Technical: This project addresses growth and further understanding of rutile heterostructures for spin-electronic applications. Objectives are to gain understanding and control over the synthesis and processing of CrO2 and related rutile materials into heterostructures using a combined experiment/theory approach. Pulsed laser deposition and laser-assisted chemical vapor deposition (LCVD) will be used for the growth of the heterostructures. In LCVD, energy from a laser is used to photochemically decompose a gas precursor to provide epitaxial CrO2 growth at low pressure with in-situ electron diffraction crystal growth monitoring to obtain spin transport quality CrO2 films and interfaces. Both Meservey- Tedrow and inelastic tunneling spectroscopy will be performed, as well as basic magneto-transport to characterize prototype device structures.
The project addresses basic research issues in a topical area of electronic/photonic materials science with high technological relevance. Research and educational activities will be integrated with involvement of both graduate and undergraduates in the research program. Collaborations with Oak Ridge National Laboratory and international collaborations provide added benefits and special opportunities to assist integration of research and education. The project includes collaborations with minority faculty members from Historically Black Colleges and Universities (HBCUs).
The goal of this project was to maintain and increase the momentum of the field of spin electronics by realizing the potential of an extremely interesting class of oxide materials, the rutile oxides. Spin electronics has a variety of exciting applications, e.g., magnetic field sensors, magnetic random access memory, and rapidly programmable logic arrays. As an offshoot of this research, we have began a detailed investigation into the fascinating metal-insulator transition in VO2. Intellectual Merit - CrO2 is a remarkable material that is simultaneously an excellent metal for majority spin electrons and an insulator for minority spin electrons. For this reason, CrO2 is called a ``half-metallic ferromagnet.'' The amazing potential of CrO2 remains untapped, however. Developing CrO2 heterostructures -- particularly given the stringent quality requirements of spin electronics -- requires a considerable step forward. Our approach has three synergistic components. First, we believe high-quality CrO2-based heterostructures can be realized by working within the rutile family. A consistent problem plaguing CrO2 heterostructures has been forming high-quality interfaces with other materials, necessary to make a realistic device, which we believe can be solved by working with chemically similar and structurally identical rutile materials. Second, efficient spin electronic devices can only be realized if appropriately electronically matched materials are employed - readily realized using rutile oxides. Third, the experience gained also allowed us to exploit the intriguing behavior of the other rutiles, such as VO2. Our results on growth and magnetic anisotropy of CrO2 films have suggested that simply using a different crystallographic orientation is a very promising route for realizing better CrO2-based devices. Our studies indicated that the structure and magnetism on certain crystal faces is much more robust. For instance, our recent synchrotron magnetism measurements point toward reduced magnetic properties of thin CrO2 films for certain orientations. We have also made substantial progress toward low-pressure synthesis of CrO2 films, which would finally allow us to able to make all "in situ" heterostructures within the same synthesis tool, a prerequisite for successful devices. Concerning VO2, the metal-insulator transition in this material is an intriguing problem more than 50 years after its discovery. More than that, this material has a great potential for future applications in such diverse areas as computer memories, "smart" window coatings, and sensitive infrared radiation measurements. With the knowledge developed in this project, in the last year of the project we successfully made high quality VO2 films and began a detailed study of their electrical properties. We found that not only the conductivity but also the dielectric properties of VO2 change dramatically near the transition. Detailed analysis indicates electrical transport in the insulating phase is dominated by the vibrational properties of the material, while metal-insulator phase coexistence dominates transport near the transition. Our recent studies of electrical noise near the transition shows that in increase in noise power betrays the phase transition well before it is visible in conductivity or optical measurements, providing a new tool for understanding the origin of the transition. Broader Impact - We have been very active in giving public lectures, incorporating research results in to current course offerings, and engaging young scientists in the laboratory. We have presented progress to industrial colleagues on many occasions, and obtained valuable feedback from colleagues in the information storage industry. During the course of this project, four students received PhD degrees, three students received masters degrees, and two more students who spent significant time on this project will receive their PhD degrees in the next year. We have also mentored several undergraduate summer students over the course of the project. Our proposal has attempted to broaden the participation of under-represented groups and build a diverse research team. We have done this by using an existing program within UA to involve minority faculty members from Historically Black Colleges and Universities (HBCUs). This program allowed us to recruit Dr. Mark Williams from the University of Maryland, Eastern Shore (an HBCU), and he and undergraduate students from UMES have been working with us on the theoretical modeling of rutile interfaces since the project's inception. He has two published papers and two conference talks as a result of his work at UA. He has also indicated that his research on this project in no small way contributed to his receiving tenure at UMES. Based on the work performed in this projet, we have initiated a new and very fruitful collaboration with Dr. E. Goering of Max-Plank-Institute für Metallforschung, Stuttgart, Germany, an expert in X-ray-based magnetic measurements. This collaboration demonstrated that there is induced magnetic moment on Ru at CrO2/RuO2 interfaces, an extremely important result that potentially explains the puzzlingly poor performance in these devices. One paper has resulted from this collaboration already, with a second under review.
