Enhancement of thermal spin signal and suppression of anomalous Nernst effect in the CoFeAl/Cu/CoFeAl lateral spin valve

Abstract

Generation of large spin current is an important issue in the operation of spintronic devices because the spin current plays a key role in spin-dependent transports and spin-transfer switching. Recently, a heat flow across a ferromagnet (FM) / nonmagnet (NM) junction is found to be able to generate and propagate the spin current. We have found that the large spin current is efficiently produced by using a heat flow across the CoFeAl /Cu junction because of the relatively large spin-dependent Seebeck coefficient for CoFeAl. The generated spin current is, in general, detected as the electrical voltage by using another ferromagnetic electrode. Using the nonlocal electrical detection scheme, one can prevent the background signal due to the spin-independent charge current. However, in the case of the thermal spin injection, the heat current reaches at the nonlocal ferromagnetic electrode and produces classical thermoelectric effects such as anomalous Nernst effect. Such spurious signals becomes serious obstacle when the large spin current is generated by the thermal spin injection. Therefore, the optimization of the device structure is indispensable for the ideal generation of the spin current using the heat. Here, we explore better geometry for generating the thermally excited spin signal in a laterally configured FM/NM hybrid nanostructure. First, we fabricated lateral spin valve consisting of multi CoFeAl strips bridged by a Cu wire shown in Fig. 1(a), 1(b). Here, the thickness for the CoFeAl is 100 nm in order to increase the heat generation due to the Joule heating. The thermally excited spin current is detected by a nonlocal ferromagnetic electrode with 2 nd harmonic lock-in technique. The field dependence of the 2 nd harmonic signal measured at room temperature exhibits a clear spin valve effect with the magnitude over 1 micro V as shown Fig. 1(c). This indicates that the CoFeAl efficiently produces the spin current. However, the obtained curve has the asymmetry with respect to the magnetic field because of the anomalous Nernst effect of the ferromagnetic electrodes. Therefore, these spurious effects complicate the proper evaluation of the spin related signal. To reduce the spurious signals and further enhancement of the spin valve signal, we develop a multi-terminal hybrid structure shown in Fig. 2(a), 2(b). Here, the thickness of the CoFeAl detector is reduced to 30 nm although that for the injector at the middle is 60 nm. This enables to reduce the influence of the Nernst effect with keeping the large heat generation. In addition, the two ferromagnetic detectors located at the top and bottom of the injectors enable the efficient electrical detection of the spin accumulation. The observed spin valve signal is shown in Fig. 2(c). Here, the magnitude of the overall spin valve signal is approximately 2 micro V. In addition, the Nernst signal can be reduced to 0.1 micro V, much smaller than that in Fig. 1(c). Thus, the optimization of the device geometry and the dimension is crucial for the efficient manipulation...