Hideo Kaiju

Magnetocapacitance effect of spin tunneling junctions

Magnetocapacitance effect of coercive differential spin tunneling junctions Co(100 A)/Al2O3(∼20 A)/Co(500 A) fabricated onto a glass substrate by ion-beam mask sputtering was investigated. The impedance was measured by a four-probe method at room temperature in the frequency range from 120 Hz to 1 MHz. It is found that the effective capacitance changes with the application of an external magnetic field. At high frequencies, the magnetocapacitance ratio is as large as the dc magnetoresistance ratio. However, at low frequencies, capacitance changes cannot be observed because the measurement sensitivity is too low. The magnetocapacitance effect was hence found to be a promising tool for high frequency magnetic sensing.

Theoretical and experimental results of electronic transport of spin quantum cross structure devices

Recently, we have proposed quantum cross structure (QCS) devices that consist of two metal thin films deposited on organic films with edge-to-edge configuration like crossed fins for switching devices. In this paper, we propose a spin quantum cross structure (SQCS) device, which is a QCS device consisting of two magnetic thin films. We show theoretical and experimental results of electronic transport characteristics regarding SQCS devices. The calculation of the I-V characteristics has been performed for the SQCS devices with the Ni magnetic thin films for both the electrodes within the framework of the Anderson model. Then, we fabricated a SQCS device with the Ni magnetic thin films and measured the I-V characteristics by a four-terminal method. Also, the calculation of the magnetoresistance ratio has been done as a function of renormalized transfer matrices including magnetostriction effects and the other effects phenomenologically.

Surface Morphology of Gold Thin Films Deposited on Poly(ethylene naphthalate) Organic Films for Quantum Cross Devices

The surface morphology of gold thin films deposited on poly(ethylene naphtalate) (PEN) organic films has been investigated for quantum cross devices. The surface roughness of gold thin films on the PEN films is 1.5–1.9 nm and the appearance of mound structures is observed. The mound grain sizes are 28.0±4.6 nm for 5-nm-thick gold films and 45.8±5.8 nm for 10-nm-thick gold films. From the result of the scaling investigation of the surface roughness, the surface roughness of 5-nm-thick gold films is 0.22 nm, corresponding to one atomic size, in the scanning scale of 5 nm. These experimental results indicate that gold thin films on PEN films are suitable for use in quantum cross devices, and may open up a novel research field on the electric characteristics of quantum cross devices using a few atoms or molecules leading to high-density memories.

Co thickness dependence of structural and magnetic properties in spin quantum cross devices utilizing stray magnetic fields

We investigate the Co thickness dependence of the structural and magnetic properties of Co thin-film electrodes sandwiched between borate glasses in spin quantum cross (SQC) devices that utilize stray magnetic fields. We also calculate the Co thickness dependence of the stray field between the two edges of Co thin-film electrodes in SQC devices using micromagnetic simulation. The surface roughness of Co thin films with a thickness of less than 20 nm on borate glasses is shown to be as small as 0.18 nm, at the same scanning scale as the Co film thickness, and the squareness of the hysteresis loop is shown to be as large as 0.96–1.0. As a result of the establishment of polishing techniques for Co thin-film electrodes sandwiched between borate glasses, we successfully demonstrate the formation of smooth Co edges and the generation of stray magnetic fields from Co edges. Theoretical calculation reveals that a strong stray field beyond 6 kOe is generated when the Co thickness is greater than 10 nm at a junction gap distance of 5 nm. From these experimental and calculation results, it can be concluded that SQC devices with a Co thickness of 10–20 nm can be expected to function as spin-filter devices.

Study of very low airborne particle count in clean-unit system platform

Cleanliness of a clean-unit system platform consisting of multiply connectable clean boxes with feedback loop is studied and the time dependence of particle count is clarified experimentally and theoretically in order to achieve highly clean environment. Based on this analysis, we have demonstrated airborne particle counts as low as 5particles∕ft3 for the diameter of 0.1μm, 0.6 particles for 0.2μm, and 0.01 particles for 0.5μm by a reduction of particle density coming out from the in-wall in the system. The clean-unit system with feedback loop can serve as a platform for cross-disciplinary experiments and production for fields such as nanotechnologies and biotechnologies.

Surface morphologies and magnetic properties of Fe and Co magnetic thin films on polyethylene naphthalate organic substrates

We have studied the surface morphologies and magnetic properties of Fe and Co thin films evaporated on polyethylene naphthalate (PEN) organic substrates toward the fabrication of spin quantum cross devices. As a result, the surface roughnesses of Co (6.1 nm)/PEN and Co (12 nm)/PEN are as small as 0.1 and 0.09 nm, respectively, corresponding to less than one atomic layer, in the same scanning scale as the thickness. As for the magnetic properties, the coercive force of the Co/PEN shows the constant value of 2 kA/m upon decreasing the Co thickness from 35 to 10 nm, and it increases up to 7 kA/m upon decreasing the Co thickness from 10 to 5 nm. It decreases when the Co thickness is less than 5 nm. These results can be explained by the competition between the shape magnetic anisotropy and the induced magnetic anisotropy.

Large magnetocapacitance effect in magnetic tunnel junctions based on Debye-Fröhlich model

The frequency dependence of tunneling magnetocapacitance (TMC) in magnetic tunnel junctions (MTJs) is investigated theoretically and experimentally. According to the calculation based on Debye-Frohlich model combined with Julliere formula, the TMC ratio strongly depends on the frequency and it has the maximum peak at a specific frequency. The calculated frequency dependence of TMC is in good agreement with the experimental results obtained in MgO-based MTJs with a tunneling magnetoresistance (TMR) ratio of 108%, which exhibit a large TMC ratio of 155% at room temperature. This calculation also predicts that the TMC ratio can be as large as about 1000% for a spin polarization of 87%, while the TMR ratio is 623% for the same spin polarization. These theoretical and experimental findings provide a deeper understanding on AC spin-dependent transport in the MTJs and will open up wider opportunities for device applications, such as highly sensitive magnetic sensors and impedance-tunable devices.

Magnetic properties on the surface of FeAl stripes induced by nanosecond pulsed laser irradiation

We demonstrate the formation of magnetic nanostripes on the surface of Fe52Al48 induced by nanosecond pulsed laser irradiation and investigate their magnetic properties. The magnetic stripe consists of a disordered A2 phase of Fe-Al alloys with Al-oxide along the [110] direction on the (111)-oriented plane. According to the focused magneto-optical Kerr effect measurement, the coercive force of the magnetic stripe obeys the 1/cos θ law, where θ is the field rotation angle estimated from the stripe direction. Also, the jump field can be observed in the magnetic hysteresis loop. These results indicate that the magnetization reversal in the magnetic stripe originates from the domain pinning, showing that the magnetization rotates incoherently.

Nanopatterns induced by pulsed laser irradiation on the surface of an Fe-Al alloy and their magnetic properties

We have studied nanopatterns induced by nanosecond pulsed laser irradiation on (111) plane surfaces of a polycrystalline iron-aluminum alloy and evaluated their magnetic properties. Multiple nanosecond pulsed laser irradiation induces a wavelength-dependent surface transformation of the lattice structure from a B2-type to a supersaturated body centered cubic lattice. The selective formation of surface nanopatterns consisting of holes, stripes, polygonal networks, and dot-like nanoprotrusions can be observed. Furthermore, focused magneto-optical Kerr effect measurements reveal that the magnetic properties of the resultant nanostructured region changes from a paramagnetic to a ferromagnetic phase in accordance with the number of laser pulses.

Surface Roughness and Magnetic Properties of Ni and

We have studied structural, electrical, and magnetic properties of Ni and Ni78Fe22 thin films evaporated on polyethylene naphtalate (PEN) organic substrates towards the fabrication of spin quantum cross (SQC) devices. As we have investigated the scaling properties on the surface roughness, the surface roughness of Ni (16 nm)/PEN is 0.34 nm, corresponding to 2 or 3 atomic layers, in the scanning scale of 16 nm, and the surface roughness of Ni78Fe22 (14 nm)/PEN is also as small as 0.25 nm, corresponding to less than 2 atomic layers, in the scanning scale of 14 nm. These facts denote that Ni/PEN and Ni78Fe22/PEN are suitable for magnetic electrodes on organic substrates used for SQC devices from the viewpoint of the surface morphology. Then, we have investigated magnetic hysteresis curve and magnetoresistance effects for Ni/PEN and Ni78Fe22/PEN. The squareness of the hysteresis loop is as small as 0.24 for Ni (25 nm)/PEN, where there is no observation of the anisotropy magnetoresistance (AMR) effect. In contrast, the squareness of the hysteresis loop is as large as 0.86 for Ni78Fe22 (26 nm)/PEN, where the AMR effect has been successfully obtained. These experimental results indicate that Ni78Fe22/PEN is a promising material for use in SQC devices from the viewpoint of not only the surface morphologies but also magnetic properties.