Magnetization Reversal Mechanism Research Articles

Anomalous magnetic behavior in half-metallic Heusler Co2FeSi alloy glass-coated microwires with high Curie temperature

In the current study, we have succeeded to fabricate Co2FeSi Heusler alloy glass-covered microwires with magnetic core nucleus diameter d = 4.36 µm and total diameter D = 17.55 µm, with high Curie temperature (Tc>1100 K) and well-defined magnetic anisotropy for high temperature spintronic devices application. The magnetic properties of as-prepared and annealed at different temperature (873 K, 973 K and 1073 K for 1 h) of Co2FeSi Heusler alloy glass-covered microwires have been investigated. Strong dependence of the magnetic properties on the annealing conditions has been indicated. Anomalous magnetic behavior for annealed samples at 873 K and 973 K has been found and structural properties of such samples have been analyzed. Critical temperatures 155 K and 250 K have been detected for annealed samples at 873 K and 973 K, respectively, where the behavior of M-H loops and coercivity changed. Below the critical point the M-H curve shows “kink or wasp-waisted” magnetic behavior and complex magnetic reversal mechanism is supposed. The anomalous magnetic behavior is due to the martensitic phases induced by annealing conditions below the room temperature. This unusual magnetization behavior provides opportunities to understand the phenomena of different types of magnetic domain structures in the preferred crystallographically oriented Co2FeSi Heusler alloy glass-coated micro-wires, essentially helpful for designing the devices based on magnetization switching.

Enhanced coercivity in Co nanowires via manipulation of head morphology

Ferromagnetic nanowires based on the assembly of shape anisotropy seem apt candidates as rare-earth free permanent magnetic materials. Here, the relation between the head morphology of the Co nanowires and their magnetic coercivity is investigated. The Co nanowires with different head morphology such as diabolo, cylinder, and capped cylinder were synthesized via the polyol process. We demonstrate that the fine manipulation of the head morphology is effective in enhancing the coercivity of Co nanowires. The highest coercivity up to 9.4 kOe was achieved in the capped cylindrical Co nanowires. The underlying magnetic reversal mechanisms of Co nanowires with different head morphology were revealed through micromagnetic simulations. The enhanced coercivity may attribute to the quasi-coherent magnetization reversal mechanism and the relatively low and uniform distribution of local demagnetization field in the capped cylindrical Co nanowires. This work can provide instructive information for the design of high-performance ferromagnetic nanowires for permanent magnetic applications.

Microscopic study on the angular dependence of coercivity at zero and finite temperatures

The angular dependence of coercivity in permanent magnets is an unsettled issue in magnetism. We investigate the angular dependences of threshold fields for magnetization reversal in a single-grain and hard-soft-hard magnet model at zero and finite temperatures. We study thermal fluctuation effects on magnetization reversal, which is a stochastic process for barrier crossing at finite temperatures. We show that the mechanism of magnetization reversal in the single grain is different between zero and finite temperatures, although the normalized threshold field at finite temperatures exhibits a similar angular dependence to that of the Stoner-Wohlfarth field. We examine the angular dependences of nucleation and pinning fields for different magnetic parameters in the soft and hard magnet phases. We find that the angular dependences of the nucleation and pinning fields differ from that of the single grain, in which an exchange field resistant to reversal plays an important role, and that the thermal fluctuation reduces the threshold fields, varies the angular dependences, and makes the difference between nucleation and pinning fields smaller. We also analyze the effect of the grain alignment distribution on the threshold fields and find that the distribution changes the angular dependences of the threshold fields in different ways. We discuss the detailed features of these phenomena and the mechanisms behind them.

Field orientation dependent magnetization reversal and dynamics in sub-100 nm wide permalloy nanowires

We have investigated the magnetization reversal mechanism and dynamic behaviour of 90 nm wide permalloy nanowires patterned over a large area as a function of thickness (t) in the range from 20 nm to 70 nm and applied field orientation. The reversal processes and dynamic properties are probed using the magneto-optical Kerr effect and broadband ferromagnetic resonance (FMR) spectroscopy respectively. We have observed that both the static and dynamic properties are markedly sensitive to the film thickness. For t= 20 nm, the magnetization reversal process is dominated by coherent rotation mode due to the strong shape anisotropy and the onset of curling mode is observed. The FMR spectra display the existence of two well-separated modes, unlike the single mode which existed in the large width nanowires of the same thickness. The two modes at high and low frequencies correspond to the excitations at the centre and edges of the wire respectively. For t ⩾ 50 nm, the magnetization reversal process is dominated by curling mode of reversal. The centre mode frequency is found to be sensitive to the film thickness and shows two fold anisotropy. The experimental results are substantiated with micromagnetic simulations. These results have potential implications in designing reconfigurable magnonic crystals for microwave devices.

Magnetization reversal mechanism for core-shell grains in GBD Nd-Fe-B sintered magnets based on in-situ domain observation

Understanding the magnetization reversal mechanism of a core-shell structure grains formed in the grain boundary diffused Nd-Fe-B magnet can further improve the utilization efficiency of heavy rare earth resources to more effectively improve the coercivity of Nd-Fe-B sintered permanent magnet. In this paper, magnetic domain structure and its dynamic evolution of core-shell structural grains in grain-boundary-diffused Nd-Fe-B magnet were in-situ observed. Wider domains are found in shell regions due to higher anisotropy constant and lower saturation magnetization. The domain motion revealed that the demagnetization process of core-shell structure has a multi-step nucleation process. The nucleation of the core-shell structural grain firstly occurs at the core regions, while another reverse nucleation appears at the outmost portion in shell-region at a larger reverse field. The micromagnetic simulation results confirm the possibility of the nucleation and reversal process observed in the experiment. Our results can help understanding the demagnetization process and the coercivity enhancement mechanism of grain-boundary-diffused Nd-Fe-B sintered magnet.

Angle selective piezoelectric strain-controlled magnetization switching in artificial spin ice based multiferroic system

The prospect of electrically controlled writing of ferromagnetic bits is highly desirable for developing scalable and energy-efficient spintronics devices. In this direction, various efforts have been made to achieve electrically controlled magnetization switching utilizing an artificial multiferroic system. To date, the magnetization switching has been realized in a diverse nanopatterned magnetic system. However, the demonstration of electric field-induced strain-controlled magnetization switching in artificial spin ice (ASI) coupled with a piezoelectric material is still unexplored. In the present work, we perform micromagnetic simulations to investigate the electric field-induced strain-mediated magnetization switching in an ASI based multiferroic system. Here, the piezoelectric strain-controlled magnetization switching has been studied by applying the electric-field pulse at different angles with respect to the axes of the system. Remarkably, magnetization switches by 180° only if the external electric-field pulse is applied at some specific angles, close to the anisotropy axis of the system (≈30°–60°). Our detailed analysis of the demagnetization energy variation reveals that the energy barrier becomes antisymmetric in such cases, facilitating complete magnetization reversal. Moreover, we have also proposed a possible magnetization reversal mechanism with two sequential electric-field pulses of a relatively smaller magnitude. We believe that the present work could pave the way for a future ASI-based multiferroic system for scalable magnetic field-free low power spintronics devices.

Effect of stacking faults on magnetic properties and magnetization reversal in Co nanowires

Ferromagnetic 3d transition metal nanowires based on the assembly of shape anisotropy seem apt candidates to replace materials based on rare-earth elements in current permanent magnets. Recently, great efforts have been devoted into enhancing the coercivity of Co nanowires, through accurate control of shape and morphology. Nevertheless, stacking faults can be formed relatively easily in close-packed metals like Co, and its effect on the magnetic properties was usually obscured by the geometrical effect and being ignored. In this work, the Co nanowires with different aspect ratios were synthesized by solvothermal chemistry processes. The effect of aspect ratio on the magnetic properties of Co nanowires was investigated, and a high magnetic property with a coercivity of 9 kOe was obtained in the Co nanowires with an aspect ratio of 10. The stacking faults were observed in the Co nanowires with an aspect ratio beyond 10, which may be a contributing factor to the lower coercivity than expected. The micromagnetic simulation was employed to reveal the effect of stacking faults on the magnetization reversal mechanism, based on which the coercivity degradation mechanism was discussed. This work is instructive for developing high-performance magnetic nanowires.

Sheet thickness dependence of magnetization properties based on domains and walls within the non-linear diffusion-like equation for grain-oriented electrical steels

The definition and development of a dynamic or transient magnetic formulation compatible with the Finite Element Method, able to take the dynamic hysteresis and classical and extra losses into account (not a posteriori but a priori), and using the most adapted state variable that makes the problem well defined and easily convergent requires the knowledge of dynamic behavioral properties of soft magnetic materials whatever the thickness and the working conditions. The dynamic hysteresis of soft magnetic materials corresponds to excess iron losses, due to dynamic magnetization reversal processes within magnetic domains and especially to microscopic eddy currents around the magnetic walls in motion and inside rotating domains. The model properties used are the internal quasi-static permeability μ and the dynamic magnetization property Λ that lumps the magnetization mechanisms (domain walls displacement, bowing, fusion, nucleation and multiplication). The latter are involved in the magnetic field damping due to microscopic eddy currents within the field diffusion that renders the magnetic behavior geometry dependent. This model does not separate the field diffusion process from the magnetization reversal mechanisms. In this paper, the sheet thickness ζ dependence of the dynamic magnetization property Λ(ζ) for a Grain Oriented Electrical Steel (GOES: steel made of 3% Silicon and Iron: SiFe) is analyzed in the Rolling Direction (RD). To this extend, it is proposed to carry out and interpret magnetic measurements on GOES samples with the Epstein frame for four different thicknesses (ζ = 0.23, 0.27, 0.30 and 0.35 mm), but with similar metallurgical and crystallographic properties for the whole specimens. Magnetic properties are first identified at low induction with linear assumptions and at higher induction with non-linearities. It makes it possible to re-compute the dynamic hysteresis loops of the material and to predict the losses whatever the working conditions with frequencies from 50 to 800 Hz. It is found that μ does not depend significantly on the sheet thickness whereas Λ depends on it significantly. Microscopic observation of the magnetic domains width are then performed thanks to the MOIF (Magneto-Optical Indicator Film) technique. It helps us discriminate between three simultaneous origins for the dependence of iron losses to the sheet thickness: a skin-like effect, a domains’ refinement and changes on the walls’ mobility. Results are discussed taking the dipolar magnetic effects, closure domains, the grains and texture and the manufacturing and coating residual stress on the metal surface into account.

Effects of Shape Anisotropy on Hard-Soft Exchange-Coupled Permanent Magnets.

Exchange-coupled magnets are promising candidates for a new generation of permanent magnets. Here, we investigated the effect of soft magnetic shell thickness and the aspect ratio of the hard magnetic core on the magnetic properties for isolated core/shell cylinder exchange-coupled magnets, as well as the packing effect of the cylindrical array via a micromagnetic simulation method. It was found that the shape anisotropy contributions to the magnetic properties in the cylindrical core/shell exchange-coupled magnets are closely related to the thickness of the soft magnetic shell. When the soft magnetic shell is thin, the magnetic properties are dominated by the hard–soft exchange coupling effects, and the contributions of shape anisotropy are quite limited. When the soft magnetic shell is relatively thick, utilizing shape anisotropy would be an effective method to improve the magnetic performance of hard–soft exchange-coupled magnets. The present work provides an in-depth fundamental understanding of the underlying magnetization reversal mechanism. This work could be useful for designing high-performance permanent magnets and avoiding pitfalls.

Wave reversal mode in permalloy wire-tube nanostructures

We have investigated hysteresis curves and magnetization reversal mechanisms in permalloy wire-tube nanostructures when an external magnetic field is applied perpendicular to the axis of the nanostructure, in the x direction. Our results show that the nanotube reverses its magnetization through the wave reversal mechanism, while the nanowire does so through pseudo-coherent rotation, according to the literature. The wire-tube nanostructure exhibits a more complex reversal mechanism, where each segment begins to revert depending on whether it is a wire or tube segment, but quickly the entire nanostructure adopts the wire segment reversal mechanism as its own, considerably reducing the area within of the hysteresis curve exhibited by the nanostructure. The possibility to alter the magnetization reversal mode in such nanostructures provides an attractive way to control the motion of the magnetic domain walls.

Microstructure, magnetic properties and magnetic hardening in 2 : 17 Sm–Co magnets

Abstract A comprehensive and systematic study has been made on Sm(Co, Fe, M, L)z magnets (M = Cu or Ni, and L = Zr or Ti) to completely understand the effects of composition and processing on the microstructure and magnetic properties of magnets. Ti-containing magnets do not have a lamellar phase but exhibit only a cellular microstructure, resulting in a much lower coercivity (below 10 kOe). Ni-containing magnets exhibit a perfect cellular/lamellar microstructure, but without a large domain wall energy gradient at the interface of the 2 : 17 and 1 : 5 phases, leading to a low coercivity. Only in the magnets containing both Cu and Zr, a uniform and stable cellular/lamellar microstructure with a high domain wall energy gradient across the 1 : 5 phase is formed, resulting in high coercivity. These results indicate that the conditions for effective magnetic hardening are: (1) Formation of a cellular/lamellar microstructure, and (2) establishment of a domain wall energy gradient at the cell boundaries. Based on all of these experimental results, the magnetization reversal mechanism of 2 : 17 Sm–Co magnets can be explained by both the domain wall pinning and nucleation models. The nucleation mechanism holds at any temperature in the Cu-rich magnets, and only above the Curie temperature of the 1 : 5 phase in the alloys with the lower Cu content. In these cases, the 2 : 17 cells become magnetically decoupled.

Magnetization reversal processes in sintered permanent magnets Sm(Co, Fe, Zr, Cu)Z

The magnetic hysteresis properties of commercially available samples of sintered permanent magnets with the composition of Sm(Co0.796–xFe0.177CuxZr0.027)6.63 (x = 0.117 – 0.131) were investigated. To analyze the processes of magnetization and magnetization reversal, the magnetization curves from the thermally demagnetized state, major hysteresis loops, recoil curves, the dependence of the reversible magnetic susceptibility vs. magnetic fields strength, Kelly plots and angular dependences of the coercivity were used. These results indicate that magnetization reversal processes are more complicated than pinning. A model of magnetization reversal processes is proposed to explain why in some experiments the magnets exhibit a pinning mechanism of magnetization reversal and in some experiments a nucleation mechanism.

Angular-dependent magnetic properties of chemically synthesized single crystalline Co nanowires

Angular-dependent measurements of magnetic properties could point the way for understating reversal modes of anisotropic nanoobjects for use in next generation of more powerful and permanent information storage devices. However, such measurements have been significantly less considered for chemically synthesized nanowires. Here, single crystalline hexagonal close-packed Co nanowires with an average length and diameter of 220 and 12 nm, respectively, are synthesized using a solvothermal method in a non-polar solvent. The angular-dependent (0° ≤θ ≤ 90°) magnetic properties are carefully studied by hysteresis loop measurements and first-order reversal curve (FORC) analysis after aligning the Co nanowires oriented along [001] direction in epoxy. While randomly oriented Co nanowires have a coercivity (Hc) and squareness (Sq) of 4.3 kOe and 0.54, respectively, they show enhanced magnetic properties (Hc = 7.9 kOe and Sq = 0.78) after alignment at θ = 0°. The FORC analysis indicates the formation of by-products such as nanoparticles and low aspect ratio nanorods in the synthesis of the Co nanowires, which are confirmed by high-resolution transmission electron microscopy. The variation of coercive field distribution (ΔHcFORC) as a function of θ is studied, demonstrating a continuous increase in (ΔHcFORC) for θ > 45°. This indicates that the aligned Co nanowires with high crystallinity may reverse their magnetization through a domain wall mode, being in accordance with the literature on the magnetization reversal mechanism taking place in electrochemically fabricated Co nanowire arrays.

Mapping the magnetic state as a function of antisite disorder in Sm2NiMnO6 double perovskite thin films

The predictability of any characteristic functional aspect in a double perovskite system has always been compromised by its strong dependence over the inevitably present anti-site disorders (ASD). Here, we aim to precisely map the quantitative and qualitative nature of ASD with the corresponding modifications in observables describing the magnetic and electronic state in epitaxial Sm$ _{2} $NiMnO$ _{6} $ (SNMO) double perovskite thin films. The concentration and distribution patterns of ASD are effectively controlled by optimizing growth conditions and estimated on both local and global scales utilizing extended X-ray absorption fine structure and bulk magnetometry. Depending upon the defect densities, the nature of disorder distribution can vary from homogeneous to partially segregated patches. Primarily, the effect of varying B-site cationic arrangement in SNMO is reflected as the competition of long range ferromagnetic (FM) and short scale antiferromagnetic (AFM) interactions originated from ordered Ni-O-Mn and disordered Ni-O-Ni or Mn-O-Mn bonds, respectively, which leads to systematic shift in magnetic transition temperature and drastic drop in saturation magnetization. In addition, we have observed that the gradual increment in density of ASD leads to significant deviation from uniaxial anisotropy character, reduction in anisotropy energy and enhancement of moment pinning efficiency. However, the observed signatures of $ Ni^{2+}+Mn^{4+} \longrightarrow Ni^{3+}+Mn^{3+} $ charge disproportionation is found to be independent of cation disorder densities. This work serves as a basic route-map to tune the characteristic magnetic anisotropy, magnetic phase transitions, and magnetization reversal mechanism by controlling ASD in a general double perovskite system.

Non-Aqueous electro-deposition of novel one-dimensional Sm2Co17 nanostructures and magnetic field annealing effect on structural and magnetic properties

Through a versatile and quite economical technique of DC electro-deposition, Sm2Co17 nanowires (NWs) have been fabricated into the nanochannels of anodic aluminum oxide (AAO) templates by using non-aqueous electrolyte. The average diameter and length of NWs are 80 nm and 3 µm respectively. The structural and magnetic properties of NWs have been investigated before and after magnetic field (MF) annealing with magnetic field strength of 1 T applied parallel to the NWs axis at temperature of 300 °C. SEM and TEM have been employed to observe the morphology of NWs after partial and full etching respectively. Deposition of SmCo alloy has been confirmed through EDX analysis whereas XRD peaks shows hexagonal closed packed (hcp-P63/mmc) as the dominant phase with (203) preferred orientation. After MF annealing the crystallinity got improved as manifested by the increase in peak sharpness. Furthermore, after the said field assisted heat treatment the magnetic parameters including coercivity (Hc) from 91 to 527 Oe for θ = 0°, from 101 to 151 Oe for θ = 90° and squareness (SQ) from 0.040 to 0.16 for θ = 0°, from 0.037 to 0.032 for θ = 90° have been observed through VSM. A switching of magnetization reversal mechanism from curling to coherent rotation has been observed after MF annealing due to the stronger inter-wire coupling which leads to parallel alignment of magnetic moments of adjacent NWs. Temperature dependent study has been carried out employing PPMS manifesting gradual increase in Hc from 67 to 440 Oe, Ms from 5.4 × 10−3 to 6.1 × 10−3 emu and SQ from 0.19 to 0.25 with the decrease in temperature up to 5 K.

Magnetic hopfions in toroidal nanostructures driven by an Oersted magnetic field

The search for magnetic hopfions has been the focus of intense research during the last years. In this direction, and using micromagnetic simulations, we studied the magnetization reversal mechanism in toroidal nanoparticles under the action of an Oersted magnetic field. Our results evidence the nucleation of four magnetic configurations as a function of geometry, two of them being hopfion-like textures. These mechanisms are preferred for large toroidal structures. The annihilation of such texture is indicated by strong changes in the energy, which characterizes a topological transition.

Magnetization reversal mechanism of chemically synthesized linear chains of α-Fe nanospheres

Magnetization reversal mechanism of chemically synthesized linear chains of α-Fe nanospheres

Micromagnetic simulation of microstructure effect for binary-main-phase Nd–Ce–Fe–B magnets

We investigate the magnetic properties of a chemically heterogeneous binary-main-phase (BMP) Nd–Ce–Fe–B magnet with a core–shell structure via micromagnetic simulation. It is found that the coercivity strongly depends on the shell thickness. The BMP magnet’s coercivity initially increases and then decreases with increasing Nd-rich shell thickness, and so there is the optimal shell thickness which shows the maximum coercivity for any given Ce concentration. The simulation shows the significant difference in coercivity and maximum energy product between the BMP and single-main-phase magnets. Notably, the magnetization reversal mechanism of the BMP magnet is revealed in the simulation. Local reversals in the BMP magnet first occur in the Ce-rich shells, followed by the Nd-rich cores. Then, the magnetization in Ce-rich core/Nd-rich shell typed grains is switched after reversed magnetization of all the Nd-rich core/Ce-rich shell typed grains. The BMP magnet represents a further increased coercivity for a larger GB thickness, which can be well explained by a maximum stray field.

In-Situ Magnetization Reversal Mechanism in Ni Nanowires Investigated by Electron Holography

In-Situ Magnetization Reversal Mechanism in Ni Nanowires Investigated by Electron Holography

Probing exchange bias at the surface of a doped ferrimagnetic insulator

With the realization of stress-induced perpendicular magnetic anisotropy, efficient spin-orbit torque switching, and room temperature topological Hall effect, interest in rare earth iron garnets has been revived in recent years for their potential in spintronic applications. In this study, we investigate the magnetic properties of micrometer-thick Bi and Ga substituted thulium iron garnets (BiGa:TmIG) grown by the liquid-phase epitaxy method. Above the magnetization compensation (MC) temperature, anomalous triple hysteresis is observed in BiGa:TmIG/Pt heterostructures by anomalous Hall effect measurements. X-ray magnetic circular dichroism and energy dispersive spectroscopy measurements reveal its origin as an internal exchange bias (EB) effect arising from inhomogeneities localized at the surface of the film. Possibly depending on the difference in thickness and defect realization of the EB layer, two types of magnetization reversal mechanisms, namely, the Stoner-Wohlfarth type and the reversible domain-wall motion type, are observed. Our results show that rich meta-magnetic phases exist in garnets close to MC, which can be robustly tuned by chemical composition engineering and conveniently probed by electrical transport measurements.