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Title: Transport properties of lateral spin valves
Author: Stefanou, Georgios Konstantinos
ISNI:       0000 0004 7225 1570
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2017
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The current study is focused on the investigation of the spin and heat properties of a Permalloy/silver/Permalloy, Permalloy/silver (doped with Iron)/Permalloy and Vanadium/silver/Vanadium spin valves in a lateral geometry. The presence of a downturn in the spin-signal of these lateral spin valves at low temperatures, below 30 K, can be avoided by depositing the Ag at a faster rate. This will promote a better microstructure that seems to have a crucial role in systems with very low or no magnetic impurities. A small part of the report is dedicated to test the two different behaviours of the spin signal that have been suggested in the literature, in the case of spin transport through an oxidised normal metal channel. According to literature, it would either remove the downturn of the spin signal at low temperatures, or form one. Our devices exhibited no downturn below 30 K, but by oxidising them, did not lead to the creation of it. By doping the normal metal channel with a dust layer of magnetic impurities (MIL) one would expect that by increasing scattering centers the spin signal and spin diffusion length would decrease no matter where the MIL would be. Interestingly that was not confirmed, as not only the spin signal and diffusion length vary depending on the position of the MIL in the NM channel, but also in the occasion of the device where the MIL was place right in the middle of the NM channel. At temperatures above 70 K the spin signal presented was bigger than that of the reference device. Moreover, the spin diffusion length presented in the particular set of devices, exceeded that of the reference set of devices for temperatures higher than 140 K. Finally, thermal effects were also expected to get picked up during the measurements as in order to create the spin accumulation, a charge current of 500 μ A is driven through the resistive Permalloy injector. That would give rise to Joule heating and Peltier effect once it reaches the Permalloy/silver junction due to the mismatch of the Peltier coefficients. Heat would then diffuse along the silver channel and through the substrate and would be detected in the second silver/Permalloy junction. The latter acted as a thermocouple and due to the difference in their Seebeck coefficients the temperature difference that was picked up, was converted to potential difference. The Scanning Thermal Microscopy technique was employed to image these effects. Based on the results of the technique, an analytical model that can predict the injector and detector junction temperature increase from that of the substrate was developed. In addition to that, since the voltage output is proportional to the product of the effective Seebeck coefficient with the temperature difference, the detector voltage could be predicted. To test the model a simple system of V/Ag/V device was used to deconvolute the thermal effects avoiding having a spin accumulation in the detector signal. After that, the thermal effects arising in a Py/Ag/Py lateral spin valve were investigated. Finally, since the Peltier effect was present in the measurements, the non-local resistance, spin signal and baseline resistance were analysed as a function of current direction to find a so far not studied effect. There was a splitting in the non-local and baseline resistance across all different devices that had temperature and current dependence. To confirm that the effect was not an artefact, three cryostats, three transport sticks, two sample holders and two more systems (Py/Cu/Py and CoFe/Cu/CoFe) were studied.
Supervisor: Hickey, Bryan J. ; Burnell, Gavin Sponsor: FP7-People-2012-ITN
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available