strain range, the strengthened interlayer TM-TM metallic bonds dominate the deformation behavior, and the corresponding metallicity of 2D TM2Ps is well preserved. Due to the competition between the d-d direct exchange and d-p-d superexchange interactions, Fe2P behaves as an antiferromagnetic material with a TN of 23 K, while Co2P is a ferromagnetic material with a TC of 580 K. Our results not only enrich the database of 2D metal shrouded crystals, but also provide novel 2D materials as promising candidates for multifunctional applications in nanoelectronics, spintronics and electrocatalysis.

Discovery of stable and intrinsic antiferromagnetic iron oxyhalide monolayers.

Two-dimensional (2-D) antiferromagnetic (AFM) materials have shown promise over their ferromagnetic (FM) counterparts for developing advanced spintronic devices; however, they have been rarely found with high Néel temperatures to date. Here, by employing first-principles calculations and Monte Carlo simulations, we demonstrate that the family of 2-D iron oxyhalides monolayers, FeOX (X = F, Cl, Br, I), are magnetic Mott insulators with their AFM ground state possessing relatively high Néel temperatures. The structural stabilities of the FeOX monolayers are proved using a set of phonon, molecular dynamics, and elastic constant calculations. The calculated Néel temperature of the FeOCl monolayer is close to that of FeOCl bulk because of the weak van der Waals interaction between the layers. More importantly, the predicted Néel temperatures of FeOX (X = F, Cl, Br, I) monolayers can be increased by biaxial compression strain. The Néel temperature of the strained FeOF and FeOI monolayers can approach 200 K, which suggests that they can be robust antiferromagnets with relatively high Néel temperatures compared with other available 2-D magnets. Our calculations show that both the in-plane and the inter-plane magnetic interactions affect the AFM coupling between Fe atoms in FeOX monolayers. The easy axis of the 2-D FeOX is found to be along the in-plane direction. The FeOX monolayers may provide an excellent platform for building novel spintronic devices at the nanoscale.

A DFT investigation on magnetoelectric coupling in PbBO3 (B = V, Cr, Mn, Co, and Cu) materials: The influence on multiferroic properties

The multiferroism is a very intriguing property discovered more than seventy years ago and largely investigated mainly after 2003 due to a strong magnetoelectric coupling observed in some materials. However, even this phenomenon has been known for so long, until now its origin at the molecular level has not been clarified for many materials yet. Thus, in this work is proposed a systematic DFT study using PbBO3 (B = V, Cr, Mn, Co, and Cu) materials, which materials show good potential as a material for multiferroic or spintronic devices. The structural properties indicate that chemical modification on B magnetic sites has not a big influence on unit cell symmetry; in addition, the investigation of energetic stability indicates that such materials are highly stable under room conditions and high pressures. As well as, the electronic properties show that all materials present metallic band gap or half-metallic ferromagnetism. In terms of magnetic property, the theoretical results show a ferromagnetic ordering for PbMnO3 and PbCuO3, while PbVO3, PbCrO3, and PbCoO3 are antiferromagnetic materials with weak-ferromagnetism. In both cases, the magnetic resultant is oriented along [1 1 1] direction. In turn, all materials presented good ferroelectric properties with anisotropic features and more pronounced along the x-direction. Once those magnetic and ferroelectric properties are tangents, such coupling is possible because of one perturbation in one property results on the immediate answer of the other property. Thus, the fact of cationic magnetic sites (magnetic property) was displaced from the center of the octahedral site (ferroelectric property) is molecular origin for magnetoelectric coupling on proposed materials.

Stability of the 2pi-vertical-Bloch-line in a canted antiferromagnetic system

We investigate topological defects in canted antiferromagnetic materials with the orthoferrite as a model system, and demonstrate that a vertical Bloch line (VBL), a topological defect within magnetic domain walls, can couple to the other VBL to form a stable antibonding state. Actually, such a 2pi-VBL is a topologically non-trivial object with a skyrmion number equivalent to that of the skyrmion itself. We demonstrate that transitions to other topological objects are possible only when a strong enough magnetic field is applied; the 2pi-VBL can transform to two-independent pi-VBLs under the magnetic field applied along the orthorhombic a-axis, and it can be annihilated with the magnetic field applied along the c-axis. As long as the 2pi-VBL is in a stable condition, it can be driven by the magnetic field to move straightly with a high speed of about 2000 m/s. This unique property of the 2pi-VBL can provide us with a possibility to exploit it as another topological magnetic defect for the spintronic applications.

Magnetotransport as a diagnostic of spin reorientation: Kagome ferromagnet as a case study

While in most ferro or antiferromagnetic materials there is a unique crystallographic direction, including crystallographically equivalent directions, in which the moments like to point due to spin-orbit coupling, in some, the direction of the spin reorients as a function of a certain physical parameter such as temperature, pressure etc. Fe3Sn2 is a kagome ferromagnet with an onset of ferromagnetism below 650 K, and undergoes a spin reorientation near 150 K. While it is known that the moments in Fe3Sn2 point perpendicular to the kagome plane at high temperatures and parallel to the kagome plane at low temperatures, how the distribution of the magnetic domains in the two different spin orientations evolve throughout the spin reorientation is not well known. Furthermore, while there have been various reports on the magnetotransport properties in the Hall configuration, the angular dependence of magnetoresistance has not been studied so far. In this paper, we have examined the spin reorientation by using anisotropic magnetoresistivity in detail, exploiting the dependence of the resistivity on the direction between magnetization and applied current. We are able to determine the distribution of the magnetic domains as a function of temperature between 360 K to 2 K and the reorientation transition to peak at 120 K. We discover that both out of plane and in plane phases coexist at temperatures around the spin reorientation, indicative of a first order transition. Although the volume of the magnetic domains in the different phases sharply changes at the spin reorientation transition, the electronic structure for a specific magnetization is not influenced by the spin reorientation. In contrast, we observe an electronic transition around 40 K, hitherto unreported, and reflected in both the zero-field resistivity and anisotropic resistivity.

Two-dimensional ferromagnetic superlattices.

Mechanically exfoliated two-dimensional ferromagnetic materials (2D FMs) possess long-range ferromagnetic order and topologically nontrivial skyrmions in few layers. However, because of the dimensionality effect, such few-layer systems usually exhibit much lower Curie temperature (TC) compared to their bulk counterparts. It is therefore of great interest to explore effective approaches to enhance their TC, particularly in wafer-scale for practical applications. Here, we report an interfacial proximity-induced high-TC 2D FM Fe3GeTe2 (FGT) via A-type antiferromagnetic material CrSb (CS) which strongly couples to FGT. A superlattice structure of (FGT/CS)n, where n stands for the period of FGT/CS heterostructure, has been successfully produced with sharp interfaces by molecular-beam epitaxy on 2-inch wafers. By performing elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally discovered that TC of 4-layer Fe3GeTe2 can be significantly enhanced from 140 K to 230 K because of the interfacial ferromagnetic coupling. Meanwhile, an inverse proximity effect occurs in the FGT/CS interface, driving the interfacial antiferromagnetic CrSb into a ferrimagnetic state as evidenced by double-switching behavior in hysteresis loops and the XMCD spectra. Density functional theory calculations show that the Fe-Te/Cr-Sb interface is strongly FM coupled and doping of the spin-polarized electrons by the interfacial Cr layer gives rise to the TC enhancement of the Fe3GeTe2 films, in accordance with our XMCD measurements. Strikingly, by introducing rich Fe in a 4-layer FGT/CS superlattice, TC can be further enhanced to near room temperature. Our results provide a feasible approach for enhancing the magnetic order of few-layer 2D FMs in wafer-scale and render opportunities for realizing realistic ultra-thin spintronic devices.

Magnetoelastic anisotropy of antiferromagnetic materials

Antiferromagnetic (AFM) materials are of great interest for spintronics. Here, we report the magnetoelastic anisotropy of an AFM IrMn thin film. An exchange-biased CoFeB/IrMn bilayer was used to obtain a single domain of the AFM thin film, and the magnetic moment arrangement of the AFM layer was deduced from the magnetic hysteresis loop of the pinned FM layer. A uniaxial compressive stress is applied on the thin film through changing the temperature due to the anisotropic thermal expansion of the polyvinylidene fluoride (PVDF) substrate. Both experimental results and theoretical calculations show that the direction of IrMn magnetic moment can be changed when a compressive stress is applied and the direction of IrMn AFM moment rotates about 10° under 2.26 GPa compressive stress. These results provide important information for the practical application of flexible spintronics based on AFM spintronic devices.

Noncollinear spintronics and electric-field control: a review

Our world is composed of various materials with different structures, where spin structures have been playing a pivotal role in spintronic devices of the contemporary information technology. Apart from conventional collinear spin materials such as collinear ferromagnets and collinear antiferromagnetically coupled materials, noncollinear spintronic materials have emerged as hot spots of research attention owing to exotic physical phenomena. In this Review, we firstly introduce two types noncollinear spin structures, i.e., the chiral spin structure that yields real-space Berry phases and the coplanar noncollinear spin structure that could generate momentum-space Berry phases, and then move to relevant novel physical phenomena including topological Hall effect, anomalous Hall effect, multiferroic, Weyl fermions, spin-polarized current, and spin Hall effect without spin-orbit coupling in these noncollinear spin systems. Afterwards, we summarize and elaborate the electric-field control of the noncollinear spin structure and related physical effects, which could enable ultralow power spintronic devices in future. In the final outlook part, we emphasize the importance and possible routes for experimentally detecting the intriguing theoretically predicted spin-polarized current, verifying the spin Hall effect in the absence of spin-orbit coupling and exploring the anisotropic magnetoresistance and domain-wall-related magnetoresistance effects for noncollinear antiferromagnetic materials.

Propagation Length of Antiferromagnetic Magnons Governed by Domain Configurations.

The compensated magnetic order and characteristic terahertz frequencies of antiferromagnetic materials make them promising candidates to develop a new class of robust, ultrafast spintronic devices. The manipulation of antiferromagnetic spin-waves in thin films is anticipated to lead to new exotic phenomena such as spin-superfluidity, requiring an efficient propagation of spin-waves in thin films. However, the reported decay length in thin films has so far been limited to a few nanometers. In this work, we achieve efficient spin-wave propagation over micrometer distances in thin films of the insulating antiferromagnet hematite with large magnetic domains while evidencing much shorter attenuation lengths in multidomain thin films. Through transport and magnetic imaging, we determine the role of the magnetic domain structure and spin-wave scattering at domain walls to govern the transport. We manipulate the spin transport by tailoring the domain configuration through field cycle training. For the appropriate crystalline orientation, zero-field spin transport is achieved across micrometers, as required for device integration.

Electrical Néel-Order Switching in Magnetron-Sputtered CuMnAs Thin Films

Antiferromagnetic materials as active components in spintronic devices promise insensitivity against external magnetic fields, the absence of own magnetic stray fields, and ultrafast dynamics at the picosecond time scale. Materials with certain crystal-symmetry show an intrinsic N\'eel-order spin-orbit torque that can efficiently switch the magnetic order of an antiferromagnet. The tetragonal variant of CuMnAs was shown to be electrically switchable by this intrinsic spin-orbit effect and its use in memory cells with memristive properties has been recently demonstrated for high-quality films grown with molecular beam epitaxy. Here, we demonstrate that the magnetic order of magnetron-sputtered CuMnAs films can also be manipulated by electrical current pulses. The switching efficiency and relaxation as a function of temperature, current density, and pulse width can be described by a thermal-activation model. Our findings demonstrate that CuMnAs can be fabricated with an industry-compatible deposition technique, which will accelerate the development cycle of devices based on this remarkable material.

Two-dimensional antiferromagnetic Dirac fermions in monolayer TaCoTe2

Dirac point in two-dimensional (2D) materials has been a fascinating subject of research. Recently, it has been theoretically predicted that Dirac point may also be stabilized in 2D magnetic systems. However, it remains a challenge to identify concrete 2D materials which host such magnetic Dirac point. Here, based on first-principles calculations and theoretical analysis, we propose a stable 2D material, the monolayers TaCoTe$_2$, as an antiferromagnetic (AFM) 2D Dirac material. We show that it has an AFM ground state with an out-of-plane N\'{e}el vector. It hosts a pair of 2D AFM Dirac points on the Fermi level in the absence of spin-orbit coupling (SOC). When the SOC is considered, a small gap is opened at the original Dirac points. Meanwhile, another pair of Dirac points appear on the Brillouin zone boundary below the Fermi level, which are robust under SOC and have a type-II dispersion. Such a type-II AFM Dirac point has not been observed before. We further show that the location of this Dirac point as well as its dispersion type can be controlled by tuning the N\'{e}el vector orientation.

Magnetism driven by the interplay of fluctuations and frustration in the easy-axis triangular XXZ model with transverse fields

We study the combined effects of fluctuations and frustration in frustrated antiferromagnets described by the spin-1/2 easy-axis XXZ model with transverse magnetic fields, and propose its promising quantum simulation with a coherently coupled binary mixture of Fermi atoms in a triangular optical lattice. We perform a cluster mean-field plus scaling analysis to show that the ground state exhibits several nontrivial magnetic phases and a spin reorientation transition caused by the quantum order-by-disorder mechanism. Moreover, we find from Monte Carlo simulations that thermal fluctuations induce an unexpected coexistence of Berezinskii-Kosterlitz-Thouless physics and long-range order in different correlators. These predictions, besides being relevant to present and future experiments on triangular antiferromagnetic materials, can be tested in the laboratory with the combination of the currently available techniques for cold atoms.

Integration of the noncollinear antiferromagnetic metal Mn3Sn onto ferroelectric oxides for electric-field control

Noncollinear antiferromagnetic materials have received dramatically increasing attention in the field of spintronics as their exotic topological features such as the Berry-curvature-induced anomalous Hall effect and possible magnetic Weyl states could be utilized in future topological antiferromagnetic spintronic devices. In this work, we report the successful integration of the antiferromagnetic metal Mn3Sn thin films onto ferroelectric oxide PMN-PT. By optimizing growth, we realized the large anomalous Hall effect with small switching magnetic fields of several tens mT fully comparable to those of bulk Mn3Sn single crystals, anisotropic magnetoresistance and negative parallel magnetoresistance in Mn3Sn thin films with antiferromagnetic order, which are similar to the signatures of the Weyl state in bulk Mn3Sn single crystals. More importantly, we found that the anomalous Hall effect in antiferromagnetic Mn3Sn thin films can be manipulated by electric fields applied onto the ferroelectric materials, thus demonstrating the feasibility of Mn3Sn-based topological spintronic devices operated in an ultralow power manner.

Introduction to antiferromagnetic magnons

The elementary spin excitations in strongly magnetic materials are collective spin deviations, or spin waves, whose quanta are called magnons. Interest in the experimental and theoretical investigation of magnons attracted many groups worldwide about 4–6 decades ago and then waned for some time. In recent years, with the advent of the field of spintronics, the area of magnonics has gained renewed attention. New phenomena have been discovered experimentally, and others have been predicted theoretically. In this tutorial, we briefly review the basic concepts of magnons in antiferromagnetic (AF) materials. Initially, we present a semiclassical view of the equilibrium spin configurations and of the antiferromagnetic resonance in AF materials with two types of magnetic anisotropy, easy-axis and easy-plane. Then, we present a quantum theory of magnons for these materials and apply the results to two important AF insulators, MnF2 and NiO. Finally, we introduce the concept of antiferromagnetic magnonic spin current that plays a key role in several phenomena in antiferromagnetic spintronics.

Carrier-Density-Induced Ferromagnetism in EuTiO_{3} Bulk and Heterostructures.

EuTiO_{3} is an antiferromagnetic (AFM) material showing strong spin-lattice interactions, large magnetoelectric response, and quantum paraelectric behavior at low temperatures. Using electronic-structure calculations, we show that adding electrons to the conduction band leads to ferromagnetism. The transition from antiferromagnetism to ferromagnetism is predicted to occur at ∼0.08 electrons/Eu (∼1.4×10^{21} cm^{-3}). This effect is also predicted to occur in heterostructures such as LaAlO_{3}/EuTiO_{3}, where ferromagnetism is triggered by the formation of a high-density two-dimensional electron gas in the EuTiO_{3}. Our analysis indicates that the coupling between Ti 3d and Eu 5d plays a crucial role in lowering the Ti 3d conduction band in the ferromagnetic (FM) phase, leading to an almost linear dependence of the energy difference between the FM and AFM ordering on the carrier concentration. These findings open up possibilities in designing field-effect transistors using EuTiO_{3}-based heterointerfaces to probe fundamental interactions between highly localized spins and itinerant, polarized charge carriers.