Teruo Kohashi

Magnetism in grain-boundary phase of a NdFeB sintered magnet studied by spin-polarized scanning electron microscopy

The magnetism in the grain-boundary phase of a NdFeB sintered magnet was measured by spin-polarized scanning electron microscopy (spin SEM). A sample magnet was fractured in the ultra-high-vacuum chamber to avoid oxidation, and its magnetizations in the exposed grain-boundary phase on the fracture surface were evaluated through the spin polarization of secondary electrons. Spin-SEM images were taken as the fracture surface was milled gradually by argon ions, and the magnetization in the grain-boundary phase was quantitatively obtained separately from that of the Nd2Fe14B phase. The obtained magnetization shows that the grain-boundary phase of this magnet has substantial magnetization, which was confirmed to be ferromagnetic.

A Spin-Polarized Scanning Electron Microscope with 5-nm Resolution.

For studying sub-10-nm-scale magnetic structures, a high-resolution spin-polarized scanning electron microscope (spin SEM) was developed. It has a specially designed, compact secondary-electron collector that produces a finer probe beam than that of a conventional spin SEM. By observing a narrow magnetic domain wall of SmCo5, the developed spin SEM was shown to have a high resolution of 5 nm.

Real Time Magnetic Imaging by Spin-Polarized Low Energy Electron Microscopy with Highly Spin-Polarized and High Brightness Electron Gun

We developed a spin-polarized low energy electron microscopy (SPLEEM) with a highly polarized and high brightness spin electron gun in the present study. Magnetic structures of Co/W(110) were observed with an acquisition time of 0.02 s with a field of view of 6 µm. We carried out a dynamic observation of magnetic structures with the SPLEEM during the growth of Co on W(110).

A spin rotator for detecting all three magnetization vector components by spin‐polarized scanning electron microscopy

A spin rotator for observing magnetic domains with all three magnetization components of a sample surface by spin‐polarized scanning electron microscopy (spin SEM) has been developed. The spin rotator is placed between the sample and the spin detector in a spin SEM, and can rotate the polarization vector of secondary electrons by π/2. Although the spin detector itself can detect only two independent polarization components, the rotation of polarization makes third‐component detection possible. The conventional spin rotator, which is a well‐known energy filter named a Wien filter, has been much improved to have a large focusing area by using hyperbolic cylindrical pole pieces as a magnet and several auxiliary electrodes. As a result, all the secondary electrons emitted from the area of a surface as large as 1 mm in diameter can pass the spin rotator with uniform spin rotation, and the distribution of all three magnetization components can be imaged successfully by spin SEM.

High-resolution spin-polarized scanning electron microscopy (spin SEM)

We have developed spin-polarized scanning electron microscopy (spin SEM) with a 5-nm resolution. The secondary electron optics is very important, as it needs to transfer a sufficient number of secondary electrons to the spin polarimeter, due to the low efficiency of the polarimeter. The optics was designed using a three-dimensional (3D) simulation program of the secondary electron trajectories, and it achieves highly efficient collection and transport of the secondary electrons even though the distance between the sample and the objective lens exit of the electron gun remains short. Moreover, the designed optics enables us to obtain clear SEM images in the spin SEM measurement and to precisely adjust the probe beam shape. These functions lead to images with high spatial resolution and sufficient signal-to-noise (S/N) ratios. This optics has been installed in an ultra-high vacuum (UHV) spin SEM chamber with a Schottky-type electron gun for the probe electron beam. We observed recorded bits on a perpendicular magnetic recording medium and visualized small irregularities in the bit shapes around the track edges and bit boundaries. The high resolution of 5 nm was demonstrated by observing the smallest domain composed by a single grain in the recording medium.

Magnetic domain structure of a La0.7Sr0.3MnO3 (001) surface observed by a spin-polarized scanning electron microscope

The magnetization vector distribution at a cleaved surface of La0.7Sr0.3MnO3 (001) crystal has been quantitatively analyzed by using a newly developed low-temperature spin-polarized scanning electron microscope. The magnetic structure essentially consists of two kinds of domains, where magnetizations are parallel or antiparallel to the [110] direction with no surface-normal component. The rhombus-shaped domains range from a few to several tens of micrometers across. The domain structure can be considered to be made by laying down the magnetization from the out-of-surface-plane easy axis to the surface plane to reduce the magnetostatic energy without forming closure domains.

A spin rotator for spin-polarized scanning electron microscopy

A Wien filter, which is a common energy analyzer, was modified as a spin rotator for use in a spin-polarized scanning electron microscope. By switching the spin rotator on and off, magnetic domain images of all three magnetization vectors can be produced in one scan. The electrodes and the magnetic pole pieces were specially designed by using a three-dimensional computer simulation for electric and magnetic fields, electron trajectories, and spin rotation; the broad beam of the secondary electrons passes through to the spin detector with a 90° rotation. The structure is simple with only two electrodes that have hyperbolically curved surfaces to create a stigmatic focusing effect, while the surfaces of the magnetic pole pieces are flat to enable a uniform rotation of all electron spins. The performance was tested and confirmed to be effective by observing the magnetic domain structures of Fe(001) with in-surface-plane magnetization and a TbFeCo magneto-optical medium with surface normal magnetization.

Observation of domains in obliquely evaporated Co–CoO films by spin-polarized scanning electron microscopy

Magnetic domains in the remanent magnetization state of obliquely evaporated recording media Co–CoO films were studied by spin-polarized scanning electron microscopy. Inverse domains densely distributed in the media were found to have a closure-domainlike structure at the surface to reduce the magnetostatic energy. The size of the inverse domains depends on the direction of the external magnetic fields that were applied when the remanent states were formed. Using these results, the relationship between the inverse domains and the carrier-to-noise ratio for this kind of recording media is discussed.

Systematic Experiment of Mott Scattering

We studied the characteristics of Mott scattering systematically by constructing a special chamber, where target film thickness and the position of electron detectors can be controlled by an engineering workstation using stepping motors. Using this instrument, key parameters such as effective Sherman function and intensity were measured as functions of the scattered electron energy, target film thickness, and electron scattering angle. In the range of the electron energies we studied, i.e., 60–120 keV, a lower energy is preferable to achieve a large figure of merit. The results are compared with previously reported simulations.

The spin-polarized scanning electron microscope observation of 0.1 μm marks recorded by magnetic field modulation on a magneto-optical recording disk

The magneto-optically recorded marks with magnetic field modulation method were studied using spin-polarized scanning electron microscopy. We confirmed that marks with as short a length as 0.1 μm could be written repeatedly even with laser spot sizes about 1.2 μm. This mark length is less than 16 that of present commercial products.