Experimental Studies of Oxide Magnetic Tunnel Junctions and Graphene.

摘要

The spin state of a ferromagnet (FM) on its surface can deviate significantly from that in the bulk. This effect could be strongly orientation-dependent in manganites due to the strong spin-orbit interaction. We have successfully fabricated high-quality (110)-oriented [La0.7Sr0.3MnO3 (t) /SrTiO3 (3ML)]n superlattices and systematically studied their crystal structure as well as interface magnetism. Compared to the (100)-oriented counterparts, LSMO has a thinner dead-layer and a higher interface moment at the (110)-orientated LSMO/STO interface. The magnetism of the manganite interface could be manipulated by taking advantage of the orientation-dependent nature of the exchange interactions.;To engineer the interface between the tunnel barrier and LSMO, we have developed a high quality Al2O3 (HfO2) tunnel barrier in LSMO/A2O3/Co stacking structure with A 2O3 (HfO2) grown ex-situ by the Atomic Layer Deposition as well as LSMO/STO/Co with LSMO and STO epitaxially grown by Pulsed Laser Deposition in both (100) and (110) orientations. Our specially designed shadow mask technique ensures a high yield and high performance of magnetic tunnel junction devices. The magneto-transport measurements show very interesting results.;The thermoelectric properties of graphene have been extensively studied both experimentally and theoretically. The exotic band structure of graphene leads to unusual thermoelectric properties which are sensitive to the carrier mobility. However, all the previous experiments were based on graphene samples with fixed mobility and make comparisons between different samples. Recently, we showed that it is possible to tune the carrier mobility of the same graphene device over a wide range. We adopted this method and systematically studied the magneto- Seebeck and Nernst effects for different mobility values. The crossover behavior of the Seebeck signal reported before around Charge Neutral Point is related to the splitting of zeroth Landau Level. Moreover, we demonstrate that the Nernst peak linearly depends on the carrier mobility in graphene.;Besides, we find that the empirical relation between the longitudinal and Hall resistivities and its counterpart between the Seebeck and Nernst coefficients hold surprisingly well for graphene in the quantum transport regime except near the Dirac point. The validity of the relations is cross-examined by independently varying the magnetic field and the carrier density in graphene. Our experimental results validate both derivative relations for massless Dirac fermions except near the Dirac point.