Weixing Xia

Skyrmion-skyrmion and skyrmion-edge repulsions in skyrmion-based racetrack memory

Magnetic skyrmions are promising for building next-generation magnetic memories and spintronic devices due to their stability, small size and the extremely low currents needed to move them. In particular, skyrmion-based racetrack memory is attractive for information technology, where skyrmions are used to store information as data bits instead of traditional domain walls. Here we numerically demonstrate the impacts of skyrmion-skyrmion and skyrmion-edge repulsions on the feasibility of skyrmion-based racetrack memory. The reliable and practicable spacing between consecutive skyrmionic bits on the racetrack as well as the ability to adjust it are investigated. Clogging of skyrmionic bits is found at the end of the racetrack, leading to the reduction of skyrmion size. Further, we demonstrate an effective and simple method to avoid the clogging of skyrmionic bits, which ensures the elimination of skyrmionic bits beyond the reading element. Our results give guidance for the design and development of future skyrmion-based racetrack memory.

High-field gradient single-pole head with an improved pole structure

The relationship between the head field and the pole structure in a single-pole head is investigated by finite-element method calculation. The calculation reveals that the regular single-pole head does show a relatively large field gradient with a maximum of 150 Oe/mn, which is crucial for high-resolution recording, but only for relatively large field strength. By placing an additive pole in the vicinity of the main pole, the head-field gradient is improved. The new pole structure increases the gradient up to 250 Oe/nm. The large field gradient is obtained in a wider range of field strength than the regular single-pole head. The calculation also shows a simulated recorded transition when the large head field gradient becomes narrow.

Investigation of magnetic structure and magnetization process of yttrium iron garnet film by Lorentz microscopy and electron holography

The micromagnetic structure and magnetization process of perpendicular Y3Fe5O12 (YIG) films were studied by Lorentz microscopy and electron holography. The closure domain structure inside the thin transmission electron microscopy specimen exhibits the same period as the magnetization pattern observed by magnetic force microscopy indicating the perpendicular anisotropy of the YIG film. Through observation of stray fields, it is concluded that the shapes of domain and domain walls are sensitive to the specimen thickness; moreover, a closure domain configuration observed in thin specimen is the stable energy state as determined by the balance between the crystalline anisotropy and shape anisotropy. Domain wall movement is observed by applying a magnetic field, in situ, inside the microscope in both horizontal and perpendicular directions; the saturation fields observed are qualitatively in agreement with the results of the hysteresis loop.

Resolution improvement of transition width with shielded pole writer

The resolution improvement of isolated magnetization transition with shielded pole writer was investigated with a two-dimensional (2-D) finite-element method (FEM) calculation using a media model. Two kinds of single-pole type (SPT) head were used in the calculation. The medium coercivity was varied from 3000 Oe to 9000 Oe. The transition parameter /spl pi/a was improved from 56.80 nm (Hc=3000 Oe) to 7.14 nm (Hc=9000 Oe) using the regular SPT head, however a significant reduction was achieved from 26.32 nm (Hc=3000 Oe) to 5.26 nm (Hc=9000 Oe) using the front yoke head with high gradient. The recording resolution can be improved with the front yoke head for a large range of media.

The microstructure and magnetic properties of anisotropic polycrystalline Nd2Fe14B nanoflakes prepared by surfactant-assisted cryomilling

A new method for fabricating rare-earth-transition metal nanoflakes and nanoparticles, surfactant-assisted cryomilling (SACM), was developed. The effects of milling temperature on the particle size, microstructure and magnetic performance of Nd2Fe14B nanoflakes were investigated systematically. Compared with Nd2Fe14B nanoflakes prepared by surfactant-assisted ball milling (SABM) at room temperature, the samples prepared by SACM showed more intriguing features such as smaller particle sizes, larger microstrain, smaller grain size and higher coercivity, which were ascribed to a higher defect concentration generated in the nanoflakes. The optimal coercivity of the samples prepared by SACM was about 50% higher than that of the samples milled at room temperature. It is demonstrated that SACM is an effective way to prepare rare-earth-transition metal nanoflakes with higher coercivity and smaller particle size. These findings are of importance for research on sintered magnets and high-performance nanocomposite magnets.

Sm2Fe17Nx nanoflakes prepared by surfactant assisted cryomilling

Hard magnetic Sm2Fe17Nx nanoflakes were prepared by surfactant assisted ball milling (SABM) at low temperature using liquid nitrogen cooling (surfactant assisted cryomilling). 2-methyl pentane was used as a milling medium and oleylamine as a surfactant. The morphology, structure, and magnetic properties of Sm2Fe17Nx were investigated using scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometer, respectively. By comparing Sm2Fe17Nx nanoflakes produced by SABM at low temperature and at room temperature, we found that the former has many interesting features: more homogeneous morphology, smaller size, especially markedly increased remanent magnetization (Mr), saturation magnetization (Ms), and remanent magnetization ratio (Mr/Ms). These Sm2Fe17Nx nanoflakes have great potential to be used for preparing high performance bonded permanent magnets.

Changes of Magnetic Anisotropy of CoPtCr Perpendicular Films Due to Ru Intermediate Layer Under High Gas Pressure

The uniaxial magnetic anisotropy K_u of CoPtCr-SiO2 granular perpendicular recording media increases when they are deposited on a Ru intermediate layer with high Ar gas pressure. In order to clarify the reason for the increase in K_u , we investigated the correlation between K_u of continuous CoPtCr film and the Ar gas pressure of the Ru layer. Six kinds of CoPtCr films with the compositions close to practical CoPtCr-SiO2 recording media were deposited on the Ru layer with the Ar gas pressure varying from 0.6 to 8.0 Pa. The increase in K_u of continuous CoPtCr films was confirmed. Through the investigation on microstructure using X-ray diffraction, we found that the stacking faults or the fraction of face center cubic (FCC) phase of CoPtCr grains were independent on the gas pressure of Ru while the ratio of hexagonal close packed (HCP) crystal constant c/a monotonously decreased with increasing gas pressure. We conclude that the increase in K_u of CoPtCr continuous film is mostly related to the decease of c/a due to high gas pressure of Ru layer.

Morphology and magnetic properties of SmCo3/α-Fe nanocomposite magnets prepared via severe plastic deformation

We report bulk SmCo3/α-Fe nanocomposite magnets prepared via high energy ball milling and warm compaction. The evolution of structure and magnetic properties with soft phase fraction have been systematically studied. Microstructural studies revealed that grain size of the nanocomposite magnets can be controlled below 20 nm with a homogeneous distribution of α-Fe phase in the matrix of hard magnetic SmCo3 phase after severe plastic deformation. The refinement of the hard and soft phases morphology in nanoscale leads to effective inter-phase exchange coupling that gives rise to single-phase-like demagnetization behavior with enhanced remanence and maximum energy product (BH)max. The (BH)max up to 13.5 MGOe in the isotropic SmCo3/α-Fe nanocomposites with 25 wt. % of the soft phase has been obtained. Magnetic characterization at elevated temperatures shows that the nanocomposite SmCo3/α-Fe magnets have improved energy product compared to the single-phase SmCo3 magnets.

Observation of Magnetization of Obliquely Evaporated Co-CoO Magnetic Recording Tape

Electron holography is effective for the study of the micromagnetic structure of various advanced magnetic materials. Although the microstructure of obliquely evaporated Co-CoO tapes has been explicitly described so far, the investigation of magnetization in the as-recorded state is indispensable for the in-depth understanding of the magnetization transition mechanism of the tape. In this study, for the first time, a clear image is shown depicting the magnetization transition of an oblique Co-CoO tape obtained by electron holography. The image is compared with those obtained by computer simulation taking into account the recording process. It is found that although the easy axes of Co crystal particles are titled at approximately 45deg to the tape plane, the recorded magnetization shows a circular shape with its center located slightly above the center plane of the tape. Using the images of the magnetization, some possible ways to improve the recording performance of the tape are discussed

Determination of Orbital Location of Electron-Induced Secondary Electrons by Electric Field Visualization

The electric field around positively charged biological specimens is studied by electron holography. By the amplitude reconstruction process for holograms, the orbits of electron-induced secondary electrons are clarified on the nanometer scale. It is found that the stationary orbit of secondary electrons can be directly located without disturbing their motions under the condition that the current of secondary electrons in stationary orbit is considerably larger than that of incident electrons. The experimental conditions for the induction of the stationary orbit of secondary electrons are discussed, and furthermore the theoretical basis of the orbital location of secondary electrons through electric field visualization is discussed in the framework of quantum mechanics.