Hirotaka Oshima

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.

SPIN-DEPENDENT CONDUCTANCE QUANTIZATION IN NICKEL POINT CONTACTS

We have observed systematic change of the quantized conductance in point contacts of macroscopic nickel wires under varying external parameters, the temperature (above and below Tc) and the applied magnetic field. The observed quantized conductance can be explained by the number of the channels for electronic transport through the point contact and the spin-dependent transmission coefficient.

Switching behavior of epitaxial perovskite manganite thin films

We have observed electric-field-induced and photoinduced switching from insulating to conducting state in stressed epitaxial Sm0.5Sr0.5MnO3 thin films fabricated by pulsed-laser deposition. Previously known only in bulk crystals of perovskite manganese oxides, the switching behavior in thin film form is more significant for potential practical utility of the strongly correlated electron systems including optical devices.We have observed electric-field-induced and photoinduced switching from insulating to conducting state in stressed epitaxial Sm0.5Sr0.5MnO3 thin films fabricated by pulsed-laser deposition. Previously known only in bulk crystals of perovskite manganese oxides, the switching behavior in thin film form is more significant for potential practical utility of the strongly correlated electron systems including optical devices.

Detecting dynamic signals of ideally ordered nanohole patterned disk media fabricated using nanoimprint lithography

The authors have fabricated ideally ordered alumina nanohole patterned disk media via anodic oxidation and nanoimprint lithography with a thermoplastic resist. The ordered arrays of alumina nanoholes with 100nm pitch, filled with Co by electrodeposition, were created over a macroscopically large area on a hard-disk substrate using these industrially applicable nanofabrication technologies. Stable flight of a perpendicular magnetic head above the media and perpendicular magnetic anisotropy of the Co nanopillars enable high-speed dynamic magnetic recording and playback. Dynamic periodic signals that matched the nanopillar periodicity were clearly observed after writing bit patterns, showing alternate reversal of magnetization of the nanopillars.

Controlled filling of Permalloy into one-end-opened carbon nanotubes

Controlled filling of Permalloy into a one-end-opened carbon nanotube (∼40 nm in inner diameter, 900 nm in length) array embedded in an anodic aluminium oxide film was achieved by an electrochemical route. Thanks to the electron-microscopically transparent carbon nanotubes, the time course of the electrochemical deposition process was readily visualized by means of transmission electron microscopy, revealing that Permalloy filling started from the pore bottom of the carbon nanotubes. This novel bottom-up filling opens a route to controllable filling of materials of interest into carbon nanotubes with one open end. The magnetic properties of the controlled filled carbon nanotubes in the array were studied. Interestingly, even in the case of isolated Permalloy nanoparticles inside the carbon nanotubes in the array, prominent magnetic anisotropy with an easy magnetization direction along the carbon nanotube axis was observed. In addition to the Permalloy-filled carbon nanotube array as embedded in the anodic aluminium oxide film, individual carbon nanotubes filled with Permalloy were also obtained.

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/.

Micromagnetic Simulations of Magnetization Reversal in Misaligned Multigrain Magnets With Various Grain Boundary Properties Using Large-Scale Parallel Computing

This paper reports on micromagnetic simulations of the magnetization reversal behavior in polycrystalline Nd-Fe-B sintered magnets using large-scale parallel computing. A multigrain model is introduced to calculate the grain alignment dependence of the coercivity of polycrystalline Nd-Fe-B magnets with various magnetic characteristics at grain boundaries (GBs). The magnetic domain wall motion is accurately treated by dividing the analyzed object into extremely small elements. The multigrain model with a soft magnetic GB phase and a reverse domain at the initial state well reproduces experimental results. The calculations of coercivity with several GB widths are also carried out to seek for the origin of the sudden decrease of coercivity with nearly perfect grain alignment.

Magnetic behavior in an ordered Co nanorod array

The magnetization reversal process of an ordered Co nanorod array is shown using the images obtained from successive in-field magnetic force microscope (MFM) measurements. The magnetization reversal model is discussed according to local and whole magnetization reversal properties measured by the polar magneto-optical Kerr effect (PMOKE) and an alternating gradient magnetometer (AGM), respectively. Additionally, the dipolar field was probed using in-field MFM measurements. By removing the effect of the dipolar field, an intrinsic switching field distribution (SFD) is shown in a map with a hexagonal array. A detailed study of the dipolar field in ordered nanorod arrays with various diameters and pitches was carried out by numerical calculations.

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.

Nanopattern transfer from high-density self-assembled nanosphere arrays on prepatterned substrates

We have fabricated nanoimprint moulds with high-density well-defined nanopatterns by pattern transfer from self-assembled nanosphere arrays on prepatterned substrates. Silica nanospheres of 100 and 25 nm diameter were regularly arranged over large areas in a self-assembling manner by capillary force via a dip-coating technique on topographically patterned substrates having 220 nm pitch line/space patterns. The nanosphere arrays were used as etching masks, and nanodot arrays with the same arrangements were created on the silica substrate surfaces by reactive ion etching (RIE). By developing a combined pattern transfer process using Ru and SiO(x) mask layers and CF4 and O2 RIE, the aspect ratio between the height and diameter of the nanodots made from the 25 nm nanospheres is improved to about two. It is demonstrated that the nanopatterns of the moulds can be inversely transferred into polymer surfaces reproducibly by UV nanoimprint process.