Antiferromagnetic System Research Articles

Modeling spin waves in noncollinear antiferromagnets: Spin-flop states, spin spirals, skyrmions, and antiskyrmions

Spin waves in antiferromagnetic materials have great potential for next-generation magnonic technologies. However, their properties and their dependence on the type of ground-state antiferromagnetic structure are still open questions. Here, we investigate theoretically spin waves in one- and two-dimensional model systems with a focus on noncollinear antiferromagnetic textures such as spin spirals and skyrmions of opposite topological charges. We address in particular the nonreciprocal spin excitations recently measured in bulk antiferromagnet $\alpha$--$\text{Cu}_2\text{V}_2\text{O}_7$ utilizing inelastic neutron scattering experiments [Phys.\ Rev.\ Lett.\ \textbf{119}, 047201 (2017)], where we help to characterize the nature of the detected spin-wave modes. Furthermore, we discuss how the Dzyaloshinskii-Moriya interaction can lift the degeneracy of the spin-wave modes in antiferromagnets, resembling the electronic Rashba splitting. We consider the spin-wave excitations in antiferromagnetic spin-spiral and skyrmion systems and discuss the features of their inelastic scattering spectra. We demonstrate that antiskyrmions can be obtained with an isotropic Dzyaloshinskii-Moriya interaction in certain antiferromagnets.

Study of effective coupling between charge degrees of freedom in low dimensional hole-doped quantum antiferromagnets

Expressions for the generalized charge stiffness constant at zero temperature are derived corresponding to low-dimensional hole-doped quantum antiferromagnets, describable by the t–J-like models, with a view to understanding fermionic pairing possibilities and charge couplings in itinerant antiferromagnetic systems. A detailed comparison between spin and charge correlations and couplings are presented for the strong and weak coupling limits in one (1D) and two (2D) dimensions. The result highlights that the charge and spin couplings show very similar behaviour in the over-doped region in both dimensions, whereas they show a completely different trend in the lower doping regimes. A qualitative equivalence of the generalized charge stiffness constant with the effective Drude weight and Coulomb interaction is established based on the comparison with other theoretical and experimental results. The fall in charge stiffness with increase in doping then implies a reduction in magnitude of the effective Coulomb repulsion between the mobile carriers. This leads to an enhanced possibility of fermionic pairing with an increase in doping concentration, in the possible presence of some other attraction-producing mechanism from a source outside the t–J-like models. Moreover, under certain conditions in the optimal doping region, the t–J-like models themselves are able to produce an attractive interaction for pairing.

Spin–phonon coupling and thermodynamic behaviour in YCrO3 and LaCrO3: inelastic neutron scattering and lattice dynamics

We report detailed temperature-dependent inelastic neutron scattering and ab initio lattice dynamics investigation of magnetic perovskites YCrO3 and LaCrO3. The magnetic neutron scattering from the Cr ions exhibits significant changes with temperature and dominates at low momentum transfer regime. Ab initio calculations performed including magnetic interactions show that the effect of magnetic interactions is very significant on the low- as well as high-energy phonon modes. We have also shown that the inelastic neutron spectrum of YCrO3 mimics the magnon spectrum from a G-type antiferromagnetic system, which is consistent with previously reported magnetic structure in the compound. The pressure-dependent ab initio lattice dynamics calculations are used to calculate the anisotropic thermal expansion behaviour in orthorhombic YCrO3, which is in excellent agreement with the available experimental data in the paramagnetic phase. We identify that the low energy anharmonic phonon modes involving Y vibrations contribute maximum to the thermal expansion behaviour.

Weak ferromagnetism in perovskite oxides

Weak ferromagnetism has been widely found in antiferromagnetic systems, including the technically important orthorhombic perovskites $R\mathrm{Fe}{\mathrm{O}}_{3}$ ($R=\mathrm{rare}\phantom{\rule{0.16em}{0ex}}\mathrm{earth}$) owing to spin canting associated with crystal symmetry. Antisymmetric exchange interaction (AEI) and single-ion anisotropy (SIA) are the essential mechanisms responsible for the development of noncollinear structures in antiferromagnetic systems. AEI and SIA share the same structural restriction to facilitate the spin canting. While both AEI and SIA originate from the spin-orbit coupling effect, they have sharply different dependences on the local structure. Consideration of the structural dependence of a canted spin motivates us to revisit the orthoferrite family. The spin canting along the $c$ axis measured on precisely oriented crystals increases monotonically from $\mathrm{La}\mathrm{Fe}{\mathrm{O}}_{3}$ to $\mathrm{Lu}\mathrm{Fe}{\mathrm{O}}_{3}$. Based on Moriya's model, AEI is sensitive to the octahedral-site distortion in the perovskite structure. However, the site distortion does not exhibit a monotonic change with the rare-earth substitution. Instead, a linear relationship has been found in both the octahedral rotation and the spin canting angle versus the rare-earth ionic radius, which indicates that SIA plays a significant role leading the spin canting in orthoferrites.

Effect of annealing on magnetic and magnetoresistance characteristics of Gd/CoFeTaB multilayer film

By using the magnetron sputtering, a Gd/CoFeTaB multilayer structure film was fabricated, and investigated the effect of annealing temperature on magnetic and magnetoresistance characteristics of the multilayer film. A magnetic compensation phenomenon was observed, which means there is an antiferromagnetic coupling in the Gd/CoFeTaB system. It was found that the annealing enhances the antiferromagnetic coupling, lead to increase the Gilbert damping coefficient. Furthermore, an unsaturated magnetoresistance effect was observed, which is explained by the electron-electron interaction in the system. It is expected that the artificial antiferromagnetic multilayer system with weak macro-magnetism and internal magnetic order will has a good application prospect for the development of spintronics devices.

Magnetotransport and magnetic properties of the layered noncollinear antiferromagnetic Cr2Se3 single crystals

We report on the growth of high-quality stoichiometric layered Cr2Se3 single crystals with metallic and noncollinear antiferromagnetic ground state using the chemical vapor transport (CVT) method. The crystals show weak ferromagnetism in the in-plane and out-of-plane directions below the Neél temperature (TN), however, the field-cooled out-of-plane magnetization at 500 Oe and 10 K (∼0.24 μB/f.u.) is approximately 15 times larger than that of the in-plane one, indicating strong c-axis easy uniaxial magnetic anisotropy, which is further supported by the in-plane and out-of-plane isothermal anisotropic magnetic hysteresis loops and the angular dependent magnetoresistance (MR). The latter also reveals a decrease of the coercive field of the crystal upon the tilting of the weak ferromagnetic easy axis away from the direction of the magnetic field. Further, the out-of-plane isothermal MR are negative below TN and show butterfly shapes for T < 10 K and couple with the magnetic hysteresis M(H) loop. These results may help researchers better understand the interplay between the weak ferromagnetism and the magnetotransport properties of 2D itinerant noncollinear antiferromagnetic systems.

Determination of spin Hall angle, spin mixing conductance, and spin diffusion length in CoFeB/Ir for spin-orbitronic devices

Iridium is a very promising material for spintronic applications due to its interesting magnetic properties such as large RKKY exchange coupling as well as its large spin-orbit coupling value. Ir is for instance used as a spacer layer for perpendicular synthetic antiferromagnetic or ferrimagnet systems. However, only a few studies of the spintronic parameters of this material have been reported. In this paper, we present inverse spin Hall effect - spin pumping ferromagnetic resonance measurements on CoFeB/Ir based bilayers to estimate the values of the effective spin Hall angle, the spin diffusion length within iridium, and the spin mixing conductance in the CoFeB/Ir bilayer. In order to have reliable results, we performed the same experiments on CoFeB/Pt bilayers, which behavior is well known due to numerous reported studies. Our experimental results show that the spin diffusion length within iridium is 1.3 nm for resistivity of 250 n$\Omega$.m, the spin mixing conductance $g_{eff}^{\uparrow \downarrow}$ of the CoFeB/Ir interface is 30 nm$^{-2}$, and the spin Hall angle of iridium has the same sign than the one of platinum and is evaluated at 26% of the one of platinum. The value of the spin Hall angle found is 7.7% for Pt and 2% for Ir. These relevant parameters shall be useful to consider Ir in new concepts and devices combining spin-orbit torque and spin-transfer torque.

A Facile Route to Synthesis of Ferromagnetic and Antiferromagnetic Phases of Iron Oxide Nanoparticles by Controlled Heat Treatment of Ferritin

Magnetic nanoparticles of ferromagnetic and antiferromagnetic phases were synthesized from controlled heat treatment of dry ferritin powder in a bottom-up approach. Heat treatment paves the way for synthesis of Fe2O3 solid phase from iron molecular complexes stored in the cavity of ferritin; the strong increase (~ 150 times) in saturation magnetization is a sign of the presence of ferromagnetic exchange coupling which exists only in solid phase. In this experimental study, vibrating sample magnetometer (VSM) was used to study magnetic induction. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed for analysis of structure and morphology of the samples. Differential scanning calorimetry (DSC) was used to search for indicators of structural phase transformation through scanning of temperature. The nanoparticle size with highly narrow distribution (mean size ~ 6 nm) was observed by SEM for sample annealed at 430 °C. As indicated by XRD measurements, the majority phase of Fe2O3 nanoparticles was amorphous up to 500 °C, whereas DSC demarcates the crystalline phase transition temperature at 545 °C. The annealing temperature range from 400 to 500 °C was found to be suitable for growing ferromagnetic nanoparticles endowed with high saturation magnetization and low coercivity. At higher range of annealing temperature (500–700 °C), XRD confirms the presence of α-Fe2O3 (haematite) phase which is an antiferromagnetic crystalline system with weak magnetization. A systematic decline of magnetization on increasing the annealing temperature beyond 500 °C was attributed to finite size effects and increased purity of antiferromagnetic phase.

Magnetic field and thermal Hall effect in a pyrochlore U(1) quantum spin liquid

The antiferromagnetic system on a rare-earth pyrochlore has been focused as a strong candidate of U(1) quantum spin liquid. Here, we study the phase transitions driven by external magnetic field and discuss the thermal Hall effect due to emergent spinon excitations with staggered gauge fields. Despite the spinons, the charge excitations of the effective action that carry spin-1/2 quantum number, do not couple to the external field, the emergent U(1) gauge field is influenced in the presence of external magnetic field. In particular, along the [111] and [110] directions, we discuss the possible phase transitions between U(1) spin liquids with different gauge fluxes are stabilized in fields. Beyond the cases where the gauge flux per plaquette is fixed to be either 0 or $\ensuremath{\pi}$, there exists a regime where the staggered gauge fluxes are stabilized without time-reversal symmetry. In such a phase, the thermal Hall conductivity ${\ensuremath{\kappa}}_{xy}/T\ensuremath{\sim}4.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{4pt}{0ex}}\text{W}/({\text{K}}^{2}\phantom{\rule{0.16em}{0ex}}\text{m})$ is expected to be measurable below 1 K.

Finite-density massless two-color QCD at the isospin Roberge-Weiss point and the 't Hooft anomaly

We study the phase diagram of two-flavor massless two-color QCD (QC$_2$D) under the presence of quark chemical potentials and imaginary isospin chemical potentials. At the special point of the imaginary isospin chemical potential, called the isospin Roberge--Weiss (RW) point, two-flavor QC$_2$D enjoys the $\mathbb{Z}_2$ center symmetry that acts on both quark flavors and the Polyakov loop. We find a $\mathbb{Z}_2$ 't Hooft anomaly of this system, which involves the $\mathbb{Z}_2$ center symmetry, the baryon-number symmetry, and the isospin chiral symmetry. Anomaly matching, therefore, constrains the possible phase diagram at any temperatures and quark chemical potentials at the isospin RW point, and we compare it with previous results obtained by chiral effective field theory and lattice simulations. We also point out an interesting similarity of two-flavor massless QC$_2$D with $(2+1)$d quantum anti-ferromagnetic systems.

Dynamics of antiferromagnetic skyrmions in the absence or presence of pinning defects

A theoretical study on the dynamics of an antiferromagnetic (AFM) skyrmion is indispensable for revealing the underlying physics and understanding the numerical and experimental observations. In this work, we present a reliable theoretical treatment of the spin current induced motion of an AFM skyrmion in the absence and presence of pinning defect. For an ideal AFM system free of defect, the skyrmion motion velocity as a function of the intrinsic parameters can be derived, based on the concept that the skyrmion profile agrees well with the 360 domain wall formula, leading to an explicit description of the skyrmion dynamics. However, for an AFM lattice containing a defect, the skyrmion can be pinned and the depinning field as a function of damping constant and pinning strength can be described by the Thiele approach. It is revealed that the depinning behavior can be remarkably influenced by the time dependent oscillation of the skyrmion trajectory. The present theory provides a comprehensive scenario for manipulating the dynamics of AFM skyrmion, informative for future spintronic applications based on antiferromagnets.

Quantum walk versus classical wave: Distinguishing ground states of quantum magnets by spacetime dynamics

We investigate the wavepacket spreading after a single spin flip in prototypical two-dimensional ferromagnetic and antiferromagnetic quantum spin systems. We find characteristic spatial magnon density profiles: While the ferromagnet shows a square-shaped pattern reflecting the underlying lattice structure, as exhibited by quantum walkers, the antiferromagnet shows a circular-shaped pattern which hides the lattice structure and instead resembles a classical wave pattern. We trace these fundamentally different behaviors back to the distinctly different magnon energy-momentum dispersion relations and also provide a real-space interpretation. Our findings point to new opportunities for real-time, real-space imaging of quantum magnets both in materials science and in quantum simulators.

Antiferromagnetics: Magnetic‐Field‐Controlled Quantum Critical Points in the Triangular Antiferromagnetic Cs2CuCl4‐xBrx Mixed System (Ann. Phys. 7/2020)

The magnetic phase diagram on the front cover summarizes experimental results reported by Natalija van Well and co-workers in article number 2000147. They use neutron scattering with and without magnetic field to explore magnetic interactions in the triangular antiferromagnetic Cs2CuCl4-xBrx mixed system. The magnetic field results show a separation of two magnetic phases, which occurs above 8.5 T for the composition Cs2CuCl0.3Br3.7.

Neuromorphic computing with antiferromagnetic spintronics

While artificial intelligence, capable of readily addressing cognitive tasks, has transformed technologies and daily lives, there remains a huge gap with biological systems in terms of performance per energy unit. Neuromorphic computing, in which hardware with alternative architectures, circuits, devices, and/or materials is explored, is expected to reduce the gap. Antiferromagnetic spintronics could offer a promising platform for this scheme. Active functionalities of antiferromagnetic systems have been demonstrated recently and several works indicated their potential for biologically inspired computing. In this perspective, we look through the prism of these works and discuss prospects and challenges of antiferromagnetic spintronics for neuromorphic computing. Overview and discussion are given on non-spiking artificial neural networks, spiking neural networks, and reservoir computing.

Current-Induced In-Plane Magnetization Switching in a Biaxial Ferrimagnetic Insulator

Ferrimagnetic insulators (FiMI) have been intensively used in microwave and magneto-optical devices as well as spin caloritronics, where their magnetization direction plays a fundamental role on the device performance. The magnetization is generally switched by applying external magnetic fields. Here we investigate current-induced spin-orbit-torque switching of the magnetization in ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ ($\mathrm{YIG}$)/$\mathrm{Pt}$ bilayers with in-plane magnetic anisotropy, where the switching is detected by spin Hall magnetoresistance. Reversible switching is found at room temperature for a threshold current density of ${10}^{7}\phantom{\rule{0.1em}{0ex}}\mathrm{A}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The $\mathrm{YIG}$ sublattices with antiparallel and unequal magnetic moments are aligned parallel or antiparallel to the direction of current pulses, which is consistent to the N\'eel-order switching in an antiferromagnetic system. It is proposed that such a switching behavior may be triggered by the antidamping torque acting on the two antiparallel sublattices of FiMI. Our finding not only broadens the magnetization switching by electrical means and promotes the understanding of magnetization switching, but also paves the way for all-electrically modulated microwave devices and spin caloritronics with low power consumption.

Oblique incidence infrared reflectance spectroscopy of phonons in cubic MgO, MnO, and NiO

The infrared (IR) reflectivity of the cubic metal oxides MgO, MnO, and NiO has been measured at room temperature using the technique of oblique incidence. The use of this technique at three angles of incidence provides multiple sets of spectra (including both s- and p-polarizations) to analyze when compared to the standard normal incidence case, which has been used extensively in the past for these compounds. It is shown that the transverse optic (TO) and longitudinal optic (LO) mode phonon parameters can be determined with greater accuracy by using the factorized model for the fits, as compared with the popular classical model used previously, and by fitting the derivative of the reflectivity. Our results for the phonon mode parameters are similar to those found earlier, as could be expected, but are generally more precise. An analysis of the difference in frequency of the TO mode in antiferromagnetic NiO at room temperature for the two polarizations of reflected light revealed a TO mode splitting of about 4 cm−1, with the p-polarized light TO mode having the higher frequency: This small splitting is in agreement with theoretical predictions. This more precise IR method for revealing the phonon mode behavior in such magnetically ordered cubic metal oxides, which are prototypes of strongly correlated electronic systems and have been found to be Mott-Hubbard insulators, may be readily applied to any similar antiferromagnetic system.

Domain walls in antiferromagnets: The effect of Dzyaloshinskii–Moriya interactions

We investigate domain walls in antiferromagnets focusing on the effect of Dzyaloshinskii–Moriya interactions (DMIs). Using spin model simulations and analytical arguments within a continuum theory, we show that Dzyaloshinskii–Moriya interactions affect static as well as dynamic properties of the domain wall. For certain configurations of the DMI vectors, the DMI can either tilt the easy plane of the domain wall, an effect that leads to a reduced domain wall width, or it can favor a certain chirality of the domain wall. Depending on the DMI configuration, the DMI may lead to an increasing or decreasing domain wall velocity. The asymmetry of the domain wall velocity observed in ferromagnets subject to DMI cannot be found in antiferromagnetic systems.

Ultrastrong coupling between a microwave resonator and antiferromagnetic resonances of rare-earth ion spins

Quantum magnonics is a new and active research field, leveraging the strong collective coupling between microwaves and magnetically ordered spin systems. To date work in quantum magnonics has focused on transition metals and almost entirely on ferromagnetic resonances in yttrium iron garnet (YIG). Antiferromagnetic systems have gained interest as they produce no stray field, and are therefore robust to magnetic perturbations and have narrow, shape independent resonant linewidths. Here we show the first experimental evidence of ultrastrong-coupling between a microwave cavity and collective antiferromagnetic resonances (magnons) in a rare earth crystal. The combination of the unique optical and spin properties of the rare earths and collective antiferromagnetic order paves the way for novel quantum magnonic applications.

Evidence for weak ferromagnetism and metamagnetic phase transitions in frustrated antiferromagnetic systems with octahedral structure

Possible source of the weak ferromagnetism and the metamagnetic phase transitions in the antiferromagnetic systems with octahedral structure, such as perovskites, is identified in the framework of the antiferromagnetic spin-1/2 Ising model on the octahedral recursive lattice. It is shown that the weak ferromagnetism of the studied antiferromagnetic model is directly related to the competition between two qualitatively different kinds of spin loops (frustrated and bipartite) realized within each elementary octahedron. The free energy of the model is found, the magnetization properties of the model are studied, and the phase diagram of the metamagnetic phase transitions in the external magnetic field is determined. The discrete system of the model ground states is found and discussed. The discontinuity of the susceptibility at the metamagnetic phase transitions is studied. The exact expression for the Néel temperature of the antiferromagnetic system is determined and compare to the Curie temperature of the ferromagnetic system.

Temperature-induced antiferromagnetic interlayer exchange coupling in (Ga,Mn)(As,P)-based trilayer structure

We present the observation of temperature-induced transition between ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange coupling (IEC) in trilayer structures consisting of two (Ga,Mn)(As,P) ferromagnetic layers and a GaAs:Be spacer. Hall resistance measurements were performed on trilayers prepared in as-grown and annealed states. Anomalous Hall resistance (AHR) measurements show two-step hysteresis loops, indicating that coercive fields of the two (Ga,Mn)(As,P) layers in the trilayer are different. Minor loops of AHR show characteristic shifts that correspond to FM IEC at low temperatures and AFM IEC at high temperatures in both as-grown and annealed samples. This temperature-induced transition from FM to AFM IEC occurs at a lower temperature in the annealed sample than in the as-grown sample. We attribute such an enhanced IEC transition effect in the annealed sample to the increase of carrier concentrations obtained by annealing, a characteristic feature of ferromagnetic semiconductors. The observation of AFM IEC in the (Ga,Mn)(As,P)-based trilayer suggests the possibility of realizing synthetic antiferromagnetic systems in such structures, which can be used for antiferromagnetic spintronic device applications.