Doctor of Philosophy
Gary M. Atkinson
Coherent injection, detection and manipulation of spins in semiconductor nansotructures can herald a new genre of information processing devices that are extremely energy-efficient and non-volatile. For them to work reliably, spin coherence must be maintained across the device by suppressing spin relaxation. Suppression can be accomplished by structural engineering, such as by confining spin carriers to the lowest subband in a semiconductor quantum wire. Accordingly, we have fabricated 50-nm diameter InSb nanowire spin valves capped with Co and Ni nanocontacts in which a single conduction subband is occupied by electrons at room temperature. This extreme quantum confinement has led to a 10-fold increase in the spin relaxation time due to dramatic suppression of the D’yakonov -Perel’ (DP) spin relaxation mechanism. We have observed the spin-valve and Hanle effects at room temperature in these systems. Observing both effects allowed us to estimate the carrier mobility and the spin relaxation length/time and we found that the latter is ~10 times larger than the value reported in bulk InSb despite a four orders of magnitude decrease in the carrier mobility due to surface roughness scattering. We ascribe this dramatic increase in spin relaxation time to the suppression of the DP relaxation mode due to single subband occupancy.
Modulation of spin relaxation rate by an external agent can open new possibilities for spintronic devices. Any agent that can excite electrons from the lowest subband to higher subbands will dramatically increase the DP spin relaxation rate. We have shown that the spin relaxation rate in InSb nanowires can be modulated with infrared light. In the dark, almost all the electrons in the nanowires are in the lowest conduction subband, resulting in near-complete absence of DP relaxation and long spin coherence length. This results in a high resistance state in a spin valve whose ferromagnetic contacts have anti-parallel spin polarizations. Under infrared illumination, higher subbands get populated and the DP spin relaxation mechanism is revived, leading to a three-fold decrease in the spin relaxation length. As a result, injected spins flip in the spacer layer of the spin valve and this causes the spin valve resistance to drop. Therefore, this effect can be exploited to implement an infrared detector.
We also studied the transport behavior of a single nanowire (~50 nm diameter) captured between two non-magnetic contact pads. The wire was attached between the pads using dielectrophoresis. A giant (∼10,000,000%) negative magnetoresistance at 39 mT field was observed at room temperature in Cu nanowires contacted with Au contact pads. In these nanowires, potential barriers form at the two Cu/Au interfaces because of Cu oxidation that results in an ultrathin copper oxide layer forming between Cu and Au. Current flows when electrons tunnel through, and/or thermionically emit over these barriers. A magnetic field applied transverse to the direction of current flow along the wire deflects electrons toward one edge of the wire because of the Lorentz force, causing electron accumulation at that edge and depletion at the other. This makes the potential barrier at the accumulated edge shorter and at the depleted edge taller. The modulation of the potential barrier height with a magnetic field dramatically alters the tunneling and/or thermionic emission rate causing a giant magnetoresistance.
Currently, effort is underway to demonstrate strain sensitive anisotropic magnetoresistance (AMR) in a single Co-Cu-Co nanowire spin valve. AMR is caused by spin-orbit coupling effects which makes the resistance of a ferromagnet depend on the angle between the direction of current flow and the magnetization. The resistance maximizes when the angle is 00 or 1800 and minimizes when the angle is 900. When an external magnetic field is applied in a direction opposite to a ferromagnet’s magnetization, the latter begins to rotate in the direction of the field and hence its resistance continuously changes. This results in a trough in the magnetoresistance of a spin valve structure between the two fields when the magnetization starts to rotate and when the magnetization completes the rotation. We have observed a magnetoresistance peak (instead of trough) in the Co-Cu-Co spin valve, which is due to the normal spin valve effect that overshadows AMR. However, when an intense infrared light source is brought close to the sample, the peak gets overshadowed by a trough, showing that the AMR effect becomes dominant. We attribute this intriguing feature to the fact that the AMR effect is strongly influenced by strain. Heating by the light source generates strain in the Co contacts owing to unequal thermal expansion of Co and the underlying substrate. We also observed that the AMR effect becomes more pronounced as the light source is brought closer to the sample, resulting in increased heating and hence increased strain generation.
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