Antiferromagnetic Insulator Research Articles

Pressure-induced transition from a Mott insulator to a ferromagnetic Weyl metal in La2O3Fe2Se2

The insulator-metal transition in Mott insulators, known as the Mott transition, is usually accompanied with various novel quantum phenomena, such as unconventional superconductivity, non-Fermi liquid behavior and colossal magnetoresistance. Here, based on high-pressure electrical transport and XRD measurements, and first-principles calculations, we find that a unique pressure-induced Mott transition from an antiferromagnetic Mott insulator to a ferromagnetic Weyl metal in the iron oxychalcogenide La2O3Fe2Se2 occurs around 37 GPa without structural phase transition. Our theoretical calculations reveal that such an insulator-metal transition is mainly due to the enlarged bandwidth and diminishing of electron correlation at high pressure, fitting well with the experimental data. Moreover, the high-pressure ferromagnetic Weyl metallic phase possesses attractive electronic band structures with six pairs of Weyl points close to the Fermi level, and its topological property can be easily manipulated by the magnetic field. The emergence of Weyl fermions in La2O3Fe2Se2 at high pressure may bridge the gap between nontrivial band topology and Mott insulating states. Our findings not only realize ferromagnetic Weyl fermions associated with the Mott transition, but also suggest pressure as an effective controlling parameter to tune the emergent phenomena in correlated electron systems.

Reversible phase transformation and comprehensive phase diagram in SrCoO2.5+x(0≤x≤0.5) with the electric field control of oxygen content

Oxygen deficiency plays an essential role in tailoring the intriguing physical properties of complex oxides that can be harnessed in the cutting-edge energy and information technologies. A fine and continuous control of the oxygen content to modulate the rich physical properties of the complex oxides is highly desirable for both basic research and modern electronic technologies. Here, we report a recently developed ionic field-effect transistor (iFET) device with a solid oxygen ionic conductor (OIC) as the gate dielectric to finely and continuously control the oxygen content of the oxides. We realize a reversible structural transformation between brownmillerite $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ and perovskite $\mathrm{SrCo}{\mathrm{O}}_{3}$ by driving oxygen ions in and out of the samples with the OICFET at room temperature. With continuously changing the oxygen content, a comprehensive phase diagram of the $\mathrm{SrCo}{\mathrm{O}}_{2.5+x}$ $(0\ensuremath{\le}x\ensuremath{\le}0.5)$ thin film from an antiferromagnetic insulator to a ferromagnetic metal is obtained in a single device. Our work provides a promising route to continuously and reversibly regulate the oxygen content and to explore unique physical states in the correlated electron oxides, and represents an important step towards the realization of a long-standing goal to develop complex oxide based devices that can be integrated with existing technologies.

Emission of coherent THz magnons in an antiferromagnetic insulator triggered by ultrafast spin–phonon interactions

Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices owing to their ultrafast spin dynamics. Manipulating coherently their spin dynamics, however, remains a key challenge that is envisioned to be accomplished by spin-orbit torques or direct optical excitations. Here, we demonstrate the combined generation of broadband THz (incoherent) magnons and narrowband (coherent) magnons at 1 THz in low damping thin films of NiO/Pt. We evidence, experimentally and through modeling, two excitation processes of spin dynamics in NiO: an off-resonant instantaneous optical spin torque in (111) oriented films and a strain-wave-induced THz torque induced by ultrafast Pt excitation in (001) oriented films. Both phenomena lead to the emission of a THz signal through the inverse spin Hall effect in the adjacent heavy metal layer. We unravel the characteristic timescales of the two excitation processes found to be < 50 fs and > 300 fs, respectively, and thus open new routes towards the development of fast opto-spintronic devices based on antiferromagnetic materials.

Consequences of average time-reversal symmetry in disordered antiferromagnetic topological insulators

Average symmetry protects the topological surface states of topological (crystalline) insulators with time-reversal symmetry from disorder-induced localization. However, the nature of such average symmetry for magnetic topological insulators and, in particular, its connection to surface transport await inspection. Here, we investigate the impact of imperfect magnetic order on an antiferromagnetic topological insulator, which is a solid with a bulk axion field $\ensuremath{\theta}=\ensuremath{\pi}$. We find that the disordered topological surfaces of an antiferromagnetic topological insulator can be generally gapped and localized. The behavior of topological surfaces is now controlled by a mesoscopic average time-reversal symmetry that requires a magnetically imperfect system to be divisible into finite and magnetically neutral slabs. In the presence of this mesoscopic average time-reversal symmetry, the topological surface states will be gapless in the thermodynamic limit and tend to delocalize at a single energy similar to the delocalization transition in the chiral universality class. The notion of average-symmetry-induced delocalization is thus extended to account for magnetic topological insulators, and the spectroscopic and transport signatures clarified herein are relevant to future experimental investigations.

Nonmagnetic Sn doping effect on the electronic and magnetic properties of antiferromagnetic topological insulator MnBi[formula omitted]Te[formula omitted

We investigated the effect of nonmagnetic Sn doping on the electronic and magnetic properties of antiferromagnetic topological insulator MnBi2Te4. We observe that the Sn doping reduces out-of-plane antiferromagnetic (AFM) interactions in Mn1−xSnxBi2Te4 for up to 68% of Sn concentration and above, the system is found to be a paramagnetic. In this way, the anomalous Hall effect observed at a very high critical field of 7.8 T in MnBi2Te4 is reduced to 2 T at 68% of Sn doping. Electrical transport measurements suggest that all compositions are metallic in nature, while the low temperature resistivity is sensitive to the AFM ordering and to the doping-induced disorder. Hall effect study demonstrates that Sn actually dopes electrons into the system, thus, enhancing the electron carrier density almost by two orders at 68% of Sn. In contrast, SnBi2Te4 is found to be a p-type metal. Angle-resolved photoemission spectroscopy (ARPES) studies show that the topological properties are intact at least up to 55% of Sn doping as the Dirac surface states are present near the Fermi level. But in SnBi2Te4 we are unable to detect the surface states due to heavy hole doping. Thus, the Sn doping significantly affects the electronic and magnetic properties of MnBi2Te4.

Interaction-driven spontaneous ferromagnetic insulating states with odd Chern numbers

Motivated by recent experimental work on moiré systems in a strong magnetic field, we compute the compressibility as well as the spin correlations and Hofstadter spectrum of spinful electrons on a honeycomb lattice with Hubbard interactions using the determinantal quantum Monte Carlo method. While the interactions in general preserve quantum and anomalous Hall states, emergent features arise corresponding to an antiferromagnetic insulator at half-filling and other incompressible states following the Chern sequence ± (2N + 1). These odd integer Chern states exhibit strong ferromagnetic correlations and arise spontaneously without any external mechanism for breaking the spin-rotation symmetry. Analogs of these magnetic states should be observable in general interacting quantum Hall systems. In addition, the interacting Hofstadter spectrum is qualitatively similar to the experimental data at intermediate values of the on-site interaction.

Ab initio materials design of superconductivity in d9 nickelates

Motivated by the recent theoretical materials design of superconducting d9 nickelates for which the charge transfer from the NiO2 layer to the block layer is completely suppressed [M. Hirayama et al., Phys. Rev. B 101, 075107 (2020)], we perform a calculation based on the dynamical vertex approximation and obtain the phase diagram of RbCa2NiO3 and A2NiO2Br2, where A is a cation with a valence of 2.5+. We show that the phase diagram of these nickelates exhibits the same essential features as those found in cuprates. Namely, superconductivity appears upon hole-doping into an antiferromagnetic Mott insulator, and the superconducting transition temperature shows a dome-like shape. This demonstrates that the electron correlations play an essential role in nickelate superconductors, and we can control them by changing block layers.

Engineering axion insulator and other topological phases in superlattices without inversion symmetry

We study theoretically the interplay between magnetism and topology in three-dimensional HgTe/MnTe superlattices stacked along the (001) axis. Our results show the evolution of the magnetic topological phases with respect to the magnetic configurations. An axion insulator phase is observed for the antiferromagnetic order with the out-of-plane N\'eel vector direction below a critical thickness of MnTe, which is the ground state amongst all magnetic configurations. Defining $T$ as the time-reversal symmetry, this axion insulator phase is protected by a magnetic two-fold rotational symmetry $C_2{\cdot}T$. The axion insulator phase evolves into a trivial insulator as we increase the thickness of the magnetic layers. By switching the N\'eel vector direction into the $ab$ plane, the system realizes different antiferromagnetic topological insulators depending on the thickness of MnTe. These phases feature gapless surface Dirac cones shifted away from high-symmetry points on surfaces perpendicular to the N\'eel vector direction of the magnetic layers. In the presence of ferromagnetism, the system realizes a magnetic Weyl semimetal and a ferromagnetic semimetal for out-of-plane and in-plane magnetization directions, respectively. We observe large anomalous Hall conductivity in the presence of ferromagnetism in the three-dimensional superlattice.

Room temperature ferromagnetic behavior in Cr2O3 nanoparticles prepared by a planetary ball mill system

Recently, room temperature ferromagnetism (RTFM) has been reported to be found in many kinds of oxide semiconductor nanoparticles. The origin of the magnetic moment is believed to be defects because RTFM strongly depends on particle size. In this study, we have focused on the antiferromagnetic insulator Cr2O3 due to the interesting core-sell structure (RTFM/antiferromagnetism). Furthermore, RTFM found in oxide nanoparticles is expected to depend on the sample preparation condition. Here, the effect of sample preparation condition on RTFM was investigated. Samples were nanoparticles prepared by milling oxide powders in Ar or air circumstances. Both samples indicated hysteresis at room temperature. At higher temperatures than the Néel temperature (308 K), hysteresis loops were found. A significant difference was observed in the saturation magnetization. The sample prepared in air has four times smaller saturation magnetization in 60 min milling time due to self-compensation of defects with air.

Monolayer and bilayer PtCl3 : Energetics, magnetism, and band topology

Two-dimensional (2D) magnetic materials hosting nontrivial topological states are interesting for fundamental research as well as practical applications. Recently, the topological state of 2D Weyl half-semimetal (WHS) was proposed, which hosts fully spin polarized Weyl points robust against spin-orbit coupling in a 2D ferromagnetic system, and single-layer ${\mathrm{PtCl}}_{3}$ was predicted as a platform for realizing this state. Here, we perform an extensive search of 2D ${\mathrm{PtCl}}_{3}$ structures, by using the particle swarm optimization technique and density-functional theory calculation. We show that the desired ${\mathrm{PtCl}}_{3}$ phase corresponds to the most stable one at its stoichiometry. The 2D structure also possesses good thermal stability up to 600 K. We suggest ${\mathrm{SnS}}_{2}$ as a substrate for the growth of 2D ${\mathrm{PtCl}}_{3}$, which has excellent lattice matching and preserves the WHS state in ${\mathrm{PtCl}}_{3}$. We find that uniaxial strains along the zigzag direction maintain the WHS state, whereas small strains along the armchair direction drives a topological phase transition from the WHS to a quantum anomalous Hall (QAH) insulator phase. Furthermore, we study bilayer ${\mathrm{PtCl}}_{3}$ and show that the stacking configuration has strong impact on the magnetism and the electronic band structure. Particularly, the $A{A}^{\ensuremath{'}}$ stacked bilayer ${\mathrm{PtCl}}_{3}$ realizes an interesting topological state---the 2D antiferromagnetic mirror Chern insulator, which has a pair of topological gapless edge bands. Our work provides guidance for the experimental realization of 2D ${\mathrm{PtCl}}_{3}$ and will facilitate the study of 2D magnetic topological states, including WHS, QAH insulator, and magnetic mirror Chern insulator states.

Unidirectional Spin Hall Magnetoresistance in Antiferromagnetic Heterostructures.

Unidirectional spin Hall magnetoresistance (USMR) has been widely reported in the heavy metal/ferromagnet bilayer systems. We observe the USMR in Pt/α-Fe_{2}O_{3} bilayers where the α-Fe_{2}O_{3} is an antiferromagnetic (AFM) insulator. Systematic field and temperature dependent measurements confirm the magnonic origin of the USMR. The appearance of AFM-USMR is driven by the imbalance of creation and annihilation of AFM magnons by spin orbit torque due to the thermal random field. However, unlike its ferromagnetic counterpart, theoretical modeling reveals that the USMR in Pt/α-Fe_{2}O_{3} is determined by the antiferromagtic magnon number with a non-monotonic field dependence. Our findings extend the generality of the USMR which pave the ways for the highly sensitive detection of AFM spin state.

Spin Hall magnetoresistance in quasi-two-dimensional antiferromagnetic-insulator/metal bilayer systems

We study the temperature dependence of spin Hall magnetoresistance (SMR) in antiferromagnetic insulator (AFI)/metal bilayer systems. We calculate the amplitude of the SMR signal by using a quantum Monte Carlo simulation and examine how the SMR depends on the amplitude of the spin, thickness of the AFI layer, and randomness of the exchange interactions. Our results for simple quantum spin models provide a useful starting point for understanding SMR measurements on atomic layers of magnetic compounds.

Layer-dependent zero-line modes in antiferromagnetic topological insulators

Recently, the magnetic domain walls have been experimentally observed in antiferromagnetic topological insulators MnBi$_2$Te$_4$, where we find that the topological zero-line modes (ZLMs) appear along the domain walls. Here, we theoretically demonstrate that these ZLMs are layer-dependent in MnBi$_2$Te$_4$ multilayers. For domain walls with out-of-plane ferromagnetism, we find that ZLMs are equally distributed in the odd-number layers. When domain walls possess in-plane magnetization, the ZLMs can also exist in even-number layers due to in-plane mirror-symmetry breaking. Moreover, the conductive channels are mainly distributed in the outermost layers with increasing layer thickness. Our findings lay out a strategy in manipulating ZLMs and also can be utilized to distinguish the corresponding magnetic structures.

Controlling Magnetism with Light in a Zero Orbital Angular Momentum Antiferromagnet.

Antiferromagnetic materials feature intrinsic ultrafast spin dynamics, making them ideal candidates for future magnonic devices operating at THz frequencies. A major focus of current research is the investigation of optical methods for the efficient generation of coherent magnons in antiferromagnetic insulators. In magnetic lattices endowed with orbital angular momentum, spin-orbit coupling enables spin dynamics through the resonant excitation of low-energy electric dipoles such as phonons and orbital resonances which interact with spins. However, in magnetic systems with zero orbital angular momentum, microscopic pathways for the resonant and low-energy optical excitation of coherent spin dynamics are lacking. Here, we consider experimentally the relative merits of electronic and vibrational excitations for the optical control of zero orbital angular momentum magnets, focusing on a limit case: the antiferromagnet manganese phosphorous trisulfide (MnPS_{3}), constituted by orbital singlet Mn^{2+} ions. We study the correlation of spins with two types of excitations within its band gap: a bound electron orbital excitation from the singlet orbital ground state of Mn^{2+} into an orbital triplet state, which causes coherent spin precession, and a vibrational excitation of the crystal field that causes thermal spin disorder. Our findings cast orbital transitions as key targets for magnetic control in insulators constituted by magnetic centers of zero orbital angular momentum.

Layer-Dependent Magnetic Structure and Anomalous Hall Effect in the Magnetic Topological Insulator MnBi4Te7.

The intrinsic antiferromagnetic topological insulator (TI) MnBi4Te7 provides a capacious playground for the realization of topological quantum phenomena, such as the axion insulator states and quantum anomalous Hall (QAH) effect. In addition to nontrivial band topology, magnetism is another necessary ingredient for realizing these quantum phenomena. Here, we investigate signatures of thickness-dependent magnetism in exfoliated MnBi4Te7 thin flakes. We observe an obvious odd-even layer-number effect in few-layer MnBi4Te7. Noticeably, we show that in monolayer MnBi4Te7 the anomalous Hall effect exhibits a sign reversal. Compared with the case of MnBi2Te4, interlayer antiferromagnetic exchange coupling, which is essential for the realization of the QAH effect, is greatly suppressed in MnBi4Te7. The demonstration of thickness-dependent magnetic properties is helpful to further explore the topological quantum phenomena in MnBi4Te7.

Twist-Induced Near-Field Radiative Heat Switch in Hyperbolic Antiferromagnets

We study the twisted control of the near-field radiative heat transfer between two hyperbolic antiferromagnetic insulators under external magnetic fields. We show that the near-field heat flux can be affected by both the twist angle $\theta$ and the magnitude of the applied magnetic field with different broken symmetries. Irrespective of twist angle, the external magnetic field causes the radiative heat flux to change nonmonotonically, and the minimum heat flux can be found with the magnetic fields of about 1.5 T. Such nonmonotonic behavior is due to the fact that the magnetic field can radically change the nature of the magnon polaritons with time reversal symmetry breaking. The field not only affects the topological structure of surface magnon polaritons, but also induces the volume magnon polaritons that progressively dominate the heat transfer as the field increases. We further propose a twist-induced thermal switch device with inversion symmetry breaking, which can severely regulate radiative heat flux through different magnetic fields. Our findings account for a characteristic modulation of radiative heat transfer with implications for applications in dynamic thermal management.

Dimensionality-driven in-plane antiferromagnetism in two-dimensional SrRuO3

Control of dimensionality is a powerful tool to unlock hidden electronic and magnetic phases. In particular, reduced dimensionality in strongly correlated oxides leads to an intriguing ``dead-layer'' behavior that a transition from (ferromagnetic) metal to (antiferromagnetic) insulator occurs as the thickness approaches the two-dimensional limit. However, the origin of such transition has been a subject of debate that both the intrinsic dimensionality effect and the extrinsic disorder effect are proposed to be the driving force. Here, we reveal a transition from ferromagnetic metal with perpendicular magnetic anisotropy to antiferromagnetic insulator with in-plane magnetic anisotropy in ${\mathrm{SrRuO}}_{3}$ epitaxial films down to the monolayer limit. Experimentally, we demonstrate that ${\mathrm{SrRuO}}_{3}$ films below 3 unit cells become magnetic insulators and the spin easy axis changes to the in-plane ⟨110⟩ directions. First-principles calculations reveal that the interplay of the orbital-selective quantum confinement on Ru $4d$ orbitals and the oxygen octahedral rotation drives the ultrathin films from ferromagnetic metal to antiferromagnetic insulator, reorienting Ru spins from the perpendicular to the ⟨110⟩ directions. Our findings demonstrate how reduced dimensionality can tailor the magnetic state and provide significant advances in one of the debated topics in complex oxide heterostructures that dimensionality effect alone can be the driving force of dead-layer phenomenon in two-dimensional systems.

Magneto-optical detection of terahertz cavity magnon-polaritons in antiferromagnetic HoFeO3

An intense THz pulse excites a high-Q magnetic resonance mode in the antiferromagnetic insulator HoFeO3 by the THz Zeeman torque. By using magneto-optical detection and sweeping the temperature, we observed an anomalous beating in the magnon dynamics for certain temperatures. The beating originates from the formation of cavity magnon-polaritons upon the intersection of the antiferromagnetic resonance frequency with the frequencies of the Fabry–Pérot modes inside the etalon formed by the sample cavity in the weak coupling limit. The validity of this idea is demonstrated by simulations using Maxwell's equations. Furthermore, the observed beating pattern depends on the polarization of the probe pulse. This dependence can be reproduced in the simulations by considering an imaginary Verdet constant, which could be a result of an interplay between the magneto-optical Faraday effect and static linear birefringence.

Strain enhanced magnetism of V-implanted CrI3 monolayer

In recent years, one of the urgent issues for two dimensional (2D) magnetic materials is to find efficient ways in enhancing the magnetic ordering temperature Tc. It is believed that an in-plane (IP) compressive strain can greatly enhance the interatomic interactions by shortening the chemical bond length if at all possible, leading to the enlarged spin exchange and possibly higher Tc. However, a large compressive strain usually favors antiferromagnetic (AFM) ordering due to growing dominance of the Pauli exclusion principle, in contradiction with the common requirement of nonzero magnetization. In compromise, ferrimagnetic (FiM) ordering can be alternated by synthesizing artificial 2D compound with two magnetic sublattices. In this work, we propose a V-implanted CrI3 monolayer, short for V-(CrI3)2, and study its FiM ordering under a series of IP biaxial strains using the first-principles calculations and Monte Carlo simulations. It is found that the V-(CrI3)2 monolayer may evolve from the stripy-type AFM insulator toward the FiM half-metal with net magnetic moment of 5.0 μB/f.u. aligned in parallel to the ab-plane upon increasing the IP biaxial strain up to ∼−3% (compressive strain) and beyond. As the IP biaxial strain increases up to ∼−5%, the Tc of the FiM state may be raised to room temperature. This work suggests that the IP strain engineering combined with spin implantation can be an alternative strategy for enhancing 2D magnetism.

The importance of the interface for picosecond spin pumping in antiferromagnet-heavy metal heterostructures

Interfaces in heavy metal (HM) - antiferromagnetic insulator (AFI) heterostructures have recently become highly investigated and debated systems in the effort to create spintronic devices that function at terahertz frequencies. Such heterostructures have great technological potential because AFIs can generate sub-picosecond spin currents which the HMs can convert into charge signals. In this work we demonstrate an optically induced picosecond spin transfer at the interface between AFIs and Pt using time-domain THz emission spectroscopy. We select two antiferromagnets in the same family of fluoride cubic perovskites, KCoF3 and KNiF3, whose magnon frequencies at the centre of the Brillouin zone differ by an order of magnitude. By studying their behaviour with temperature, we correlate changes in the spin transfer efficiency across the interface to the opening of a gap in the magnon density of states below the Néel temperature. Our observations are reproduced in a model based on the spin exchange between the localized electrons in the antiferromagnet and the free electrons in Pt. Through this comparative study of selected materials, we are able to shine light on the microscopy of spin transfer at picosecond timescales between antiferromagnets and heavy metals and identify a key figure of merit for its efficiency: the magnon gap. Our results are important for progressing in the fundamental understanding of the highly discussed physics of the HM/AFI interfaces, which is the necessary cornerstone for the designing of femtosecond antiferromagnetic spintronics devices with optimized characteristics.