Observation Of Magnetic Domains Research Articles

Effect of pulsed magnetic field treatment on the residual stress of 20Cr2Ni4A steel

Abstract To study the effect of a pulsed magnetic field on the residual stress of 20Cr2Ni4A steel, the material was treated for 120 s with a 1 T pulsed magnetic field. The microstructure of the materials was studied using residual stress measurement, transmission electron microscopy (TEM) observations, X-ray diffraction analysis, and magnetic domain observations. Experimental results revealed that the residual stress of the 1 T sample was reduced by 14.26% compared with that of the 0 T sample. The results show that the residual stress is effectively reduced by the pulsed magnetic field. The mechanism of magnetic field action involves improving the flexibility of dislocation movement, proliferating and entangling the dislocation structure, and inducing slippage. During the pulsed magnetic treatment, the labyrinth domain with larger stress changes to a lamellar domain with less stress and the residual stress decreases.

Development of an X-ray tomographic technique for three-dimensional observation of internal magnetic domains

Development of an X-ray tomographic technique for three-dimensional observation of internal magnetic domains

Magnetostrictive Property and Magnetic Domain Structure of Fe-Ga Alloy Single Crystal under Tensile Strain

The saturation value of the magnetostriction curve in the [100] direction of a Fe-Ga alloy single crystal was decreased from 226 to 55 ppm by applying the tensile strain of 533 ppm to the measured direction. By magnetic domain observation using a magneto-optic Kerr effect microscope, a complex structure composed of various magnetic domains was observed under zero applied strain. On the other hand, a stripe structure composed of magnetic domains with the magnetization direction in two kinds of <100> magnetic easy directions parallel to the tensile direction, which were separated by straight 180° domain walls, was observed under the tensile strain of 533 ppm. The characteristic magnetic domain structure due to the tensile strain was successfully observed as a cause of the significant decrease of the saturation value of the magnetostriction curve.

Observation of the magnetoelectric reversal process of the antiferromagnetic domain

We investigated the switching process of the perpendicular exchange bias, which is driven by the magnetoelectric effect, by conducting magnetic domain observations using scanning soft X-ray magnetic circular dichroism microscopy. Isothermal and simultaneous application of magnetic and electric fields switches the perpendicular exchange bias polarity. The switching process proceeds by the nucleation and growth of reversed domains. The correspondence among the ferromagnetic/antiferromagnetic domains and exchange bias polarity indicates that interfacial antiferromagnetic spin/domain reversal is responsible for the magnetoelectric switching of the perpendicular exchange bias polarity.

Frustration and relaxation of antiferromagnetic domains reversed by magneto-electric field cooling in a Pt/Co/Au/Cr2O3/Pt-stacked film

Using magnetic domain observations, we investigated the reversal process of the perpendicular exchange bias polarity resulting from the antiferromagnetic Cr2O3 domain reversal driven by magneto-electric field cooling (MEFC). The exchange bias polarity changed from negative to positive with increasing electric field during MEFC. The relevant change in the magnetic domain revealed the stochastic appearance of the reversed magnetic domains that exhibit the positive exchange bias. The local magnetization curves suggest that the antiferromagnetic domain state after MEFC was frustrated because of energy competition between the interfacial exchange coupling and the bulk magneto-electric effect. The frustrated nature of the magnetic domain structure is supported by the training effect of the exchange bias after MEFC.

Antiferromagnetic domain wall creep driven by magnetoelectric effect

We observed the magnetoelectric induced domain wall propagation in a Pt/Co/Au/Cr2O3/Pt stacked thin film based on magnetic domain observations using scanning soft X-ray magnetic circular dichroism microscopy. The antiferromagnetic (Cr2O3) domain wall velocity was estimated by a quasi-static approach using a pulsed voltage. At a pulse voltage amplitude of −12 V, corresponding to an electric field of −8.0 × 102 kV/cm, the domain wall velocity was very low, at 0.3 m/s. The domain wall velocity increased with increasing voltage amplitude, reaching 22 m/s at −20 V (−1.3 × 103 kV/cm). The change in the domain wall velocity with the applied voltage amplitude indicates the creep motion of the domain wall. Using a phenomenological model, we estimated the domain wall depinning energy, and found that the bulk and interface terms of the magnetic anisotropy affect the effective magnetic field to the same degree, suggesting that the magnetic domain wall motion may be controllable by the antiferromagnetic layer thickness.

Symmetry Modulation and Enhanced Multiferroic Characteristics in Bi1‐xNdxFeO3 Ceramics

AbstractBiFeO3 is recognized as the most important room temperature single phase multiferroic material. However, the weak magnetoelectric (ME) coupling remains as a key issue, which obstructs its applications. Since the magnetoelectric coupling in BiFeO3 is essentially hindered by the cycloidal spin structure, here efforts to improve the magnetoelectric coupling by destroying the cycloidal state and switching to the weak ferromagnetic state through symmetry modulation are reported. The structure is tuned from polar R3c to polar Pna21, and finally to nonpolar Pbnm by forming Bi1‐xNdxFeO3 solid solutions, where two morphotropic phase boundaries (MPBs) are detected. Greatly enhanced ferroelectric polarization is obtained together with the desired weak ferromagnetic characteristics in Bi1‐xNdxFeO3 ceramics at the compositions near MPBs. The change of magnetic state from antiferromagnetic (cycloidal state) to ferromagnetic (canted antiferromagnetic) is confirmed by the observation of magnetic domains using magnetic force microscopy. More interestingly, combining experiments and first‐principles‐based simulations, an electric field‐induced structural and magnetic transition from Pna21 back to R3c is demonstrated, providing a great opportunity for electric field‐controlled magnetism, and this transition is shown to be reversible with additional thermal treatment.

Realization of a scanning soft X-ray microscope for magnetic imaging under high magnetic fields.

For the purpose of imaging element- and shell-specific magnetic distributions under high magnetic fields, a scanning soft X-ray microscope has been developed at beamline BL25SU, SPring-8, Japan. The scanning X-ray microscope utilizes total electron yield detection of absorbed circularly polarized soft X-rays in order to observe magnetic domains through the X-ray magnetic circular dichroism effect. Crucially, this system is equipped with an 8 T superconducting magnet. The performance and features of the present system are demonstrated by magnetic domain observations of the fractured surface of a Nd14.0Fe79.7Cu0.1B6.2 sintered magnet.

Minor hysteresis patterns with a rounded/sharpened reversing behavior in ferromagnetic multilayer

Hysteresis of ferromagnetic system exhibits a fundamental stimulus-response behavior, thereby casting all the important macromagnetic system parameters such as coercivity, nucleation field, saturation magnetization, and hysteresis loss. Recently, increasing attention has been paid to exploration of relatively less understood minor loop behavior, since faster operation of magnetic devices is inevitably accompanied by minor hysteresis behavior from cycling among unsaturated ferromagnetic states. Here, we report our microscopic investigation of unusual minor hysteresis loop behavior, represented by rounded or sharpened response of minor hysteresis loop of (CoFeB/Pd)4 multilayer film. It is observed that rounded and sharpened response in the minor hysteresis response could be manifested under proper conditions. The minor loop behavior has been systematically investigated by direct microscopic magnetic domain observation using magneto-optical Kerr microscopy. The rounded response of magnetization at the reversing external field along the minor hysteresis curve, so far neglected or considered as one of ‘unusual’ behaviors, has been found to be elaborately controllable by tuning the reversing field strength and the field sweep rate for multilayers with low repeat numbers. Variable roundedness of the minor hysteresis loop is understandable based on the analysis of magnetic domain dynamics such as domain nucleation and the domain wall velocity.

Investigation of coercivity for electroplated Fe-Ni thick films

We have already reported Fe-Ni firms with good soft magnetic properties prepared by using an electroplating method. In our previous studies, we prepared the Fe-Ni films from citric-acid-based baths (CA-baths) and ammonium-chloride-based ones (AC-baths), and confirmed that the coercivity for the AC-baths was lower than that for the CA-baths. In the present study, we investigated reasons for the lower coercivity for the AC-baths to further improve the soft magnetic properties. From an observation of magnetic domains of the Fe22Ni78 films, we found that Fe22Ni78 film for AC-bath had a magnetic anisotropy in the width direction, and also found that the coercivity in the width direction was lower than the longitudinal one for the AC-bath. As an annealing for a stress relaxation in the films reduced the difference in the coercivity, we considered that the anisotropy is attributed to the magneto-elastic effect.

Effect of Silicon Content on Iron Loss and Magnetic Domain Structure of Grain-Oriented Electrical Steel Sheet

Increasing the silicon content in non-grain-oriented (NO) electrical steel sheets is known to decrease iron loss by changing electrical resistivity and magnetostriction behavior. However, few studies on the effect of high silicon contents in grain-oriented (GO) electrical steel have been reported. One of the major differences in the iron loss of GO and NO is that excess eddy current loss, which has a strong relationship with the magnetic domain structure, is more dominant in total iron loss in GO. In this paper, high silicon GO was prepared by the CVD siliconizing method, which has the advantage that the silicon content can be changed without changing the crystal grain size and crystallographic texture of the specimen. It was found that iron loss was reduced as the silicon content was increased, and 6.5 mass% silicon steel showed about half the hysteresis loss of conventional 3 mass% silicon steel, but the eddy current loss in 6.5 mass% silicon steel was equivalent to or slightly larger than that in 3 mass% silicon steel. Magnetic domain observation and calculations suggested that this difference was a result of coarsening of the magnetic domains in the high silicon steel.

Influence of Misorientation Angle Between Adjacent Grains on Magnetization Reversal in Nd-Fe-B-Based Sintered Magnet

To clarify the difference between the degree of alignment dependence of coercivity and the angular dependence of coercivity, the crystal orientation distribution and demagnetization curve of Nd-Fe-B-based sintered magnets with different degrees of alignment were compared. It is suggested that the increase in coercivity due to a low degree of alignment cannot be explained only by the angular dependence of coercivity. A crystal orientation analysis and in-situ observation of magnetic domains that were performed in the same area clarified that the ratio of the grain boundary where the magnetization reversal stopped became larger when the misorientation angle between adjacent grains became larger. This suggests that a grain boundary having a larger misorientation angle is one of the factors that suppresses magnetization reversal.

Coercivity enhancement in Dy-free sintered Nd-Fe-B magnets by effective structure optimization of grain boundaries

In this study, we presented an effective approach for the substantial coercivity enhancement of Dy-free sintered Nd-Fe-B magnets with Ga-doping. The coercivity was substantially enhanced from 10.1 kOe to 19.8 kOe by effective microstructure optimization and phase constitution regulation in antecedent alloys. A good isolation of 2:14:1 grains by Nd-rich intergranular phase is obtained in final magnets accompanying with the formation of intermetallic Nd6Fe13Ga phase. However, when dehydrogenating above 500 °C, masses of preformed Nd6Fe13Ga phases are detected in the initial strip casting alloys and the homologous magnet suffers an unanticipated degradation of coercivity. Magnetic domain observation and thermal magnetic analysis imply a pronounced difference in the magnetism of grain boundary (GB) phases, which closely associated with the composition homogeneity of Nd-rich phases in antecedent alloys. The mechanism of the coercivity variation in the sintered magnets with different GBs magnetism is discussed based on magnetic behavior analysis.

Observation and manipulation of magnetic domains in sol gel derived thin films of spinel ferrites

Thin films of spinel ferrites, namely zinc substituted nickel, cobalt ferrite, and manganese substituted cobalt ferrite, were synthesized using sol-gel derived spin-coating techniques. The films were characterized using x-ray diffraction, field emission scanning electron microscopy, Fourier transform infrared spectroscopy and Raman spectroscopy techniques for the analysis of structural, morphological and vibrational band transition properties, which confirm the spinel phase formation of the films. The magnetic force microscopy (MFM) technique was used to observe the magnetic domain structure present in the synthesized films. Further, the films were subjected to an external DC magnetic field of 2 kG to orient the magnetic domains and analyzed using an ex situ MFM technique.

Using Kerr Microscopy for Direct Observation of Magnetic Domains in Ni–Mn–Ga Magnetic Shape Memory Alloy

Giant magnetic-field-induced strain up to 12% in Ni–Mn–Ga single crystal is due to magnetically-induced reorientation of the twinned martensitic low symmetry crystal structure. An important interplay is arising from the interaction between ferroelastic domains (twin domains) and magnetic domains. Here, we studied the magnetic domains in an Ni50Mn28Ga22 single crystal exhibiting 6% strain by using a magneto-optical indicator film and for the first time also directly by the Kerr magneto-optical microscopy. To compare the magnetization behavior of bulk and surface, we measured the magnetization loops by the Kerr effect (MOKE) and by the vibrating sample magnetometry. We demonstrate that using a particular arrangement of the Kerr microscopy enables visualizing the magnetic domains structure and its evolution in the magnetic field.

Evolution of antiferromagnetic domains in the all-in-all-out ordered pyrochlore Nd2Zr2O7

We report the observation of magnetic domains in the exotic, antiferromagnetically ordered all-in-all-out state of Nd$_2$Zr$_2$O$_7$, induced by spin canting. The all-in-all-out state can be realized by Ising-like spins on a pyrochlore lattice and is established in Nd$_2$Zr$_2$O$_7$ below 0.31 K for external magnetic fields up to 0.14 T. Two different spin arrangements can fulfill this configuration which leads to the possibility of magnetic domains. The all-in-all-out domain structure can be controlled by an external magnetic field applied parallel to the [111] direction. This is a result of different spin canting mechanism for the two all-in-all-out configurations for such a direction of the magnetic field. The change of the domain structure is observed through a hysteresis in the magnetic susceptibility. No hysteresis occurs, however, in case the external magnetic field is applied along [100].

Direct observation of magnetic domains by Kerr microscopy in a Ni-Mn-Ga magnetic shape-memory alloy

The magnetic domains in a magnetic shape-memory Ni-Mn-Ga alloy were observed by magneto-optical Kerr microscopy using monochromatic blue LED light. The domains were observed for both single- and multivariant ferroelastic states of modulated martensite. The multivariant state with very fine twins was spontaneously formed after transformation from high-temperature austenite. For both cases, bar domains separated by ${180}^{\ensuremath{\circ}}$ domain walls were found and their dynamics was studied. A quasidomain model was applied to explain the domains in the multivariant state.

Correlation of microchemistry of cell boundary phase and interface structure to the coercivity of Sm(Co0.784Fe0.100Cu0.088Zr0.028)7.19 sintered magnets

We revisited the microstructure of Sm2Co17-type sintered magnets to understand the underlying mechanism of substantial decrease of coercivity from 2.56 T for the optimally annealed sample to 0.14 T for the sample re-annealed at aging temperature of 850 °C for just 1 min and quenched (quenched sample). This low coercivity can be recovered by annealing of the magnet at 850 °C, slow cooling to 400 °C followed by quenching (slowly cooled sample) and with this is fully reversible between the two states. Magnetic domain observations of thermally demagnetized samples with c-axis out-of-plane showed an ultra-fine maze-like domain pattern comparable to the cell size in the slowly cooled sample. However, annealing at 850 °C, even for a short time, followed by quenching resulted in a substantial increase in the size of maze-like domain pattern. Microchemistry of the SmCo5 cell boundary phase for two samples was analyzed using 3D atom probe. 8.6 at. % Cu and 7.7 at. % Fe was found in the cell boundary of the quenched sample while the SmCo5 cell boundary phase of the slowly cooled sample contains 15.4 at. % Cu and 3.0 at. % Fe. Unlike the slowly cooled sample, a broader distribution of Cu than Sm was found in the cell boundary of the quenched sample. High resolution STEM-HAADF results showed SmCo5/Sm2Co17 interface is sharp and defect free for the slowly cooled sample while it is rough and zigzag shaped for the quenched sample. Diffused Cu into the matrix phase from the cell boundary results in a gradual increase of magnetocrystalline anisotropy form matrix to the cell boundary phase. Large Fe concentration in the cell boundary phase reduces its magnetocrystalline anisotropy. Micromagnetic simulations showed that these microstructural features of the quenched sample cause a weak pinning strength of the cell boundary phase explaining its low coercivity.

Photographic observation of magnetic domain structure with three-dimensional local magnetization direction

The direction of magnetization of a magnetic material is possibly oriented three-dimensionally because of the presence of magnetic anisotropy field, self-demagnetizing field, and stray field. Therefore, the three-dimensional detection of the direction of magnetization is required. The method of magnetic domain observation by photographic imaging utilizing the Kerr effect is widely used. If the perpendicular magnetization components exist, there is a problem that obliquely incident light has superimposed longitudinal Kerr and polar Kerr effects. To perform the three-dimensional detection of magnetization direction, it is necessary to eliminate the influence of the polar Kerr effect from the Kerr effect of obliquely incident light. We report the photographic observation of the magnetic domain structure and the detection of the three-dimensional local magnetization direction using the Kerr effect, applying only an in-plane saturation magnetic field.

ChemInform Abstract: Recent Progress in the Thermodynamics of Ferrotoroidic Materials

AbstractReview: 78 refs.