Masamitsu Hayashi

Masamitsu Hayashi

Winner in 2015 | 16th Winner

Senior research scientist, National Institute for Materials Science

Effective fields measurements and spin torque dynamics in magnetic nanostructures

Abstract When Prize Awarded

The control of electrons spin orientation in solids is the holy grail of Spintronics research. A particular focus has been placed on manipulating the electrons’ spin orientation by electrical means, that is, to control the magnetization direction of ferromagnets by passing current or applying electric field to the system. The "spin transfer torque" allows such electrical control of magnetization: a spin polarized current generated by passing current into a ferromagnetic layer can be supplied to another ferromagnetic layer to switch its magnetization direction.

We have studied the current induced motion of magnetic domain walls in magnetic nanowires. Magnetic domain walls are boundaries that separate regions magnetized in different directions. The motivation of this study is to build the Racetrack Memory proposed my supervisor Dr. Stuart Parkin. Using nanosecond current pulses, we demonstrated in-sync motion of multiple domain walls. The direction to which the domain walls moved was against the current flow, consistent with the theory of spin transfer torque. This work was press released by IBM and was published in the journal of Science in 2008.

The use of spin polarized current to manipulate magnetization triggered efforts to develop means of magnetization control in a more efficient way. Of particular interest was the use of "spin current". Spin current consists of electrons possessing opposite spins moving in opposite direction and is considered to provide technologically viable approaches to controlling magnetization.

Key to the generation of spin current is the strong spin orbit interaction of the material. Using a heterostructure that consists of a paramagnetic metal layer with strong spin orbit interaction and a few atomic layers thick ferromagnetic layer, we have studied the current induced effects on the magnetization. Spin current generated from the paramagnetic layer via the "spin Hall effect" exerts torque on the magnetization of the ferromagnetic layer, causing magnetization switching and motion of domain walls. Using unique transport measurements, we revealed the unique characteristics of the torque that occur at the paramagnetic/ferromagnetic layer interface. In addition, we find that the chirality of the magnetic order of such system can be controlled by materials engineering of the bilayer system owing to the interfacial anti-symmetric exchange interaction known as the Dzyaloshinskii-Moriya interaction. These works were published in Nature Materials and Nature Communications in 2013, 2014.