3. Electron spin control
(1)Magnetoimpedance characteristics in ferromagnetic tunnel junction
A ferromagnetic tunnel junction consisting of ferromagnet-insulator-ferromagnet layer structure has drawn global attention due to its giant magnetoresistance effect (a phenomenon in which resistance varies significantly in response to a magnetic field) at room temperature.
In contrast, there are few reports on the magnetoimpedance effect (a phenomenon in which impedance varies in response to a magnetic field) in a ferromagnet tunnel junction.
This laboratory has studied the magnetoimpedance effect, focusing on AC impedance characteristics of a ferromagnet tunnel junction.
In fact, a Co-AlOx-Co-layered ferromagnet tunnel junction was fabricated and the impedance characteristics was examined. The results clarified that not only the real part, but also the imaginary part of impedance changes in response to the magnetic field.
Such a magneto impedance effect is applicable to future magnetic devices including high-sensitive magnetic sensors for high frequencies and the magnetic head of hard disk drives (HDD).
(2)Fabrication of ferromagnetic nano-scale junction and its structure, electrical conduction and magnetism properties.
In a nano-scale junction of metal, magnetic body, and molecules, curious diverse phenomena are observed. These include the quantization of conductance by ballistic conduction, emergence of an electrical switching effect, and the generation of giant magnetoresistance effect, among others.
As a way to fabricate such a nano-scale junction, various methods such as lithography, breakjunction and nanoindentation have been proposed.
Under such an environment, this laboratory has recently proposed a new fabrication method of nano-scale junction using edges of magnetic thin films. In this method, as edges of two magnetic thin films meet at right angles to each other, the junction area (S = d x d, where d is the film thickness) is determined by the thickness of the magnetic thin film.
Accordingly, when the film thickness is from 1 to 20 nm, the junction area is from 1x1 to 20x20 nm2. Thus, ultrafine nano-scale junctions can be fabricated.
In fact, employing this fabrication method, a variety of nano-scale junctions were fabricated. The results showed success in observing an ormic behavior in Ni-Ni nano junction, a nano-scale tunnel phenomenon in Ni-NiO-Ni nanojunction, intramolecular ballistic conduction in Ni-P3HT and PCBM-Ni nanojunctions. (joint research with Ishibasi Laboratory and Kondo Laboratory of Hokkaido University, Hirotsu Laboratory of Osaka University, and Ishimaru Laboratory of Kyushu Institute of Technology)
This device has the potential to find further new phenomena, and thereby an attempt will be made to create a next-generation switching device and a beyond-CMOS device.
(3)Surface and internal structures and magnetism properties of pulse-laser-irradiated magnetic alloy
Regarding magnetic nanostructures comprised of magnetic body such as nanodot, nanostripe, nanowire and nanotube, interesting phenomena including coercivity enhancement and current-driven domain wall have been expressed.
Various methods to fabricate such magnetic nanostructures such as lithography, shadow mask and ion irradiation are proposed.
Under such circumstances, the laboratory has recently revealed that nano-second irradiation of pulse laser to FeAl alloy creates change in the surface morphology and brings about paramagnet-ferromagnet magnetism transition. (joint research with Watanabe Laboratory of Hokkaido University and Yoshimi Laboratory of Tohoku University)
In terms of surface morphology, interesting structures such as nanostripe structures, nanonetwork structures and nanodot patterns can be obtained.
Subsequently with this structural phase transition, the magnetism property changes from paramagnetism to ferromagnetism.
These results imply potential to control not only surface and internal structures, but also magnetism through light irradiation.
At present, using the same irradiation method, the laboratory is propelling an unprecedented exploratory research on nanostructured magnetic material.
Moreover, in this system, the spin transport is also studied to pioneer this research field of interdisciplinary new frontier.