We discovered phenomena to detect spin current. Spin current refers to a flow of electron spin angular momentum in condensed matter, which is a spin analogue of electric current. An electron is a particle which carries two quantities: electric charge and internal autorotation called a spin. Spin information is quite fragile in condensed matter; in fact, it disappears within a very short length scale, such as one thousandth of 1mm in Cu. So, it used to be unimportant. However, this length scale is no longer negligible today; contemporary electronic devices are composed of nanometre-scale structures, much smaller than the spin disappearance length scale. In such a small world, spin current and electric current should be considered on equal footing. However, although physics of electric current in matter is well written in the electromagnetism in matter, no laws were provided for spin current (macroscopic effective theory for spin current was missing); which means that there was no compass to search for spin current phenomena. Our work was the finding of a series of fundamental phenomena of spin current. In particular, the discovery of inverse spin Hall effects [1], the generation of electric fields from spin current, made measurements of spin current possible. By using the effect as a spin current probe, we then explored physical phenomena induced by spin current. One typical outcome is the discovery of the spin Seebeck effect [2] in YIG/Pt: the conversion of heat current into spin current. Surprisingly, we found spin current can flow even in insulators such as YIG when spin excitation gap in the insulators is small enough [3].

[1] E. Saitoh et al. Appl. Phys. Lett. 88 (2006) 182509.
[2] K. Uchida et al. Nature 455 (2008) 778-781.
[3] Y. Kajiwara et al. Nature 464 (2010) 262-266.

Fig.1 (a) Conduction electron spin current.

Fig.1 (b) Two-particle model of inverse spin Hall effect (ISHE)

Fig.1 (c) Electric-field generation from spin current via ISHE.