arXiv subjects

Ken Harada

Publications and source records attributed to Ken Harada.

10 recordsLinked to original sources

arXiv2022

Magnetization controlled by crystallization in soft magnetic Fe-Si-B-P-Cu alloys

Soft magnetic materials have low coercive fields and high permeability. Recently, nanocrystalline alloys obtained using annealing amorphous alloys have attracted much interest since nanocrystalline alloys with small grain sizes of tens of nanometers exhibit low coercive fields comparable to that of amorphous alloys. Since nanocrystalline soft magnetic materials attain remarkable soft magnetic properties by controlling the grain size, the crystal grains' microstructure has a substantial influence on the soft magnetic properties. In this research, we examined the magnetic properties of Fe-Si-B-P-Cu nanocrystalline soft magnetic alloys obtained by annealing amorphous alloys. During crystallization, the observation findings reveal the correlation between the generated microstructures and soft magnetic properties.

cond-mat.mtrl-sci↗

arXiv2021

Two types of magnetic bubbles in MnNiGa observed via Lorentz microscopy

Magnetic bubbles are remarkable spin structures that developed in uniaxial magnets with strong magnetocrystalline anisotropy. Several contradictory reports have been published concerning the magnetic bubble structure in a metallic magnet MnNiGa: Biskyrmions or type-II bubbles. Lorentz microscopy in polycrystalline MnNiGa was used to explain the magnetic bubble structure. Depending on the connection between the magnetic easy axis and the observation plane, two types of magnetic bubbles were formed. Magnetic bubbles with 180{\deg} domains were formed if the easy axis was away from the direction perpendicular to the observation plane. The contrast of biskyrmion is reproduced by this form of a magnetic bubble. When the easy axis was approximately perpendicular to the observing plane, type-II bubbles were observed in the same specimen. The findings will fill a knowledge gap between prior reports on magnetic bubbles in MnNiGa.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic textures in a hexaferrite thin film and their response to magnetic fields revealed by phase microscopy

We investigated magnetic textures in a Sc-doped hexaferrite film by means of phase microscopy (PM) with a hole-free phase plate in a transmission electron microscope. In a zero magnetic field, the stripe-shaped magnetic domains coexist with magnetic bubbles. The magnetization in both magnetic domains was oriented perpendicular to the film and the domain walls have an in-plane magnetization. In the remnant state at 9.2 mT, several magnetic bubbles were formed with the formation of stripe-shaped magnetic domains, and the out-of-plane component in the stripe-shaped domains gradually appeared as the film thickness increased. As the film thickness increases further, the magnetic bubbles with clockwise or counter-clockwise spin helicities formed a triangular lattice. These results in the remnant state suggest that the domain wall energy in the magnetic bubble domains is lower in the thicker region.

cond-mat.mtrl-sci↗

arXiv2020

Magnetic bubbles in an M-type hexagonal ferrite observed by hollow-cone Foucault imaging and small-angle electron diffraction

We report hollow-cone imaging and small-angle electron diffraction of nanoscale magnetic textures such as magnetic-striped domains and magnetic bubbles of M-type hexagonal ferrite BaFe$_{10.35}$Sc$_{1.6}$Mg$_{0.05}$O$_{19}$. The advantage of the hollow-cone Foucault method is that magnetic domains with various directions of magnetization can be visualized under an infocus condition. Moreover, the contrast of magnetic domain walls in magnetic bubbles depends on the inclination angle of the illumination beam. The combination of small-angle electron diffraction and hollow-cone Foucault imaging proves that magnetization at domain walls exhibits in-plane directions in the magnetic-striped domains and magnetic bubbles.

cond-mat.mtrl-sci↗

arXiv2020

Observation of magnetic domains in uniaxial magnets via small-angle electron diffraction and Foucault imaging

Observation of magnetic domains is important in understanding the magnetic properties of magnetic materials and devices. In this study, we report that the magnetic domains of M-type hexaferrites with uniaxial anisotropy can be visualized via small-angle electron diffraction and Foucault imaging. The position of the diffraction pattern spots has the same period as that of magnetic domains in a Sc-substituted hexaferrite (BaFe$_{12-x-\delta}$Sc$_x$Mg$_\delta$O$_{19}$). Conversely, the spots were observed four times longer than the period of magnetic domains in hexaferrite without substitution (BaFe$_{12}$O$_{19}$), demonstrating the long-range order of the Bloch walls. When the specimen was tilted, the magnetic deflection effect, as well as the periodic spots of magnetic domains, occurred. Thus, we were able to visualize the magnetic domains with different magnetization directions and domain orientations by selecting deflection spots. The results indicate that the technique utilized in this study is useful in observing the magnetic materials with uniaxial anisotropy.

cond-mat.mes-hall↗

arXiv2019

Foucault imaging and small-angle electron diffraction in controlled external magnetic fields

We report a method for acquiring Foucault images and small-angle electron diffraction patterns in external magnetic fields using a conventional transmission electron microscope without any modification. In the electron optical system that we have constructed, external magnetic fields parallel to the optical axis can be controlled using the objective lens pole piece under weak excitation conditions in the Foucault mode and the diffraction mode. We observe two ferromagnetic perovskite-type manganese oxides, La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ and Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$, in order to visualize magnetic domains and their magnetic responses to external magnetic fields. In rhombohedral-structured La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, pinning of magnetic domain walls at crystallographic twin boundaries was found to have a strong influence on the generation of new magnetic domains in external applied magnetic fields.

cond-mat.mtrl-sci↗

arXiv2019

Electron diffraction covering a wide angular range from Bragg diffraction to small-angle diffraction

We construct an electron optical system to investigate Bragg diffraction (the crystal lattice plane, $10^{-2}$-$10^{-3}$ rad) with the objective lens turned off by adjusting the current in the intermediate lenses. A crossover was located on the selected-area aperture plane. Thus, the dark-field imaging can be performed by using a selected-area aperture to select Bragg diffraction spots. The camera length can be controlled in the range of 0.8 to 4 m without exciting the objective lens. Furthermore, we can observe the magnetic-field dependence of electron diffraction using the objective lens under weak excitation conditions. The diffraction mode for Bragg diffraction can be easily switched to a small-angle electron diffraction mode having a camera length of more than 100 m. We propose this experimental method to acquire electron diffraction patterns that depict an extensive angular range from 10$^{-2}$ to 10$^{-7}$ rad. This method is applied to analyze the magnetic microstructures in three distinct magnetic materials, i.e., a uniaxial magnetic structure of BaFe$_{10.35}$Sc$_{1.6}$Mg$_{0.05}$O$_{19}$, a martensite of a Ni-Mn-Ga alloy, and a helical magnetic structure of Ba$_{0.5}$Sr$_{1.5}$Zn$_{2}$Fe$_{12}$O$_{22}$.

cond-mat.mtrl-sci↗

arXiv2019

Formation process of skyrmion lattice domain boundaries: The role of grain boundaries

We report on the formation process of skyrmion lattice (SkL) domain boundaries in FeGe using Lorentz transmission electron microscopy and small-angle electron diffraction. We observed that grain boundaries and edges play an important role in the formation of SkL domain boundaries; The SkL domain boundary is stabilized at the intersection of two grains. A micromagnetic simulation using the Landau-Lifshitz-Gilbert equation revealed that the SkL domains separated by a boundary represent the lowest energy configuration. Conversely, in a wide area, SkL domain boundaries were not formed and SkL domains with different orientations rotated to form a single SkL domain.

cond-mat.mes-hall↗

arXiv2006

Domain Nucleation and Annihilation in Uniformly Magnetized State under Current Pulses in Narrow Ferromagnetic Wires

We investigate the current-driven magnetization dynamics in narrow Permalloy wires by means of Lorentz microscopy and electron holography. Current pulses are found to transform the magnetic structure in the uniformly magnetized state below the Curie temperature. A variety of magnetic states including reversed magnetic domains are randomly obtained in low probability. The dynamics of vortices found in most of observed magnetic states seems to play a key role in triggering the magnetization reversal.

cond-mat.mtrl-sci↗

arXiv2006

Current-excited magnetization dynamics in narrow ferromagnetic wires

We investigate the current-excited magnetization dynamics in a narrow ferromagnetic Permalloy wire by means of Lorentz microscopy, together with the results of simultaneous transport measurements. A detailed structural evolution of the magnetization is presented as a function of the applied current density. Local structural deformation, bidirectional displacement, and magnetization reversal are found below the Curie temperature with increasing the current density. We discuss probable mechanisms of observed features of the current-excited magnetization dynamics.

cond-mat.mtrl-sci↗