The Flow of Spin?

Kazuya Ando (Senior Assistant Professor, Department of Applied Physics and Physico-Informatics)

From smartphones and personal computers to countless other devices, we are now surrounded by an immense number of electronic gadgets. The advancement of semiconductor technology that has supported this world has been tremendous, and its growth continues to be remarkable. So, can we assume that the development of electronic technology will continue smoothly into the future? In fact, it does not seem to be that simple. The physical foundation of current electronic technology was established several decades ago, and recent improvements in reducing power consumption and increasing the speed of electronic devices have been primarily supported by the miniaturization of device structures. However, since atoms have a finite size, there is an inevitable limit to how small we can make things. Furthermore, along with the limits of device miniaturization, energy loss due to Joule heating from electric currents has become a serious problem. To overcome these limitations of current electronic technology, or "electronics," a more fundamental reconsideration seems necessary.

Amidst this situation, "spintronics," which utilizes the flow of electron spin known as "spin current," has recently been gaining attention. In addition to charge, which is an electrical property, electrons also possess spin, a magnetic property. Let's consider the situation shown on the left side of the figure. Electron spin has two states, up and down. In the left diagram, two electrons with opposite spins are flowing in the same direction. If we focus only on the spin at this time, since opposite spins are moving in the same direction, it results in no net flow of spin. However, if we look at the charge, a unidirectional flow is generated, indicating that an electric current is flowing. On the other hand, if we could move these two electrons in opposite directions, as shown on the right side of the figure, we could create a flow of only spin, unaccompanied by a flow of charge. This flow of spin is what we call a spin current. Unlike electric current, spin current does not have an energy dissipation mechanism corresponding to Ohm's law. For this reason, spin current is expected to be an inherently energy-efficient information transport carrier. Spintronics aims to utilize this to develop new device functionalities and realize ultra-low-power electronic technologies.

Compared to the electric current we use so readily today, only a fraction of the nature of spin current is understood. We are currently building the physical foundation of spintronics by pioneering the fundamental physics of spin current. The core of electronic technology using spin current is to generate, control, and measure it. We have established a method for generating spin current in all kinds of materials (Nature Materials 2011) and, by utilizing this, have succeeded in the electrical detection of spin current due to relativistic effects in Si, one of the most important materials in modern semiconductor technology (Nature Communications 2012). Furthermore, it has recently been revealed that, contrary to expectations, highly efficient conversion between spin current and electrical signals can be achieved in conductive plastics (Nature Materials 2013).

Realizing ultra-low-power electronic information processing devices that operate solely on spin current still requires several breakthroughs, and researchers at universities and companies around the world are fiercely competing in this field. Can we freely control the diverse physical phenomena generated by spin current in materials and create a new era of electronic technology that surpasses current-based electronics? Perhaps the answer will be found as soon as tomorrow.