Experimental studies of asymmetric dipolar coupling effect and graphene spin transfer torque
Spin-based devices for post-complementary metal-oxide-semiconductor (CMOS) applications is a widely discussed proposition. Among the spin-based device ideas, all-spin-logic (ASL) and charge-coupled spin logic (CSL) are the two promising devices due to their potential for lower power consumption and downscaling capability. The objective of this dissertation is to implement the key elements of the ASL and CSL devices. Key results for ASL: Toward the single stage of the ASL device, graphene is an ideal channel material for spin transport due to its long spin diffusion length. To develop a graphene-based ASL device, it is therefore important to demonstrate the spin transfer torque in graphene; and in this dissertation, experimental measurement of spin transfer torque in graphene non-local spin valve devices are conducted. Assisted by a small external in-plane magnetic field, the magnetization reversal of the receiving magnet was induced by pure spin diffusion currents from the injector magnet. The magnetization switching was reversible between the parallel and antiparallel configurations by controlling the polarity of the applied charged currents. Current-induced heating and the Oersted field from the nonlocal charge flow were then excluded from the dissertation. Next, the spin angular momentum absorption at the interface of the receiving magnet and graphene channel were further enhanced by removing the tunneling barrier in the receiving magnet. The device with a tunneling barrier only at the injector magnet showed a comparable nonlocal spin valve signal but lower electrical noise. Moreover, in the same preset condition, the critical charge current density for spin torque in the single tunneling barrier device showed a substantial reduction compared to the double tunneling barrier device. Key results for CSL: The proposed CSL device will use giant-spin-Hall-effect material as the Write unit and a magnetic tunnel junction as the Read unit, and a magnetic dipolar coupling effect in a vertical stack is the key element for communication between them. This dissertation demonstrates the magnetic dipolar coupling effect with directionality for the first time, which is the key element to ensuring non-reciprocal spin information propagation. It is also shown that this directionality in a vertical stack can be strengthened by optimizing the size of the input and output magnets and positioning the output magnet at the edge of the input magnet, which is also quantitatively in agreement with the coupled-LLG simulation.
Chen, Purdue University.
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