Novel Rutile Heterostructures for Spin-electronics Applications
Funding: NSF Focused Research Group (DMR 0706280; Electronic Materials / Ceramics)
Personnel
William Buter Arunava Gupta Patrick LeClair Manjit Pathak Xueyu Zhang Xing Zhong Hunter Sims
Overall Goal
The goal of the proposed research is to maintain and increase the momentum of the young field of spin electronics by realizing the potential of an extremely interesting class of oxide materials, the rutile structure oxides. The work aims at combining the half metallic ferromagnet CrO2 and the other rutiles into epitaxial heterostructures that are expected to exhibit novel phenomena and have exciting applications, e.g., magnetic field sensors, magnetic random access memory, and rapidly programmable logic arrays.
Intellectual Merit
CrO2 is a remarkable material that is simultaneously an excellent metal for majority spin electrons and an insulator for minority spin electrons — it possesses perfect 100% spin polarization (P). For this reason, CrO2 is called a “half-metallic ferromagnet.” The amazing potential of CrO2 remains thus far untapped, however. Developing CrO2 heterostructures — particularly given the stringent interface requirements of spin electronics — will require a considerable step forward. In this proposal, we put forward an approach we believe will make this step.
Our approach has three unique and synergistic components. First, we believe that high-quality CrO2-based heterostructures can be realized by working within the rutile family. A consistent problem plaguing CrO2-based heterostructures has been forming high-quality interfaces, which we believe will be alleviated by working with isostructural, chemically similar rutile materials, using novel synthetic strategies. Second, the half metallic nature of CrO2 is only one part of a heterostructure. Efficient spin electronic devices can only be realized if appropriately band matched materials are employed. Working within the rutile oxides, this is readily realized. Third, working with the rutile oxides will not only help us realize the potential of CrO2 and prototype devices, but it will also allow us to exploit the intriguing behavior of the other rutile oxides (e.g., metal-insulator transition in VO2) for even more novel devices. Overall, we believe that experiment and theory both drive each other, and in each task we emphasize a combined approach, bringing all our strengths to bear.
The objectives of the proposed research are to obtain control over the synthesis and processing of these materials and structures, using theory to guide experiments and vice versa. We are developing, fabricating, and theoretically studying novel rutile heterostructures built around CrO2, focusing initially on structures we predict theoretically to have exciting properties. We anticipate that all-rutile structures, combined with novel synthetic strategies, will solve the interface problems thus far plaguing heterostructures. A demonstration of the possibilities of half metals — so far lacking — would have an extraordinary impact and open up new possibilities in spintronics. In order for “half metals” to live up to their promise, it is crucial at least one viable example be clearly demonstrated. We believe CrO2 can fill this vital role.
Our recent results on growth and magnetic anistropy of CrO2 (110) films have suggested that simply using a different crystallographic orientation is a very promising route for realizing CrO2-based devices, particularly since (110) films can be grown essentially strain-free. Surface studies and ab-initio calculations have shown the TiO2 (110) surface to be more stable than (100), implying that material properties in the (110) direction could, in principle, be more promising than (100). Further, our recent X-ray magnetic circular dichroism results point toward a reduced magnetization of thin (100)-oriented CrO2 films which is absent for (110)-oriented films. Finally, we have made substantial progress toward low-pressure synthesis of CrO2 films, which would finally allow us to bring the powerful tools of surface science to bear on our rutile heterostructures, not to mention the obvious advantage of at long last being able to make all in situ heterostructures, a prerequisite for ultimately successful spintronics devices.
Broader Impact
Education of the next generation of young scientists will play a prominent role in the proposed study. In this first year of the project, we have been very active in giving public lectures, incorporating research results in to current course offerings, and engaging young scientists in the laboratory. During the summer and the coming semester, one new undergraduate will be performing independent research on this project, and the past year has see us graduate two Ph.D. students who have worked on this project at least part time, and one master's degree.
We have initiated a new collaboration with the Synchrotron Radiation Center at the University of Wisconsin-Madison to perform X-ray magnetic circular dichroism experiments. Already, nine weeks of beam time have been awarded and used for this project, with an upcoming six weeks before the end of the year. As a result of our ongoing work, we have become regular users of the SRC, and added an important state-of-the-art tool to our repertoire. With the success of our experiments at SRC thusfar (two publications in preparation, four graduate students trained in XMCD), we will be applying for time at higher flux photon sources, such as the Advanced Photon Source and the Advanced Light Source. To this end, we have also initiated a collaboration with Dr. E. Goering of Max-Plank-Institute für Metallforschung, Stuttgart, Germany. Preliminary qualitative investigations suggest there is induced magnetic moment on Ru at CrO2/RuO2 interfaces, an extremely important result if it can be verified. Further investigation and data analysis are being carried on in this new international collaboration.
We have also leveraged, at every opportunity, the existing and considerable resources within the MINT center at UA for the maximum impact of outreach and dissemination of results specifically related to this work, particularly with regard to colleagues in industry. We have presented progress on this project at our MINT reviews, both at UA and industrial research sponsor sites, and obtained valuable feedback from colleagues in the information storage industry.
Our project has attempted to broaden the participation of under-represented groups and build a diverse research team. The most successful way in which we have done this is 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) even before the initiation of this project, and he and undergraduate students from UMES have been working with us on the theoretical modeling of rutile interfaces since the project's inception.
If you're interested in reading even more, you can take a gander at the first and second annual reports we wrote for NSF.