Shin Eguchi

Transmission electron microscopy study on the crystallization and boron distribution of CoFeB/MgO/CoFeB magnetic tunnel junctions with various capping layers

High-resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS) were used to study the microstructural properties of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with various capping layers. Crystallization of CoFeB layers was strongly dependent on the capping materials, and was affected by B diffusion. With NiFe-cap MTJs, CoFeB crystallized from the cap interface and formed a fcc structure; on the other hand, with Ta- and Ti-cap MTJs, CoFeB crystallized from the MgO interface and formed a bcc structure. EELS analysis showed that B mainly diffused to the capping layers and rarely to the MgO layers with increasing temperature. With Ti-cap MTJs, B diffusion caused hcp-Ti crystals to form an amorphous structure and CoFeB crystallized at lower temperature.

Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density. recording

We have investigated the magnetoresistance properties of spin valves in the current-perpendicular-to-plane (CPP) mode. Conventional single spin-valve (SV) film shows resistance area product RA=81 m/spl Omega//spl mu/m/sup 2/, and resistance change area product /spl Delta/RA=0.35 m/spl Omega//spl mu/m/sup 2/. For a CPP element of a single SV with oxide layers, /spl Delta/RA dramatically increased to 9.0 m/spl Omega//spl mu/m/sup 2/. Micromagnetics simulation revealed that the readback performance of the CPP head with low-impedance conventional SV film was much affected by sense current. On the other hand, the medium impedance CPP head, such as the SV film with oxide layers, shows several advantages compared with the low impedance head, such as higher output voltage, less influence by magnetic field of the sense current, and narrower core width.

Lower-temperature crystallization of CoFeB in MgO magnetic tunnel junctions by using Ti capping layer

Effects of capping materials on magnetoresistance (MR) properties of MgO magnetic tunnel junctions (MTJs) with a CoFeB free layer were investigated. MR ratios of samples with various capping materials showed a difference in annealing temperature dependence. MTJ with a Ti capping layer annealed at 270°C showed a MR ratio 1.4 times greater than that with a conventional Ta or Ru capping layer. Secondary ion mass spectroscopy and high-resolution transmission electron microscopy images revealed that crystallization of CoFeB was remarkably affected by adjacent materials and the Ti capping layer adjoining CoFeB acted as a boron-absorption layer. These results suggest that the crystallization process can be controlled by choosing proper capping materials. Ti is one of the effective materials that accelerate the crystallization of CoFeB layers at lower annealing temperature.

Readout performance of confined-current-path current-perpendicular-to-plane heads

Current-perpendicular-to-plane (CPP) spin valve (SV) films with nano-oxide layers have been developed. The enhancement of giant magnetoresistance by inserting the current confining layer between free and pinned layers was confirmed by the calculation of the current-flow distribution using a finite-element model. For the bottom-type spin valve films with the structure of PdPtMn/CoFeB/Ru/CoFeB/Cu/oxidized CoFeB(t nm)/Cu/CoFeB, MR ratio of 5.0% and RA of 0.6 /spl Omega//spl middot//spl mu/m/sup 2/ were obtained at t=1.2 nm. The merged head using the CPP-SV film with a nano-oxide layer whose MR ratio was 3.2% was fabricated. The magnetic read core width was 130 nm when the physical free-layer width was 100 nm. 0.91 mV/sub p-p/ output voltage was obtained at the sense current of 4.8 mA which corresponds to the average current density of 19 MA/cm/sup 2/.

Real-time fingerprint sensor using a hologram

A holographic fingerprint sensor has been developed for a system that identifies a person by his or her fingerprints. The sensor uses a laser as its light source and consists of a light-conducting plate, which is a transparent glass plate with a plain grating-type hologram, and a focusing lens system just under the hologram. Since the sensor uses a plane-parallel plate, all the optical paths from each point of a fingerprint to the hologram are equal, and a bright fingerprint can be created without the trapezoidal distortion that is inherent in conventional prism-type sensors.

Perpendicular giant magnetoresistance of CoFeB/Cu single and dual spin-valve films

We present giant magnetoresistance measurements in the current-perpendicular-to-plane mode on single and dual spin valves whose areas are from 0.1 to 1.0 μm2. Dependence on the magnetic layer thickness and the structure was investigated. It was revealed that magnetoresistance change increases 20% by insertion of a nonmagnetic spacer layer in the free layer. The results were analyzed within the framework of the two-current series-resistor model, and proved to be consistent with the theory.

Holographic Fingerprint Sensor

The fingerprint sensor that we developed uses a hologram. Two requirements are important for actual use; laser safety and high-contrast images. The illumination method we developed uses total reflection and a new type of detection. For safety, total-reflection lighting ensures that laser beams cannot enter an operators eyes. To obtain high-contrast images, signal and noise light were separated.

The temperature dependence of exchange anisotropy in ferromagnetic/PdPtMn bilayers

We have studied the differences in the temperature dependence of exchange anisotropy of PdPtMn/CoFeB, NiFe, and NiFeTa bilayers to understand the role of ordering temperatures. The three different ferromagnetic layers [Curie temperatures: TCNiFeTa(400 °C)<TCNiFe(570 °C)<TCCoFeB(1040 °C)] exchange biased by the same antiferromagnet [Neel temperature: TNPdPtMn(600 °C)] showed significantly different behavior: the exchange bias field was monotonically decreasing with temperature for the CoFeB and showed distinct, broad peak for NiFe and NiFeTa at a lower temperature before it decreased above 200 °C. The temperature dependence of exchange anisotropy, calculated from the measured exchange bias field and saturation magnetization, was also different for the three bilayers as a result of the differences in Hua and MFM. The results could be understood by the modified thermal fluctuation aftereffect model, which includes the temperature dependence of the ferromagnetic magnetization and the assumption that the order...

Writer pole remanence of a tapered main pole head

We used a micromagnetic model to study the head remanence of a perpendicular tapered main pole head. To reduce the head pole tip remanence, a single magnetic domain structure forming a vortex state from the pole tip to the tapered region is preferred. A tapered main pole structure with a trailing gap depth less than about 30 nm and a tapered angle (TGθ) equal to about 30° improved the writing efficiency. This was achieved with a head field value more than 1.3 times higher and with a head field gradient about 1.15 times higher than that of the conventional structure. Moreover, the pole tip remanence was reduced to a value closer to that of the conventional structure. The reduction in the throat height had less effect on the reduction in the head remanence in the tapered head than in the conventional head. We also discuss both single pole heads and shielded heads.

Compact holographic head-up display

We have developed a head-up display (HUD) using a holographic combiner. A HUD superimposes information in front of the driver. The HUD must be unaffected by sunlight and should be small. A holographic combiner eliminates the VFD wash-out, and compensates for the distortion of concave mirror. The size of unit is reduced by folding the optical path. The image distance is extended to 1.2 m by using a powered element. The dimensions of the HUD unit is 155 X 135 X 77 mm, and the thickness of the optical unit is 40 mm.