Electrical signal transmission via spin waves

When an electric current flows in a material, the coupling of the electrons’ spin and orbital angular momentum causes spin-up electrons to bend in one direction and spin-down electrons in the other, both transverse to the charge current. That phenomenon is known as the spin Hall effect (see Physics Today, February 2005, page 17). Eiji Saitoh (Tohoku University), Sadamichi Maekawa (now at the Japan Atomic Energy Agency), and their colleagues have used that effect and its inverse—the creation of a charge current from a transverse spin current—as the basis for a counterintuitive demonstration: the transmission of a DC electrical signal a macroscopic distance through an insulator. Although the bandgap of a magnetic insulator prevents charge conduction, it doesn’t prevent the excitation of a spin wave, the collective precession of localized magnetic moments. For their demonstration, the researchers used a heterostructure composed of a micron-thick slab of yttrium iron garnet on which sits two thin rectangular platinum electrodes a millimeter apart, as pictured here. An electric current JC applied to one electrode is first converted to a spin current that flows downward. Provided it’s large enough, that current, in turn, induces in Y3Fe5O12 a spin wave JS, which propagates to the second electrode. There the wave is reconverted to a spin current of conduction electrons and, thanks to the inverse spin Hall effect, is detectable as a small voltage signal. The work sets the stage for exploiting a rich class of materials—ferromagnetic insulators—in spintronics research. (Y. Kajiwara et al., Nature 464, 262, 2010.)—R. Mark Wilson