Domain Wall Damping Research Articles

Recent advances in nanocrystalline soft magnetic materials: A critical review for way forward

In this report, shortcomings in the recent trend of alloy development in nanocrystalline soft magnetic materials are discussed critically and suggestions for further improvement of the soft magnetic properties are given. Owing to the strong power dependence of the coercivity on the mean grain size, much emphasis tends to be placed primarily on reducing the grain size. While this approach is quite effective in reducing the effect of the magnetocrystalline anisotropy (K1) and the domain wall pinning effect is suppressed, this approach may have limited impact on the domain wall damping which governs the dynamic energy dissipation. Thus, the core loss could not be reduced solely by nano-scale grain refinement. Our recent study shows that the excess loss is suppressed dramatically by reducing the saturation magnetostriction (λs) to near zero. Thus, much attention should be paid to λs in addition to the common grain refinement approach in nanocrystalline soft magnetic alloys.

The positive correlation of the strain hysteresis of textured PMN–24%PT ceramics with domain wall damping

High strain hysteresis is a stumbling block for the application of piezoelectric ceramics. This research affords a new insight into the origin of the ultra-low strain hysteresis of textured 76%Pb(Mg1/3Nb2/3)O3–24%PbTiO3 ceramics (PMN–24%PT). Highly textured PMN–24%PT ceramic system with some doping was successfully fabricated. This system shows unipolar strain curve approximately matching a straight line. The maximum hysteresis falls in 7%–11%. Compared to other representative piezoceramic system, current PMN–24%PT textured ceramics possess the least strain hysteresis. In addition, Mn2+ substitution for Nb5+ can inhibit domain wall mobility. The positive correlation between the ultra-low strain hysteresis and the domain wall damping is demonstrated by experiment and finite element simulation. In a conclusion, this work can enhance the understanding of strain hysteresis and contribute to high-precision piezoelectric actuators.

The Dynamics of Domain Wall Motion in Spintronics

A general equation describing the motion of domain walls in a magnetic thin film in the presence of an external magnetic field has been reported in this paper. The equation includes all the contributions from the effects of domain wall inertia, damping and stiffness. The effective mass of the domain wall, the effects of both the interaction of the DW with the imperfections in the material and damping have been calculated.

Permeability Spectral Analysis and Determination of Microstructural Parameters for Mn0.7+xZn0.3SixFe2-2xO4 Series

In this communication, the compositional dependence of the real part (μ′) and an imaginary part (μ″) of complex permeability for polycrystalline compositions of ferrite series, Mn0.7+xZn0.3SixFe2-2xO4 (x = 0.1–0.3), over the wide frequency and temperature ranges has been discussed. The μ′ decrease with Mn-Si concentration (x) is mainly due to the reduction in saturation magnetization (Ms) and average grain size (D). The D decreases due to smaller solid solubility in the spinel structure and segregation of SiO2 at grain boundaries. The μ′(f) plots at 300 K show a relaxed type of spectra. The observed maximum in the vicinity of Neel temperature in μ′(T) curves is due to the relative rate of change of anisotropy constant and Ms values, whereas in μ″(T) plots it is because of the domain wall damping. The derived microstructural parameters related to grain structure have been discussed in a systematic manner.

Dependence of magnetic and microwave loss on evolving microstructure in yttrium iron garnet

The parallel magnetic and microwave loss dependence on microstructural evolutions in several polycrystalline yttrium iron garnet samples were studied in detail, focusing on the attendant occurrence of their relationships. In this study, polycrystalline YIG samples were synthesized by employing the mechanical alloying technique and sintering toroidal compacts at temperatures from 600 to 1400 °C. The samples were characterized for their evolution in crystalline phases, structure, microstructure, magnetic hysteresis parameters, microwave losses and electrical resistivity. The results showed an increasing tendency of the saturation magnetization with grain size, which is attributed to crystallinity increase in the grains. The M–H hysteresis loop results showed a transition from disordered-to-ordered magnetism which belongs to different magnetically dominant stages of formation. The starting appearance of room temperature ferromagnetic order suggested by the sigmoid-shaped loops seems to be dependent on crystallinity, phase purity and a sufficient number of large enough magnetic domain-containing grains having been formed in the microstructure. An increasing trend of transmission loss with grain size may be attributed to increment of loss contribution from hysteresis and domain wall resonance of the samples. The changes in crystallinity and microstructure, and the associated processes of microwave resonance and relaxation due to domain wall movements and damping of spin rotation contributes to the variations in transmission loss and ferromagnetic linewidth of the samples. The increased electrical resistivity while the microstructure was evolving is believed to strongly indicates improved phase purity and compositional stoichiometry.

The profile of the domain walls in amorphous glass-covered microwires

We have studied the domain wall dynamics in Joule-annealed amorphous glass-covered microwires with positive magnetostriction in the presence of an electric current, in order to evaluate the profile and shape of the moving domain wall. Such microwires are known to present magnetic bi-stability when axially magnetized. The single domain wall dynamics was evaluated under different conditions, under an axially applied stress and an electric current. We have observed the well known increasing of the domain wall damping with the applied stress due to the increase in the magnetoelastic anisotropy and, when the current is applied, depending on the current intensity and direction, a modification on the axial domain wall damping. When the orthogonal motion of the domain wall is considered, we have observed that the associated velocity present a smaller dependence on the applied current intensity. It was observed a modification on both the domain wall shape and length. In a general way, the domain wall evolves from a bell shape to a parabolic shape as the current intensity is increased. The results were explained in terms of the change in the magnetic energy promoted by the additional Oersted field.

Chiral damping of magnetic domain walls.

Structural symmetry breaking in magnetic materials is responsible for the existence of multiferroics, current-induced spin-orbit torques and some topological magnetic structures. In this Letter we report that the structural inversion asymmetry (SIA) gives rise to a chiral damping mechanism, which is evidenced by measuring the field-driven domain-wall (DW) motion in perpendicularly magnetized asymmetric Pt/Co/Pt trilayers. The DW dynamics associated with the chiral damping and those with Dzyaloshinskii-Moriya interaction (DMI) exhibit identical spatial symmetry. However, both scenarios are differentiated by their time reversal properties: whereas DMI is a conservative effect that can be modelled by an effective field, the chiral damping is purely dissipative and has no influence on the equilibrium magnetic texture. When the DW motion is modulated by an in-plane magnetic field, it reveals the structure of the internal fields experienced by the DWs, allowing one to distinguish the physical mechanism. The chiral damping enriches the spectrum of physical phenomena engendered by the SIA, and is essential for conceiving DW and skyrmion devices owing to its coexistence with DMI(ref.).

Effect of a DC transverse magnetic field on the magnetization dynamics in FeCuNbSiB ribbons and derived nanostructured powder cores

The paper aims to give a comprehensive account of DC transverse bias magnetic field influence on the dynamic magnetization processes in Fe73Cu1Nb3Si16B7 soft magnetic ribbons and nanostructured powder cores based on ribbon precursor. These materials were extensively investigated using an approach of impedance spectroscopy from the viewpoint of complex permeability spectra, which offer the indirect insight into magnetization mechanisms and their frequency relaxation. In order to analyze the dynamic nature of domain wall magnetization mechanism, we carried out complex permeability measurements under combined influence of three independent variables, i.e. magnetizing frequency, various driving AC and DC bias magnetic fields. From this ample matrix of results, it has been found that the DC bias smaller than some critical field can increase the real part of complex permeability in the case of as-quenched and annealed ribbon. Nonetheless, the higher fields play a role of additional anisotropy and consequently domain wall damping mechanism. For nanocrystalline compacted powder cores, only the damping influence of DC transverse field was observed. The results are compared in detail and possible mechanisms of domain wall movement damping are proposed and discussed.

Competition of Peierls relief and structural defects in the damping of domain walls in [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 ferrimagnet

A reverse ordering of changes in the modes of domain wall movement in the molecular ferrimagnet [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 is observed with rising temperature in ac magnetic fields with frequencies of 0.04–1400 Hz. The appearance of a relaxation mode against the background of domain wall creep during heating of the crystals indicates that these modes apply to two different types of barriers. The existence of a threshold amplitude for the ac magnetic field confirms that the periodic Peierls relief contributes to the damping of domain walls along with the traditional contribution from pinning of the walls on structural defects.

Iron Based Soft Magnetic Compacted Materials

Soft magnetic materials play an important role in broad applications, such as transformers and electrical motors. There is an interest in bulk soft magnetic materials because of the demand for miniaturization of cores. We have prepared bulk samples in the form of the small cylinders with good soft magnetic properties. The frequency dependence of magnetic properties is studied, and it is attributed mainly to the structure of the initial powder and domain wall damping. The good combination of various shapes and good soft magnetic properties indicates the possibility of future development as a new soft magnetic compacted material.

Domain Wall Dynamics in Thin Magnetic Wires

We are dealing with the fast domain wall dynamics in glass-coated microwires. Three main parameters are recognized that results in a very high domain wall velocity that can reach more than 10 km/s. Firstly, it is a low Gilbert damping that decreases the domain wall damping. Secondly, it is the presence of two perpendicular anisotropies (that can be modified by proper thermal treatment). Finally, the radial domain structure shields the domain wall from the pinning at the surface.

Stress dependence of the domain wall dynamics in the adiabatic regime

In this work, we determine the domain wall velocity in the low field region and study the domain dynamics in as-cast and annealed bi-stable amorphous glass-covered Fe 77.5Si 7.5B 15 microwires. In particular, from the relation between the domain wall velocity and magnetic field in the adiabatic regime, the power-law critical exponent β, the critical field H 0 and the domain wall damping η were obtained. It has been verified that the main source of domain wall damping is the eddy current and spin relaxation, both with a strong relation with the magnetoelastic energy. This energy term is changed by the axial applied stress, which, by its time, modifies the damping mechanisms. It was also verified that the domain wall damping terms present different behavior at low (mainly eddy currents) and high applied stress (spin relaxation).

Domain wall dynamics during the devitrification ofFe73.5CuNb3Si11.5B11magnetic microwires

We have studied the evolution of domain wall dynamics during the devitrification of amorphous and nanocrystalline ${\text{Fe}}_{73.5}{\text{Cu}}_{1}{\text{Nb}}_{3}{\text{Si}}_{11.5}{\text{B}}_{11}$ microwires. Depending on the annealing temperature range, three different regimes in the domain wall dynamics can be considered for these materials. Annealing below the Curie temperature, ${T}_{\text{c}}$, of the alloy leads to the stabilization of the domain structure and a strong temperature dependence of the domain wall dynamic parameters. However, annealing above ${T}_{\text{c}}$ leads to a destabilization of domain patterns and a decrease in the domain wall damping in one order of magnitude. Finally, annealing above the crystallization temperature, ${T}_{\text{x}}$, leads to the appearance of the nanocrystalline state that is structurally very stable. In this case, the domain wall velocity is very fast due to the lack of magnetic anisotropy and extremely stable with the temperature.

Soft Magnetic Properties of Nanostructured Vitroperm Alloy Powder Cores

Magnetic properties of amorphous and nanocrystalline samples have been experimentally investigated. Rapidly quenched ribbons of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">73</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> (Vitroperm 800) were ball-milled and cryomilled to powder and warm-consolidated (at a pressure of 700 MPa) to get bulk compacts. It was found by investigating the influence of mechanical milling on the magnetic properties of powder samples prepared by milling that the alloy remains amorphous during the whole milling process. The frequency dependence of the coercivity and total core losses were studied. Investigation of the dc coercivity, magnetostriction, and electrical resistivity were done. Annealing at higher temperature causes a deterioration of dc soft magnetic properties. The higher coercivity of the as-prepared samples is mainly due to interfaces of the powder elements and internal stresses created by milling and consolidation that were decreased greatly after annealing. The frequency dependence of magnetic properties is also illustrated, and it is attributed mainly to the domain wall damping. The absolute values of losses and coercivity of Fe-based compacts are similar to that for Co-based compacts. We have prepared bulk samples in the form of the small cylinders with coercivity down to 13 A/m. These materials have more degrees of freedom for tailoring their magnetic properties due to their flexibility in shape and dimensions.

Locally induced domain wall damping in a thin magnetic wire

The damping mechanisms affecting the motion of a single domain wall were studied in a thin bistable magnetic wire. It was found that the overall damping is frequency and temperature dependent through the locally induced anisotropy via structural relaxation. This phenomenon can increase the overall damping by one order of magnitude and enables an effective tailoring of the domain wall dynamics according to required application.

Micromagnetic simulation of domain wall dynamics in Permalloy nanotubes at high frequencies

The formation and motion of a single cross-tie type domain wall (DW) was studied in a Ni80Fe20 Permalloy nanotube of 50 nm thickness and 500 nm length by means of micromagnetic simulations. Circular magnetization curves, calculated with circumferential ac magnetic fields applied on the nanotube, showed that the propagation of the DW along the nanotube length occurred for hrms values as low as 166 A/m at a frequency of 250 MHz. In general, the observed DW motion exhibited an out-of-phase oscillating character, relative to the applied hac field, with a normalized phase shift of 0.20 being independent of hrms. The DW damping was observed at 10 GHz.

Magnetization processes in thin magnetic wires

Amorphous magnetic microwires are novel materials, which are characterized by the unique magnetic properties. Their magnetization process runs through the depining and subsequent propagation of the single-domain wall. This allows us to study the magnetization processes of the single-domain wall either in static (when the domain wall lies in its potential) or dynamic (when the domain wall propagates along the wire) mode. In the given work, we present surprising results that were found during the single-domain wall switching and propagation in microwires. The negative critical propagation field during the propagation of the single-domain wall in microwires has been found. Moreover, new contribution (based on the structural relaxation) to the domain wall damping during its propagation in microwire was found. The complex shape of the single-domain wall potential, which consists of two contributions, has been found in microwires. The magnetoelastic one coming from the magnetoelastic interaction of the domain wall with the stresses applied on microwires and the stresses introduced during the microwire's production and stabilization one coming from the structural relaxation on atomic level.

High-temperature AC magnetic properties of FeCo-based soft magnets

The temperature dependence of AC magnetic properties of soft magnetic Co27Cr0.6Ni0.6Febal alloys has been studied. Microstructural characterization indicates that the alloy has a disordered BCC structure which is insensitive to heat-treatment. At low frequencies, the alloy shows a low coercivity Hc, high saturation magnetization Ms, and high magnetic permeability μm. As the frequency increases, all those properties deteriorate due to the damping of magnetic domain walls. On the other hand, Hc and Ms decrease, and μm increases with increasing temperature in the range of 20–600oC. The change of Hc and μm with temperature is attributed to the variation of magnetocrystalline anisotropy K1, which also improves the core loss at high temperature.

The analysis of magnetoimpedance by equivalent circuits

Simple circuits formed by resistors, inductors and capacitors can be used to model giant magnetoimpedance (GMI) with a good approximation. Three different circuits are found to represent GMI in the whole frequency range (100 Hz–9 GHz) and the AC magnetic field amplitude range (0.01–7.22 A/m RMS): a series R s C s L s, a parallel R p L p and a parallel R H L H, associated with the magnetization processes of spin rotation (leading to ferro-magnetic spin resonance), domain wall bulging (initial permeability) and domain wall displacement (magnetic hysteresis), respectively. We also propose a correlation between these elements and physical parameters of the sample, such as permeabilities, domain wall damping parameters and domain wall effective mass.

Giant magnetoimpedance in CoFeBSi wires and polycrystalline ferrites

Measurements of Giant magnetoimpedance (GMI) in Co-rich amorphous wires are compared with magnetic measurements in polycrystalline ZnMn ferrites as a function of frequency (5 Hz-13 MHz) and a superimposed dc magnetic field, aiming to simulate GMT conditions. It is found that, except for the scale differences expected in these two different materials, frequency phenomena are essentially similar. In particular, both properties set can be represented by using the same equivalent-circuit approach. These results point to a domain wall damping effect of the dc field, and a reduced role of macroscopic eddy currents.