The objective of this research is to synthesize, characterize, and engineer high spin polarization ferromagnetic/organic interfaces for high-efficiency spin injection, and to fabricate lateral spin valve devices using organic semiconductors as spacer. The approach is to synthesize and fabricate devices with manganite-based half-metallic ferromagnetic electrodes which possess nearly 100% spin polarization. To characterize the half-metallic property of the spin injecting electrodes, two unique methods will be employed: second harmonic generation and tunneling magnetoresistance, both probing the electronic and magnetic properties of the ferromagnetic/organic interfaces.
Intellectual merit: Ideal half-metallic ferromagnets should have 100% spin polarization, which can serve as the most efficient spin injector in spintronic devices. In real devices, the interface can significantly differ from the bulk. In particular, the interfacial magnetism has not yet been adequately addressed. In addition, spin diffusion and decoherence in organic materials have not been systematically studied for lacking effective experimental tools. These topics are the focus of this proposed research. Advances in synthesis and characterization of the half-metallic ferromagnetic materials and in fundamental understanding of the spin-dependent process in organic materials will not only undoubtedly enrich our knowledge of materials, but also benefit other disciplines of condensed matter physics and materials science.
Broader impact: The proposed research is related to high-efficiency spin injection in spintronic devices such as recording heads, non-volatile memories, and light-emitting devices. If the spin injection efficiency is dramatically increased by using half-metallic ferromagnetic injectors, the resulting devices will perform superior to the existing devices. High-efficiency devices can lead to a significant saving in energy and a lower production cost, which will positively impact the high-technology industry and the society as a whole. The proposed education plan will involve more women and under-represented minorities in research, continue the outreach activities to local middle and high schools and the surrounding communities, and enhance information exchange between the research community and the general public. These proposed activities will also raise public?s interest level in science and encourage the college-bound students to choose science as their careers.
This project deals with the synthesis and characterization of ferromagnetic oxide materials (e.g. La1-xSrxMnO3 (x=1/3) or LSMO) that have high spin polarization mainly for carbon-based spintronic devices. LSMO has been successfully demonstrated as an efficient spin injector in organic spin valves and carbon nanotube spin valves. However, an outstanding issue in LSMO-based spintronic devices (organic and inorganic alike) is that the spin valve magnetoresistance disapprears at temperatures much lower than the Curie temperature of LSMO. A possible reason, as suggested by other researchers, is the spin-canting state at the LSMO surface which determines the spin valve magnetoresistance. This research was set out to explore the surface and interface spin state and its effect on the electronic and magnetic properties in LSMO devices. High-quality materials and devices such as thin films, superlattices, and magnetic tunnel structures, etc., are prepared with laser molecular beam epitaxy in PI's group (Fig. 1). In particular, [110]-oriented films, superlattices and magnetic tunnel junction devices have been fabricated and investigated in comparison with those prepared in [100]-orientation. It has been found that the [110]-oriented surfaces/interfaces have higher Curier temperatures and higher magnetic moments than those of the structures prepared in the [100] direction (Fig. 2), which indicates that the spin state in [110]-oriented structures is dominated by the ferromagnetic one. This arises mainly because the [110] surface/interface has a lower effective doping level that that of the [100] surface/interface which in turns reduces the antiferromagnetic interaction. This is verified in the temperature dependence of magnetic tunnel junction devices prepared in both orientations (Fig. 3). More direct studies of interface magnetism was investigated in parallel in the co-PI's group using non-linear magneto-optic Kerr effect measurements. In addition to the crystal orientation effect, systematic studies have also been carried out for different manganite materials such as La1-xBaxMnO3, and for various doping levels. Another unsolved issue in LSMO-based magnetic tunnel junctions as well as in LSMO-based organic spin valve devices is the sign of the magnetoresistance. In this research, it is found that different tunnel barriers (e.g. SrTiO3, Al2O3, HfO2) yield different signs in the magnetoresistance. In the organic spin valve devices, the Alq3/Co combination always gives rise to an inverted (negative) magnetoresistance no matter whether LSMO is covered by a tunnel barrier or which barrier is next to LSMO. The results indicate that the bonding at the ferromagnetic/non-magnetic interface is important in determining the sign of the magnetoresistance. The findings in this research have improved the fundamental understanding of several important issues such as the effect of the spin state in ferromagnetic oxide materials and the sign of magnetoresistance, and have significant relevance to spintronics using half-metallic ferromagnetic materials. To explore spin transport/diffusion in carbon-based materials for spintronics, the PI's group has studied charge transport properties in graphene devices. Magnetic nanoparticles (γ-Fe2O3 and Fe3O4) are placed on graphene and the effect on carrier mobility in graphene is measured. It is found that the modification of the charge environment of graphene leads to a wide range in tunability in carrier mobility and therefore the charge transport (Fig. 4). Subsequently, the effect on spin transport has been investigated in non-local graphene spin valve devices. The findings in this research provide a pathway for effective tuning of charge and potentially spin transport using nanoparticles. The PI has actively engaged in education/outreach activities during the funding period. Among various activities, the most successful one is the participation in training high school physics teachers through the annual Summer Academy for Physics Teachers sponored by PI's department. Besides providing hands-on experiments, lab tours and demos, the PI and his students have given short lectures in both classrooms and laboratories every year. Such education/outreach activities have made a positive impact in improving high school physics education in mainly under-represented minority schools.
