Domain Magnetization Reversal Research Articles

Effects of surface acoustic waves on reversal of magnetic domains in patterned films with perpendicular magnetic anisotropy

In this work, the magnetization reversal of patterned Pt/Co/Pt multilayers influenced by surface acoustic waves is investigated. For each patterned cell, the magnetization reversal involves formation of a reversal nucleus followed by rapid motion of a domain wall. The nucleation probability of the reversal nucleus increases with the power of the applied surface acoustic wave, which is explained by the magnetic droplet model, and fitting the experimental results with this model gives important parameters such as the domain-wall energy density and the reduction rate of local perpendicular anisotropy. The fitting results for different patterned cells are consistent with the experimental results, which validates this model for dealing with such issues.

In-situ observation of magnetic domain reversal during magnetic viscosity of permanent magnets

Magnetic viscosity analysis is a widely studied topic in understanding the stability and magnetization mechanisms of magnets. However, observing the minute changes in magnetic domains during the magnetic viscosity process poses a challenge in research. To address this issue, a self-made in-situ magnetized sample holder was used in this study to observe the dynamic evolution of domain structures in alnico alloy during magnetic viscosity. The findings indicate that the domain reversal during magnetic viscosity is largely irreversible, with only minor reversible changes observed in the inhomogeneous region. The magnetization reversal in alnico alloy occurs through the reversal of α1 phases, which follows a single domain particle reversal mode. The zero-field activation energy (E0) of alnico alloy is determined as 3.30 × 10−17 J, and the coercivity field without thermal disturbance (H0) is measured at 1779.19 Oe. This research method proves to be a valuable tool in the study of magnetic materials.

Magnetization reversal and direct observation of magnetic domains on FePt thin films

Magnetization reversal and direct observation of magnetic domains on FePt thin films

Segregation of Al and its effect on coercivity in Nd-Fe-B

Doping a small amount of Al can effectively enhance coercivity in Nd-Fe-B magnets. We investigated the partitioning behavior of Al and its effect on coercivity in Nd-Fe-B using first principles DFT (density functional theory) calculation and micromagnetic simulation. The calculated substitution energies of Fe by Al are negative at the crystallographic sites of 4c and 8j2 while they are positive values at the other sites in Nd2Fe14B (2:14:1), implying a small solubility of Al in 2:14:1. Further, Al prefers to segregate at grain boundary (GB) and stabilize the Nd-Fe-Al phase with a Nd6Fe13Si-type tetragonal structure (6:13:1). The formation of the antiferromagnetic or weak ferrimagnetic 6:13:1-like phase depletes Fe and reduces the amount of ferromagnetic Nd–Fe type grain boundary phase (GBP), which weakens the inter-grain magnetic interaction. Micromagnetic simulations indicate that the 6:13:1-like GBP increases the pinning field of magnetic domain wall at GB and suppresses the nucleation of reversal magnetic domain on the grain surface of 2:14:1 during demagnetization process. The formation of Al-rich shell on 2:14:1 grain surface can further moderately increase the domain pinning field at GB and the nucleation field of 2:14:1 grain. Developing novel processing method to tailor Al segregation and promote formation of 6:13:1-like phase at GB can be a promising approach to improve coercivity in Nd-Fe-B magnet.

Development of an Optical Probe Current Sensor for Local and Narrow Area Measurement Using Magnetic Domain Reversal in Bismuth-Substituted Rare-Earth Iron Garnet Crystal

Development of an Optical Probe Current Sensor for Local and Narrow Area Measurement Using Magnetic Domain Reversal in Bismuth-Substituted Rare-Earth Iron Garnet Crystal

Direct Observation of Magnetic Domain and Magnetization Reversal on Prussian Blue-Based Magnetic Films.

Knowledge of the magnetic domain is indispensable for understanding the magnetostatic properties of magnets. However, to date, the magnetic domain has not yet been reported in the field of molecule-based magnets. Herein, we study the magnetic domains of molecule-based magnets. Two magnetic films of iron/chromium hexacyanidochromate FexCr1-x[Cr(CN)6]2/3·5H2O (x = 0; Film 1 and x = 0.2; Film 2) were prepared for investigation. The temperature evolution of surface magnetization was measured using magnetic force microscopy. Film 1 showed a magnetic domain below Curie temperature (TC) and its positive-magnetic polarization increased monotonously with decreasing temperature, while Film 2 showed positive magnetic polarization below TC and switches from positive to negative magnetization through a demagnetization state at 146 K. This study originally reports the temperature variation of the magnetization state at the magnetization reversal. The magnetic domains appeared as a maze pattern with an approximate domain size of one-to-several micrometers. This work shows that research on molecule-based magnets can be expanded from magnetochemistry to the magnetostatic engineering of bulk magnets, molecule-based magnetostatic engineering.

Recent Unrevealing on Magnetic Hysteresis of Permanent Magnets

Magnetic hysteresis and coercivity mechanism of a permanent magnet have been long discussed, and their understandings have been significantly deepened during the last decade. Magnetization reversal in a permanent magnet proceeds with thermally activated nucleation and domain wall depinning processes followed by magnetic domain expansions. To treat theoretically the thermally activated magnetization reversal process, atomistic spin model and continuum calculations have been performed. Through these multi-scale calculations, the physical meanings of fluctuation field and activation volume which have been used phenomenologically become very clear. Experimental approaches for both microscopic and macroscopic thermally activated magnetization reversal processes and magnetic domain growths have been also performed. It is expected that these theoretical and experimental multi-scale studies will be integrated to fully understand the magnetic hysteresis and coercivity mechanism with the aid of data science technologies in the future.

Effects of Ni and Cu Residuals on the Magnetic Properties and Microstructure of SmCo5 Magnets.

The effect of Ni/Cu-coating residuals on the magnetic properties and microstructures of samarium-cobalt (SmCo5) magnets was studied. SmCo5 magnets with 0.0, 0.5, 1.0, 2.0, 3.0 and 4.0 wt.% of added Ni/Cu (85 wt.% Ni/15 wt.% Cu) were prepared using a conventional sintering route. The magnetic properties of the magnets were found to be consistent up to 2 wt.% Ni/Cu. Any further increase in the Ni/Cu content resulted in a significant reduction in the magnetic properties, to lower than values that would be commercially acceptable. SEM/EDS studies showed that two major phases, i.e., the SmCo5 matrix phase and Sm2O3 were present in all the sintered SmCo5 magnets. The presence of Sm2Co7 as a minor phase fraction was detected in the sintered SmCo5 magnets containing up to 2 wt.% Ni/Cu. A 2 wt.% Ni/Cu addition to magnets resulted in the presence of two new phases with compositions close to SmCo and Sm2Co17 in addition to SmCo5 and Sm2O3 as major phases in the SEM-observed microstructure. These newly formed phases are present in small fractions and are presumably homogenously distributed at the grain boundaries of the magnets. As they are known to act as nucleation sites for reverse magnetic domains, they effectively reduce the intrinsic grain boundary magnetic strength, leading to a drop in the coercivity. We concluded that the sintered SmCo5 magnets could be recycled with up to 2 wt.% Ni/Cu as a residual from the coating under our sintering and heat treatment conditions.

Magnetic domain reversal induced by thermal activation in SmCo alloy

The SmCo alloys are widely used in the field of precision instruments and high temperatures due to their excellent temperature stability. At present, the effect of high temperature on the domain structure of SmCo alloy lacks sufficient study. In this paper, the effect of temperature on the magnetic domain of type 2:17 SmCo alloy was investigated by domain structure observation and micromagnetic simulation. After heating at 240 ℃ and 300 ℃, the reverse domains could be observed on the surface of the magnet, and the domains nucleated at the center region of the grains and then expanded into maze domains. The in-situ heating test using Lorentz electron microscope confirms, from a microscopic point of view that thermal activation causes irreversible movement of the walls of the domains. It was found by the comparison between batch heating tests and continuous heating tests that part of the inverted domains are unstable during the cooling from high temperature down to room temperature, and the micromagnetic simulation results verified that without the temperature field and magnetic field, the unstable inverted domains would disappear under the influence of the interaction, and the domains pinned by the phase boundary will be stably retained. The micromagnetic simulation results reveal that the nucleation sites of the reverse domains in the SmCo alloy are at the intersection of the 2:17R phase and Zr-rich phase, which is due to the lower pinning field of the Zr-rich phase.

Rotation behavior of individual magnetic moment with uniaxial magnetocrystalline anisotropy in magnetic field

The dynamic process of individual magnetic moment is closely related to the formation or persistence of reverse nucleus in regions where there are usually low anisotropy and complicated stray fields. It is also associated with the propagation of reverse magnetic domain and therefore affects the coercivity of magnets. In this work, we studied the detailed rotation process of individual moment in changing magnetic field. The dependence of the direction of magnetic moment on the direction of anisotropy axis and magnetic field is interestingly complicated. With the angle between the anisotropy axis and magnetic field increasing for different value of the field, the direction of magnetic moment will show three different changing processes, that is, continuously increasing process, sine-like changing process and discontinuously changing process.

Tremendous enhancement of magnetic performance for Sm(CoFeCuZr)z magnet based on multiscale copper redistribution

Microstructure and magnetic properties were studied for the commercial Sm(CoFeCuZr)z magnets before and after post annealing treatment. The results show that the phases composition and orientation of the magnet do not change after post annealing treatment, but the substantial redistribution of Cu element within multiscale (the microscale crystal grain and the nanoscale cellular structure) is observed simultaneously. In detail, along with the Cu redistribution, the thickness of the Cu-rich Sm(Co,Cu)5 cell boundary becomes thinner, and the Cu concentration in the boundary increases sharply. The pinning field of domain walls and corresponding coercivity increase remarkably with slight remanence and maximum energy product loss, and the overall magnetic performance of (BH)max (MGOe)+Hcj (kOe) increases by 54.3% as a result. Moreover, the thermal stability of the magnet improves as well. On the other hand, Cu-lean phenomenon was observed along the grain boundary region, triggering to magnetic domain reversal process and slightly undermining the squareness of the demagnetization curve of the magnet.

Effect of hydrogenation-disproportionation-desorption-recombination powder processing on the demagnetization process of Nd-Fe-B sintered magnets analyzed by soft X-ray magnetic circular dichroism microscopy

The demagnetization process of a variety of Nd-Fe-B-based magnets produced using different manufacturing techniques are studied by soft X-ray magnetic circular dichroism microscopy. The four magnets studied include a hot deformed magnet, sintered magnets produced from either a N2 jet-milled or a He jet-milled fine powder, and a magnet produced by hydrogenation-disproportionation-desorption-recombination (HDDR) followed by He jet-milling. The coercivity of the HDDR processed magnet is lower than the magnets produced with N2 jet-milled powder even though the size of the grains is smaller. By following the magnetic domain reversal behavior during the demagnetization process, it was found that, while the magnets produced with N2 jet-milled powder and the hot deformed magnet presents a predominance of the reversed magnetic domain propagation, in the HDDR processed magnet the nucleation of individual grains occurs before the reversed magnetic domains propagate. This behavior is associated with a decreased nucleation field of each grain.

Nanoscale Magnetization Reversal by Magnetoelectric Coupling Effect in Ga0.6Fe1.4O3 Multiferroic Thin Films.

The control of magnetism by electric means in single-phase multiferroic materials is highly desirable for the realization of next-generation magnetoelectric (ME) multifunctional devices. Nevertheless, most of these materials reveal either low working temperature or antiferromagnetic nature, which severely limits the practical applications. Herein, we selected room-temperature multiferroic Ga0.6Fe1.4O3 (GFO) with ferrimagnetism to study electric-field-induced nanoscale magnetic domain reversal. The GFO thin film fabricated on the (111)-orientated Nb-doped SrTiO3 single-crystal substrate was obtained through the pulsed laser deposition method. The test results indicate that the thin film not only exhibits ferroelectricity but also ferrimagnetism at room temperature. More importantly, reversible and nonvolatile nanoscale magnetic domains reversal under pure electrical fields is further demonstrated by taking advantage of its ME coupling effect with dependent origins based on iron ions. When providing an appropriate applied voltage, clear magnetic domain structures with large size can be easily manipulated. Meanwhile, the change ratio of the electrically induced magnetizations in the defined areas can reach up to 72%. These considerable merits of the GFO thin film may provide a huge potential in the ME multifunctional devices, such as the multi-value, low-energy-consuming, and nonvolatile memory and beyond.

Extreme anti-reflection enhanced magneto-optic Kerr effect microscopy

Magnetic and spintronic media have offered fundamental scientific subjects and technological applications. Magneto-optic Kerr effect (MOKE) microscopy provides the most accessible platform to study the dynamics of spins, magnetic quasi-particles, and domain walls. However, in the research of nanoscale spin textures and state-of-the-art spintronic devices, optical techniques are generally restricted by the extremely weak magneto-optical activity and diffraction limit. Highly sophisticated, expensive electron microscopy and scanning probe methods thus have come to the forefront. Here, we show that extreme anti-reflection (EAR) dramatically improves the performance and functionality of MOKE microscopy. For 1-nm-thin Co film, we demonstrate a Kerr amplitude as large as 20° and magnetic domain imaging visibility of 0.47. Especially, EAR-enhanced MOKE microscopy enables real-time detection and statistical analysis of sub-wavelength magnetic domain reversals. Furthermore, we exploit enhanced magneto-optic birefringence and demonstrate analyser-free MOKE microscopy. The EAR technique is promising for optical investigations and applications of nanomagnetic systems.

Coercivity enhancement mechanism of grain boundary diffused Nd-Fe-B sintered magnets by magnetic domain evolution observation

The grain boundary diffusion (GBD) technology was used to prepare high performance Nd-Fe-B sintered magnets by NdH3 and TbH3 nanoparticle diffusion. The factors affecting the coercivity of GBD magnets include distribution of rare earth rich grain boundary phase and substitution of the heavy rare earth. In order to distinguish the influence of various factors on the coercivity, the microstructure and magnetic domain evolution of the original, reference, Nd-diffused, and Tb-diffused magnets were analyzed. The core-shell structure formed by heavy rare earth substitution is the main factor of coercivity enhancement, and it can transform the magnetic domain reversal mode from easy-nucleation (EN) to difficult-nucleation (DN). With increasing the diffusion depth, the shell of the core-shell structure gradually becomes thinner, DN grains gradually decrease while the EN grains gradually increase, indicating that the magnetic domain reversal mode is directly related to the core-shell structure.

Coercivity enhancement mechanism of Tb-diffusion Nd–Fe–B sintered magnets studied by magneto-optical Kerr optical microscope

The coercivity, microstructure, and magnetic domain structure of Nd–Fe–B sintered magnets by grain boundary diffusion process (GBDP) with TbH3 nanoparticles were systematically investigated. Compared to the original magnet, the coercivity (Hci) of the GBDP magnets improved from 1702 to 2374 kA·m−1 with few remanence reduced from 1.338 to 1.281 T. Electron probe micro-analysis (EPMA) analysis showed that Tb diffused along grain boundary, mainly concentrated in the boundary layer of the main phase, and formed a core–shell structure. Magneto-optical Kerr optical microscope (MOKE) analysis showed that there were two types of magnetic domain reversal in one grain: gradual reversal (GR) and abrupt reversal (AR). When the applied field decreased from saturated magnetic field, the reversal magnetic domain nucleated and then spread over the whole grain gradually, which was called GR. However, some grains kept the single domain state until Hh which was a value of reverse direction applied field in second quadrant in hysteresis loops. When the applied field increased above Hh, reversed magnetic domain would suddenly appear and occupy most of the area of the grain, which was called AR. That is because AR grains have higher reversed magnetic domain nucleation field (HRN2) than GR grains (HRN1). After GBDP, the area of AR region increased obviously and GR region decreased accordingly, indicating that the core–shell structure could change GR grain into AR grain. The core–shell structure could suppress flipping of the magnetization of the grains due to the large magnetic anisotropy of Tb-rich shell. Therefore, large AR area led to high coercivity.

MM-Fe-B based gap magnet with excellent energy density

TbHx nanoparticles were applied to prepare sintered MM-Fe-B magnet by using grain boundary diffusion technique. Under optimal processing conditions, the coercivity and maximum energy product substantially increase by 383% and 151%, reaching to 6.81 kOe and 20.18 MGOe, respectively. Further investigation indicates that the interchange between Tb atoms and Ce atoms leads to a core/shell structure with (MM,Tb)2Fe14B phase in the surface region of MM2Fe14B matrix grains in the magnet. The modified structure successfully restrains the magnetic domain reversal process, resulting in remarkable enhancement of coercivity and maximum energy product of the magnet.

A mechanism study on influence of strong external magnetic field on fracture properties of a ferromagnetic steel

For some ferromagnetic metallic materials, measurable influence of strong external magnetic field on their fracture properties has been observed experimentally, i.e., an external magnetic field of large intensity can promote the propagation of the crack. In order to clarify the mechanism of this phenomenon, a fracture model is put forward in this paper based on reversal magnetic domain theory and minimum free energy algorithm and verified through observations of magnetic domains structure. Compact tensile (CT) specimens which were stretched in strong magnetic field are adopted to observe their magnetic domains structure with the powder grain method in practice. It is found that the crack in the CT specimen propagated mainly along the magnetic domain wall rather than passing through the domain body. Together with the experimental results, the fracture model for the rupture and propagation mechanism under strong magnetic field was discussed.

Magnetic properties improvement of hot-deformed Nd–Fe–B permanent magnets by Pr–Cu eutectic pre-diffusion process

Pre-diffusion grain boundary strategy was proposed to fabricate die-upset Nd-Fe-B magnets with excellent magnetic properties. Microstructure modification of die-upset magnets played a key role in determining the magnetic properties. The effective coercivity enhancement of die-upset magnets through pre-diffusion process was enhanced from 0.076 T/wt%Pr to 0.114 T/wt%Pr with a slight remanence loss. Driven by heat, the Pr–Cu eutectic alloys were diffused into melt-spun ribbons with weakened exchange couple of matrix phase, which led to enhanced coercivity. The uniform distribution of intergranular phase was obtained in die-upset magnet with pre-diffusion process and the remanence reduction was limited with a high squareness factor. Microstructure analysis confirmed that the pre-diffusion process suppressed the longitudinal/lateral ratio of platelet-shaped grains and grain growth in die-upset process. Simultaneously, the pre-diffusion process facilitated the formation of continuous and uniform grain boundaries (GBs). The grain size was distributed over a narrow range of value with similar local critical nucleation field, which resulted in the improved squareness. The thick and continuous intergranular phase strengthened the magnetic isolation between neighboring Nd2Fe14B phases and offered more “pinning” sites for domain wall shift. The modified structure hindered the nucleation and spread of reverse domains in a low magnetic field in favor of the coercivity enhancement. The view-direct time-dependent behavior of reverse magnetic domains indicated that the coercivity mechanism for die-upset magnet was a combination of “pinning” effect and nucleation model.

Magnetic Domain Structures and Magnetic Properties of Lightly Nd-Doped Sm–Co Magnets With High Squareness and High Heat Resistance

The relationship between magnetic domain structures and magnetic properties of Nd-doped Sm(Fe, Cu, Zr, Co)7.5 was investigated. In the preparation process, slow cooling between sintering and solution treatment was employed to promote homogenization of microstructures. The developed magnet achieved a maximum energy product, [BH]m, of 33.8 MGOe and coercivity, Hcb, of 11.2 kOe at 25 °C, respectively. Moreover, B-H line at 150 °C was linear, which means that irreversible demagnetization does not occur even at 150 °C. Temperature coefficients of remanent magnetic flux density, Br, and intrinsic coercivity, Hcj, were 0.035%/K and 0.24%/K, respectively, as usual the conventional Sm-Co magnet. Magnetic domain structures were observed with a Kerr effect microscope with a magnetic field applied from 0 to -20 kOe, and then reverse magnetic domains were generated evenly from grain boundaries. Microstructures referred to as “cell structures” were observed with a scanning transmission electron microscope. Fe and Cu were separated to 2-17 and 1-5 phases, respectively. Moreover, without producing impurity phases, Nd showed the same composition behavior with Sm in a cell structure.