Efficient thermal spin injection using CoFeAl nanowire

Nanoelectronic devices based on electron spin can overcome the physical limitations of the present semiconductor technology because of their low power consumption while exploiting the spin degree of freedom of electrons. Although enhancing the efficiency of generation of the spin current is imperative and a primary issue for the practical application of spin-based electronics, seamless device integration with the conventional complementary metal-oxide semiconductor technology is another important milestone for developing spin-based nanoelectronics. In particular, the preparation of nanosized, magnetic, multilayered structures with electrical connections to individual complementary metal-oxide semiconductor circuits significantly complicates the fabrication procedure of nanoelectronic devices. Thermal spin injection, which is a recently discovered unique characteristic of spin current, may be an innovative method for simplifying device integration without the need for electricity, namely wireless spintronics. However, the feasibility of using the thermal spin injection method is poor because of its extremely low-generation efficiency. Here, we demonstrate that a highly spin-polarized, ferromagnetic CoFeAl electrode with a favorable band structure has excellent properties for thermal spin injection. The spin-dependent Seebeck coefficient is approximately 70 μV K−1, which facilitates highly efficient generation of the spin current from heat. The heat generates approximately 100 times more spin voltage than a conventional ferromagnetic injector at room temperature. This innovative demonstration may open a new route for spin-device integration and its applications. Takashi Kimura from Kyushu University in Japan and colleagues have developed an ferromagnetic alloy for generating spin current without using electric fields. The researchers found that the Seebeck coefficient — a measure of how temperature changes can create electric voltages — of their ferromagnetic, iron-cobalt-aluminum alloy strongly depends upon the spin states in the alloy. By assembling this material into a nanoscale spintronic device then subjecting the device to a temperature gradient, the subsequent heat flow produced a thermal spin injection current 100 times greater than seen in typical magnets. This activity is particularly appealing because thermal spin generators are self-powered, wireless and can work in combination with conventional electric spin currents to boost the efficiency of nanoscale spintronic circuits. Efficient thermal spin injection can be achieved by using CoFeAl alloy in which the sign of the Seebeck coefficients for up and down spins are opposite each other.