Magnetoelastic Modes Research Articles

Effect of magnon dissipation in the caustic pattern of magnetoelastic waves in ferromagnets

The effects of magnon dissipation on the properties of magnetoelastic waves in ferromagnets in the vicinity of magnetoelastic resonance are investigated. The magnetoelastic waves in this region are strongly anisotropic, and the directions with high intensity (caustic waves) are formed. Dissipation leads to the disappearance of the gap between different magnetoelastic modes, finiteness of amplification factor, and to the damping of caustic waves in some directions. The caustic patterns formed by the caustic waves are transformed due to dissipation. In elastically isotropic and slightly anisotropic ferromagnets, caustics disappear when even a small dissipation is taken into account, while in strongly elastically anisotropic ferromagnets the main features of caustic patterns are retained in a wide region of the external magnetic field. • Magnetoelastic waves in ferromagnets form caustics near magnetoelastic resonance. • Set of caustic directions form caustic patterns. • Structure of caustic patterns is determined by magnitude of constant magnetic field. • Even a small dissipation can destroy caustics in elastically isotropic ferromagnets. • In anisotropic ferromagnets, dissipation retain main features of caustic pattern.

Thermoelastic enhancement of the magnonic spin Seebeck effect in thin films and bulk samples

A non-uniform temperature profile may generate a pure spin current in magnetic films, as observed for instance in the spin Seebeck effect. In addition, thermally induced elastic deformations may set in that could affect the spin current. A self-consistent theory of the magnonic spin Seebeck effect including thermally activated magneto-elastic effects is presented and analytical expressions for the thermally activated deformation tensor and dispersion relations for coupled magneto-elastic modes are obtained. We derived analytical results for bulk (3D) systems and thin magnetic (2D) films. We observed that the displacement vector and the deformation tensor in bulk systems decay asymptotically as $u\sim1/R^{2}$ and $\varepsilon\sim1/R^{3}$, respectively, while the decays in thin magnetic films proceed slower following $u\sim1/R$ and $\varepsilon\sim1/R^{2}$. The dispersion relations evidence a strong anisotropy in the magnetic excitations. We observed that a thermoelastic steady state deformation may lead to both an enchantment or a reduction of the gap in the magnonic spectrum. The reduction of the gap increases the number of magnons contributing to the spin Seebeck effect and offers new possibilities for the thermoelastic control of the Spin Seebeck effect.

Review of Ultrafast Demagnetization After Femtosecond Laser Pulses: A Complex Interaction of Light with Quantum Matter

When a magnetic film is excited by a femtosecond laser pulse, either with THz or with optical frequencies, then there is at least a partial demagnetization within a few hundred femtoseconds, followed by a remagnetization to the original state on a bit longer time scale. This phenomenon is caused by a complex interaction of light with quantum matter. This paper gives a review of the present knowledge of the underlying physics. It discusses first the situation of a direct change of the magnetization by its interaction with the electromagnetic wave of the laser pulse, which appears during THz laser pulses with small field amplitudes. Then it considers the situation of an indirect change which appears after THz laser pulses with large field amplitudes and after optical laser pulses. In these cases the laser photons primarily excite electrons, with subsequent modifications of their spin-angular momenta by spin-flip scatterings of these electrons at quasiparticles, either at other electrons or at phonons or at magnons. The contributions of these various spin-flip scatterings to demagnetization are investigated. Then the transfer of angular momentum from the electronic spin system to the lattice during ultrafast demagnetization is discussed by describing the lattice vibrations in terms of magnetoelastic spin-phonon modes. Finally, the effect of electronic correlations in the sense of the density-matrix theory is investigated.

Role of nuclear physics in oscillations of magnetars

Strong magnetic fields have important effects on the crustal properties of magnetars. Here we study the magneto-elastic oscillations of magnetars taking into consideration the effect of strong magnetic fields on the crustal composition (magnetised crust). We calculate global magneto-elastic (GME) modes as well as modes confined to the crust (CME) only. The composition of the crust directly affects its shear modulus which we calculate using three different nucleon-nucleon interactions: SLy4, SkM and Sk272. The shear modulus of the crust is found to be enhanced in strong magnetic fields $\geq 10^{17}$ G for all those Skyrme interactions. It is noted that the shear modulus of the crust for the SLy4 interaction is much higher than those of the SkM and Sk272 interactions in presence of magnetic fields or not. Though we do not find any appreciable change in frequencies of fundamental GME and CME modes with and without magnetised crusts, frequencies of first overtones of CME modes are significantly affected in strong magnetic fields $\geq 10^{17}$ G. However, this feature is not observed in frequencies of first overtones of GME modes. As in earlier studies, it is also noted that the effects of crusts on frequencies of both types of maneto-elastic modes disappear when the magnetic field reaches the critical field ($B > 4 \times 10^{15}$ G). Frequencies of GME and CME modes calculated with magnetised crusts based on all three nucleon-nucleon interactions, stellar models and magnetic fields, are compared with frequencies of observed quasi-periodic oscillations (QPOS) in SGR 1806-20 and SGR1900+14. As in earlier studies, this comparison indicates that GME modes are essential to explain all the frequencies as CME modes can explain only the higher frequencies.

Ultrafast demagnetization after femtosecond laser pulses: Transfer of angular momentum from the electronic system to magnetoelastic spin-phonon modes

During ultrafast demagnetization after the excitation of ferromagnetic films with femtosecond laser pulses, the angular momentum of the electronic system is transferred to the lattice via electron-phonon scatterings. The actual amount of transfer is calculated for Ni and Fe by considering spin-phonon eigenmodes, which have a sharp angular momentum. Because the considered Hamiltonian is not isotropic, the total angular momentum is not conserved.

A critical analysis of the feasibility of pure strain-actuated giant magnetostrictive nanoscale memories

Concepts for memories based on the manipulation of giant magnetostrictive nanomagnets by stress pulses have garnered recent attention due to their potential for ultra-low energy operation in the high storage density limit. Here, we discuss the feasibility of making such memories in light of the fact that the Gilbert damping of such materials is typically quite high. We report the results of numerical simulations for several classes of toggle precessional and non-toggle dissipative magnetoelastic switching modes. Material candidates for each of the several classes are analyzed and forms for the anisotropy energy density and ranges of material parameters appropriate for each material class are employed. Our study indicates that the Gilbert damping as well as the anisotropy and demagnetization energies are all crucial for determining the feasibility of magnetoelastic toggle-mode precessional switching schemes. The roles of thermal stability and thermal fluctuations for stress-pulse switching of giant magnetostrictive nanomagnets are also discussed in detail and are shown to be important in the viability, design, and footprint of magnetostrictive switching schemes.

Magnetoelastic modes and lifetime of magnons in thin yttrium iron garnet films

We calculate the effects of the spin-lattice coupling on the magnon spectrum of thin ferromagnetic films consisting of the magnetic insulator yttrium-iron garnet. The magnon-phonon hybridisation generates a characteristic minimum in the spin dynamic structure factor which quantitatively agrees with recent Brillouin light scattering experiments. We also show that at room temperature the phonon contribution to the magnon damping exhibits a rather complicated momentum dependence: In the exchange regime the magnon damping is dominated by Cherenkov type scattering processes, while in the long-wavelength dipolar regime these processes are subdominant and the magnon damping is two orders of magnitude smaller. We supplement our calculations by actual measurements of the magnon relaxation in the dipolar regime. Our theory provides a simple explanation of a recent experiment probing the different temperatures of the magnon and phonon gases in yttrium-iron garnet.

Magnetostriction and thermal expansion of HoFe11−xCoxTi intermetallic compounds

The thermal expansion and magnetostriction of the HoFe11−xCoxTi (0≤x≤11) alloys have been investigated, using the strain gauge technique in the temperature range 77–600K. Both thermal expansion and thermal expansion coefficient exhibit an anomalous behaviour and Invar effect below Curie temperature (TC=516K) in sample with x=0. The increase of x in HoFe11−xCoxTi system leads to disappearing of the Invar effect and increasing of the average thermal expansion coefficient from 6.370×10−6K−1 in x=0 to 10.735×10−6K−1 in x=11 below room temperature. In addition, the spontaneous volumemagnetostriction decreases with Co content. The magnetostriction compensation point is observed in the anisotropic magnetostriction curve of all samples. The maximum value of anisotropic magnetostriction (Δλ≈50×10−6) at room temperature is observed in sample with x=9. The saturation behaviour only appears in samples with x=5, 7 and 9. In samples with x=3, 5 and 7, a noticeable volume magnetostriction (ΔV/V) is observed in low temperature due to first-order magnetisation process. Moreover, ΔV/V exhibits a large anomaly about 45×10−6 and 20×10−6 around room temperature related to spin reorientation transition in samples with x=9 and 11, respectively. The results are discussed based on the local magnetic moment model and irreducible magnetoelastic coupling modes.

Thermal expansion anomaly and magnetostriction of Nd2Fe14Si3 intermetallic compound

The thermal expansion and magnetostriction of Nd2Fe14Si3 polycrystalline intermetallic compound was measured, using the strain gauge method in the temperature range 77–590K under applied magnetic fields up to 1.5T. Results show that both linear thermal expansion and linear thermal expansion coefficient exhibit anomalies below Curie temperature. A pronounced positive spontaneous volume magnetostriction has been observed below the Curie temperature. The spontaneous volume magnetostriction decreases with Si substitution in Nd2Fe17 compound. The saturation trend of the anisotropic magnetostriction below 115K depends on the saturation of the λγ mode. In the low field region (μ0H<0.3T), a minimum appears in the isothermal curves of the volume magnetostriction that may be attributed to the domain walls motion and rotation of the magnetic moments of domains toward the applied field direction. The results of the spontaneous and anisotropic magnetostriction are discussed based on the local magnetic moment model and irreducible magnetoelastic coupling modes, respectively.

Present status of theoretical modeling the magnetoelectric effect in magnetostrictive-piezoelectric nanostructures. Part II: Magnetic and magnetoacoustic resonance ranges

We presented here the theoretical analysis of high frequency magnetoelectric (ME) effects for a ferrite-piezoelectric bilayer and a detailed treatment for electric field induced resonance field shift for ferromagnetic resonance (FMR) in layered structures. ME effects in a single-crystal ferrite-piezoelectric bilayer in the magnetoelastic resonance region are considered. The theory predicts a giant ME effect at magnetoacoustic resonance. The enhancement in ME effect predicted by our theory arises from interaction between elastic modes and the uniform precession mode, resulting in magnetoelastic modes. The peak ME voltage coefficient appears at the coincidence of acoustic resonance and FMR frequencies. In our calculations, we suppose that the layer thickness is sufficiently large to neglect the influence of strain relaxation on average stresses in the structures that determine the ME voltage coefficient. The work presented here will certainly be of interest for the design and analysis of electrically controlled high-frequency devices. Microwave devices of magnetic type with electrical control have unique advantages over traditional ferrite and semiconductor analogs.

On the Magnetic Field Effect in Electroconductive Plates Under Nonconservative Loading

This work investigates the behavior of an electroconductive plate under the action of a nonconservative load and subjected to a transversal magnetic field. The governing equation of the bending vibrations of an electroconductive plate, subjected to a transverse magnetic field and a follower type force at one edge, is presented. The assumption of an elongated plate leads to a simplified equation, which is conveniently written in dimensionless terms. For a cantilevered configuration, the characteristic equation relative to the magnetoelastic modes of vibration of the system is derived. Approximate solutions based on Galerkin method and an adjoint formulation are also presented and compared with the semi-analytical results. Root loci plots are computed as a function of the proper dimensionless parameters. The behavior of the system is very similar to the one exhibited by other structures subjected to nonconservative loads when damping is present. A relaxed definition of stability is used to regain continuity in the instability envelope.

Domain fluctuations near the field-induced incommensurate-commensurate phase transition ofTbMnO3

We present temperature- and field-dependent inelastic light-scattering studies of multiferroic ${\text{TbMnO}}_{3}$. By carefully examining the evolution of magneto-elastic modes in various phases of ${\text{TbMnO}}_{3}$, our study reveals several features of the field-induced incommensurate-commensurate (IC-C) transition in ${\text{TbMnO}}_{3}$. We find that, for fields applied along the $b$ axis, there is a coexistence of distinct structural phases in the intermediate field regime $(H=4--7\text{ }\text{T})$ around the IC-C-phase transition. We present evidence for the existence of dynamical fluctuations of the C phase for fields lower than the critical field for $\mathbf{H}$ along the $b$ axis, $H&lt;{H}_{c}^{b}$. Furthermore, we present evidence for strong spin-lattice coupling effects in ${\text{TbMnO}}_{3}$ in the form of zone-boundary phonon modes that strongly couple to the spins by modulating the exchange interaction in this material.

Microwave magnetoelectric interactions in ferrite–piezoelectric nanobilayers: Theory of electric field induced magnetic excitations

A theory for magnetic excitations in a yttrium iron garnet (YIG)–lead zirconate titanate (PZT) nanobilayer due to microwave electric field and magnetoelectric (ME) interactions is discussed. The magnetic response is described by ME susceptibility and a technique has been proposed for its determination. The electric field excites elastic modes in PZT that would result in magnetoelastic modes in YIG. The model predicts maximum ME susceptibility when the microwave field is at the coincidence of electromechanical and magnetic resonance. The theory is of importance for new devices such as magnetoelectric spin-acoustic wave generators and power limiters.

Observation of indirect parallel pumping of magneto-elastic modes in layered YIG/GGG structures

In this work we report new experimental results on nonlinear excitation of magnetoelastic (ME) modes in layered YIG/GGG waveguide structures at GHz frequencies, obtained by a guided-wave light scattering technique. It is shown that the fundamental spin-dipole wave (SDW) mode induces a secondary microwave field at double frequency that can efficiently excite shear elastic modes of the structure. The distinctive feature of a mechanism for ME coupling proposed is that it is free from the selection rules and provides continuous excitation of elastic modes within a wide frequency range by means of the standard microstripe line excitation system.

Covariant Newtonian and relativistic dynamics of (magneto)-elastic solid model for neutron star crust

This work develops the dynamics of a perfectly elastic solid model for application to the outer crust of a magnetised neutron star. Particular attention is given to the Noether identities responsible for energy-momentum conservation, using a formulation that is fully covariant, not only (as is usual) in a fully relativistic treatment but also (sacrificing accuracy and elegance for economy of degrees of gravitational freedom) in the technically more complicated case of the Newtonian limit. The results are used to obtain explicit (relativistic and Newtonian) formulae for the propagation speeds of generalised (Alfven type) magneto-elastic perturbation modes.

Electromagnetic activity of a pulsating paramagnetic neutron star

The fact that neutron star matter possesses the capability of maintaining a highly intense magnetic field has been and still is among the most debatable issues in pulsar astrophysics. Over the years, there were several independent suggestions that the dominant source of pulsar magnetism is either the field-induced or the spontaneous magnetic polarization of the baryon material. The Pauli paramagnetism of degenerate neutron matter is one of the plausible and comprehensive mechanisms of the magnetic ordering of neutron magnetic moments, promoted by a seed magnetic field inherited by the neutron star from a massive progenitor and amplified by its implosive contraction due to the magnetic flux conservation. Adhering to this attitude and based on the equations of magnetoelastic dynamics underlying continuum mechanics of single-axis magnetic insulators, we investigate electrodynamics of a paramagnetic neutron star undergoing nonradial pulsations. We show that the suggested approach regains a recent finding of Akhiezer et al. [1] that the spin-polarized neutron matter can transmit perturbations by low-frequency transverse magnetoelastic waves. We found that nonradial torsional magnetoelastic pulsations of a paramagnetic neutron star can serve as a powerful generator of a highly intense electric field producing the magnetospheric polarization charge whose acceleration along the open magnetic field lines leads to the synchrotron and curvature radiation. Analytic and numerical estimates for periods of non-radial torsional magnetoelastic modes are presented and are followed by a discussion of their possible manifestation in currently monitored activity of pulsars and magnetars.

Magnetization of FeBO3 by microwave pumping

Changes in the magnetic moment of crystalline FeBO3 when an antiferromagnetic resonance is excited in it are studied. This done using a SQUID magnetometer in combination with a microwave spectrometer. At temperatures T>15 K a reduction in the total magnetic moment of the sample is observed when an antiferromagnetic resonance is excited in it. At liquid helium temperatures, an induced rise in the sample magnetic moment was observed. This type of magnetization of the sample can be explained qualitatively if it is assumed that under antiferromagnetic resonance excitation conditions, magnetoelastic modes with high wave numbers are excited along with magnons. Efficient excitation of magnetoelastic modes under antiferromagnetic resonance conditions is confirmed by the experimental observation of a size effect in thin, high quality single crystal slabs of FeBO3.

Paramagnetic and ferromagnetic resonance imaging with a tip-on-cantilever magnetic resonance force microscope

A magnetic resonance force microscope with a “tip-on-cantilever” configuration was used to compare imaging characteristics of paramagnetic and ferromagnetic samples. Three-dimensional electron paramagnetic resonance (EPR) imaging of diphenylpicrylhydrazil (DPPH) particles was accomplished by scanning the sample in two dimensions while stepping an external field. The EPR force map showed broad response reflecting the size and shape of the sample, allowing a three-dimensional real-space magnetization image to be successfully reconstructed. In contrast to the EPR case, ferromagnetic resonance imaging of a micron-scale yttrium iron garnet sample showed no significant line broadening despite the strong field gradient (∼10 G/μm). Two-dimensional force maps revealed spatial dependence of magnetostatic and magnetoelastic modes.

Nonlinear conversion of magnetoelastic modes in a magnetostrictive plate

Longitudinal deformation of magnetostrictive plate induced by resonance transversal ultrasonic oscillations is studied on a sample of hematite single crystal. Excitation of the fundamental longitudinal mode of frequency 81.565 kHz by modulated pumping of shear modes in the band 1.06–1.16 MHz was observed. The experimental value of the modes conversion efficiency κ exp=12 ppm −1 is in agreement with the theory of giant magneto-elastic anharmonicity. Application of magneto-acoustic detection for driving of magnetostrictive mechanical systems is discussed.

Magneto Elastic Modes in Ferro or Ferrimagnetic Polycristals

We propose a new method to compute the dimensional variations of ferri or ferro magnetic materials induced by an electromagnetic wave. The algorithm used derives from a mechanical finite element method to solve stress and strain states of a solid. Magnetostriction is written using a quadratic relationship between deformation and magnetization. The results of the numerical computation for nickel samples show the complete frequency spectrum of magneto elastic modes of free standing or clamped sample. Experimental data obtained by measuring frequency dispersion permeabilities shows the coupling rays which are within 3.5% agreement with the computed frequencies of magneto elastic modes.