Magnon Excitations Research Articles

Magnetostructural coupling in RFeO3 (R = Nd, Tb, Eu and Gd)

We investigate the interplay of magnetization and lattice vibrations in rare-earth orthoferrites RFeO3, with a specific focus on non-symmetry-breaking anomalies. To do so, we study the magnetization, magnon excitations and lattice dynamics as a function of temperature in NdFeO3, TbFeO3, EuFeO3 and GdFeO3. The magnetization shows distinct temperature anomalous behavior for all investigated rare-earth orthoferrites, even in the compounds with no phase transitions occurring at those temperatures. Through spin–phonon coupling, these magnetic changes are mirrored by the FeO6 rotation mode for all the studied RFeO3, revealing a common magnetostructural effect associated with the octahedra rotations. The R3+ oscillation modes evidence a Fe3+/R3+ spins cross-talk for the NdFeO3 and TbFeO3 cases. Our work sheds light into the common magnetostructural coupling in rare-earth orthoferrites, and the important role of magnetic anisotropy and spin–orbit coupling strength of the R–Fe interactions on the spin-reorientation transition at high temperatures.

Electric field induced parametric excitation of exchange magnons in a CoFeB/MgO junction

Inspired by the success of field-effect transistors in electronics, electric field controlled magnetization dynamics has emerged as an important integrant in low-power spintronic devices. Here, we demonstrate electric field induced parametric excitation for CoFeB/MgO junctions by using interfacial in-plane magnetic anisotropy (IMA). When the IMA and the external magnetic field are parallel to each other, magnons are efficiently excited by electric field induced parametric resonance. The corresponding wavelengths are estimated to be tuned down to exchange interaction length scales by changing the input power and frequency of the applied voltage. A generalized phenomenological model is developed to explain the underlying role of the electric field torque. Electric field control of IMA is shown to be the origin for excitation of both uniform and parametric resonance modes in the in-plane magnetized sample, a crucial element for purely electric field induced magnetization dynamics. Electric field excitation of exchange magnons, with no Joule heating, offers a good opportunity for developing nanoscale magnonic devices and exploring various nonlinear dynamics in nanomagnetic systems.

Transition of laser-induced terahertz spin currents from torque- to conduction-electron-mediated transport

Spin transport is crucial for future spintronic devices operating at bandwidths up to the terahertz (THz) range. In F|N thin-film stacks made of a ferro/ferrimagnetic layer F and a normal-metal layer N, spin transport is mediated by (1) spin-polarized conduction electrons and/or (2) torque between electron spins. To identify a cross-over from (1) to (2), we study laser-driven spin currents in F|Pt stacks where F consists of model materials with different degrees of electrical conductivity. For the magnetic insulators YIG, GIG and maghemite, identical dynamics is observed. It arises from the THz interfacial spin Seebeck effect (SSE), is fully determined by the relaxation of the electrons in the metal layer and provides an estimate of the spin-mixing conductance of the GIG/Pt interface. Remarkably, in the half-metallic ferrimagnet Fe3O4 (magnetite), our measurements reveal two spin-current components with opposite direction. The slower, positive component exhibits SSE dynamics and is assigned to torque-type magnon excitation of the A- and B-spin sublattices of Fe3O4. The faster, negative component arises from the pyro-spintronic effect and can consistently be assigned to ultrafast demagnetization of e-sublattice minority-spin hopping electrons. This observation supports the magneto-electronic model of Fe3O4. In general, our results provide a new route to the contact-free separation of torque- and conduction-electron-mediated spin currents.

Generalized model of magnon kinetics and subgap magnetic noise

Magnetic noise spectroscopy provides a noninvasive probe of spin dynamics in magnetic materials. We consider two-dimensional magnetically ordered insulators with magnon excitations, especially those supporting long-distance magnon transport, where nitrogen-vacancy (NV) centers enable the access to (nearly) ballistic transport regime of magnons. We develop a generalized theory to describe the magnon transport across a wide range of length scales. The longitudinal dynamic spin susceptibility is derived from the Boltzmann equation and extended to a Lindhard form, which is modified by both the spin-conserving magnon collisions and spin relaxation. Our result is consistent with the diffusive (ballistic) model for the length scale much larger (smaller) than the magnon mean free path, and provides a description for the intermediate regime. We also give a prediction for the NV transition rate in different magnon transport regimes.

Quantum theory of magnon excitation by high energy electron beams

The role of magnon inelastic scattering in high energy electron diffraction of spin unpolarised electron beams, including vortex beams, is investigated theoretically for a Heisenberg ferromagnet. The interaction is between the atomic magnetic dipoles in the specimen and orbital angular momentum (OAM) of the electron beam. Magnon inelastic scattering by vortex beams is allowed despite many atoms along the magnon spin wave experiencing mixed OAM states. The scattering cross-section is however independent of the vortex beam winding number. In the case of planes waves in ferromagnetic iron, the magnon diffuse scattered intensity is significantly smaller than phonons in the energy loss range currently accessible by state-of-the-art monochromated electron energy loss spectroscopy (EELS). Nevertheless, it is shown that the long-range magnetic field of the atomic dipoles has a similar role to dipole scattering in phonon excitation. This means that magnons could, in principle, be detected using aloof beam EELS, where long acquisition times can be realised without any specimen beam damage, an important pre-requisite for detecting the weak magnon signal.

Phonon, magnon and magnetic properties of Cu‐doped NiO nanostructures

AbstractWe have extensively studied the phonon, magnon and magnetic properties of copper (Cu)‐doped nanocrystalline nickel(II) oxide (NiO) using room‐temperature Raman spectroscopy. The required nanomaterials were synthesized using a simple sol–gel method. The phonon modes of NiO nanoparticles have been observed in a large background of magnon excitations, and the wavenumbers of phonon modes were deviated from the theoretical predictions due to the phonon–magnon interaction. The variations of magnon excitations with the Cu doping concentrations were investigated based on reduced spin correlation length and superexchange interaction. The two‐magnon (2M) excitations were suppressed and blue shifted with Cu doping concentration indicating the variations in the magnetic behaviour. The modification of antiferromagnetic nature and the formation of multiphase magnetic properties of NiO nanoparticles were investigated using a vibrating sample magnetometer (VSM) and discussed in detail. The variations in the phonon and magnon excitations can be used to probe the magnetic nature and spintronic applications of NiO nanoparticles.

Ground-state phase diagram and thermodynamics of coupled trimer chains

The density matrix renormalization group and quantum Monte Carlo method are used to describe coupled trimer chains in a magnetic field h. The Hamiltonian contains exchange terms involving the intra-trimer coupling J1 (taken as the unit of energy) and the inter-trimer coupling J2, plus the Zeeman interaction for a magnetic field h along the z direction. Results for the magnetization per trimer m are calculated in regimes of positive and negative values of the ratio J = J2 / J1, from which the rich field-induced ground state phase diagram h vs. J is derived, with the presence of Luttinger liquid, the 1/3-plateau (m = 1/2), and the one of fully polarized magnetization (m = 3/2). Also, the zero-field Lanczos calculation of spin-wave dispersion from the 1/3-plateau for $S^z = 1$ is shown at the previous regimes of J values. In addition, we also report on the decay of correlation functions of trimers along open chains, as well as the average two-magnon distribution. The ground state is ferrimagnetic for $0<J\leq 1$, and is a singlet for $-1\leq J<0$. In the singlet phase, the spin correlation functions along the legs present an antiferromagnetic power-law decay, similar to the spin-1/2 linear chain, thus suggesting that the ground state is made of three coupled antiferromagnetically oriented chains. In the singlet phase, the dimensionless thermal magnetic susceptibility per site normalized by 1/|J| gets closer to $1/\pi^2$ as the temperature $T\rightarrow 0$. For the ferrimagnetic phase, we fit the susceptibility to the experimental data for the compound Pb$_3$Cu$_3$(PO$_4$)$_4$ and estimate the model exchange couplings: J1=74.8 K and J=0.4. These values imply a range of energies for the magnon excitations that are in accord with the data from neutron scattering experiments on Pb$_3$Cu$_3$(PO$_4$)$_4$ for two excitation modes. The 1/3-plateau closes only at 1/|J|=0 with J<0.

Oxygen nonstoichiometry and magnetic properties of doped manganites La0.7Sr0.3Mn0.95Fe0.05O3-δ

In this work, solid solutions of La0.7Sr0.3Mn0.95Fe0.05O3-δ with different oxygen content were obtained by the solid-phase reactions technique. Based on the investigation of the dynamics of changes in the oxygen index (3 – δ) during heating of the samples, the formation of a stressed state in their grains as a result of annealing was established. This results in a decrease in the mobility of oxygen vacancies during the reduction of cations according to the Mn4+ + e– → Mn3+ scheme and explains the decrease of released oxygen amount with an increase of δ as well as the heating rate of the samples. When studying the magnetic properties of the obtained samples, it was found that the temperature dependence of the magnetization obeys the Curie–Weiss law and as the oxygen defficiency increases, the Curie temperature for solid solutions decreases. It was found that the particles are in a frozen ferromagnetic state when measured in the low-temperature region of the М(Т) dependence in “zero-field mode” at Т ˂ ТВ. The presence of ferromagnetism at Т ˃ ТВ leads to a magnetically ordered state, in which the resulting magnetic moment of the magnetic particle is influenced by thermal fluctuations. When considering the temperature values of the magnetization of lanthanum-strontium manganite samples, it was found that with an increase of temperature in the low-temperature region, magnetic ordering is disturbed due to the excitation of magnons with a quadratic dependence of the energy from the wave vector, the number of which increases in proportion to T3/2. This results in a decrease in the manganite magnetization. The observed temperature dependence of the magnetization measured in the “field-cooling mode” was approximated taking into account the quadratic and non-quadratic dispersion laws of the magnon spectrum.

Important Heat Contribution by Tunneling Spin Scattering in Magnetic Tunnel Junction

We investigated the heat contribution by tunneling spin scattering of magnetic tunnel junction in parallel (P) and antiparallel (AP) states using finite element simulation. It showed a maximum temperature increase of 23.2&#x0025; from P to AP, which is even more significant than the heating asymmetry caused by different current directions. A remarkable enhancement of the temperature gradient was confirmed for contributing to the extra thermal spin-transfer torque to the free layer under a specific current direction. Both the contributions of enhanced temperature and temperature gradient should be the possible reasons for the asymmetry of the critical switching current density from P to AP and AP to P. We also extended the research to a double-barrier structure, clarifying its high performance from the aspect of heat generation. Our demonstration may offer efficient tunneling magnon excitation by using the tunneling spin scattering heat.

Multi‐Center Magnon Excitations Open the Entire Brillouin Zone to Terahertz Magnetometry of Quantum Magnets

AbstractDue to the small photon momentum, optical spectroscopy commonly probes magnetic excitations only at the center of the Brillouin zone; however, there are ways to override this restriction. In case of the distorted kagome quantum magnet Y‐kapellasite, Y3Cu9(OH)19Cl8, under scrutiny here, the spin (magnon) density of states (SDOS) can be accessed over the entire Brillouin zone through three‐center magnon excitations. This mechanism is aided by the three different magnetic sublattices and strong short‐range correlations in the distorted kagome lattice. The results of THz time‐domain experiments agree remarkably well with linear spin‐wave theory (LSWT). Relaxing the conventional zone‐center constraint of photons gives a new aspect to probe magnetism in matter.

Optical sensing of magnons via the magnetoelastic displacement

We show how to measure a steady-state magnon population in the magnetostatic mode of a ferromagnet or ferrimagnet, such as yttrium iron garnet. We adopt an optomechanical approach and utilize the magnetoelasticity of the ferromagnet. The magnetostrictive force dispersively couples magnons to the deformation displacement of the ferromagnet, which is proportional to the magnon population. By further coupling the mechanical displacement to an optical cavity that is resonantly driven by a weak laser, the magnetostrictively induced displacement can be sensed by measuring the phase quadrature of the optical field. The phase shows excellent linear dependence on the magnon population for a not very large population and can thus be used as a ``magnometer'' to measure the magnon population. We further study the effect of thermal noises and find a high signal-to-noise ratio even at room temperature. At cryogenic temperatures, the resolution of magnon excitation numbers is essentially limited by the vacuum fluctuations of the phase, which can be significantly improved by using a squeezed light.

A perspective on magnon spin Nernst effect in antiferromagnets

Magnon excitations in antiferromagnetic materials and their physical implications have helped to facilitate the emergence of device concepts not presently available in ferromagnets. A unique characteristic of antiferromagnetic magnons is the coexistence of opposite spin polarization, which mimics the electron spin in a variety of transport phenomena. Among them, the most prominent spin-contrasting phenomenon is the magnon spin Nernst effect (SNE), which refers to the generation of a transverse pure magnon spin current through a longitudinal temperature gradient. We introduce selected recent progress in the study of magnon SNE in collinear antiferromagnets with focus on its underlying physical mechanism entailing profound topological features of magnon band structures. By reviewing how the magnon SNE has inspired and enriched the exploration of topological magnons, we offer our perspective on this emerging frontier that holds potential in future spintronic nano-technology.

Thermally driven magnon valve with perpendicular magnetic anisotropy

The active manipulation of quasiparticles, other than electrons, is a feasible alternative for developing the next generation of devices for information processing. Exploring magnons is advantageous as they can travel far and fast due to their low dissipation and high group velocity, transferring spin without charge transport, thus reducing the Joule heating. Moreover, magnon currents can switch a film's magnetization via a magnon torque facilitated by a perpendicular magnetic anisotropy (PMA). We demonstrate the proof of principle for three states' memories via transport studies of thermally excited magnon currents at room temperature in ferrimagnetic insulating magnon valves TmIG/Au/TmIG with PMA. While varying the relative TmIG magnetizations orientation, magnon currents excited in TmIG films are detected as a voltage in a top platinum electrode film due to the inverse spin Hall effect. The magnon transmission is maximum in the parallel state where the two signals sum up. Possibilities are seen for wave-based nonvolatile magneto-resistive random-access memory, sensing, and logic devices.

Type-II Weyl Excitation in Vortex Arrays

Weyl-like magnon excitations in ordered magnets have attracted significant recent attention. Despite the tantalizing physics and application prospects, the experimental observation of Weyl magnons is still challenging owing to their extraordinarily high frequency that is not accessible to the microstrip antenna technique. Here we predict gigahertz Weyl excitations in the collective dynamics of dipolar-coupled magnetic vortices arranged in a three-dimensional stacked honeycomb lattice. It is found that inversion symmetry breaking leads to the emergence of the type-II Weyl semimetal (WSM) state with tilted dispersion. We derive the full phase diagram of the vortex arrays that support WSMs with both single and double pairs of Weyl nodes, and the topological insulator phase. We observe robust arc surface states in a dual-segment fashion due to the tilted nature of type-II WSMs. Our findings uncover the low-frequency WSM phase in magnetic-texture-based crystals that are indispensable for future Weyltronic applications.

Cavity magnon-polaritons in cuprate parent compounds

Cavity control of quantum matter may offer new ways to study and manipulate many-body systems. A particularly appealing idea is to use cavities to enhance superconductivity, especially in unconventional or high-$T_c$ systems. Motivated by this, we propose a scheme for coupling Terahertz resonators to the antiferromagnetic fluctuations in a cuprate parent compound, which are believed to provide the glue for Cooper pairs in the superconducting phase. First, we derive the interaction between magnon excitations of the Ne\'el-order and polar phonons associated with the planar oxygens. This mode also couples to the cavity electric field, and in the presence of spin-orbit interactions mediates a linear coupling between the cavity and magnons, forming hybridized magnon-polaritons. This hybridization vanishes linearly with photon momentum, implying the need for near-field optical methods, which we analyze within a simple model. We then derive a higher-order coupling between the cavity and magnons which is only present in bilayer systems, but does not rely on spin-orbit coupling. This interaction is found to be large, but only couples to the bimagnon operator. As a result we find a strong, but heavily damped, bimagnon-cavity interaction which produces highly asymmetric cavity line-shapes in the strong-coupling regime. To conclude, we outline several interesting extensions of our theory, including applications to carrier-doped cuprates and other strongly-correlated systems with Terahertz-scale magnetic excitations.

Entanglement of magnon excitations in spin chains

We calculate exactly the entanglement content of magnon excited states in the integrable spin-1/2 XXX and XXZ chains in the scaling limit. In particular, we show that as far as the number of excited magnons with respect to the size of the system is small one can decompose the entanglement content, in the scaling limit, to the sum of the entanglement of particular excited states of free fermionic or bosonic theories. In addition we conjecture that the entanglement content of the generic translational invariant free fermionic and bosonic Hamiltonians can be also classified, in the scaling limit, with respect to the entanglement content of the fermionic and bosonic chains with the number operator as the Hamiltonian in certain circumstances. Our results effectively classify the entanglement content of wide range of integrable spin chains in the scaling limit.

Effective field theory of dark matter direct detection with collective excitations

We develop a framework for computing light dark matter direct detection rates through single phonon and magnon excitations via general effective operators. Our work generalizes previous calculations focused on spin-independent interactions involving the total nucleon and electron numbers $N$ (the usual route to excite phonons) and spin-dependent interactions involving the total electron spin $\mathbit{S}$ (the usual route to excite magnons), leading us to identify new responses involving the orbital angular momenta $\mathbit{L}$, as well as spin-orbit couplings $\mathbit{L}\ensuremath{\bigotimes}\mathbit{S}$ in the target. All four types of responses can excite phonons, while couplings to electron's $\mathbit{S}$ and $\mathbit{L}$ can also excite magnons. We apply the effective field theory approach to a set of well-motivated relativistic benchmark models, including (pseudo) scalar mediated interactions, and models where dark matter interacts via a multipole moment, such as a dark electric dipole, magnetic dipole or anapole moment. We find that couplings to pointlike degrees of freedom $N$ and $\mathbit{S}$ often dominate dark matter detection rates, implying that exotic materials with orbital $\mathbit{L}$ order or large spin-orbit couplings $\mathbit{L}\ensuremath{\bigotimes}\mathbit{S}$ are not necessary to have strong reach to a broad class of DM models. We highlight that phonon based crystal experiments in active R (such as SPICE) will probe light dark matter models well beyond those having a simple spin-independent interaction, including e.g., models with dipole and anapole interactions. Lastly, we make publicly available a code, $\mathsf{PhonoDark}$, which computes single phonon production rates in a wide variety of materials with the effective field theory framework.

Dynamical Mass Generation for Ferromagnetic Skyrmions in Two Dimensions

Magnetic skyrmions are topological magnetization textures that are characterized by the homotopy group of two dimensional spheres. Despite years of intensive research on skyrmions, the fundamental problem of the inertia of a skyrmion in driven motion remains unresolved. By properly taking into account a direct coupling between skyrmion motion and the correspondingly excited magnons, we identify a dynamical mass for the skyrmion in motion. The direct coupling between skyrmion motion and magnons can be employed to engineer skyrmion dynamics with magnons through ingenious material and geometry design.

Oxygen nonstoichiometry and magnetic properties of doped manganites La0.7Sr0.3Mn0.95Fe0.05O3-δ

In this work, solid solutions of La0.7Sr0.3Mn0.95Fe0.05O3-δ with different oxygen content were obtained by the solid-phase reactions technique. Based on the investigation of the dynamics of changes in the oxygen index (3 – δ) during heating of the samples, the formation of a stressed state in their grains as a result of annealing was established. This results in a decrease in the mobility of oxygen vacancies during the reduction of cations according to the Mn4+ + e– → Mn3+ scheme and explains the decrease of released oxygen amount with an increase of δ as well as the heating rate of the samples. When studying the magnetic properties of the obtained samples, it was found that the temperature dependence of the magnetization obeys the Curie–Weiss law and as the oxygen defficiency increases, the Curie temperature for solid solutions decreases. It was found that the particles are in a frozen ferromagnetic state when measured in the low-temperature region of the М (Т) dependence in “zero-field mode” at Т ˂ ТВ. The presence of ferromagnetism at Т ˃ ТВ leads to a magnetically ordered state, in which the resulting magnetic moment of the magnetic particle is influenced by thermal fluctuations. When considering the temperature values of the magnetization of lanthanum-strontium manganite samples, it was found that with an increase of temperature in the low-temperature region, magnetic ordering is disturbed due to the excitation of magnons with a quadratic dependence of the energy from the wave vector, the number of which increases in proportion to T3/2. This results in a decrease in the manganite magnetization. The observed temperature dependence of the magnetization measured in the “field-cooling mode” was approximated taking into account the quadratic and non-quadratic dispersion laws of the magnon spectrum.

Tunable magnon antibunching in a hybrid ferromagnet-superconductor system with two qubits

We explore a scheme to investigate the tunable magnon antibunching characteristics of the conventional magnon blockade (CMB) and the unconventional magnon blockade (UMB) in a hybrid ferromagnet-superconductor system. The magnon excited in the yttrium iron garnet is indirectly coupled to two qubits by eliminating two mutually perpendicular microwave cavity modes. In our paper, we relax the magnon-qubits detuning constraint. In two cases with and without magnon-qubits detuning, it is found that in the strong coupling regime CMB and UMB coexist in our system, and they can be regulated simultaneously by varying the magnon-qubits coupling strength ratio and the qubits detuning ratio. In the weak coupling regime, there exists only the UMB in the system. Without magnon-qubits detuning, the UMB shows a nonmonotonic dependence on the magnon-qubits coupling strength ratio. With magnon-qubits detuning, for the UMB, the magnon bunching effect and antibunching effect can be converted mutually by adjusting the magnon-qubits coupling strength ratio and the qubits detuning ratio. Moreover, changing the magnon-qubits coupling strength ratio and the qubits detuning ratio has a significant impact on the antibunching effect against the thermal magnon excitation. Our research results provide a theoretical scheme for the manipulation of magnon antibunching characteristics, and have important practical significance for the construction of a single magnon source.