Yoshihiko Togawa

Double-biprism electron interferometry

Electron holography based on two electron biprisms was developed. The upper biprism was installed just on the image plane of the objective lens, and the lower one was set between the crossover point and image plane of the magnifying lens. This system was able to control two important parameters of the hologram—fringe space and width of interference region—independently. The system enabled us to perform electron holography and interferometry more flexibly. We confirmed the good performance of the system and did preliminary applications using a 1-MV field-emission electron microscope.

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.

Direct observation of the washboard noise of a driven vortex lattice in a high-temperature superconductor, Bi2Sr2CaCu2Oy

We studied the conduction noise spectrum in the vortex state of a high-temperature superconductor, Bi(2)Sr(2)CaCu(2)O(y), subject to a uniform driving force. Two characteristic features, a broad-band noise (BBN) and a narrow-band noise (NBN), were observed in the vortex-solid phase. The origin of the large BBN was determined to be plastic motion of the vortices, whereas the NBN was found to originate from the washboard modulation of the translational velocity of the driven vortices. We believe this to be the first observation of washboard noise of dc driven vortices in any superconductor.

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.

Current-excited magnetization reversal under in-plane magnetic field in a nanoscaled ferromagnetic wire

We microscopically demonstrate that the magnetic domain is controllably nucleated and erased in the uniformly magnetized wire using a current pulse in small magnetic fields. Lorentz microscopy is performed in Permalloy nanowires with in-plane anisotropy. The stochastic nature of the magnetization reversal due to spin wave and thermal excitations in the absence of magnetic field completely disappears and turns into deterministic in the presence of small magnetic field, which enables the magnetization reversal control. We interpret that the phenomena are associated with Zeeman energy stabilization.

Discrete Change in Magnetization by Chiral Soliton Lattice Formation in the Chiral Magnet Cr1/3NbS2

In the chiral magnet Cr1/3NbS2, discrete changes in the magnetization (M) caused by the formation of a chiral soliton lattice (CSL) were observed in magnetization curve measurements using a single crystal of submillimeter thickness. When M is measured with a minimal increment of the magnetic field H, 0.15 Oe, discrete changes in M are observed in the H region that exhibits definite magnetic hysteresis. In particular, enormous discrete changes in M are observed as H decreases from the field above the saturation field, while fine M steps are also found in the intermediate H range independently of the sweeping direction of the field. The former is considered as a type of enormous Barkhausen effect associated with the CSL formation. The latter originates from the change in soliton number during the CSL formation.

Laser-Induced Skyrmion Writing and Erasing in an Ultrafast Cryo-Lorentz Transmission Electron Microscope

We demonstrate that light-induced heat pulses of different duration and energy can write Skyrmions in a broad range of temperatures and magnetic field in FeGe. Using a combination of camera-rate and pump-probe cryo-Lorentz transmission electron microscopy, we directly resolve the spatiotemporal evolution of the magnetization ensuing optical excitation. The Skyrmion lattice was found to maintain its structural properties during the laser-induced demagnetization, and its recovery to the initial state happened in the sub-μs to μs range, depending on the cooling rate of the system.

Small angle electron diffraction and deflection

Electron optical system is constructed in order to obtain small angle diffraction and Lorentz deflection of electrons at the order of down to 10-6 radian in the reciprocal space. Long-distance camera length up to 3000 m is achieved in a conventional transmission electron microscope with LaB6 thermal emission type. The diffraction pattern at 5 × 10-6 radian is presented in a carbon replica grating with 500 nm lattice spacing while the magnetic deflection pattern at 2 × 10-5 radian is exhibited in Permalloy elements. A simultaneous recording of electron diffraction and Lorentz deflection is also demonstrated in 180 degree striped magnetic domains of La0.825Sr0.175MnO3.

High-resolution observation by double-biprism electron holography

High-resolution electron holography has been achieved by using a double-biprism interferometer implemented on a 1MV field emission electron microscope. The interferometer was installed behind the first magnifying lens to narrow carrier fringes and thus enabled complete separation of sideband Fourier spectrum from center band in reconstruction process. Holograms of Au fine particles and single-crystalline thin films with the finest fringe spacing of 4.2pm were recorded and reconstructed. The overall holography system including the reconstruction process performed well for holograms in which carrier fringes had a spacing of around 10pm. High-resolution lattice images of the amplitude and phase were clearly reconstructed without mixing of the center band and sideband information. Additionally, entire holograms were recorded without Fresnel fringes normally generated by the filament electrode of the biprism, and the holograms were thus reconstructed without the artifacts caused by Fresnel fringes.

Location-sensitive measurement of the local fluctuation of driven vortex density in Bi 2 Sr 2 CaCu 2 O y

To investigate the dynamics of driven vortices in superconductors, noise in the local vortex density was investigated in the mixed state of a high-