Magnon Energy Research Articles

Quantum and classical spin dynamics across temperature scales in the S=1/2 Heisenberg antiferromagnet

Using the framework of semiclassical Landau-Lifshitz dynamics (LLD), we conduct a systematic investigation of the temperature-dependent spin dynamics in the S=1/2 Heisenberg square-lattice antiferromagnet (SAFM). By performing inelastic neutron scattering measurements on Zn2VO(PO4)2 (ZVPO) and corresponding finite-temperature spin dynamics simulations based on LLD, we present a comprehensive analysis that bridges quantum and classical spin dynamics over a broad temperature range. First, remarkable agreement between experimental data and LLD simulations is found in the paramagnetic phase of ZVPO, demonstrating the capability of LLD in accurately determining the spin Hamiltonian of S=1/2 systems and capturing the quantum-to-classical crossover of their spin dynamics. Second, by analyzing the discrepancies between the experimental data and the LLD simulations at lower temperatures, we determine the experimental temperature dependence of the quantum effects in the excitation spectrum of the S=1/2 SAFM: the quantum renormalization factor for the magnon energies and the quantum continuum above the one-magnon bands. Notably, the emergence of each quantum effect is found to correlate with the formation of three-dimensional long-range order. This work demonstrates the utility of LLD in gaining experimental insights into the temperature-induced modifications of quantum spin dynamics and their convergence towards classical expectations at higher temperatures. This motivates further applications to more challenging quantum antiferromagnets dominated by stronger quantum fluctuations. Published by the American Physical Society 2024

[formula omitted]-symmetry induced bi-stability in non-Hermitian cavity magnomechanics

We study the steady-state non-Hermitian magnomechanical system driven by a transverse magnetic field directly interacting with YIG sphere yielding in magnons excitation, which later interacts with cavity microwave photons. To make the system non-Hermitian, we use a traveling field directly interacting with magnons generating gain to the system. We start by illustrating PT-configuration of the system containing two PT broken regions around exceptional point and PT protected region along the axis of exceptional point. We discover that the steady-state population of cavity photons and magnons show bistable behavior depending upon the PT configuration, which becomes more significant as the values of the magnon–photon coupling and traveling field strength increases. We illustrate that steady-state photon only shows bistable behavior when the system in lossy PT broken configuration, means strength of traveling field is less than the magnon–photon coupling. Otherwise, it will just contain a single stable state because of bistability suppression with gain in the system, which is unlike with any other investigation in this direction. Further, a larger magnon–photon coupling increases photon intensity and decreases magnon intensity, because of photon and magnon energy exchange, leading to enhanced photon bistablity and decreased magnon bistability. However, in case of increasing strength of traveling field, both photon as well as magnon bistability is appeared to be decreasing. We also study the steady-state effective potential of the system and illustrate the occurrence of bistability with nonlinear interactions between contour trajectories, which similarly depends on the PT broken configuration of the system.

Emergence of Interfacial Magnetism in Strongly-Correlated Nickelate-Titanate Superlattices.

Strongly-correlated transition-metal oxides are widely known for their various exotic phenomena. This is exemplified by rare-earth nickelates such as LaNiO3, which possess intimate interconnections between their electronic, spin, and lattice degrees of freedom. Their properties can be further enhanced by pairing them in hybrid heterostructures, which can lead to hidden phases and emergent phenomena. An important example is the LaNiO3/LaTiO3 superlattice, where an interlayer electron transfer has been observed from LaTiO3 into LaNiO3 leading to a high-spin state. However, macroscopic emergence of magnetic order associated with this high-spin state has so far not been observed. Here, by using muon spin rotation, x-ray absorption, and resonant inelastic x-ray scattering, direct evidence of an emergent antiferromagnetic order with high magnon energy and exchange interactions at the LaNiO3/LaTiO3 interface is presented. As the magnetism is purely interfacial, a single LaNiO3/LaTiO3 interface can essentially behave as an atomically thin strongly-correlated quasi-2D antiferromagnet, potentially allowing its technological utilization in advanced spintronic devices. Furthermore, its strong quasi-2D magnetic correlations, orbitally-polarized planar ligand holes, and layered superlattice design make its electronic, magnetic, and lattice configurations resemble the precursor states of superconducting cuprates and nickelates, but with an S→1 spin state instead.

Nanoscaled magnon transistor based on stimulated three-magnon splitting

Magnonics is a rapidly growing field, attracting much attention for its potential applications in data transport and processing. Many individual magnonic devices have been proposed and realized in laboratories. However, an integrated magnonic circuit with several separate magnonic elements has yet not been reported due to the lack of a magnonic amplifier to compensate for transport and processing losses. The magnon transistor reported in Chumak et al. [Nat. Commun. 5, 4700 (2014)] could only achieve a gain of 1.8, which is insufficient in many practical cases. Here, we use the stimulated three-magnon splitting phenomenon to numerically propose a concept of magnon transistor in which the energy of the gate magnons at 14.6 GHz is directly pumped into the energy of the source magnons at 4.2 GHz, thus achieving the gain of 9. The structure is based on the 100 nm wide YIG nano-waveguides, a directional coupler is used to mix the source and gate magnons, and a dual-band magnonic crystal is used to filter out the gate and idler magnons at 10.4 GHz frequency. The magnon transistor preserves the phase of the signal, and the design allows integration into a magnon circuit.

On the Mechanical Concept of Self-Assembly of Nanomaterials

The article presents the mechanical concept of self-assembly of nanoparticles. It is assumed that nanoparticles are deformable stamps in a plane dynamic contact problem, lying on the boundary of a multilayer deformable medium. The constant vibration in the microcosm is caused by the oscillatory mode by the energy of phonons and magnons. Earlier, in the works of the authors, the mechanical concept of self-organization of nanoparticles was presented. It is based on high-frequency resonance, which causes the formation of standing waves. They localize the available aggregates of nanoparticles on the crest of standing waves. The self-assembly of nanoparticles is based on resonance, previously predicted by Academician I. I. Vorovich and inherent only in deformable dies in contact problems on a multilayer medium. Deformable nanoparticles are modeled by fractals representing packed block elements described by the Helmholtz equation. The resonance of the deformable dies allows the capture of nanoparticles, dictated by the Coulomb forces of attraction. It is shown that the combination of two fractals generates a new fractal with a combined carrier, and in the case of multiple association, a fragment of a nanomaterial is obtained. To implement the study, for the first time it was possible to construct a high-precision approximate solution of a plane contact problem on the action of a stamp of any finite size on a multilayer base. This result is dictated by the need for an analytical construction of the theory of self-assembly of nanomaterials.

Spin–orbit coupling and magnetism in

With one electron in the degenerate orbital sector, the compound exhibits active orbital degree of freedom, resulting in strongly enhanced orbital and spin-orbital correlations due to Coulomb interaction induced renormalization of the otherwise weak bare spin–orbit coupling (SOC) in this 3d transition metal compound. Finite temperature orbital fluctuations strongly reduce spin-orbital correlations, effective SOC strength, and magnon excitation energy. Orbital and magnetic transition temperatures estimated from the calculated orbiton and (renormalized) magnon energies are in good agreement with the experimental values obtained from susceptibility and resistivity anomalies in recent high-pressure studies.

Two-dimensional intrinsic ferromagnetism in CrBr3 monolayer

The CrBr3 monolayer provides an ideal experimental platform to study the two-dimensional ferromagnetism. In this work, we theoretically study the intrinsic ferromagnetism of CrBr3 monolayer based on the anisotropic Heisenberg model with exchange interactions beyond the nearest neighbor and employing the double-time Green's function method. The Hamiltonian parameter values of the model are taken from the recently reported first-principles calculations. The spontaneous magnetization, Curie temperature, hysteresis loop, and magnon energy spectrum are calculated numerically, and the theoretical results are compared with the available experimental results as much as possible. The calculated Curie temperature of 30.5 K is well consistent with the two experimental measurements of 27 K and 34 K, which in turn support the magnetic parameters obtained from the first-principles calculations. Moreover, the theoretically calculated results of critical exponent describing the critical behavior of spontaneous magnetization and hysteresis loops agree qualitatively well with the experimental measurements.

Magnetic excitations beyond the single- and double-magnons

A photon carrying one unit of angular momentum can change the spin angular momentum of a magnetic system with one unit (ΔMs = ±1) at most. This implies that a two-photon scattering process can manipulate the spin angular momentum of the magnetic system with a maximum of two units. Herein we describe a triple-magnon excitation in α-Fe2O3, which contradicts this conventional wisdom that only 1- and 2-magnon excitations are possible in a resonant inelastic X-ray scattering experiment. We observe an excitation at exactly three times the magnon energy, along with additional excitations at four and five times the magnon energy, suggesting quadruple and quintuple-magnons as well. Guided by theoretical calculations, we reveal how a two-photon scattering process can create exotic higher-rank magnons and the relevance of these quasiparticles for magnon-based applications.

Effect of magnon-magnon interaction on ferromagnetism in hexagonal manganese pnictide monolayers

We have studied the finite-temperature magnetic phase transition using a combination of first-principles calculations and the second-order Holstein-Primakoff approximation of the anisotropic Heisenberg model for hexagonal $\mathrm{Mn}X$ ($X=\mathrm{N}$, P, As, and Sb) monolayers. The $\mathrm{Mn}X$ monolayers are all half-metal with dynamically and thermally stable atomic structures at $T=300$ K. The hexagonal MnN is an out-of-plane easy-axis ferromagnetically ordered monolayer with the Curie temperature close to the room temperature. However, the other three $\mathrm{Mn}X$ ($X=\mathrm{P}$, As, and Sb) monolayers have an easy axis inside the plane with Curie temperature close to zero. The $\mathit{ab}$ initio energy difference between spin configurations is mapped into an anisotropic Heisenberg spin Hamiltonian which is solved using the second-order Holstein-Primakoff approximation. The anharmonic (magnon-magnon) interaction softens the magnetic excitation energy and reduces the magnon energy gap at the $\mathrm{\ensuremath{\Gamma}}$ point, which is crucial for the finite-temperature long-range magnetic order in two dimensions. As a result, the inclusion of magnon-magnon interaction dramatically reduces the Curie temperature of hexagonal $\mathrm{Mn}X$ monolayers. Although the magnon-magnon interaction is a perturbation in energy dispersion, it has an important effect on the finite-temperature long-range magnetic order in two-dimensional monolayers.

Exciton-assisted low-energy magnetic excitations in a photoexcited Mott insulator on a square lattice

The photoexcitation of a Mott insulator on a square lattice weakens the intensity of both single- and two-magnon excitations as observed in time-resolved resonant-inelastic X-ray scattering and time-resolved Raman scattering, respectively. However, the spectral changes in the low-energy regions below the magnons have not yet been clearly understood. To uncover the nature of the photoinduced low-energy magnetic excitations of the Mott insulator, we numerically investigate the transient magnetic dynamics in a photoexcited half-filled Hubbard model on a square lattice. After turning off a pump pulse tuned for an absorption edge, new magnetic signals clearly emerge well below the magnon energy in both single- and two-magnon excitations. We find that low-energy excitations are predominantly created via excitonic states at the absorption edge. These exciton-assisted magnetic excitations may provide a possible explanation for the low-energy spectral weight in a recent time-resolved two-magnon Raman scattering experiment on insulating YBa2Cu3O6.1.

Transversal transport of magnons in a modified Lieb lattice

We studied a two-band magnon insulating model whose geometry is that of a modified Lieb lattice in which one of the sites was removed. Anisotropic ferromagnetic exchange interactions exist between the three nearest neighbors, and the anisotropy opens a gap in the magnon energy band structure. A non-vanishing Berry curvature is induced by a Dzyaloshinskii–Moriya interaction (DMI). The topology of the bands is trivial (in the sense of a null Chern number), but the finite Berry curvature induces Hall-like transport effects whose coefficients were calculated. Their dependence on temperature was studied and shows a resemblance with other magnon insulating systems found in the literature. The dependence on exchange couplings, DMI parameter, and external magnetic field was also investigated.

Magnetization Dynamics in FexCo1-x in Presence of Chemical Disorder

In this paper, we present a theoretical formulation of magnetization dynamics in disordered binary alloys, based on the Kubo linear response theory, interfaced with a seamless combination of three approaches: density functional-based tight-binding linear muffin-tin orbitals, generalized recursion and augmented space formalism. We applied this method to study the magnetization dynamics in chemically disordered FexCo1−x (x = 0.2, 0.5, 0.8) alloys. We found that the magnon energies decreased with an increase in Co concentration. Significant magnon softening was observed in Fe20Co80 at the Brillouin zone boundary. Magnon–electron scattering increased with increasing Co content, which in turn modified the hybridization between the Fe and Co atoms. This reduced the exchange energy between the atoms and softened down the magnon energy. The lowest magnon lifetime was found in Fe50Co50, where disorder was at a maximum. This clearly indicated that the damping of magnon energies in FexCo1−x was governed by hybridization between Fe and Co, whereas the magnon lifetime was controlled by disorder configuration. Our atomistic spin dynamics simulations show reasonable agreement with our theoretical approach in magnon dispersion for different alloy compositions.

Determining Kitaev interaction in spin- S honeycomb Mott insulators

The Kitaev interaction in a honeycomb lattice with higher-spin $S$ has been one of the central attractions, as it may offer quantum spin liquids. A microscopic theory showed that when the Hund's coupling at the transition metal generates $S>\frac{1}{2}$, the spin-orbit coupling at the heavy ligands provides a route to the Kitaev interaction. However, there have been debates over its strength compared to other symmetry-allowed interactions. Investigating the symmetry of the Hamiltonian for general $S$, we show the magnon energies at two momentum points related by a broken mirror symmetry reflect the Kitaev interaction when a magnetic field is in the mirror plane. Applying the symmetry analysis to $\mathrm{Cr}{\mathrm{I}}_{3}$ with $S=\frac{3}{2}$ together with the available angle-dependent ferromagnetic resonance data, we estimate the Kitaev interaction out of the full Hamiltonian and find that it is subdominant. Our theory can be tested by inelastic neutron scattering on candidate materials under the proposed magnetic field direction, which will advance the search for general $S$ Kitaev materials.

Enhancement of Low‐k Spin‐Wave Transmission Efficiency with a Record‐High Group Velocity in YIG/Nonmagnetic Metal Heterojunctions

AbstractImprovement of transmission efficiency of spin waves in magnonic devices is crucial for high‐efficiency magnon‐based operations. However, existing techniques for enhancing spin‐wave detection signals, which either improve waveguide material quality or convert other forms of external energy into magnon energy, are still not efficient and convenient enough. Here, it is experimentally demonstrated that low‐k spin waves (where k is the wavenumber) transmission efficiency can be enhanced through yttrium iron garnet (YIG) surface metallization owing to the significantly increased group velocity and low spin‐pumping efficiency at low k. Given that the shielding effect of the conductive metal on YIG increases the spin‐wave group velocity from 55 to 132 km s−1, the weak magnon‐dissipation mechanism of low‐k spin waves provides the foundation for signal enhancement. Through a detailed theoretical analysis on dispersion relations and spin‐wave amplitude, the detected signal strength is increased to 2 times for copper and 2.24 times for gold is verified. In detail the competition between the spin‐pumping effect is discussed and the enhanced group velocity is introduced by the metal decoration. This competition describes the essence of partial signal enhancement in the spin‐wave transmission spectrum. The work provides a practical way to achieve spin‐wave manipulation and low‐loss transmission.

Room-temperature antiferromagnetic order in monolayer Fe2C

We have studied the room-temperature long-range antiferromagnetic order in the ${\mathrm{Fe}}_{2}\mathrm{C}$ monolayer using the combination of first-principles calculations and the Green's function analysis within the anisotropic Heisenberg model. The ${\mathrm{Fe}}_{2}\mathrm{C}$ monolayer is a semimetal with the out-of-plane antiferromagnetic order between two $\mathrm{Fe}$ planes. The Green's function approach within the random phase approximation is formulated to calculate the temperature-dependent antiferromagnetic magnon energy and the spin correlation function in a two-dimensional antiferromagnetic monolayer. The correlation function is used to evaluate the sublattice magnetization and study the magnetic phase transition in monolayer ${\mathrm{Fe}}_{2}\mathrm{C}$. Moreover, the spin Hamiltonian and the Green's function formalism are developed to investigate the antiferromagnetic ${\mathrm{Fe}}_{2}\mathrm{C}$ monolayer in the presence of an external magnetic field applied along the easy axis. The H-T phase diagram shows that the antiferromagnetic to spin-flop and the spin-flop to paramagnetic phase transitions occur in low temperatures. Finally, we estimate the N\'eel temperature and the critical values of the magnetic field strength for these two phase transitions.

Giant stress response of terahertz magnons in a spin-orbit Mott insulator

Magnonic devices operating at terahertz frequencies offer intriguing prospects for high-speed electronics with minimal energy dissipation However, guiding and manipulating terahertz magnons via external parameters present formidable challenges. Here we report the results of magnetic Raman scattering experiments on the antiferromagnetic spin-orbit Mott insulator Sr2IrO4 under uniaxial stress. We find that the energies of zone-center magnons are extremely stress sensitive: lattice strain of 0.1% increases the magnon energy by 40%. The magnon response is symmetric with respect to the sign of the applied stress (tensile or compressive), but depends strongly on its direction in the IrO2 planes. A theory based on coupling of the spin-orbit-entangled iridium magnetic moments to lattice distortions provides a quantitative explanation of the Raman data and a comprehensive framework for the description of magnon-lattice interactions in magnets with strong spin-orbit coupling. The possibility to efficiently manipulate the propagation of terahertz magnons via external stress opens up multifold design options for reconfigurable magnonic devices.

Thickness dependence of spin Seebeck resistivity in polycrystalline YIG films grown by metal organic decomposition method

In this work, we investigated the dependence of spin Seebeck effect (SSE) voltages on the thickness of yttrium iron garnet (Y3Fe5O12-YIG) thin films grown on a silicon (Si) substrate fabricated by a metal-organic decomposition (MOD) method. The thickness of films was controlled by repeated spin coating and annealing cycles. Based on grazing incidence X-ray diffraction measurements, the YIG thin films displayed high crystallinity without secondary peaks, despite the increase in thickness. Scanning electron microscopy revealed uniform, highly dense films for samples having a thickness in the range of 37–260 nm. Magnetic hysteresis loops showed similar coercivities of <36 Oe for all samples, confirming the high quality of the YIG films, regardless of mismatch between the lattice constants of YIG and the substrate. Spin Seebeck resistivity results revealed a strong dependence on the thickness of the samples and the local maxima for 138-nm-thick YIG. This was attributed to the effect of the magnon energy relaxation length (ξL), which is shorter in comparison to that of a single-crystal YIG film grown on gadolinium gallium garnet. In this study, facile fabrication of hundreds of nanometers-thick YIG films on a Si substrate was achieved, for applications involving thermoelectric energy harvesting.

High-Pressure Tuning of Magnon-Polarons in the Layered Antiferromagnet FePS3.

Magnetic layered materials have emerged recently as promising systems to introduce magnetism in structures based on two-dimensional (2D) materials and to investigate exotic magnetic ground states in the 2D limit. In this work, we apply high hydrostatic pressures up to P ≈ 8.7 GPa to the bulk layered antiferromagnet FePS3 to tune the collective lattice excitations (phonons) in resonance with magnetic excitations (magnons). Close to P = 4 GPa, the magnon-phonon resonance is achieved, and the strong coupling between these collective modes leads to the formation of new quasiparticles, the magnon-polarons, evidenced in our low-temperature Raman scattering experiments by a particular avoided crossing behavior between the phonon and the doubly degenerate antiferromagnetic magnon. At the pressure-induced magnon-phonon resonance, three distinct coupled modes emerge. As it is mainly defined by intralayer properties, we show that the energy of the magnon is nearly pressure-independent. We additionally apply high magnetic fields up to B = 30 T to fully identify and characterize the magnon excitations and to explore the different magnon-polaron regimes for which the phonon has an energy lower than, equal to, or higher than the magnon energy. The description of our experimental data requires introducing a phonon-phonon coupling not taken into account in actual calculations.

Spin-wave theory in a randomly disordered lattice: A Heisenberg ferromagnet

Starting from the hamiltonian for the Heisenberg ferromagnet which comprise randomly distributed nonmagnetic ions as impurities in a Bravais lattice, we express the spin operators by means of the Dyson–Maleev transformation in terms of the Bose operators of the second quantization. Then by using methods of quantum statistical field theory, we derive the partition function and the free energy for the system. We adopt the Matsubara thermal perturbation method to a portion of the hamiltonian which describes the interaction between magnons and the stationary field of nonmagnetic ions. Upon averaging over all possible distributions of impurities, we express the free energy of the system as a function of the mean impurity concentration. Subsequently, we set up the double-time single particle Green function at temperature T in the momentum space in terms of magnon operators and derive the equation of motion for the Green function through the Heisenberg equation of motion and then solve the resulting equation. From this, we calculate the self-energy and then the spectral density function for the system. We apply the formalism to the case of the simple cubic lattice and compute the density of states, the spectral density function and the lifetime of the magnons as a function of energy for several values of the mean concentration of nonmagnetic ions in the ferromagnetic lattice. We calculate the magnon energy spectrum as a function of the average impurity concentration fraction c, which shows that for low lying states, the excitation energy increases continuously with c in the studied range 0.1≤c≤0.7. We also use the spectral density function to compute some thermodynamical quantities through the magnon occupation number. We have obtained closed form expressions for the configurationally averaged physical quantities of interest in a unified fashion as functions of the mean concentration of nonmagnetic impurities c to any order of c applicable below a critical percolation concentration cp. The quantities of interest comprise the thermodynamic potential (free energy), the spin-wave self-energy and the spectral density function from which other quantities can be derived.

Strong momentum-dependent electron–magnon renormalization of a surface resonance on iron

The coupling of electrons to spin excitations and the generation of magnons is essential for spin mixing in the ultrafast magnetization dynamics of 3d ferromagnets. Although magnon energies are generally much larger than phonon energies, until now their electronic band renormalization effect in 3d ferromagnets suggests a significantly weaker quasiparticle interaction. Using spin- and angle-resolved photoemission, we show an extraordinarily strong renormalization leading to two-branch splitting of an iron surface resonance at ∼200 meV. Its strong magnetic linear dichroism unveils the magnetic nature and momentum dependence of the energy renormalization. By determining the frequency- and momentum-dependent self-energy due to generic electron–boson interaction to compute the resultant electron spectral function, we suggest that the surface-state splitting can be described by strong coupling to an optical spin wave in an iron thin film.