Ballistic spin transport in four-terminal NiFe/In_0.75Ga_0.25As structure

Spin-injection experiments in NiFe/two-dimensional electron gas (2DEG)/NiFe four-terminal devices are described. The 2DEG was confined at a modulation-doped In_0.75Ga_0.25As/In_0.75Al_0.25As heterojunction interface. NiFe source-drain electrodes were located ∼ 1 μm apart and two voltage probes were fabricated between them. The separation between the NiFe electrodes was thus almost less than both the mean free path (∼ 2 μm) and the spin-diffusion length (> 1 μm) of the 2DEG. In nonlocal four-terminal measurements, where a constant current (i) was sent through one NiFe to one ohmic electrode, the two-terminal magnetoresistance (R_2t = V_2t/i) exhibited a spin-valve-like effect, while R_4t = V_4t/i showed a resistance hysteresis. The amplitude of the latter amounted to almost 12% of the R_4t (B = 0) at 1.5 K, the extent of which was one order of magnitude larger than those so far reported. These results suggest the importance of a quasi-ballistic as well as a spin-coherent coupling between the two NiFe electrodes, which may crucial in the future operation of spin field-effect transistors (FETs).