Electronic spin transport and thermoelectric effects in graphene

Abstract

Spintronics and spin caloritronics in graphene are recently very active fields of research, and this thesis is a contribution to both. The main topic is the study of spin currents in graphene non local spin valves via means of electrical spin injection and detection. In a preliminary work, we analytically investigate the tunneling process of conduction electrons between ferro- and non magnetic materials. On the experimental side, we report on spin precession in freely suspended graphene spin valves. In this context, we have developed a novel method for the fabrication of freely suspended graphene devices, which additionally is beneficial for the spin injection/detection efficiency of the devices. In order to investigate these enhanced spin signals, we have performed bias dependent measurements, which lead to the experimental demonstration of a spin thermocouple in graphene. In order to investigate tunneling of conduction electrons between ferro- and non magnetic electrodes, we have developed a theoretical model based on the analytical solution of the one-dimensional, time-independent Schrˆdinger equation. The model shows that a complex behavior of the polarization is intrinsic to the tunneling process of electrons between ferro- and non magnetic materials. Spin relaxations times of several tens of micrometers in graphene have been predicted. A promising approach to studying the intrinsic properties of graphene is to suspend the flakes, thus eliminating the influence of the substrate and enabling cleaning methods. In order to achieve this, we have developed a method to fabricate freely suspended graphene non local spin valves that involves a minimal number of steps and chemicals. Since the method is acid free, the yield of high quality, as-processed devices is notably improved when comparing to the standard fabrication process. Therefore, our as-processed devices exhibit excellent mobility, as high as 20000 cm^2/(Vs) at room temperature. We demonstrate electrical detection of spin precession, allowing us to extract the spin relaxation length in these devices, finding values of a few micrometers. We expect that by applying cleaning methods to freely suspended spin valves, it will be possible to investigate the origins of spin relaxation in intrinsic graphene. We have further observed enhanced spin injection/detection efficiency in our devices. We attribute the enhancement to the formation of an amorphous carbon layer at the interface between graphene and ferromagnet due to electron beam induced deposition. The interfaces are stable even for large applied bias current densities. We obtain a 10000x enhancement of the spin signal as compared to Ohmic contacts, but expect further increase after optimizing the deposition method. The increased contact resistance and spin accumulation suggests that the interface has a combination of Ohmic and tunneling properties. The simplicity and transferability of the fabrication process is in contrast to those of the conventional insulators used in spintronics. Therefore, we expect that amorphous carbon barriers are a viable alternative, which might improve the spin injection/detection efficiency in other materials as well.

Finally, we have performed bias dependent measurements in our samples, observing a novel phenomenon which is due to the particular properties of graphene such as its energy dependent mobility. We demonstrate an anomalous enhancement of the spin accumulation at the Dirac point, which is caused by heating in the injector contacts. Because of this higher order contribution to the spin accumulation, the electrochemical potentials of the spin sub bands exhibit supralinear behavior as a function of the bias current. The spin splitting becomes so large that at the Dirac point we observe a huge quantity of carriers of opposite spin and charge. We show that this constitutes a spin thermocouple, where the thermoelectric voltage between spin up and spin down enhances the total spin accumulation.