Epitaxial Films of Chromium Dioxide from a New Precursor (Cr₈O₂₁) and Research on Their Application in Spin-Electronic Devices

There are theoretical predictions that chromium dioxide (CrO2) is a half metallic ferromagnet. Accordingly, CrO2 would be expected to be very suitable for spin electronic devices. We started the present work in an attempt to prove (or disprove) this theoretical prediction, and if possible, to make a magnetoresistive device (spin-valve) from CrO2-containing multilayers. First we needed to be able to prepare high quality epitaxial films of CrO2. When we began our work, the best epitaxial thin films of CrO2 were made by chemical vapor deposition (CVD) in a two-zone furnace from a CrO3 precursor. Later, some research groups started growing CrO2 from a CrO2Cl2 precursor, and we started using Cr8O21 precursor. The growth mode has previously been described as CrO3 vaporizing in the first zone and thermally decomposing at higher temperature in the second zone onto a substrate. In the published papers, the focus has been on the properties of the obtained layers, rather than on the deposition mechanisms. In our experimental work, we attacked the CrO2 growth mechanism by two completely different methods, namely by molecular beam epitaxy (MBE) and by CVD. We attempted to understand the CVD growth from CrO3. By considering in parallel our"""failed"""attempts to deposit CrO2 by MBE, and our experiments with the CVD process, we concluded that CrO3 does not decompose directly to CrO2 and oxygen, as it had been previously thought. We showed the existence and the importance of an intermediate compound (Cr8O21). We demonstrated that it is not necessary to start the CVD from CrO3; instead one can prepare Cr8O21 ex situ, and use it directly for the growth of high quality CrO2 epitaxial layers, avoiding any contamination caused by the decomposition of CrO3 to Cr8O21. We proposed a hypothesis that the role of Cr8O21 in the CVD process is to exude unstable molecules of CrO4, and that the reaction on the substrate is the decomposition CrO4 --> CrO2 + O2. Then we used our CVD-from-Cr8O21 method and prepared a variety of samples containing CrO2 film to probe the effect of magnet ordering on the electrical transport. We started with Fe3O4(polycr.)/NiO(polycr.)/CrO2(100)/TiO2(100) type structures. They did not show the anticipated spin valve effect. We continued by a"""one step at a time"""strategy. The first logical step was to make a simple tunneling barrier on a CrO2 film, and to probe the spin polarization of the tunneling current. We tested many approaches for making a tunneling barrier in a structure of the form: Superconductor/Insulator/CrO2 (SC/I/ CrO2). We produced our best tunneling barriers by attacking the CrO2 surface with bromine-methanol solution, which produces a structure SC/CrOx/CrO2. We performed Meservey-Tedrow type measurements with such tunneling barriers and found tunneling current from CrO2 that has a spin polarization close to 100% at 400 mK. The next step was expected to be straightforward ¨C grow another ferromagnetic film on the top of the same barrier, and the spin valve must be ready. It did not work so easily, but after reinforcing the barrier with oxidized aluminum, we measured -24% magnetoresistance at 5K for samples with a multilayer structure of the form: Co(polycr.)/AlOx/CrOx/CrO2(100)/substrate-TiO2(100).