Easy-axis Antiferromagnet Research Articles

Ghost surface magnon-plasmon polariton in antiferromagnets covered with graphene monolayer.

We predicted one ghost surface magnon-plasmon polariton (GSMPP) in antiferromagnets covered a graphene monolayer in the out-plane geometry where both an external magnetic field and the antiferromagnetic easy axis are normal to the surface. This GSMPP exists in the frequency range that linearly increases with the external magnetic field. Due to the coupling between the external magnetic-field and electrons in the graphene monolayer, we found some interesting features of the GSMPP. (1) The frequency range of the GSMPP occupies the whole interval between the two antiferromagnetic resonant frequencies and linearly increases with the external magnetic field. (2) There two necessary conditions for the existence of GSMPP, or one is the existence of external magnetic field and the other is that the GSMPP frequency must be larger than the electronic-cyclotron frequency in the graphene monolayer so that the GSMPP exists in an enclosed triangular region in the frequency-field space. (3) The oscillatory behavior of GSMPP field and Poynting vector along the surface normal are very easily seen. (4) The surface conductivity of graphene monolayer and the antiferromagnetic permeability are controlled by the Femi-energy of graphene and external magnetic field, so the GSMPP is a highly-tunable surface polariton. The numerically-simulated attenuated total reflection (ATR) spectra prove that the GSMPP is a real-world existence and more easily observed in experiment. The results are interesting in spintronics and surface optics, as well as the relevant technology fields.

High-Angular Momentum Excitations in Collinear Antiferromagnet FePS3.

We report on magneto-optical studies of the quasi-two-dimensional van der Waals antiferromagnet FePS3. Our measurements reveal an excitation that closely resembles the antiferromagnetic resonance mode typical of easy-axis antiferromagnets; nevertheless, it displays an unusual, four-times larger Zeeman splitting in an applied magnetic field. We identify this excitation with an |Sz| = 4 multipolar magnon─a single-ion 4-magnon bound state─that corresponds to a full reversal of a single magnetic moment of the Fe2+ ion. We argue that condensation of multipolar magnons in large-spin materials with a strong magnetic anisotropy can produce new exotic states.

Field-induced spin reorientation transitions in antiferromagnetic ring-shaped spin chains

Easy axis antiferromagnets are robust against external magnetic fields of moderate strength. Spin reorientations in strong fields can provide an insight into more subtle properties of antiferromagnetic materials, which are often hidden by their high ground state symmetry. Here, we investigate theoretically effects of curvature in ring-shaped antiferromagnetic achiral anisotropic spin chains in strong magnetic fields. We identify the geometry-governed helimagnetic phase transition above the spin-flop field between vortex and onion states. The curvature-induced Dzyaloshinskii-Moriya interaction results in the spin-flop transition being of first- or second-order depending on the ring curvature. Spatial inhomogeneity of the N\'eel vector in the spin-flop phase generates weak ferromagnetic response in the plane perpendicular to the applied magnetic field. Our work contributes to the understanding of the physics of curvilinear antiferromagnets in magnetic fields and guides prospective experimental studies of geometrical effects relying on spin-chain-based nanomagnets.

Field-assisted sub-terahertz spin pumping and auto-oscillation in NiO

Spin pumping converting sub-terahertz electromagnetic waves to DC spin currents has recently been demonstrated in antiferromagnets (AFMs) with easy-axis magnetic anisotropy. However, easy-plane AFMs such as NiO, which are easier to prepare experimentally, are considered to be bad candidates for spin pumping because the Néel vector oscillation is linearly polarized, placing a major restriction on the material choice for practical applications. Through a case study of NiO, we show that an applied magnetic field below the spin–flop transition can substantially modify the polarization of the resonance eigenmodes, which enables coherent sub-terahertz spin pumping as strong as that in easy-axis AFMs. In addition, we find that an applied magnetic field can significantly reduce the threshold of Néel vector auto-oscillation triggered by spin-transfer torques. These prominent field-assisted effects can greatly facilitate spintronic device engineering in the sub-terahertz frequency regime.

Interplay of magnetic field and trigonal distortion in the honeycomb Γ model: Occurrence of a spin-flop phase

In candidate Kitaev materials, the off-diagonal $\Gamma$ and $\Gamma'$ interactions are identified to come from the spin-orbit coupling and trigonal distortion, respectively. They have generated intense research efforts because of their intimate relation to the field-induced magnetically disordered state reported in $\alpha$-RuCl$_3$. Theoretically, while a plethora of field-induced phases has been proposed in the honeycomb lattice, a stable intermediate phase that can survive in a wide parameter region regardless of the underlying phases is still lacking. Here we focus on the interplay of an out-of-plane magnetic field and a symmetry-allowed $\Gamma'$ term due to trigonal distortion in the dominant antiferromagnetic $\Gamma$ region. By using multifaceted approaches ranging from classical Monte Carlo and semiclassical spin-wave theory to density-matrix renormalization group, we identify an intriguing spin-flop phase in the presence of magnetic field and antiferromagnetic $\Gamma'$ interaction, before it eventually enters into a fully polarized state. As the $\Gamma'$ interaction approaches the size of $\Gamma$ one, the $\Gamma$-$\Gamma'$ model maps to the easy-axis XXZ antiferromagnet, where the spin-flop phase can be understood as a superfluid phase in the extended Bose-Hubbard model. Our finding thus demonstrates an exciting path from the honeycomb $\Gamma$ model towards a $U(1)$-symmetric XXZ antiferromagnet in a magnetic field.

Unique surface polaritons and their transitions in metamaterials.

We investigated surface polaritons in a metamaterial composed of polar-crystal layers and antiferromagnetic layers. In a specific geometry, two surface polaritons were predicted, which are a unique ghost surface polariton (GSP) and surface hybrid-polarization polariton (SHP). The two surface polaritons occupy different segments of one smooth dispersion curve and are magnetically tunable. An external magnetic field along the antiferromagnetic easy axis can bring about the switch or transition between the two surface polaritons and meanwhile performs the necessary condition for the existence of two surface polaritons. In the metamaterial, either surface polariton consists of two branch waves. The branch waves of the GSP are coherent and have the same amplitude and different phases, but those of the SHP are not coherent and have different amplitudes and phases. The main characteristic of the GSP is that its fields oscillate and attenuate with the distance away from the metamaterial surface and exhibit interferent fringes on the plane normal to the surface.

Magnon polarons in spin Seebeck effect of easy axis antiferromagnets

The formation of magnon polarons, quanta of magnetoelastic waves, was found to be able to stimulate an enhancement or suppression in the magnetic field dependence of the spin Seebeck effect when the dispersion curve of the magnon becomes tangential to those of acoustic phonons. In the present work, we systematically analyzed the properties of the magnon spectrum in body-centered cubic easy-axis antiferromagnets with varying strength of exchange interaction and magnetic anisotropy, which allowed us to classify the antiferromagnets according to the number of solutions for the tangential condition between the dispersion curves of magnons and acoustic phonons. The anomaly features were found to occur only in the relatively weak magnetic field regime before spin flop transition. The manifestation of magnon–polaron-induced anomaly on the longitudinal spin Seebeck coefficient was also calculated directly from which a triple-peak feature, never observed or proposed before, was predicted. Our analysis also works for antiferromagnets with other magnetic lattices.

Magnetic-field tuning of the spin dynamics in the magnetic topological insulators(MnBi2Te4)(Bi2Te3)n

We report a high frequency/high magnetic field electron spin resonance (HF-ESR) spectroscopy study in the sub-THz frequency domain of the two representatives of the family of magnetic topological insulators (MnBi$_{2}$Te$_{4}$)(Bi$_{2}$Te$_{3}$)$_{n}$ with $n = 0$ and 1. The HF-ESR measurements in the magnetically ordered state at a low temperature of $T = 4$ K combined with the calculations of the resonance modes showed that the spin dynamics in MnBi$_{\text{4}}$Te$_{\text{7}}$ is typical for an anisotropic easy-axis type ferromagnet (FM) whereas MnBi$_{\text{2}}$Te$_{\text{4}}$ demonstrates excitations of an anisotropic easy-axis type antiferromagnet (AFM). However, by applying the field stronger than a threshold value $\sim 6$ T we observed in MnBi$_{\text{2}}$Te$_{\text{4}}$ a crossover from the AFM resonance modes to the FM modes which properties are very similar to the ferromagnetic response of MnBi$_{\text{4}}$Te$_{\text{7}}$. We attribute this remarkably unusual effect unexpected for a canonical easy-axis AFM, which, additionally, can be accurately reproduced by numerical calculations of the excitation modes, to the closeness of the strength of the AFM exchange and magnetic anisotropy energies which appears to be a very specific feature of this compound. Our experimental data evidences that the spin dynamics of the magnetic building blocks of these compounds, the Mn-based septuple layers (SLs), is inherently ferromagnetic featuring persisting short-range FM correlations far above the magnetic ordering temperature as soon as the SLs get decoupled either by introducing a nonmagnetic quintuple interlayer, as in MnBi$_{\text{4}}$Te$_{\text{7}}$, or by applying a moderate magnetic field, as in MnBi$_{\text{2}}$Te$_{\text{4}}$, which may have an effect on the surface topological band structure of these compounds.

Magnon spin transport around the compensation magnetic field in easy-plane antiferromagnetic insulators

In this work, we theoretically study the magnon spin transport in easy-plane antiferromagnetic insulators in the presence of an in-plane magnetic field. By exactly calculating the magnon spectrum, we find that the band splitting due to magnetic anisotropy can be fully compensated by the external field at a particular strength, which makes the dynamics nearly equivalent to an easy-axis antiferromagnet. As a result, the intrinsic magnon spin Hall effect due to the dipole–dipole interaction, previously predicted in easy-axis antiferromagnets, is activated in easy-plane antiferromagnets. The compensation feature also allows the field control of magnon spin lifetime and hence the spin diffusion length. The compensation feature is robust against biaxial anisotropy.

Long-distance spin-transport across the Morin phase transition up to room temperature in ultra-low damping single crystals of the antiferromagnet \u03b1-Fe2O3

Antiferromagnetic materials can host spin-waves with polarizations ranging from circular to linear depending on their magnetic anisotropies. Until now, only easy-axis anisotropy antiferromagnets with circularly polarized spin-waves were reported to carry spin-information over long distances of micrometers. In this article, we report long-distance spin-transport in the easy-plane canted antiferromagnetic phase of hematite and at room temperature, where the linearly polarized magnons are not intuitively expected to carry spin. We demonstrate that the spin-transport signal decreases continuously through the easy-axis to easy-plane Morin transition, and persists in the easy-plane phase through current induced pairs of linearly polarized magnons with dephasing lengths in the micrometer range. We explain the long transport distance as a result of the low magnetic damping, which we measure to be ≤ 10−5 as in the best ferromagnets. All of this together demonstrates that long-distance transport can be achieved across a range of anisotropies and temperatures, up to room temperature, highlighting the promising potential of this insulating antiferromagnet for magnon-based devices.

Spin Seebeck effect near the antiferromagnetic spin-flop transition

We develop a low-temperature, long-wavelength theory for the interfacial spin Seebeck effect (SSE) in easy-axis antiferromagnets. The field-induced spin-flop (SF) transition of N\'eel order is associated with a qualitative change in SSE behavior: Below SF, there are two spin carriers with opposite magnetic moments, with the carriers polarized along the field forming a majority magnon band. Above SF, the low-energy, ferromagnetic-like mode has magnetic moment opposite the field. This results in a sign change of the SSE across SF, which agrees with recent measurements on Cr$_2$O$_3$/Pt and Cr$_2$O$_3$/Ta devices [Li $\textit{et al.,}$ $\textit{Nature}$ $\textbf{578,}$ 70 (2020)]. In our theory, SSE is due to a N\'eel spin current below SF and a magnetic spin current above SF. Using the ratio of the associated N\'eel to magnetic spin-mixing conductances as a single constant fitting parameter, we reproduce the field dependence of the experimental data and partially the temperature dependence of the relative SSE jump across SF.

Driving a magnetized domain wall in an antiferromagnet by magnons

We theoretically study the interaction of magnons, quanta of spin waves, and a domain wall in a one-dimensional easy-axis antiferromagnet in the presence of an external magnetic field applied along the easy axis. To this end, we begin by obtaining the exact solution for spin waves in the background of a domain wall magnetized by an external field. The finite magnetization inside the domain wall is shown to give rise to reflection of magnons scattering off the domain wall, deviating from the well-known result of reflection-free magnons in the absence of a magnetic field. For practical applications of the predicted reflection of magnons, we show that the magnon reflection contributes to the thermally driven domain-wall motion. Our work leads us to envision that inducing a finite magnetization in antiferromagnetic solitons such as vortices and skyrmions can be used to engender phenomena that do not occur in the absence of magnetization.

Dipolar spin waves in uniaxial easy-axis antiferromagnets: A natural topological nodal-line semimetal

The existence of the magnetostatic surface spin waves in ferromagnets, known as Damon-Eshbach mode, was recently demonstrated to originate from the topology of the dipole-dipole interaction. In this work, we study the topological characteristics of magnons in easy-axis antiferromagnets with uniaxial anisotropy. The dipolar spin waves are found to be, driven by the dipole-dipole interaction, in a topological nodal-line semimetal phase, which hosts Damon-Eshbach-type surface modes due to the bulk-edge correspondence. The long wavelength character of dipolar spin waves makes our proposal valid for any natural uniaxial easy-axis antiferromagnet, and thus enriches the candidates of topological magnonic materials. In contrast to the nonreciprocal property in ferromagnetic case, the surface modes with opposite momentum coexist at each surface, but with different chiralities. Such a chirality-momentum or spin-momentum locking, similar to that of electronic surface states in topological insulators, offers the opportunity to design novel chirality-based magnonic devices in antiferromagnets.

Electric field control of ordered oxygen vacancy planes and antiferromagnetic structures in strontium cobaltite

The polarized ionic liquids (ILs) could generate intense electric fields on the surface of solid-state materials and create functional defects by ion migration within them, resulting in phase transitions of metal–insulator or paramagnet–ferromagnet, etc. Such a strong electric field even provides an opportunity for the control of spin ordering in antiferromagnetic (AFM) crystal which is difficult to be manipulated due to the strong exchange coupling between antiparallel spins in the whole bulk. Here we find that the ferromagnetic SrCoO3 of 40 nm could be transformed to AFM SrCoO2.5 with ordered oxygen vacancy planes either vertical (V-SrCoO2.5) or parallel (P-SrCoO2.5) to the surface by IL gating. The spin Hall magnetoresistances suggest that the AFM easy axes of V- and P-SrCoO2.5 are along [010] and , respectively. The orientations of gating induced oxygen vacancy planes are related to the oxygen framework rotation in the parent SrCoO3 and could be controlled by the strain engineering. Our results not only supply a novel way to manipulate the AFM spins by creating functional ordered defects, but also reveal the effect of oxygen framework rotation on the formation of oxygen vacancies under ionic liquid gating.

Controlling antiferromagnetic domains in patterned La0.7Sr0.3FeO3 thin films

Transition metal oxide thin films and heterostructures are promising platforms to achieve full control of the antiferromagnetic (AFM) domain structure in patterned features as needed for AFM spintronic devices. In this work, soft x-ray photoemission electron microscopy was utilized to image AFM domains in micromagnets patterned into La0.7Sr0.3FeO3 (LSFO) thin films and La0.7Sr0.3MnO3 (LSMO)/LSFO superlattices. A delicate balance exists between magnetocrystalline anisotropy, shape anisotropy, and exchange interactions such that the AFM domain structure can be controlled using parameters such as LSFO and LSMO layer thickness, micromagnet shape, and temperature. In LSFO thin films, shape anisotropy gains importance only in micromagnets where at least one extended edge is aligned parallel to an AFM easy axis. In contrast, in the limit of ultrathin LSFO layers in the LSMO/LSFO superlattice, shape anisotropy effects dominate such that the AFM spin axes at micromagnet edges can be aligned along any in-plane crystallographic direction.

Improvement and stabilization of exchange bias in ferromagnet/antiferromagnet/ferromagnet trilayers

Exchange bias (EB) in ferromagnet/antiferromagnet bilayers, which has been extensively studied and applied for several decades, is sensitive to many factors such as layer thickness, texture and crystallization. Various factors in an antiferromagnet may counterbalance each other to limit and deteriorate EB. We used an unbiased Monte-Carlo method based on a modified Metropolis algorithm to predict that dependence of EB properties on antiferromagnetic anisotropy (KAF) are highly improved by attaching a soft ferromagnet on the other side of the antiferromagnet. On one hand, target ferromagnet in trilayers displays a pronounced and stabilized EB plateau at high KAF, on the contrary, EB is completely removed and instead a high coercivity is observed at low KAF, exhibiting a roughly KAF-modulated EB switching effect. On the other hand, EB is identified with no training in trilayers and well axially symmetric about antiferromagnetic easy axis. Meanwhile, in trilayers EB versus angle (θ) which is between antiferromagnetic easy axis and the direction of magnetic field is a roughly linear relationship in the intermediate θ range. Microscopic explorations found that a fully uncompensated magnetization in antiferromagnet may appear in trilayers and its rotatability is precisely controlled by KAF, designating that the antiferromagnet/seed-ferromagnet bilayers resemble a ferromagnet with changeable hardness to induce a maximized coercivity of target ferromagnet at low KAF to a maximized EB at high KAF. Finally, a phenomenological model reveals that antiferromagnetic spins change from a fluctuated state to a blocked state due to the existence of seed ferromagnet, and thus in this work we conceived an artificial pinning layer to establish and regulate EB.

Crystal structure and thermodynamic properties of dinickel diphosphate dihydrate Ni2(H2O)2[P2O7

Single crystals of dinickel diphosphate dihydrate, Ni2(H2O)2[P2O7], have been synthesized by a hydrothermal method. Its structure was refined in the monoclinic P21/n space group (unit cell parameters a = 6.2517(1) Å, b = 13.7892(3) Å, c = 7.2894(2) Å, β = 94.507(2)°, V = 626.45(2) Å3, and Z = 4) based on low-temperature X-ray diffraction data until R- 0.016. Corrugated chains of NiO5(H2O) octahedra sharing edges are aligned in the [101[combining macron]] direction. They are linked into a three-dimensional framework through diphosphate groups and hydrogen bonds. A detailed crystal chemical analysis of the family Me2(H2O)2[X2O7] revealed correlations between the unit-cell parameters of the isotypic transition metal phosphates and arsenates, their structural features and the sizes of structure forming cations. Despite the isolation of the cis and trans edge-sharing infinite zigzag chains of Ni-centered octahedra from each other no pronounced low dimensionality is seen in the magnetic response of Ni2(H2O)2[P2O7]. The magnetic susceptibility χ evidences a short range correlation maximum at Tmax = 11.9 K accompanied by the onset of long-range magnetic order at TN = 9.4 K. Below TN, the title compound exhibits the features of an archetype three-dimensional easy-axis antiferromagnet which experiences a sequence of spin-flop and spin-flip phase transitions. Basing on specific heat Cp and magnetization M studies, the magnetic phase diagram of Ni2(H2O)2[P2O7] has been established.

Micromagnetic model of exchange bias: effects of structure and AF easy axis dispersion for IrMn/CoFe bilayers

A micromagnetic model of an exchange bias bilayer is used to examine the impact of the physical structure and the easy axis dispersion of the antiferromagnetic (AF) layer on the exchange bias field (HEB) in an IrMn/CoFe system. Because of the different timescales, the magnetization dynamics of the IrMn and CoFe layers are modelled using respectively a kinetic Monte Carlo (kMC) approach and Landau–Lifshitz–Gilbert (LLG) equation. The easy axis dispersion is modelled using a Gaussian distribution. The calculations show that HEB increases with increasing IrMn thickness and grain size, in agreement with experimental work. Moreover, the model allows the visualization of the switching process at the micromagnetic level to reveal the reversal mechanism. We find that the effect of AF easy axis distribution not only strongly affects the reduction of HEB but also drives non-coherent behaviour in the reversal mechanism. This confirms that the easy axis distribution is an important factor with strong impact on the magnetic properties and exchange bias field of an exchange bias system.

Hematite: Morin temperature of nanoparticles with different size

Abstract A spin-reorientation transition from a weakly ferromagnetic (WF) to an antiferromagnetic (AF) spin ordering in hematite (α-Fe2O3) during cooling occurs at Morin temperature (TM∼264 K for bulk). The transition is strongly size dependent and TM generally decreases with the decreasing volume of the particles. For particles smaller than approximately ∼20 nm, the Morin transition may be even suppressed and disappears entirely as near-surface spins deviate strongly from the antiferromagnetic easy axis. We report an investigation on nanoparticles prepared by hydrothermal method and sol–gel technique (in silica) of pure α-Fe2O3 phase as confirmed by XRD (space group R-3c, lattice parameters a = 5.038(2) A, c = 13.772(12) A) differing in the median size derived by TEM: 5.6 nm, 26 nm, 42 nm and 103 nm. By means of Mossbauer spectra acquired between 4.2 and 300 K, we determined the relative concentrations of magnetic phases (WF and AF) within the 57Fe enriched sample and searched for the best finite-scaling theoretical model (mean-field, 3D Heisenberg, Ising) describing the derived size dependence of Morin temperature of the nanoparticles with a log-normal size distribution. The comparison of relevant parameters derived from the fit of experimental data by theoretical model is consistent with the 3D Heisenberg model with scaling parameter λ = 1.4, Morin temperature of bulk material TM(∞) = 265(1) K and correlation length ξ0 = 8.1(2) nm or Ising model with λ = 1.6, TM(∞) = 265(1) K and ξ0 = 9.4(2) nm.

Noninteger-spin magnonic excitations in untextured magnets

Interactions are responsible for intriguing physics, e.g. emergence of exotic ground states and excitations, in a wide range of systems. Here we theoretically demonstrate that dipole-dipole interaction leads to bosonic eigen-excitations with average spin ranging from zero to above $\hbar$ in magnets with uniformly magnetized ground states. These exotic excitations can be interpreted as quantum coherent conglomerates of spin $\hbar$ magnons, the eigen-excitations when the dipolar interactions are disregarded. We further find that the eigenmodes in an easy-axis antiferromagnet are spin-zero quasiparticles instead of the widely believed spin $\pm \hbar$ magnons. The latter re-emerge when the symmetry is broken by a sufficiently large applied magnetic field. The average spin greater than $\hbar$ is accompanied by vacuum fluctuations and may be considered to be a weak form of frustration.