First- and second-harmonic detection of spin accumulation in a multiterminal lateral spin valve under high-bias ac current

We have investigated the transport properties of electrically and thermally excited spin currents in a lateral spin valve consisting of a spin injector and detector with a middle ferromagnetic wire by detecting the first- and second-harmonic voltages. The first-harmonic spin signal was significantly suppressed by the middle ferromagnetic wire because of the spin absorption effect. On the other hand, in the second-harmonic signal, a small signal related to the middle ferromagnetic wire was observed in addition to a conventional spin signal with a reduced magnitude. This indicates that the additional ferromagnetic wire acts not only as the spin absorber but also as another spin injector under thermal spin injection, because the heat current caused by direct spin injection propagates to the middle ferromagnetic wire and creates another temperature gradient. By using this effect, we show that the magnetization direction of a ferromagnetic nanodot embedded in a nonmagnetic Cu wire becomes measurable.

Figure 1

Scanning electron microscope (SEM) image of the fabricated lateral spin valve consisting of three CoFeAl wires bridged by a Cu strip, together with a schematic illustration of the device geometry.

Figure 2

(a) Room-temperature first-harmonic spin valve signal using a CFA1 injector and CFA2 detector. (b) First-harmonic spin valve signal detected by CFA1 under spin injection from CFA2 to CFA3. (c) Second-harmonic spin valve signal detected by CFA2 under thermal spin injection using CFA1. The right-hand sides for each figure correspond to the probe configuration for the measurements.

Figure 3

Room-temperature first-harmonic spin valve signal for a standard CFA/Cu lateral spin valve (a) without the middle CFA wire and (b) with the middle CFA wire. In both devices, the center-to-center distance between the injector and the detector is 600 nm.

Figure 4

(a) Schematic illustration of the thermal spin injection and detection in a lateral spin valve with the middle CFA wire, together with the probe configuration for the measurement. (b) Heat flow under thermal spin injection in the triple CFA/Cu junction and induced spin accumulation in the Cu strip. (c) Room-temperature second-harmonic spin valve signal observed in the CFA/Cu lateral spin valve with the middle CFA wire.

Figure 5

Numerically simulated temperature distribution of the lateral spin valve under a bias current of 0.63 mA at ambient temperature. (a) Top view of the temperature profile in the $x\text{−}y$ plane. (b) Cross-sectional view of the temperature profile in the $x\text{−}z$ plane. Line profiles of the temperature distributions along the $z$ axis (c) at the center of the CFA1/Cu junction and (d) that at the center of the CFA2/Cu junction.

Figure 6

(a) SEM image of the fabricated lateral spin valve with a ferromagnetic nanodot embedded in a Cu wire. (b) Second-harmonic voltage as a function of the magnetic field, which is swept from the negative to the positive direction. The inset shows a schematic illustration of the probe configuration for the measurement.