This award supports experimental research and education in the field of spintronics. The project entails synthesis of epitaxial magnetic nanostructures for systematic control of the transport and energy spectra of spin-polarized current. The research will be directed toward the understanding of outstanding issues in spintronics, which are both fundamental and essential to applications. In particular, measurements of the single magnetic-domain-wall resistance in single crystal half-metals will be carried out as well as the development of nanostructures that will take advantage of the expected large domain-wall resistance. The project also aims at investigating nanostructures with periodic magnetic domain walls, and using itinerant spin currents to stimulate the modified spin-wave excitations arising from this periodicity. Experimental methods include variable-temperature magnetotransport studies, epitaxial chemical vapor deposition, magnetron sputtering, and submicron lithography. Students will be trained and they will acquire cutting edge skills in advanced experimental techniques and materials processing. There will be efforts to recruit students from underrepresented groups in science, including women and minorities, to participate in this research.
Semiconductor technology is facing severe challenges in miniaturization. Future electronics will likely rely on the fundamental attribute of electron spin. Spintronics concerns itself with several, equally important issues, related to highly spin-polarized solids and spin transport in microscopic systems. This individual investigator award supports a project to harnesses the electron's spin to create new electronics devices. In particular, measurements of the magnetic-domain-wall resistance in half-metals will be carried out as well as the development of nanostructures that will take advantage of the expected large domain-wall resistance. The project also aims at investigating nanostructures with periodic magnetic domain walls, and using itinerant spin currents to stimulate the modified spin-wave excitations arising from this periodicity. The primary impact of the activity will be the training of human resources. Students will be trained and they will acquire cutting edge skills in advanced experimental techniques and materials processing. There will be efforts to recruit students from underrepresented groups in science, including women and minorities, to participate in this research. This research will generate new knowledge and data on novel spintronic nanostructures. New spintronic devices will be invented and adapted to applications areas where existing solutions are inadequate.
The objective of this project was to fabricate, study, and explore spintronic nanostructures, for systematic control of the spin-dependent electron transport. We directed our research toward the understanding of outstanding physics issues which are both fundamental and essential to applications. In particular, we measured the single magnetic domain-wall (DW) resistance in single crystal half-metals. Magnetic domain-wall resistance can be potentially utilized in spintronic devices such as magnetic sensors for electronic compasses and non-volatile magnetic memory or logic devices for computers. Currently, spintronic devices tend to be complicated, multilayered structures. Thus, achieving a large spintronic effect in a single-layer ferromagnetic film has been an attractive goal, both because of its capacity to reveal unexplored physical phenomena as well as from a practical production perspective. We foresee the scenario where DWs form a major component in future spintronic devices. We have grown high quality CrO2 epitaxial films using atmospheric chemical vapor deposition (CVD) with CrO3 as the precursor. In order to grow lateral epitaxial CrO2 nanocontacts to confine DWs, we adopted a unique selective-area growth technique. In this method, a nanostructure is naturally grown during CVD without any of the post-deposition patterning that inherently produces structural defects. On a (100)-TiO2 substrate, we deposited an amorphous SiO2 film and patterned it using electron-beam lithography into a desired geometry, thereby opening a nanocontact window to expose the TiO2 substrate (see Fig.1). We then formed the CrO2 nanocontact by exploiting CrO2’s naturally high affinity for the TiO2 substrate and low affinity (i.e. zero sticking-coefficient) for the SiO2 substrate. On each substrate, CrO2 nanocontacts with their symmetry axes oriented along various crystalline axes can be readily obtained. Fig. 1 shows one of our epitaxial CrO2 nanocontacts where a single magnetic domain is localized near the neck area (seen with scanning magnetic microscopy). Manipulating the domain walls using a DC current (based on the STT effect) applied to the contact area yields a large resistance change of 25% due to domain-wall-resistance (DWR) (see Fig.1). Single DWR was determined to be three orders of magnitude larger than that of conventional 3d ferromagnets as a result of the material's half-metallicity. We also have measured DWR and the STT effect along different crystallographic axes and at varying temperatures. Our results show that lateral spintronic devices incorporating DWs not only can exhibit very large magnetoresistive properties but since DWs can be manipulated by STT, potential sensors and logical devices are readily possible. During the funding period, the PI has also studied MgO based magnetic tunneling junctions (MTJs). We have relied on our understanding of the micromagnetics, quantum tunneling, magnetic coupling, and materials engineering in MTJs to develop magnetic sensors with superior performance. Magnetic sensors are an enabling technology in many areas of science and engineering and have found a role in multiple industries. Magnetic sensors are essential components in the emerging technology of magnetic random access memory. For semiconductor manufacturers, magnetic sensors have made diagnostic analysis of integrated circuits (ICs) possible by sensing the high-frequency magnetic fields emitted from transistors and interconnects. Magnetic sensors are able to detect cracks below the surface of an aircraft or engine turbine by sensing electromagnetic waves originating from the defective area. Under this project, the PI has supported 3 Ph.D. students. These students not only carried out the bulk of the research, but also learned to supervise and mentor undergraduates and high school students. The PI has trained the students in materials synthesis and characterization, in data acquisition and analysis, and in building theoretical models to understand experimental results. As a team, the PI’s group has disseminated research results by timely publication via professional journals and by presentations at scientific conferences. To broaden the impact of this project, the PI has given public lectures on nanotechnology and its relevance to career development to several hundred high school students, with the goal to increase enrollment in science and math courses.
