Observation Of Magnetic Domains Research Articles

Interface Structures on Mechanically Polished Surface of Spinel Ferrite and Its Effect on the Magnetic Domains.

Soft magnetic spinel ferrites are indispensable parts in devices such as transformers and inductors. Mechanical surface processing is a necessary step to realize certain shapes and surface roughness in producing the ferrite but also has a negative effect on the magnetic properties of the ferrite. In the past few years, a new surface layer was always believed to form during the mechanical surface processing, but the change of atomic structure on the surface and its effect on the magnetic structure remain unclear. Herein, an interface structure consisting of a rock-salt sublayer, distorted NiFe2O4 sublayer, and pristine NiFe2O4 was found to form on mechanically polished single-crystal NiFe2O4 ferrite. Such an interface structure is produced by phase transformation and lattice distortion induced by the mechanical processing. The magnetic domain observation and electrical property measurement also indicate that the magnetic and electrical anisotropy are both enhanced by the interface structure. This work provides deep insight into the surface structure evolution of spinel ferrite by mechanical processing.

Influence of ferromagnetic interlayer exchange coupling on current-induced magnetization switching and Dzyaloshinskii-Moriya interaction in Co/Pt/Co multilayer system.

This paper investigates the relationship among interlayer exchange coupling (IEC), Dzyaloshinskii-Moriya interaction (DMI), and multilevel magnetization switching within a Co/Pt/Co heterostructure, where varying Pt thicknesses enable control over the coupling strength. Employing Brillouin Light Scattering to quantify the effective DMI, we explore its potential role in magnetization dynamics and multilevel magnetization switching. Experimental findings show four distinct resistance states under an external magnetic field and spin Hall effect related spin current. We explain this phenomenon based on the asymmetry between Pt/Co and Co/Pt interfaces and the interlayer coupling, which, in turn, influences the DMI and subsequently impacts the magnetization dynamics. Numerical simulations, including macrospin, 1D domain wall, and simple spin wave models, further support the experimental observations of multilevel switching and help uncover the underlying mechanisms. Our proposed explanation, supported by magnetic domain observation using polar-magnetooptical Kerr microscopy, offers insights into both the spatial distribution of magnetization and its dynamics for different IECs, thereby shedding light on its interplay with DMI, which may lead to potential applications in storage devices.

Analysis on coercivity enhancement mechanism of grain-boundary-diffused and Dual-alloyed Nd-Fe-B Magnets

With the rapid development of application fields, such as the green energy industry, has resulted in increased demand for magnetic characteristics, especially the coercivity (Hcj). In this paper, two types of (Nd, Dy)-Fe-B magnets with comparable Hcj values were prepared using by grain boundary diffusion (GBD) method and dual alloy (DA) method. The composition, microstructure, domain structure, and magnetization reversal process of GBD and DA magnets were subjected to comparison. Additionally, an analysis of the specific coercivity mechanism was also conducted by fitting the Brown's equation. The results of composition and microstructure analysis revealed the formation of core-shell structural grains were formed in both GBD and DA methods. Specifically, the GBD method exhibited a higher concentration of Dy in the shell, constituting 26.8 % of the total rare earth (RE) content. In contrast the DA magnets had Dy accounting for 15.8 % of the total RE content in the shell. This discrepancy was identified as the primary factor contributing to the superior efficiency in enhancing the coercivity (Hcj) observed in the GBD method. The fitting results showed that GBD magnets were more inclined to nucleate in the core region in grain, whereas DA magnets nucleate at the edge of the grain shell layer. Both the observation of magnetic domains and magnetization reversal processes analysis revealed that the demagnetization process in GBD magnets is more uniform, which leads to better squareness. Our finding can provide theoretical and experimental basis for the preparation of high-performance Nd-Fe-B magnets.

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

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

Infrared imaging of magnetic octupole domains in non-collinear antiferromagnets

ABSTRACT Magnetic structure plays a pivotal role in the functionality of antiferromagnets (AFMs), which not only can be employed to encode digital data but also yields novel phenomena. Despite its growing significance, visualizing the antiferromagnetic domain structure remains a challenge, particularly for non-collinear AFMs. Currently, the observation of magnetic domains in non-collinear antiferromagnetic materials is feasible only in Mn3Sn, underscoring the limitations of existing techniques that necessitate distinct methods for in-plane and out-of-plane magnetic domain imaging. In this study, we present a versatile method for imaging the antiferromagnetic domain structure in a series of non-collinear antiferromagnetic materials by utilizing the anomalous Ettingshausen effect (AEE), which resolves both the magnetic octupole moments parallel and perpendicular to the sample surface. Temperature modulation due to AEE originating from different magnetic domains is measured by lock-in thermography, revealing distinct behaviors of octupole domains in different antiferromagnets. This work delivers an efficient technique for the visualization of magnetic domains in non-collinear AFMs, which enables comprehensive study of the magnetization process at the microscopic level and paves the way for potential advancements in applications.

Formation of helical spin alignment in the AFM/FM/AFM trilayers by spin–orbit torque controlled exchange bias

Non-collinear spin structures can exhibit unusual magnetic properties that cannot be expected in an ordinary collinear ferromagnet (FM) due to the chiral alignment of magnetic moments, offering new opportunities for applications in the field of spintronics. In the present study, we demonstrate that exchange bias pinning can be applied to a single FM layer in two different directions simultaneously, resulting in modified magnetic behaviors due to the formation of non-collinear helical spin structures in the multilayers of Co0.7Ni0.3O (antiferromagnet, AFM)/Co0.7Ni0.3 (FM)/Co0.7Ni0.3O (AFM)/Pt (heavy metal, HM). The pinning of spins at one interface between FM and AFM/HM was controlled by spin Hall current originating from the electrical current through the HM layer at room temperature, while the spins at the other interface between FM and AFM were pinned in a fixed direction, hence allowing for the formation of a helical spin structure along the FM layer thickness with controllable chirality at room temperature. Modified magnetic behaviors of a helical spin structure were confirmed from measurements of magnetic hysteresis and magnetoresistance, as well as direct observation of magnetic domains.

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.

Coercivity mechanism of high-performance anisotropic heterostructure SmCo5 magnets

Due to its large magnetocrystalline anisotropy field and high Curie temperature, SmCo-based magnets are irreplaceable in high-temperature applications. The hot-deformed SmCo-based magnets are especially attractive for their excellent corrosion resistance, making them more suitable for harsh environments. However, it is still challenging for hot-deformed SmCo-based magnets to obtain high magnetization and high coercivity simultaneously. Therefore, it is necessary to analyze the relationship between its microstructure and magnetic properties and explore its magnetic hardening mechanism to guide the further improvement of magnetic properties. In this paper, the anisotropic heterostructure SmCo5 magnet was prepared by combining large height-reduction and introducing a nano-grained Sm-rich phase. Strong c-axis texture, (BH)m of 16.1 MGOe, and Hci of 9.6 kOe were obtained via the heterostructure with SmCo5 micron-grain and Sm-rich nano-scale precipitates. The magnetic domain observation and technical magnetization analysis demonstrate that the coercivity mechanism is dominated by nucleation. The results also show that the smooth and Sm-rich precipitated grain boundary provides weak pinning effects. It provides a reference for the development of high-performance nanocrystalline SmCo5 magnets.

Direct observation and stochastic analysis on thermally activated nucleation and growth of individual magnetic domain

The nucleation and subsequent growth of the reversed magnetic domain at a finite temperature are stochastic processes, which are often analyzed using an ensemble of magnetic elements. In this study, we investigate the stochastics of magnetization reversal in multiple trials for the sole magnetic domain. Specifically, we utilize the robust magnetic domain structure formed in a dual-exchange-biased Pt/Co/Au/Cr2O3/Pt thin film. Our investigation encompasses the latency of reversed domain nucleation and the subsequent domain wall motion based on time-lapse magnetic domain observations. The time evolution of the magnetic domain is observed after applying a pulsed magnetic field superimposed on a DC field. Magnetization reversal is triggered by the nucleation of a small embryo with finite latency, followed by domain wall propagation. The nucleation probability of the embryo increases exponentially with the DC field, thus indicating that the nucleation process obeys the Poisson process. An analysis of the relaxation time for nucleation provides a suitable expression for the energy barrier. The nucleated domain wall propagates with temporal stops, thus indicating creep motion. The temperature dependence of the relaxation time and domain wall creep motion reveal that the magnetic anisotropy in the antiferromagnetic layer significantly affect both the energy barrier for nucleation and the depinning potential of domain wall propagation. This study provides comprehensive understanding into the coercivity mechanism and contributes to the thermal stability of magnetic/spintronic devices.

Observation of Magnetic Domains in Amorphous Magnetic Wires with a Diameter of 10 μm Used in GSR Sensors.

The core of a Gigahertz Spin Rotation (GSR) sensor, a compact and highly sensitive magnetic sensor, is composed of Co-Fe-based amorphous magnetic wire with a diameter of 10 μm. Observations of the magnetic domain structure showed that this magnetic wire has unusual magnetic noise characteristics. Bamboo-shaped magnetic domains a few hundred micrometers in width were observed to form inside the wire, and smaller domains a few micrometers across were observed to form inside these larger domains. The magnetic domain pattern changed abruptly when an external magnetic field was applied to the wire. Herein is shown how these changes may be a source of magnetic noise in the wire.

Analysis on the inhomogeneous magnetization reversal in the sintered Sm-Co magnet

Domain-wall-pinning is one of the ways to realize magnetic hardening for permanent magnet materials. The sintered 2:17 Sm-Co magnets, a most typical pinning-controlled magnet, have complex nano-scaled cellular structures including cellular matrix phase, cell wall phase, and lamellar Zr-rich phase. In this paper, a multiple domain-wall-pinning with cell wall phase dominating and Zr-rich phase participating was proposed through micromagnetic simulation. The simulation results were supported by magnetic domain observation and macroscopic magnetic measurement analysis, which provided a key reference for the correlation between the magnetic properties and microstructure of this kind of magnet. Furthermore, the squareness of the demagnetization curve for the magnet was affected by multiple pinning, including multiple pinning phases and the internal inhomogeneity inside a pinning phase. Our finding can enrich the magnetic hardening mechanism and provide theoretical guidance for the optimization of magnetic properties of the pinning-controlled permanent magnets.

Interface change of Pr-Fe-B thin films with different ferromagnetic compositions

The thin film structure of the Si3N4/Pr-Fe-B/Si3N4/glass substrate was fabricated by the sputtering process followed by an in-situ annealing process at 650 °C. The changes at the interface of magnetic layer/underlayer were investigated with simultaneous addition of Fe content in the range of 68.9 to 76.61 at.% during the sputtering process. With ferromagnetic properties and more reactive and movable capacity of Fe, the addition of Fe created significant changes in the crystal structure, surface morphology, microstructure, and magnetic properties in Pr-Fe-B thin films. X-ray diffraction results confirmed the presence of two Pr2Fe14B and Pr2Fe23B3 magnetic phases in the thin films. The surface morphology of the thin films observed through SEM pictures and AFM images showed the blossom phenomenon with various grain sizes and surface roughness, respectively. The microstructure of the thin films through HR-TEM pictures revealed the discernable change at the interface of the Pr-Fe-B layer/Si3N4 underlayer. The new Fe3Si underlayer formed different thicknesses at the interface corresponding to various Fe contents. The stacking of (100) and (010) planes of Pr2Fe14B phase with different d-spacings was observed at higher magnification of HR-TEM. Results of magnetic measurement showed that the thin film at 73.32 at.% Fe content had the better growth potential of perpendicular magnetic anisotropy (PMA) and obtained a higher coercivity of approximately 4.5 kOe. MFM images through magnetic domain observation showed that the thin films had the same magnetic domain pattern but varied in magnetic domain size. Analysis of saturation magnetization and effective uniaxial anisotropy of the thin films showed the contribution of the interface magnetic anisotropy (to the PMA with the value of Ki = 122.25 × 106 erg/m2.

Competing Easy-Axis Anisotropies Impacting Magnetic Tunnel Junction-Based Molecular Spintronics Devices (MTJMSDs).

Molecular spintronics devices (MSDs) attempt to harness molecules' quantum state, size, and configurable attributes for application in computer devices-a quest that began more than 70 years ago. In the vast number of theoretical studies and limited experimental attempts, MSDs have been found to be suitable for application in memory devices and futuristic quantum computers. MSDs have recently also exhibited intriguing spin photovoltaic-like phenomena, signaling their potential application in cost-effective and novel solar cell technologies. The molecular spintronics field's major challenge is the lack of mass-fabrication methods producing robust magnetic molecule connections with magnetic electrodes of different anisotropies. Another main challenge is the limitations of conventional theoretical methods for understanding experimental results and designing new devices. Magnetic tunnel junction-based molecular spintronics devices (MTJMSDs) are designed by covalently connecting paramagnetic molecules across an insulating tunneling barrier. The insulating tunneling barrier serves as a mechanical spacer between two ferromagnetic (FM) electrodes of tailorable magnetic anisotropies to allow molecules to undergo many intriguing phenomena. Our experimental studies showed that the paramagnetic molecules could produce strong antiferromagnetic coupling between two FM electrodes, leading to a dramatic large-scale impact on the magnetic electrode itself. Recently, we showed that the Monte Carlo Simulation (MCS) was effective in providing plausible insights into the observation of unusual magnetic domains based on the role of single easy-axis magnetic anisotropy. Here, we experimentally show that the response of a paramagnetic molecule is dramatically different when connected to FM electrodes of different easy-axis anisotropies. Motivated by our experimental studies, here, we report on an MCS study investigating the impact of the simultaneous presence of two easy-axis anisotropies on MTJMSD equilibrium properties. In-plane easy-axis anisotropy produced multiple magnetic phases of opposite spins. The multiple magnetic phases vanished at higher thermal energy, but the MTJMSD still maintained a higher magnetic moment because of anisotropy. The out-of-plane easy-axis anisotropy caused a dominant magnetic phase in the FM electrode rather than multiple magnetic phases. The simultaneous application of equal-magnitude in-plane and out-of-plane easy-axis anisotropies on the same electrode negated the anisotropy effect. Our experimental and MCS study provides insights for designing and understanding new spintronics-based devices.

Core loss reduction through a laser scribe on Fe-based amorphous ribbon surface for transformer cores

Core loss reduction of an Fe-based amorphous ribbon for distribution transformer cores by laser scribe is researched and the efficacy of core loss reduction by laser scribe is compared with that by fish scale-formation during ribbon casting. Core loss reduction by laser scribe is found to be more effective than fish scale and the reduction is prominent in a 53 kg single phase core tested. Core loss reduction is realized through the reduction of anomalous eddy current loss by domain refinement. Although a fish scale ribbon shows refined magnetic domains in static magnetic domain observation, effective magnetic domains are not refined and the reduction of anomalous eddy current loss by fish scale is small. Core building factor, the ratio of core loss of a transformer core to that of a strip ribbon, is found to be increased due to higher anomalous eddy current loss in a 53 kg single phase core tested. Building factor is reduced from about 2 to 1.5 by reducing anomalous eddy current loss by laser scribe.

Magnetic performance and microstructure of NdFeB sintered magnet by diffusing Tb10Pr90-x(Cu,Al,Ga)x alloys

Commercial NdFeB sintered magnets were processed by grain boundary diffusion process with serial Tb10Pr90-x(Cu,Al,Ga)x (at%) alloys, in order to study the influence of rare earths and non-rare earth metallic elements proportion change on the diffusion effect for alloy diffusion sources with low Tb content. The coercivity increased significantly from 14.20 kOe to 24.10 kOe, 25.46 kOe and 25.36 kOe respectively for the diffused magnets treated with Tb10Pr80(Cu,Al,Ga)10, Tb10Pr60(Cu,Al,Ga)30 and Tb10Pr40(Cu,Al,Ga)50. However, the coercivity optimization effect deteriorated drastically when the total amounts of metallic elements (Cu, Al and Ga) in diffusion resources reached 70 at%. Microstructure analyses demonstrated that the remarkable coercivity enhancements achieved by the relatively low metallic content sources were mainly attributed to the excellent diffusion effects, in which the Tb diffusion depths were rather deeper and the core-shell structures were easily distinguishable. Meanwhile, due to certain differences in thermodynamic characteristics and diffusion behaviors for these serial of diffusion sources, the effective introductions of Tb and Pr from the original coated sources to the inner of the magnets were respectively different. Especially, the utilization rate of Tb obtained an optimum value of 94.74% using Tb10Pr60(Cu,Al,Ga)30 alloy and the utilization rate of Pr reached the peak value of 81.76% in the use of Tb10Pr40(Cu,Al,Ga)50 alloy, providing experiment support for the further design of alloy diffusion sources with low Tb content. In addition, in-situ magnetic domain observation for the typical diffused magnets in this work was carried out to study the demagnetization behavior and coercivity mechanism of the magnet.

Fabrication of Samples for Observation of Magnetic Domains in Ferromagnetic Materials Using by Electro-Chemical Etching

Fabrication of Samples for Observation of Magnetic Domains in Ferromagnetic Materials Using by Electro-Chemical Etching

FARADAY EFFECT

This article describes a magneto-optical study of the faraday effect in rare-earth ferrite garnets. The Faraday effect is an odd magneto-optical effect in terms of magnetization, that is, the sign of the Faraday rotation depends on whether the direction of the light propagating in the crystal coincides with the direction of the magnetization vector M, or is antiparallel to the vector M. This circumstance underlies the well-known method of visual observation of magnetic domains in rare-earth ferrites - grenades.

Observation of domain structure in non-collinear antiferromagnetic Mn3Sn thin films by magneto-optical Kerr effect

We perform hysteresis-loop measurement and domain imaging for (11¯00)-oriented D019-Mn3+xSn1-x (−0.11≤x≤0.14) thin films using the magneto-optical Kerr effect (MOKE) and compare it with the anomalous Hall effect (AHE) measurement. We obtain a large Kerr rotation angle of 10 mdeg, comparable with bulk single-crystal Mn3Sn. The composition x dependence of AHE and MOKE shows a similar trend, suggesting the same origin, i.e., the non-vanishing Berry curvature in the momentum space. Magnetic domain observation at the saturated state shows that x dependence of AHE and MOKE is explained by the amount of the reversible area that crucially depends on the crystalline structure of the film. Furthermore, in-depth observation of the reversal process reveals that the reversal starts with nucleation of sub-micrometer-scale domains dispersed in the film, followed by domain expansion, where the domain wall preferentially propagates along the [112¯0] direction. Our study provides a basic understanding of the spatial evolution of the reversal of the chiral-spin structure in non-collinear antiferromagnetic thin films.

Magnetic properties and coercivity mechanism of high Ce-content CeNdFeB film with Tb diffusion

Magnetic properties and coercivity mechanism of an anisotropic Ce–Nd–Fe–B film with a high Ce content (70 at. % Nd is replaced by Ce) have been investigated. After grain boundary diffusion with Tb layers, an enhancement of coercivity from 4150 to 9250 Oe is observed. Combining the initial magnetization curves, micromagnetic theory, and in-siut observation of magnetic domains in the demagnetization process, it is confirmed that the coercivity mechanism for the high-Ce-content magnets is the mixed type dominated by the pinning mechanism. Moreover, as the thickness of the Tb diffusion layer increases, the pinning center of domain walls changes from narrow planar inhomogeneities to wide planar inhomogeneities. A growing role of pinning plays in determining the coercivity of samples with increasing the thickness of Tb layer due to the increase in magnetocrystalline anisotropy after Tb substitution in the RE2Fe14B phase. In addition, the CeFe2 intergranular phase leads to the enhancement of coercivity due to decoupling the hard magnetic phase grains. Our results provide an insight into the coercivity mechanism of high-Ce-concentration Ce–Nd–Fe–B magnetic materials and promote the comprehension of the effect of Tb diffusion in the magnetization reversal process.

Grain refinement leading to the ultra-high coercivity in L1-Mn1.33Ga bulk magnet via hot deformation

We achieved ultra-high coercivity of 5.65 kOe in L10-Mn1.33Ga alloy via the hot deformation (HD) method and revealed the hardening mechanism. Hot deformation led to recrystallization and grain refinement, where the average grain size was reduced to about 1.5 μm for HD-88% magnet. The coercivity mechanism indicated a weak pinning model by magnetic mini-loop analysis. It was found that small grains were formed, accompanied by a certain number of {111} ⟨11-2⟩ twins for HD magnets. The magnetic domain observation showed that both the sub-micro grain boundary and the twin boundary were acted as the pinning center of the domain wall, but the latter had weaker pinning effect. The ultra-high coercivity of the HD-88% Mn1.33Ga magnet originated mainly from the pinning of the grain boundary enhanced by grain refinement. Further grain refinement and twin structure inhibition will be promising approaches in order to obtain higher magnetic properties for L10-MnxGa bulk magnets.