Research
The quantity of digitalized information generated in the world has increased approximately ten-fold in the past five years, in the modern information society. An enormous amount of electronic devices are embedded in various kinds of electric instruments and used for performing information processing. As a consequence, the total amount of electric power that electronic devices consume has enlarged rapidly. Since the information society is still in its earliest days, the rapid progress of digitalization of information will have a significant and continuous social impact in the future. Minimizing power consumption of electronic devices is a critical issue in modern society and needs to be achieved as quickly as possible through technological innovations. In this respect, it is quite important to understand and manipulate two characteristic properties of electrons responsible for electronics, namely charge and spin, in the viewpoint different from the conventional one.
In our research group, we explore the physical properties of the spin degree of freedom, which has hardly been utilized in electronics. It is our ambition to design and create novel device applications using electron spins. We tackle seriously the fundamental physical issues on how to manipulate electron spins as the origin of magnetism and enlighten the physical properties unique to quantum spins. At the same time, we are challenging to meet the demand by the industry on how to make a breakthrough in the technological limit of the information processing as found in a hard disk drive. We are also investigating electronic and structural properties as well as spin properties widely found in various kinds of electronic materials, taking advantage of experimental methods we are developing in the laboratory.
Chiral Material Science
It is found recently that chiral materials exhibit macroscopic spin response. In particular, chiral crystals induce polarization phenomena over the crystal because a spatial inversion symmetry breaking imbedded in the unit cell spreads throughout the crystal. Namely, chirality in materials could be the key ingredient for developing the giant physical response on a macroscopic length scale. Given the diversity of chiral materials, it will be an interdisciplinary research subject involving a wide range of researchers in biology, chemistry, and physics. We are exploring such a new research field, which should be called “Chiral Material Science”.
Spin Phase Electronics (Spin electronics using spin phase coherence in chiral magnetism)
We are exploring a novel research field called ‘Spin Phase Electronics.’ Here, we focus on ">the spin phase order which appears coherently in chiral magnetic materials. Utilizing quantum functions arising from the spin phase coherence that the spin phase order exhibits in a macroscopic scale, we try to generate the fundamental principle. This will contribute directly to the fields of data processing technology and to open up a novel route to the application of innovative magnetic devices. Our challenge has attracted much attention as a novel and promising approach in spin electronics. For instance, we have demonstrated that magnetoresistance (MR) becomes giant, discretized and multi-valued. We have also shown that magnetic resonance occurs in discretized, broadband, multi-mode frequencies. These material responses reflect the coherent and topological nature of spin phase order and have attracted much attention in fundamental and application viewpoints. This research field is sometimes called chiral spin electronics recently.
Spin Electronics
One of our main research topics is a research field called Spin Electronics. In conventional electronics, the charge degree of freedom that electrons possess is mainly utilized, while the spin degree of freedom becomes a key factor in spin electronics. Then, what is the benefit from spin electronics? For instance, based on the technology developed in spin electronics, we are able to alter the magnetic state of a tiny magnet (magnetic material) simply by applying an electric current to the magnet. This is achieved by the flow of spins, frequently called ‘spin current’, involved in the flow of electrons (corresponding to electric current) in the material. Before this innovative development, we have only noticed that the application of large external magnetic field to the material is useful to change the magnetic state. There are many reports that may alter our conventional understanding of the physical phenomena in magnetic materials. Spin electronics has attracted much attention as a research field that can contribute to the further development of electronics. The following topics are examined in our research group.
/ Microscopic analysis of magnetization dynamics driven by current and voltage in metallic magnetic wires.
/ Microscopic analysis of magnetization dynamics induced by pure spin current, spin-wave spin current and thermal spin current.
Transmission Electron Microscopy (TEM): Development of new experimental techniques including in-situ Lorentz microscopy and small-angle electron scattering (SAES) method
We are currently focused on achieving direct visualization and detailed analysis of electromagnetic fields inside or outside materials. In the environment of the TEM we can control the experimental conditions, such as temperature, magnetic field, and electric current. For instance, this technique allows us to investigate, with sub-nanometer spatial resolution, how certain magnetic structures change when in the presence of an external force such as electric current or magnetic field. Taking advantage of in-situ observation techniques developed in the laboratory, we are trying to discover novel physical phenomena and to develop technology from the viewpoint of spin electronics. Using these advanced techniques, we are also performing in-situ analyses of ferroelectric domains dynamics and ionic conduction in all-solid state battery. Based on microscopic knowledge obtained, we aim at establishing the novel principle of device operation and the design guidelines for the next generation of magnetic and electric devices. The research carried out using our TEMs is regarded as original and of great importance in the worldwide community of spin electronics.
Electron Physics
Electron beams used in TEM are accelerated by a voltage as high as several hundreds kilovolt (kV) and have a wavelength as short as several picometers (pico: 10-12). Conventional electron beams are widely utilized as a probe in the analysis of crystalline structure and electromagnetic fields. However, there remains considerable potential in electron beams. We are looking for new directions of Electron Physics including the observation of novel kinds of physical phenomena using electron beams. For instance, the detection of chirality using electron beams is very challenging but promising.