K. Shigeto

Injection of a magnetic domain wall into a submicron magnetic wire

Two types of magnetic wires (150 nm width) with trilayer structure consisting of NiFe (20 nm)/Cu (20 nm)/Co (20 nm) were prepared. One was connected to a square pad (0.5×0.5 μm2) at one end, while the other has a symmetrical shape with two flat ends. Magnetization reversal was detected sensitively by magnetoresistance measurement. Switching field of the Co layer for the wire with a pad was much smaller than that for the wire without a pad. This indicates that a domain wall nucleates initially in the pad and is injected into the wire at the switching field. This model for the magnetization reversal process is supported by the angular dependence of the switching field.

Magnetization reversal in submicron magnetic wire studied by using giant magnetoresistance effect

The magnetization reversal phenomenon in a submicron magnetic wire with a trilayer structure consisting of NiFe(200 A)/Cu(100 A)/NiFe(50 A) was investigated by measuring the electric resistance in an external magnetic field. A giant magnetoresistance (GMR) effect of about 0.8% was observed when the magnetizations in two NiFe layers are oriented antiparallel. It is demonstrated that magnetization reversal phenomena can be very sensitively investigated by utilizing the GMR effect.

Magnetic force microscopy observation of antivortex core with perpendicular magnetization in patterned thin film of permalloy

The cross-tie wall is a kind of magnetic domain wall composed of a main straight wall and crossing subwalls and observed in magnetic thin films. This wall contains two kinds of magnetic vortex structures: “circular vortex” and “antivortex.” At the cores of both vortices, the existence of a spot with perpendicular magnetization has been theoretically predicted. We have detected the perpendicular magnetization spots at each vortex core and identified the direction of it by applying magnetic force microscopy imaging to cross-tie walls in patterned rectangular thin permalloy (Ni80Fe20) films. We also fabricated magnetic structures that contain only antivortex by engineering the shape of thin films.

Dynamics of a magnetic domain wall in magnetic wires with an artificial neck

Magnetization reversal in submicron magnetic wires consisting of a NiFe/Cu/NiFe trilayer with an artificial neck was investigated by utilizing the giant magnetoresistance effect. A magnetic domain wall was injected into the wire by a local magnetic field applied at the end of the wire. Pinning and depinning of the magnetic domain wall were detected as sharp changes in resistance. It was found that the neck works as a pinning site of a domain wall and that the depinning field increases with a decrease of the neck width.

Geometrical confinement of a domain wall in a nanocontact between two NiFe wires

A nanocontact structure (typically 22×34 nm2) between two NiFe wires was fabricated by an electron-beam lithography and a lift-off method, and the magnetoresistance was measured. The magnetization switching process was artificially controlled by engineering the sample geometry to realize a magnetic structure with a single domain wall (DW) trapped in the nanocontact area. This domain structure was confirmed by magnetic force microscopy observations. The magnetization rotation of 180° was realized within the nanocontact area. The contribution of the DW to the resistance was negative, which can be understood on the basis of anisotropic magnetoresistance.

Kerr microscopy observations of magnetization process in microfabricated ferromagnetic wires

The magnetization process in microfabricated NiFe wires was observed using a Kerr microscope. Magnetic wires were made from a 20-nm-thick NiFe film by using lift-off techniques. Their width W and length L were designed as W=0.5, 1.0 and 2.0 μm and L=50 μm, respectively. One end of the wire was connected to a square shaped head with a side of 2W, which is designed to act as a domain wall source. In each wire, necks with different width of 0.2W, 0.6W, and 0.8W were introduced as artificial pinning sites of a domain wall. By using an oil-immersion lens (NA=1.3) and a Hg lamp, magnetization reversals in very narrow wires, as narrow as 0.5 μm, were clearly observed. It is confirmed that domain wall penetration, pinning, depinning, and also the direction of wall motion are controllable using square shaped head and necks with optimized width.

Organic light-emitting diode driven by organic thin film transistor on plastic substrates

A method for fabricating an organic light-emitting diode (OLED) connected to an organic thin film transistor (OTFT) on plastic substrates without heating is proposed. A three-dimensional pixel structure consisting of an OLED and an OTFT is prepared by the proposed method, and the characteristics of the device are tuned by refinement of structural parameters. By room-temperature fabrication, the OTFT with passivation film can be formed on a poly(ethylene naphthalate) plastic substrate, and the transparent anode of the OLED can be fabricated on the passivation film directly. OLED emission is thus generated directly by the current flowing through the OTFT, and the emission intensity is fully controllable by the gate voltage.

Propagation of the magnetic domain wall in submicron magnetic wire investigated by using giant magnetoresistance effect

The magnetization reversal phenomenon in a submicron magnetic wire with a trilayer structure consisting of NiFe(400 A)/Cu(200 A)/NiFe(50 A) was investigated by measuring the electric resistance in external magnetic fields. It is shown that the magnetization reversal can be very sensitively investigated by utilizing the giant magnetoresistance effect. The time variation of resistance during the magnetization reversal was also measured and the velocity of the magnetic domain wall propagating in the wire was determined at 77 K.

Magnetization switching of a magnetic wire with trilayer structure using giant magnetoresistance effect

The switching fields of magnetic wires with trilayer structure consisting of NiFe/Cu/Co were investigated using giant magnetic resistance effect. The switching fields of both magnetic layers were observed to be inversely proportional to wire width (150–520 nm). We found that the magnetization of the NiFe layer switches under much lower applied field than in the case of single layer structure by the assistance of the stray field from the magnetic charge of Co at the edge of the wire. Attaching a pad at one end of the wire causes drastic decrease of the switching field. We investigated pad shape dependence of the switching field of the Co layer. For the sample with a square pad we measured the temperature dependence of the switching field between 5 and 300 K. The dependence at low temperatures between 5 and 50 K can be described by the model on thermally assisted magnetization reversal over a simple potential barrier.

Magnetization reversal and electric transport in ferromagnetic nanowires

Abstract A method for magnetization reversal measurements in submicron magnetic wires was developed by utilizing the giant magnetoresistance effect, which enables us to determine a domain wall position as a function of time, and allows to evaluate the propagation velocity of the domain wall easily. It was found that the magnetization reversal in a wire occurs in association with the propagation of a single domain wall and that an artificial neck introduced into the wire acts as a pinning site of the magnetic domain wall. Injection of the magnetic domain wall from one end of the wire is demonstrated. Results on electric resistance measurements down to 20 mK for ultranarrow Ni wires are also presented. An increase of resistance proportional to T−1/2 was observed in a temperature range from 10 K to 80 mK. The result is discussed in terms of the electron–electron interaction and the weak localization effects.