University of Twente Student Theses


Spintronic devices based on the graphene-nickel(111) interface

Smits, R. (2016) Spintronic devices based on the graphene-nickel(111) interface.

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Abstract:Because of possible applications in next generation electronics, there is a lot of interest in research on carbon-based spintronics. An interesting development in this field is a theoretical study done by Karpan et al., which predicts perfect spin filtering at the interface between Ni(111) and graphene. A spin filter like this could, for example, enhance the on/off ratio in a magnetic random access memory. This thesis presents the results of a MSc project performed at the NanoElectronics group at the University of Twente. The goal of this project is the fabrication and characterization of spintronic devices based on the graphene-nickel(111) interface. The initial focus of the project was on the fabrication of a proper insulating layer out of sputtered SiO2 which is employed in the production of new spintronic devices. A recipe has been developed for sputtering a 200nm layer which has a resistance in the order of 10GΩ when cooled down to temperature below 150K. A spintronic junction can be fabricated by using a lithography step and a buffered hydrofluoric acid etching of 95s. With this SiO2 layer, spintronic devices have been successfully fabricated. Subsequently, initial characterization has been performed by doing transport measurements at room- and cryogenic temperature. At ambient temperature an ohmic behaviour is observed while the measurement at 10K shows a tunnelling like characteristic. Also these devices can be used to study spin-orbit coupling (SOC) at this interface by performing magnetoresistance measurements, in which electrical characterization is performed under influence of a high magnetic field. By rotating the field, magnetic anisotropies in the density of states can be probed (tunneling anisotropic magnetoresistance, TAMR). Initial magnetoresistance measurements have been performed by applying a 9T magnetic field in-plane and out-of-plane of the sample surface. A difference of about 5% in resistance between these two configurations has been observed in the range -0.05 µA and 0.05 µA. This non-zero magnetoresistance might be an indication of SOC at the interface between Ni(111) and graphene. However, further measurements are required before firm conclusions can be drawn. Furthermore, the predicted spin filtering effect can be examined by doing Tedrow-Meservey measurements. Therefore, a tunnelling experiment needs to be performed using a counter electrode of superconducting Al, under influence of a high magnetic field. From the conductance versus applied bias characteristic, the spin polarization of the tunnelling current can be derived. It has been attempted to induce and detect superconductivity in a top electrode consisting of Al2O3(3.5nm)/Al(8nm)/Cu(10nm). Four-point measurements over the electrode, at a temperature of 314mK, have been performed. This tests did not shown any sign of superconductivity. The explanation of this might lie in the fact that the underlying layers have a very large surface roughness, which might avoid the formation of a continuous Al layer thereby destroying superconductivity already at low bias current.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics MSc (60436)
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