Efficient thermal spin injection using CoFeAl nanowires and its application

Abstract

The manipulation of spin current is a central issue in the operation of spintronic devices because the spin current plays key role both in spin-dependent transports and spin-transfer switching. Recently, a heat utilization for creating the spin current has been paid considerable attention, leading to an emerging field, spin caloritronics. One of the representative phenomena is thermal spin injection, in which excess heat can be used to produce the spin current because of the spin-dependent Seebeck coefficient [1]. However, the generation efficiency of the thermally excited spin current was quite low because of the low spin-dependent Seebeck coefficient of conventional ferromagnetic metals. The Seebeck coefficient is strongly correlated to the band structure around the Fermi level, and, under the simple approximation in metals, the coefficient is known to be proportional to the energy derivative of the logarithmic density of state (DOS) at the Fermi level. In a ferromagnetic material, since the DOS shows different features between the up and down spins, one can separately consider the moving directions of the up-spin electrons and the down-spin electrons. Therefore, in the ferromagnetic material with a large difference in the DOS between up and down spins, the sign of the Seebeck coefficient for the up-spin electron can be reversed from that for the down-spin electron. In such a situation, the up-spin and down-spin electrons flow in opposite directions, as shown in Fig. 1(a, b). Therefore, the generation efficiency of the spin current due to thermal spin injection is significantly enhanced by the large spin-dependent Seebeck coefficient.