Magnetic Damping Coefficients Research Articles

Acoustic attenuation in magnetic insulator films: effects of magnon polaron formation

A magnon and a phonon are the quanta of spin wave and lattice wave, respectively, and they can hybridize into a magnon polaron when their frequencies and wavenumbers match close enough the values at the exceptional point. Guided by an analytically calculated magnon polaron dispersion, dynamical phase-field simulations are performed to investigate the effects of magnon polaron formation on the attenuation of a bulk acoustic wave in a magnetic insulator film. It is shown that a stronger magnon–phonon coupling leads to a larger attenuation. The simulations also demonstrate the existence of a minimum magnon–phonon interaction time required for the magnon polaron formation, which is found to decrease with the magnetoelastic coupling coefficient but increase with the magnetic damping coefficient. These results deepen the understanding of the mechanisms of acoustic attenuation in magnetic crystals and provide insights into the design of new-concept spin interconnects that operate based on acoustically driven magnon propagation.

Influences of Anisotropic Equivalent Field and Magnetic Damping Coefficient on Giant Magnetoimpedance Effect of Cylindrical Alloy Fibers: Theoretical Magnetoimpedance Calculations

In this paper, the giant magneto-impedance (GMI) model of a cylindrical alloy fiber was established by the Maxwell equation and Landau–Lifshitz equation to simulate the influence of physical parameters of cylindrical alloy fiber on GMI under different control parameters. MATLAB was employed to calculate the magneto-impedance of cylindrical fibers and draw its curves. We found that when the anisotropic equivalent field of the fiber changes from 10Oe to 50Oe, the peak position of the GMI ratio also moves from about 10Oe to 50Oe, and the peak value gradually increases from 100% to 300%. The GMI ratio increased rapidly with the decrease in the magnetization damping coefficient. Our findings could further guide the design of supersensitive micro GMI sensors by optimally regulating the magnetic damping coefficient, the angle between the external magnetic field and easy axis and the anisotropic equivalent field of cylindrical alloy fibers.

Unveiling transport properties of Co2MnSi Heusler epitaxial thin films with ultra-low magnetic damping

The family of Co-based Heusler compounds contains promising candidates for spintronic applications regarding their predicted Half-Metal-Magnetic nature, ultra-low magnetic damping coefficients, high curie temperatures and tunable electronic properties. Here we focused on the transport properties of Co2MnSi thin films with thickness in the range of 4-44 nm exhibiting magnetic damping in the 10-4 - 10-3 range. The goals of this study are to examine the impact of the peculiar electronic band structure on the transport properties, to identify the temperature-dependent scattering process, and to extract robust conduction parameters to exploit this material in magnetoelectric devices. In order to undoubtedly correlate all results, the full study has been performed on the same series of samples. Scanning transmission electron microscopy experiments were performed to check the chemically-ordered L21 phase in our films, and also allowed us to identify misfit dislocations generated at the interface with the substrate. The variation of the resistivity with film thickness was measured at different temperatures. The results are examined under the Fuchs and Sondheimer model which allowed us to extract the electron mean free path in Co2MnSi in the temperature range 5 – 300 K. Values for the residual resistivity, Debye temperature, and distance between the Fermi energy and the conduction band for minority spins were obtained from the fit of the resistivity versus temperatures curves. A negative AMR ratio was measured for all the samples which confirmed the Half-metallic nature of the Co2MnSi films. The determination of the ordinary Hall coefficient alloys allowed us to extract the carrier concentration and carrier mobility and their dependency on the temperature. Finally, scaling of the anomalous Hall coefficient with the longitudinal resistivity was performed indicating that skew scattering is the dominant temperature-dependent scattering mechanism in our films.

Numerical Simulation and Experimental Analysis of the Magnetic Damping Effect generated by a Moving Magnet

Nowadays, to reduce vibrations of machines, damping devices utilize the eddy current damping ef-fect being increasingly investigated for its advantages of no mechanical contact, no viscous liquid required, high reliability, and good damping capacity. This article studied the main principle of the eddy current damping effect for a moving permanent magnet in a stationary and electrically conduc-tive nonmagnetic cylindrical tube. The magnetic damping coefficient is investigated experimentally, analytically, and by numerical simulations in a steady-state. The numerical simulation is performed in the ANSYS Maxwell programme. The obtained results indicate that the damping force affecting the moving magnet has a viscous form. The experimentally measured and computed results are in good agreement. The effect of varying tube diameter and the tube wall thickness on the magnetic damping coefficient is shown. The contribution of this article consists in the development and a comparison of the obtained results of three approaches for determining the magnetic damping coefficient for a mov-ing magnet in a cylindrical tube.

Vibration energy harvesting using magnetic spring based nonlinear oscillators: Design strategies and insights

The rising technologies of wearable electronics resulted in urgent demand for developing eco-friendly power sources that can utilize free energies around us including ambient vibrations. Magnetic springs are amongst the most common techniques used to build vibration energy harvesting systems that convert vibrational energy into useful electric power. Here, we present an experimental and theoretical platform for design guidelines and analysis of magnetic springs encountered in vibration energy harvesting systems. An energy harvester prototype consisting of an oscillating solid magnet levitated between two stationary ring magnets is constructed and used for experimental evaluation. Results show excellent agreement between model and experiment. The use of the analytical force model to represent magnetic force nonlinearities is essential at high accelerations. While the magnetic damping coefficient varies during dynamic operation and is dependent on the position of the levitated magnet, it is shown that approximating this coefficient as a constant provides accurate prediction of the dynamic behavior of the system. Approximate analytical expressions for linear and nonlinear stiffness coefficients are obtained. Results suggest that linear and nonlinear stiffness coefficients are coupled. The outer diameter of the stationary ring magnet can be used to tune the nonlinearity of the energy harvesting system to obtain linear, hardening nonlinear, or softening nonlinear response. This work serves as a tool for designers to understand the behavior of magnetic spring based harvesting systems and evaluate their performance in light of their design parameters. This work also can serve other energy systems that utilize magnetic springs including energy sinks.

Polycrystalline Co2Mn-based Heusler thin films with high spin polarization and low magnetic damping

Spin-polarization and magnetic damping are measured for several polycrystalline films with each of them being made of a different single Co2Mn-based Heusler compound. As several epitaxial Co2Mn-based Heusler compounds are shown to be half-metal magnetic materials with full spin-polarization and ultralow magnetic damping, we explore here these properties but in polycrystalline films. Co2MnSi, Co2MnGe, and Co2MnGa thin films were grown on glass substrates and analyzed in situ by electron diffraction and spin-resolved photoemission and ex situ by transmission electron microscopy and ferromagnetic resonance. Despite the polycrystalline state of the films, they still exhibit high spin polarizations and very low magnetic damping coefficients. The latter are at least of the same order as the best epitaxial films using regular ferromagnetic materials. The key point to achieve such properties is to control the Heusler stoichiometry as best as possible.

Effect of Dilute Rare-Earth Doping on Magnetodynamic Properties of Permalloy Films

Tunable magnetodynamic properties of magnetic materials will allow significant design freedom in spintronic devices. One of the most efficient methods to tailor magnetic properties is by element doping. Here, we study the effect of rare-earth (RE) doping on Permalloy (Py) films to modify the magnetic damping and exchange stiffness by both the ferromagnetic resonance uniform precession mode and the first perpendicular standing spin-wave mode. Results show that both static and dynamic magnetic properties of Py films are adjusted by doping with Gd, Tb, and Nd. The saturation magnetization is decreased by 59.4% with Tb, Gd, and Nd at concentrations of 10 at.%. The magnetic damping coefficient (α) is enhanced mostly by Tb dopant, followed by Nd and Gd, indicating that spin-orbit coupling may be one of the main mechanisms in determining α of RE-doped films. A 44.2% reduction of the exchange stiffness was achieved by doping with 3.3 at.% Nd. The results demonstrate that RE doping may be a viable technique for the control of magnetodynamic properties for spintronics applications.

Magnetic properties tailored by inserting ultrathin Cr layer in Gd/Ni80Fe20 film

The magnetic properties of Gd(4 nm)/Cr()/(12 nm) films with different thicknesses of Cr layer prepared by magnetron sputtering were investigated. X-ray diffraction proves the coexistence of hexagonal closed-packed (hcp) (0 0 2) and face-centred cubic (fcc) (1 1 1) peaks in the Gd layer. Magnetic measurements show a modulation akin to RKKY coupling in the saturation magnetization () as a result of the coupling sign between Gd and Py layers oscillating with . The strained hcp phase and, in particular, the fcc phase lead to poor magnetic moments and strongly reduced saturation of Gd, which can also be affected by the possible magnetically dead layers, the intermixing at interfaces and a presumably more gradual transition to adjacent phases. Theoretical fittings of the relationship between the ferromagnetic resonance field () and the microwave frequency yield the effective magnetization (4) and perpendicular anisotropy constant (), which are all enhanced by introducing Gd and Cr layers as a result of the increased interfaces. Interestingly, with the increasing thickness of Cr spacer layer, an obvious decrease in the magnetic damping coefficient () is observed, from which we infer that both the spin–orbit interaction due to the interlayer couplings and the electron density of states (DOS) at the Fermi energy (EF) resulting from the intermixing at the interfaces of Gd/Cr/ film are the major origins. In addition, all magnetic parameters are approaching to those of a Py film with increasing to 3 nm.

The Enhancement of Magnetic Damping in Fe4N Films with Increasing Thickness

The film thickness dependence of the magnetic damping coefficient (α) was investigated by measuring the ferromagnetic resonance (FMR) of Fe4N/NM (NM: Pt and Cu) bilayer films at room temperature. We observed that α in Fe4N/Pt films increased with increasing Fe4N film thickness. Moreover, the enhancement of α, which is represented by the difference in the α values of Fe4N/Pt and Fe4N/Cu films, also increased as the Fe4N film thickness increased. The behavior observed in the Fe4N system was the opposite of that observed for the conventional Ni–Fe system. We speculated two mechanisms occurring in Fe4N/Pt films, which are less magnetic and/or nonmagnetic impurities in the Fe4N films and enhancement of the coercive force (Hc) with increasing Fe4N film thickness.

Investigation of one-dimensional photonic bandgap structures containing lossy double-negative metamaterials through the Bloch impedance

The Bloch impedance is studied and used to understand the properties of the absorption loss in one-dimensional photonic crystals (PCs) composed of air and metal-based double-negative metamaterials. We find that as the frequency increases across the zero-¯ n gap of the considered structures, the modulus of the Bloch impedance always decreases from a maximum to a minimum value. On the other hand, the frequency dependence of the phase angle of the Bloch impedance is greatly influenced by the ratio of the electric to the magnetic damping coefficient γe∕γm of the metamaterials. When the phase angle of the Bloch impedance reaches maximum inside the zero-¯ n gap, the impedance mismatch between the incident medium and the considered PC becomes greatest, the reflection will be strongest and a minimum absorption will be observed. As γe∕γm increases, the frequency corresponding to the minimum absorption shifts from the lower to the upper gap edge. We also show that the main characteristics of both the Bloch impedance and the absorption loss are insensitive to the geometrical parameters. Our study offers a valuable reference in the designs of zero-¯ n gap with optimized properties. © 2013 Optical Society of America

Magnetic damping and stiffness effects on rod translation by active magnetic bearing

The purpose of this article is to apply the wavelet transform algorithm to identify the magnetic damping and magnetic stiffness coefficients of the drive rod with which a set of 4-pole active magnetic bearing (AMB) is equipped. By taking advantage of time–frequency analysis feature, the ridge curve of rod free response after wavelet transformation can be extracted to find the natural frequency of the rod/AMB system. In other words, due to the influence of magnetized field by AMB, the stiffness of the rod dynamics is not linear any more and can be estimated from the curve of the amplitude versus frequency by wavelet transformation. On the other hand, the non-linear damping coefficients can be estimated from the derivative of amplitude versus amplitude by wavelet transformation of rod free vibration. It is found that the non-linear magnetic damping coefficients are up to second order in polynomial and the stiffness coefficient is mainly of third order, respectively. In addition, the identified second-order damping coefficient is negative and hence implies that under specific rod displacement and speed, the dynamic of rod/AMB system in axial direction is unstable.

Identification of rod dynamics under influence of Active Magnetic Bearing

The purpose of this paper is to apply the wavelet transform algorithm to identify the magnetic damping and magnetic stiffness coefficients of the drive rod with which a set of 4-pole Active Magnetic Bearing (AMB) is equipped. By taking advantage of time–frequency analysis feature, the ridge curve of rod free response after wavelet transformation can be extracted to find the natural frequency of the rod/AMB system. In other words, due to the influence of magnetized field by the AMB, the stiffness of the rod dynamics is not linear any more and can be estimated from the curve of the amplitude versus frequency by wavelet transformation. On the other hand, the nonlinear damping coefficients can be estimated from the derivative of amplitude versus amplitude by wavelet transformation of rod free vibration. It is found that the nonlinear magnetic damping coefficients are up to 2nd-order in polynomial and the stiffness coefficient is mainly of 3rd-order respectively. In addition, the identified 2nd-order damping coefficient is negative and hence implies that under specific rod displacement and speed, the dynamic of the rod/AMB system in axial direction is unstable.

Left-handed materials in magnetized metallic magnetic thin films

The authors’ theoretical investigation on the high-frequency response of magnetized metallic magnetic films showed that magnetic films may become left-handed materials (LHMs) near the ferromagnetic resonance frequency of incident waves with right-handed circular polarization (RCP) and linear polarization (LP). The frequency range where LHM exists depends on the waves polarization, the magnetic damping coefficient, and the ferromagnetic characteristic frequency ωm of the film. There also exists a critical damping coefficient αc, above which the left-handed properties disappear completely.

Wave Polarization and Left-handed Materials in Metallic Magnetic Thin Films

We have theoretically investigated the high-frequency response of magnetized metallic magnetic thin films. We found that magnetic films may become left-handed materials (LHM) in the vicinity of the ferromagnetic resonance frequency for incident waves with right-hand circular polarization (RCP) and linear polarization (LP). The frequency range where LHM exists depends on the waves polarization, and the magnetic damping coefficient, the ferromagnetic characteristic frequency ofthe film. There also exists a critical damping coefficient αc, above which the left-handed properties disappear completely.

Magnetic Damping For Maglev

Magnetic damping is one of the important parameters that control the response and stability of maglev systems. An experimental study to measure magnetic damping directly is presented. A plate attached to a permanent magnet levitated on a rotating drum was tested to investigate the effect of various parameters, such as conductivity, gap, excitation frequency, and oscillation amplitude, on magnetic damping. The experimental technique is capable of measuring all of the magnetic damping coefficients, some of which cannot be measured indirectly.

Magnetic damping forces in figure-eight-shaped null-flux coil suspension systems

This paper discusses magnetic damping forces in figure-eight-shaped null-flux coil suspension systems, focusing on the Holloman Maglev rocket system. The paper also discusses simulating the damping plate, which is attached to the superconducting magnet by two short-circuited loop coils interacting with the figure-eight-shaped null-flux coils in the guideway. Closed-form formulas for the magnetic damping coefficient as functions of heave-and-sway displacements are derived by using a dynamic circuit model. These formulas are useful for dynamic stability studies.

Magnetic forces for type II superconductors in a levitation field

A complete loop of hysteretic force has been calculated for hard superconductors in an almost-constant-gradient magnetic suspension system, which consists of a pair of oppositely wound superconducting coils. The dependences of levitation forces on the sample size, critical current density, external field strength, field gradient, and the magnetic history were investigated. Dynamic spring constants as well as magnetic damping coefficients were inferred from minor loop calculations. The minor loops, similar to the Rayleigh loops for ferromagnetic materials, could be described in quadratic terms. The major loops strongly depend on the magnetic passage, a flux-trap effect which is responsible for the subtlety in obtaining reproducible hysteresis loop. The basic physics of levitation and inverse levitation will be quantitatively illustrated.