Excitations In Ferromagnet Research Articles

On-Chip Coherent Transduction between Magnons and Acoustic Phonons in Cavity Magnomechanics

Magnons, namely spin waves, are collective spin excitations in ferromagnets, and their control through coupling with other excitations is a key technology for future hybrid spintronic devices. Although strong coupling has been demonstrated with microwave photonic structures, an alternative approach permitting high density integration and minimized electromagnetic crosstalk is required. Here we report a planar cavity magnomechanical system, where the cavity of surface acoustic waves enhances the spatial and spectral power density to thus implement magnon-phonon coupling at room temperature. Excitation of spin-wave resonance involves significant acoustic power absorption, whereas the collective spin motion reversely exerts a back-action force on the cavity dynamics. The cavity frequency and quality-factor are significantly modified by the back-action effect, and the resultant cooperativity exceeds unity, suggesting coherent interaction between magnons and phonons. The demonstration of a chip-scale magnomechanical system paves the way to the development of novel spin-acoustic technologies for classical and quantum applications.

Topological Spin Excitations in Honeycomb Ferromagnet .

In two-dimensional honeycomb ferromagnets, bosonic magnon quasiparticles (spin waves) may either behave as massless Dirac fermions or form topologically protected edge states. The key ingredient defining their nature is the next-nearest-neighbor Dzyaloshinskii-Moriya interaction that breaks the inversion symmetry of the lattice and discriminates chirality of the associated spin-wave excitations. Using inelastic neutron scattering, we find that spin waves of the insulating honeycomb ferromagnet ( ) have two distinctive bands of ferromagnetic excitations separated by a ∼4 meV gap at the Dirac points. These results can only be understood by considering a Heisenberg Hamiltonian with Dzyaloshinskii-Moriya interaction, thus providing experimental evidence that spin waves in can have robust topological properties potentially useful for dissipationless spintronic applications.

Novel nonlinear excitations in ferromagnet excited by all-magnonic spin-transfer torque

We report the novel nonlinear excitations, such as Akhmediev breathers solution and Kuznetsov-Ma soliton caused by a spin wave passing through a magnetic soliton. The former case demonstrates a spatial periodic process of a magnetic soliton. The other case shows a localized process of the spin-wave background. In the limit case, we get the novel rogue waves with high magnon density distribution and clarify its formation mechanism.

Spatially resolved detection of complex ferromagnetic dynamics using optically detected nitrogen-vacancy spins

We demonstrate optical detection of a broad spectrum of ferromagnetic excitations using nitrogen-vacancy (NV) centers in an ensemble of nanodiamonds. Our recently developed approach exploits a straightforward CW detection scheme using readily available diamond detectors, making it easily implementable. The NV center is a local detector, giving the technique spatial resolution, which here is defined by our laser spot, but in principle can be extended far into the nanoscale. Among the excitations, we observe the propagating dipolar and dipolar-exchange spinwaves, as well as dynamics associated with the multi-domain state of the ferromagnet at low fields. These results offer an approach, distinct from commonly used optically detected magnetic resonance techniques, for spatially resolved spectroscopic study of magnetization dynamics at the nanoscale.

Antiferromagnetic Skyrmion: Stability, Creation and Manipulation.

Magnetic skyrmions are particle-like topological excitations in ferromagnets, which have the topo-logical number Q = ± 1, and hence show the skyrmion Hall effect (SkHE) due to the Magnus force effect originating from the topology. Here, we propose the counterpart of the magnetic skyrmion in the antiferromagnetic (AFM) system, that is, the AFM skyrmion, which is topologically protected but without showing the SkHE. Two approaches for creating the AFM skyrmion have been described based on micromagnetic lattice simulations: (i) by injecting a vertical spin-polarized current to a nanodisk with the AFM ground state; (ii) by converting an AFM domain-wall pair in a nanowire junction. It is demonstrated that the AFM skyrmion, driven by the spin-polarized current, can move straightly over long distance, benefiting from the absence of the SkHE. Our results will open a new strategy on designing the novel spintronic devices based on AFM materials.

Spin Seebeck Power Conversion

Spin caloritronics is the science and technology to control spin, charge, and heat currents in magnetic nanostructures. The spin degree of freedom provides new strategies for thermolelectric power generation that have not yet been fully explored. After an elementary introduction into conventional thermoelectrics and spintronics, we give a brief review of the physics of spin caloritronics. We discuss spin-dependent thermoelectrics based on the the two-current model in metallic magnets as well as the spin Seebeck and Peltier effects that are based on spin wave excitations in ferromagnets. We derive expressions for the efficiency and figure of merit ZT of several spin caloritronic devices.

Magnetic skyrmion logic gates: conversion, duplication and merging of skyrmions.

Magnetic skyrmions, which are topological particle-like excitations in ferromagnets, have attracted a lot of attention recently. Skyrmionics is an attempt to use magnetic skyrmions as information carriers in next generation spintronic devices. Proposals of manipulations and operations of skyrmions are highly desired. Here, we show that the conversion, duplication and merging of isolated skyrmions with different chirality and topology are possible all in one system. We also demonstrate the conversion of a skyrmion into another form of a skyrmion, i.e., a bimeron. We design spin logic gates such as the AND and OR gates based on manipulations of skyrmions. These results provide important guidelines for utilizing the topology of nanoscale spin textures as information carriers in novel magnetic sensors and spin logic devices.

Landau–Zener tunneling in multiferroic composites

For a coupled ferroelectric/ferromagnetic composite we demonstrate theoretically a mechanism for ferroelectric–ferromagnetic excitation conversion. The switching occurs upon sweeping an external homogeneous magnetic field that modifies the spectrum. We show analytically and by direct numerical simulations that the excitation transfer proceeds via Landau–Zener tunneling: in the case of an abrupt application of the magnetic field (a diabatic regime) there is no transition of excitation energy between the electric and magnetic systems, while increasing the magnetic field slowly (an adiabatic process) an almost complete excitation transfer takes place.

Odd- and Even-Frequency Superconductivities Mediated by Ferromagnetic Magnons

On the basis of a generalized Kondo-lattice model (i.e., spin-fermion model), we examine herein superconductivity mediated by magnetic excitations in ferromagnets. By applying the perturbation theory, we derive effective pairing interactions between conduction electrons. Pairing fluctuations are numerically calculated by fully accounting for the momentum and frequency dependences in quasi-one dimension. The results indicate that the normal state is unstable against even- and odd-frequency pairing fluctuations, depending on the temperature and model parameters. In a few cases, we obtain phase diagrams. The two-magnon exchange interaction is found to favor odd-frequency pairing at high temperatures (but below the ferromagnetic transition temperature); however, at low temperatures, triangle processes involving two magnons and one electron induce even- and odd-frequency pairings depending on the signs of the coupling constants of the Kondo interaction. Finally, we discuss the possibility of odd-frequency supe...

Intense low-energy ferromagnetic fluctuations in the antiferromagnetic heavy-fermion metal CeB6.

Heavy-fermion metals exhibit a plethora of low-temperature ordering phenomena . Among these are the so-called hidden-order phases that, in contrast to conventional magnetic order, are invisible to standard neutron diffraction experiments. One of the structurally most simple hidden-order compounds, CeB6, has been intensively studied for an elusive phase that was attributed to the antiferroquadrupolar ordering of cerium-4f moments . As the ground state of CeB6 is characterized by a more conventional antiferromagnetic (AFM) order , the low-temperature physics of this system has generally been assumed to be governed solely by AFM interactions between the dipolar and multipolar Ce moments . Here we overturn this established picture by observing an intense ferromagnetic (FM) low-energy collective mode that dominates the magnetic excitation spectrum of CeB6. Inelastic neutron-scattering data reveal that the intensity of this FM excitation significantly exceeds that of conventional spin-wave magnons emanating from the AFM wavevectors, thus placing CeB6 much closer to a FM instability than previously anticipated. This propensity for ferromagnetism may account for much of the unexplained behaviour of CeB6, and should lead to a re-examination of existing theories that have so far largely neglected the role of FM interactions.

Temperature gradient assisted magnetodynamics

Energy excitations in ferromagnets are studied in case a temperature gradient is coupled to the local magnetization. Due to different time scales only the coupling between the spatially varying part of the temperature field and the magnetization is relevant. The magnetothermal coupling with a definite sign breaks the time inversion symmetry and lead to damped spin waves. Applying the model to nanowires it is demonstrated that the energy spectrum is significantly affected by the boundary conditions as well as the initial temperature distribution. The magnetothermal coupling is several orders of magnitude stronger for open wires compared to isolated ones.

Studies on the superconducting state of NaxCoO2·yH2O – Overview

Results of experimental studies carried out to clarify the superconducting state of NaxCoO2·yH2O (x∼0.3; y∼1.3) are presented. Various means such as NMR, neutron magnetic scattering, and specific-heats and magnetic-susceptibility measurements have been applied to both poly- and single-crystalline samples to obtain internally consistent data. Effects of non-magnetic impurities and oxygen isotope substitution on the superconducting transition temperature Tc have also been studied. Data obtained for the mother system NaxCoO2 have also presented useful information on the superconducting state of NaxCoO2·yH2O. In the course of these studies, we have shown by measuring Knight shifts of 59Co and 23Na of aligned crytalline samples that the Cooper pairs are in the singlet state. Neutron inelastic scattering measurements on large single crystals have presented the firm evidence for the disappearance of the low energy ferromagnetic excitation with decreasing T, excluding the triplet Cooper pairing in NaxCoO2·yH2O consistently with the Knight shift. In the Tc–νQ phase diagram, it has been confirmed that there is a nonsuperconducting phase, which divides the superconducting region into two, νQ being the nuclear quadrupole frequency. The possible different symmetries between the two superconducting phases suggested by this characteristic νQ dependence shown in the phase diagram, has been carefully examined mainly by specific heat measurements and NMR. However, any difference between the pair states of these phases has not been found. The appearance of the nonsuperconducting phase should be considered due to an instability which appears within a single superconducting phase.

Ferromagnetic Spin Coupling as the Origin of 0.7 Anomaly in Quantum Point Contacts

We study one-dimensional itinerant electron models with ferromagnetic coupling to investigate the origin of the 0.7 anomaly in quantum point contacts. Linear conductance calculations from the quantum Monte Carlo technique for spin interactions of different spatial range suggest that 0.7(2e;{2}/h) anomaly results from a strong interaction of low-density conduction electrons to ferromagnetic fluctuations formed across the potential barrier. The conductance plateau appears due to the strong incoherent scattering at high temperature when the electron traversal time matches the time scale of dynamic ferromagnetic excitations.

Skyrmion Excitations in Planar Ferromagnets

By making use of the ϕ-mapping topological current theory and the decomposition of gauge potential theory, we investigate the (2+1)-dimensional skyrmion excitations in ferromagnets. We also discuss the branch processes of these skyrmions and the generation and annihilation of skyrmion-antiskyrmion pairs.

Laser-Induced Orbital and Spin Excitations in Ferromagnets: Insights from a Two-Level System

A recent time-resolved measurement showed that laser-induced orbital and spin excitations proceed in unison and the spin-orbit ratio is held constant during demagnetization. Here a two-level model shows that these orbital and spin excitations originate from state population and state interference effects. For an addressed state, spin and orbital dynamics are solely from the state interference, where the spin and orbital momenta oscillate with the laser frequency and match the dipole moment exactly, an unambiguous test case for the time-resolved magneto-optical Kerr effect. For an undressed state, the interference effect introduces a rapid beating in orbital momentum, which is observed in the first-principles calculation in fcc Ni. The state population change leads to a constant spin-orbit ratio, which explains the linear dependence between spin and orbital momentum changes within 2 ps upon the arrival of a pump pulse in Fe.

Magnetic mechanism of quasiparticle pairing in hole-doped cuprate superconductors

We have computed ${\ensuremath{\alpha}}^{2}F$'s for the hole-doped cuprates within the framework of the one-band Hubbard model, where the full magnetic response of the system is treated properly. The $d$-wave pairing weight ${\ensuremath{\alpha}}^{2}{F}_{d}$ is found not only to contain a low-energy peak due to excitations near $(\ensuremath{\pi},\ensuremath{\pi})$ expected from neutron-scattering data but also to display substantial spectral weight at higher energies due to contributions from other parts of the Brillouin zone as well as pair-breaking ferromagnetic excitations at low energies. The resulting solutions of the Eliashberg equations yield transition temperatures and gaps comparable to the experimentally observed values, suggesting that magnetic excitations of both high and low energies play an important role in providing the pairing glue in the cuprates.

Excitation spectrum gap and spin-wave velocity ofXXZHeisenberg chains: Global renormalization-group calculation

The anisotropic $XXZ$ spin-$\frac{1}{2}$ Heisenberg chain is studied using renormalization-group theory. The specific heats and nearest-neighbor spin-spin correlations are calculated throughout the entire temperature and anisotropy ranges in both ferromagnetic and antiferromagnetic regions, obtaining a global description and quantitative results. We obtain, for all anisotropies, the antiferromagnetic spin-liquid spin-wave velocity and the Ising-like ferromagnetic excitation spectrum gap, exhibiting the spin-wave to spinon crossover. A number of characteristics of purely quantum nature are found: The in-plane interaction ${s}_{i}^{x}{s}_{j}^{x}+{s}_{i}^{y}{s}_{j}^{y}$ induces an antiferromagnetic correlation in the out-of-plane ${s}_{i}^{z}$ component, at higher temperatures in the antiferromagnetic $XXZ$ chain, dominantly at low temperatures in the ferromagnetic $XXZ$ chain, and, in-between, at all temperatures in the $XY$ chain. We find that the converse effect also occurs in the antiferromagnetic $XXZ$ chain: an antiferromagnetic ${s}_{i}^{z}{s}_{j}^{z}$ interaction induces a correlation in the ${s}_{i}^{xy}$ component. As another purely quantum effect, (i) in the antiferromagnet, the value of the specific heat peak is insensitive to anisotropy and the temperature of the specific heat peak decreases from the isotropic (Heisenberg) with introduction of either type (Ising or $XY$) of anisotropy; and (ii) in complete contrast, in the ferromagnet, the value and temperature of the specific heat peak increase with either type of anisotropy.

Antiferromagnetic correlation in the half-filled strongly correlated electron models at nonzero temperature: A rigorous result

As an extension of our previous rigorous investigation on the spin correlations in the ground states of the half-filled Hubbard model and the periodic Anderson model [G. S. Tian, Phys. Rev. B 50, 6246 (1994)], in the present paper we study the behavior of these correlation functions at finite temperature. We show rigorously that, at any $T\ensuremath{\ne}0,$ the predominant spin correlations in these systems are antiferromagnetic. Furthermore, based on this result, we also show that a quasi-one (or two)-dimensional itinerant electron ferrimagnet must have a gapless branch of ferromagnetic excitations. This conclusion is consistent with the previous results derived by the spin-wave theory.

Long wavelength spin dynamics in La0.53Ca0.47MnO3

The magnetic structure and dynamics in the colossal magnetoresistance (CMR) class perovskite La0.53Ca0.47MnO3 have been studied by elastic and inelastic neutron scattering. This composition is near the 0.5 Ca transition from a metallic ferromagnet to an insulating antiferromagnet. Powder neutron diffraction on these samples showed coexisting ferromagnetic and antiferromagnetic phases at low temperature and a splitting of the lattice parameters of the antiferromagnetic phase near TN, reflecting the onset of the charge ordered state. Inelastic scattering measurements of the ferromagnetic excitations exhibited well-defined spin waves of resolution width below Tc. The spin waves exhibited conventional Heisenberg behavior with dispersion of the form E=Dq2+Δ with spin stiffness D(T=0 K)≈135 meV Å2 and an energy gap Δ≈0.1 meV. The value of the spin wave stiffness D is similar to that found for other ferromagnetic perovskite materials, and it renormalized with temperature in a manner consistent with the magnetization.

Microscopic theory of magnetic excitations in ferromagnets with random anisotropy axes

We present a microscopic theory of the harmonic magnetic excitations in ferromagnets with random anisotropy axes. The theory is analogous to the theory of excitations in spin glasses developed by Walker and Walstedt, with the difference that the disorder arises from the anisotropy rather than the exchange interaction. We report the results of numerical calculations of magnon density of states and the ferromagnetic resonance frequency for a model system where the spins occupy sites on a fcc lattice. The exchange interaction is between nearest neighbors, and the anisotropy axis varies randomly from site to site with no correlation between different sites. Data are obtained with a range of values of the ratio of the anisotropy to the exchange interaction in both the zero-field and high-field limits.