Nonreciprocity of electrically excited thermal spin signals in CoFeAl-Cu-Py lateral spin valves

Electrical and thermal spin currents excited by an electric current have been systematically investigated in lateral spin valves consisting of CoFeAl and ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ (Py) wires bridged by a Cu strip. In the electrical spin signal, the reciprocity between the current and voltage probes was clearly confirmed. However, a significant nonreciprocity was observed in the thermal spin signal. This provides clear evidence that a large spin-dependent Seebeck coefficient is more important than the spin polarization for efficient thermal spin injection and detection. We demonstrate that the spin-dependent Seebeck coefficient can be simply evaluated from the thermal spin signals for two configurations. Our experimental description paves a way for evaluating a small spin-dependent Seebeck coefficient for conventional ferromagnets without using complicated parameters.

Figure 1

(a) SEM image and the schematic illustration of the present lateral spin valve. (b) Measurement configuration for the electrically and thermally driven spin injections. In electrically driven spin injection, when the bias current flows through the F/N junction, the generated spin current is detected by measuring the first harmonic responding voltage. In thermally driven spin injection, when the bias current flows through the ferromagnetic wire without passing through the junction, the spin current generated by the temperature gradient due to Joule heating is detected by measuring the second harmonic responding voltage.

Figure 2

Electrically driven nonlocal spin valve curves for the device with 350 nm interval distance together with the measurement probe configuration. (a) Bias current flows through the Py/Cu junction, and the electrically driven spin current will be detected by the first harmonic voltage with sweeping magnetic field at the CoFeAl/Cu(CFA/Cu) junction. The spin signal $\mathrm{\Delta }{R}_S^{\mathrm{ele}}$ is defined as the normalized value of the voltage difference between the parallel and antiparallel by the injection current. (b) The CoFeAl/Cu junction works as the spin injector and the Py/Cu junction works as the spin current detector. The dashed and solid arrows correspond to the directions of the sweeping field.

Figure 3

Interval distance dependence of the electrical spin signals in different lateral spin valves. CFA-Cu-Py and Py-Cu-CFA correspond to the reversed injector and detector for the same lateral spin valve containing two kinds of ferromagnets CFA and Py.

Figure 4

Nonlocal spin curve for thermally driven spin injection together with the schematic illustration of the measurement probe configurations. The two solid arrows represent the direction of the magnetization for the two ferromagnets. (a) Thermally induced nonlocal signals with the Py wire as an injector and the CoFeAl wire as a detector. We refer to this configuration as “configuration A.” Here, the bias current in the Py wire is 0.71 mA. (a) Thermally induced nonlocal signals with the CoFeAl wire as an injector and the Py wire as a detector. We refer to this configuration as “configuration B.” Here, the bias current is 0.60 mA.

Figure 5

Interval distance dependence of the thermal spin signals for various types of lateral spin valves.