Published papers

The tunnel magnetocapacitance (TMC) effect in two-dimensional single nanogranular layered Fe/MgF2 films is investigated both experimentally and theoretically. We measured the frequency dependence of TMC ratios in a frequency range of 20Hz-1MHz and discovered that TMC ratios strongly depend on the frequency, reaching a peak value at a specific frequency. We observe that the largest TMC ratios occur at lower frequencies and that TMC values steadily reduce with increasing frequency. Notably, we achieved a maximum TMC ratio of 1.45%, which is the largest low-field TMC ever reported for granular films. A combination of the Debye-Frohlich (DF) model and the Julliere formula is used to fit the experimental data to theory, and an excellent agreement between the calculated values and the experimental data is obtained. To perfectly fit the experimental data, the conventional DF model is extended to a composite model in which three capacitors (with three different relaxation times) are introduced. Our findings will give further insights into the exact mechanism of the TMC effect in nanogranular films and will open broader opportunities for device applications, such as magnetic sensors and impedance tunable devices.

One of the most important achievements in the field of spintronics is the development of magnetic tunnel junctions (MTJs). MTJs exhibit a large tunneling magnetoresistance (TMR). However, TMR is strongly dependent on biasing voltage, generally, decreasing with applying bias. The rapid decay of TMR was a major deficiency of MTJs. Here we report a new phenomenon at room temperature, in which the tunneling magnetocapacitance (TMC) increases with biasing voltage in an MTJ system based on Co40Fe40B20/Mgo/Co40Fe40B20 . We have observed a maximum TMC value of 102% under appropriate biasing, which is the largest voltage-induced TMC effect ever reported for MTJs. We have found excellent agreement between theory and experiment for the bipolar biasing regions using Debye-Frohlich model combined with quartic barrier approximation and spin-dependent drift-diffusion model. Based on our calculation, we predict that the voltage-induced TMC ratio could reach 1100% in MTJs with a corresponding TMR value of 604%. Our work has provided a new understanding on the voltage-induced AC spin-dependent transport in MTJs. The results reported here may open a novel pathway for spintronics applications, e.g., non-volatile memories and spin logic circuits.

The paper presents room-temperature magnetoresistance in molecular spintronic nanodevices fabricated using magnetic thin-film edges.

Oxidation states of AlOx in Co/AlOx/Co spin tunneling junctions were studied using X-ray photoelectron spectroscopy. Co(10nm)/AlOx/Co(50nm) spin tunneling junctions were fabricated on glass substrates by ion-beam mask sputtering. The Al layers were deposited in various thicknesses and were oxidized in pure O-2 gas for 24-48 hours at room temperature. Co/AlOx bilayers simultaneously fabricated were analyzed by XPS. The thicker the Al layer is deposited, the more the unoxidized Al remains. MR ratio of the junctions increased as the unoxidized Al decreased, and this result is qualitatively in agreement with the LMTO calculation result. The O/Al ratio of AlOx layer is about 1.9-2.0, which means existence of the higher order oxides than Al2O3. XPS depth profiles showed that the unoxidized Al remains not only near the interface with Co but also near the surface when the deposited Al layer thickness is, thick.

We investigate the laser wavelength dependence of structural and magnetic transitions on the surface of an iron-aluminum (FeAl) alloy induced by nanosecond pulsed laser irradiation. The formation of self-organized FeAl stripes with a wavelength-dependent period is observed in a local area on the (111)-oriented plane. Focused magneto-optical Kerr effect measurements reveal that the coercivity reaches up to 1.2 kOe with increasing the magnetic field rotation angle, which is estimated from the stripe direction, in FeAl stripes irradiated at 355 nm, and its magnetization reversal can be explained by the domain-wall motion model. On the other hand, the magnetization reversal agrees with the Stoner-Wohlfarth model in FeAl stripes irradiated at 1064 nm. This magnetic transition originates from the B2-to-A2 phase transition in stripe structures and bulk regions. These results indicate that the magnetic transition from the incoherent to coherent mode as well as the structural transformation of stripe patterns can be controlled by the incident laser wavelength. (C) 2015 AIP Publishing LLC.

We investigate the structural and magnetic properties of Ni78Fe22 thin films sandwiched between low softening-point (LSP) glasses, which can be used in spin quantum cross (SQC) devices utilizing stray magnetic fields generated from magnetic thin-film edges. We also calculate the stray magnetic field generated between the two edges of Ni78Fe22 thin-film electrodes in SQC devices and discuss the applicability to spin-filter devices. Using the established fabrication technique, we successfully demonstrate the formation of LSP-glass/Ni78Fe22/LSP-glass structures with smooth and clear interfaces. The coercivity of the Ni78Fe22 thin films is enhanced from 0.9 to 103 Oe by increasing the applied pressure from 0 to 1.0 MPa in the thermal pressing process. According to the random anisotropy model, the enhancement of the coercivity is attributed to the increase in the crystal grain size. The stray magnetic field is also uniformly generated from the Ni78Fe22 thin-film edge in the direction perpendicular to the cross section of the LSP-glass/Ni78Fe22/LSP-glass structures. Theoretical calculation reveals that a high stray field of approximately 5 kOe is generated when the distance between two edges of the Ni78Fe22 thin-film electrodes is less than 5 nm and the thickness of Ni78Fe22 is greater than 20 nm. These experimental and calculation results indicate that Ni78Fe22 thin films sandwiched between LSP glasses are useful as electrodes for SQC devices, serving as spin-filter devices. (C) 2016 Elsevier B.V. All rights reserved.

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 5 particles/ft(3) for the diameter of 0.1 mu m, 0.6 particles for 0.2 mu m, and 0.01 particles for 0.5 mu m 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. (c) 2005 American Institute of Physics.

We have investigated structural and magnetic properties of Ni and Ni 75Fe25 thin films evaporated on polyethylene naphtalate (PEN) organic substrates, which can be expected as electrodes of our proposed nanoscale junctions utilizing thin-film edges. As a result, there is no diffusion of Ni and Fe atoms into PEN substrates, resulting in clear and smooth formation of the interface. The surface roughness is also as small as 0.28-0.37 nm in the same scanning scale as the film thickness. As for the magnetic properties, the squareness of the hysteresis loop is as small as 0.24 for Ni/PEN, where there is no observation of the anisotropy magnetoresistance (AMR) effect. In contrast, the squareness of the hysteresis loop is as large as 0.95 for Ni75Fe25/PEN, where the AMR effect has been successfully obtained. These experimental results indicate that Ni 75Fe25/PEN is a promising material for use in electrodes of nanoscale junctions from the viewpoint of structural and magnetic properties.

We investigated surface morphologies of Ni thin films evaporated on polyethylene naphtalate (PEN) organic substrates for the fabrication of spin quantum cross structures and discussed its feasibility toward ferromagnetic nanojunctions from a viewpoint of the Ni grain size and the Ni surface roughness. The grain size for Ni films of sub-10-nm thickness is ∼30 nm, which is larger than the Ni thickness, and the surface roughness, in the scanning scale of the film thickness, is less than 0.25 nm, corresponding to one-atomic-layer thickness. These experimental results indicate that spin quantum cross structures which consist of Ni thin films on PEN substrates can be expected as a candidate of ferromagnetic nanojunctions, which may lead to large magnetoresistance effect.

We have investigated surface roughnesses and magnetic properties of Co ferromagnetic thin films evaporated on polyethylene naphtalate (PEN) organic substrates. As a result, the surface roughness decreases from 1.3 to 0.55 nm with increasing the Co thickness up to 55 nm, where a two-step smoothing phenomenon can be seen. As for magnetic properties, the coercive force and the squareness of the hysteresis loop show the maximum values at a Co thickness of 5.3 nm. This experimental result can be explained by the competition between the shape magnetic anisotropy and the induced magnetic anisotropy of Co ferromagnetic thin films. Copyright © 2002 - 2012 Hogrefe Publishing.

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.

Quantum cross (QC) devices which consist of two Ni thin films deposited on polyethylene naphthalate substrates with their edges crossing have been fabricated and their current-voltage characteristics have been investigated. The cross-sectional area between the two Ni electrodes, which was obtained without the use of electron-beam or optical lithography, can be as small as 17 nm x 17 nm. We have successfully obtained ohmic current-voltage characteristics, which show good agreement with calculation results within the framework of the modified Anderson model. The calculated results also predict a high switching ratio in excess of 100000:1 for QC devices having the molecule sandwiched between the Ni electrodes. This indicates that QC devices having the molecule can be expected to have potential application in novel switching devices.

The rf magneto-impedance effect of spin tunneling junctions Co/Al-oxide/Co fabricated on glass substrate by ion-beam sputtering is investigated. To measure junction impedance, a measurement method is established. Junction impedance can be determined from the voltages of two cobalt electrodes and a standard resistor at room temperature over a frequency range of 0.1-100 MHz using a lock-in amplifier by taking input impedance into account. The real part of magneto-impedance ratio exhibits a positive value at frequencies less than the roll-off frequency and a negative value at frequencies higher than the roll-off frequency. Furthermore, the imaginary part of the magneto-impedance ratio is found to be positive across the entire frequency region measured and to approach zero at high frequencies. (C) 2004 American Institute of Physics.