Spin-Polarized Currents for Spintronic Devices: Point-Contact Andreev Reflection and Spin Filters

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The emerging field of spintronics deals with the control and manipulation of the spin of the elec¬trons rather than the charge. The first chapter of this thesis gives an overview of the areas of research in spintronics and puts the goals of the present work into the perspective of the work on spintronic devices. Amongst others, the generation of highly spin-polarized currents, i.e., currents with a nonequilibrium spin population, is of high importance for spintronics. In this thesis two different approaches to the generation of spin-polarized currents are investigated. In the second chapter the ferromagnetic Heusler alloy Ni2MnIn is investigated. It is a candidate as a half-metallic spin-injector. We use point-contact Andreev reflection as a method to determine the spin polariza¬tion of thin films of Ni2MnIn. A detailed description of the theoretical basics of the method is given. The setup for this technique has been optimized within this work. Measurements are per¬formed on Au, Ni, and Ni2MnIn films. The experimental data is analyzed using three different the¬oretical models. Chapter 3 deals with an alternative approach to generate spin-polarized currents: the realization of a full-semiconductor spin filter. In such a device the spin polarization originates from the spin-orbit interaction in the two-dimensional electron gas in an InAs heterostructure. To obtain one-dimensional electron channels in the filter we have designed quantum point contacts. The chapter is divided into one part that presents the theoretical basics of quantum point contacts and of spin filters and a second part that gives the experimental results. The thesis ends with a conclusion and an outlook.