Electrons possess a fundamental property known as spin, which makes the electron equivalent to a small magnet. The long term goal of this project is to develop a powerful new type of computer known as a "spin computer" that uses the electron spin to store and process data. The design of this type of computer has been developed for electron spins in semiconductors, but such efforts have been hampered by small signals and the need for cryogenic (very low) operating temperatures. The advent of a new electronic material known as graphene, a single atomic sheet of carbon, is making the prospects of a spin computer more realistic because large spin signals have been demonstrated at room temperature. This project will advance the fundamental knowledge of electron spin in graphene by performing experiments that investigate the role of imperfections such as impurities, vacancies, and ripples in the graphene sheet. These studies are crucial because imperfections are believed to be responsible for the loss of information held by the electron spin ("decoherence"), although it is currently unclear which type of imperfection is the main source of the problem. The experiments will systematically address this critical issue and also explore new methods of using the imperfections to control the alignment of the spins ("magnetism"). This project will support the training of a PhD student, an undergraduate student, and a high school student in condensed matter physics using state-of-the-art instrumentation. The knowledge gained in these studies will greatly broaden the scientific and technological impact of spintronics.
The origin of spin scattering in graphene is a central issue of graphene spintronics, while tunable magnetism is a fascinating collective phenomena predicted for doped and/or defective graphene. This project investigates spin scattering and tunable magnetism by systematically introducing impurities, vacancies, and ripples into graphene spin valves and Hall bar devices. The experimental approach combines the techniques of molecular beam epitaxy (MBE) and magnetotransport measurements in a unique manner. Impurities will be systematically introduced atom-by-atom through MBE deposition while vacancies will be generated by Ar-ion sputtering in an ultrahigh vacuum chamber. Their effect on charge and spin transport will be measured in the same chamber via in situ magnetotransport measurements. Ripples will be controlled through the use of atomically flat substrates produced by MBE. The effects of these types of disorder on spin lifetimes will be measured by spin precession (Hanle) measurements on spin valves and will elucidate the mechanism of spin scattering in graphene. The Kondo effect and gate tunable magnetic ordering in doped and/or defective graphene will be investigated through a combination of magnetotransport measurements and magnetization measurements. These experiments will greatly expand the knowledge of spin-dependent interactions in solid-state systems and provide excellent training for a PhD student, an undergraduate student, and a high school student in condensed matter physics.
In this project, Dr. Kawakami’s research team made seminal advances in a new research area known as "graphene spintronics." These advances helped establish the international scientific agenda within the field and the results demonstrated superior spintronic properties to make graphene the material of choice for potential spintronic applications in integrated logic and memory. Indeed, this project has served as the foundation for two subsequent industry-guided grants through the semiconductor industry’s Nanoelectronics Research Initiative (in conjunction with NSF and DARPA), which seek to develop the technological aspects of graphene spintronics and develop career pathways for participating students. This project has involved the participation of graduate students, undergraduate students, high school students, and high school teachers. The research revolves around a fascinating material known as graphene, a single atomic sheet of carbon (see image 1) that has excellent electrical, mechanical, and thermal properties. Results from this NSF grant have established that graphene is also excellent for spintronics, an alternative form of electronics that utilizes the electron’s spin. Conceptually, spin is a property that makes each electron into a tiny magnet with "north" and "south" poles and the goal is to use the direction of the magnetic poles to carry information (see image 2). This lies at the heart of magnetic information storage technologies like computer hard drives, but the new twist is to try to also use this for logic operations in computer chips. The performance advantages would come through combining logic and non-volatile storage into a single chip. The key physics process is called "spin transport," where electrons with aligned spins are shuttled across graphene between different ferromagnetic electrodes (see image 3). The ferromagnets themselves store information according to the direction of their magnetic poles and act as a reservoir for aligned electron spins. By applying a small voltage to the electrode, the aligned spins are injected into the graphene, where they travel until they are detected by another ferromagnetic electrode. What makes graphene special is that it can perform this lateral spin transport at room temperature far better than any other material already, and theoretical projections suggest that it could substantially improve further. By considering spin transport, the important scientific questions become clear: How efficiently can the spins be injected from the ferromagnet into graphene ("spin injection efficiency")? How far can the spins travel in graphene before their directions randomize ("spin diffusion length")? For how much time can the spins retain their alignment before their directions randomize ("spin lifetime")? This last question is particularly important because theory predicts much longer lifetimes than observed, so a central challenge is to understand what is limiting the spin lifetime. The work of Kawakami’s group has led the way in helping address these important issues. First, Kawakami developed a method for depositing atomically flat, atomically thin insulators on graphene, which immediately led to record high values for spin injection efficiency into graphene. The study also showed that poorly isolated magnetic electrodes caused major reduction of the spin lifetime. Second, by comparing the behavior of spins in a single atomic sheet of graphene vs. two stacked sheets (i.e. bilayer graphene), it showed that bilayer graphene could show much longer spin lifetimes. This result highlighted the possibility of so-called "inhomogeneous spin-orbit interaction" as an important factor in limiting the spin lifetime. Finally, Kawakami’s group resolved a highly controversial issue regarding the ability of point defects such as missing atoms ("vacancies") or extra atoms ("adsorbates") to produce magnetism in graphene. Kawakami’s novel method used one type of atomic sized magnet (the electron spin) to detect another type of atomic sized magnet (coming from the point defect), as shown in image 4. With this method, Kawakami was able to establish that point defects do indeed generate magnetic dipoles, and therefore places the field of magnetism in graphene on solid footing. It also highlights the possibility that magnetic interactions may be responsible for limiting the spin lifetime. While these questions are still open, the work of Kawakami’s group has aggressively pushed this field forward. The forefront research has enabled substantial progress in workforce development and outreach activity with high school students and teachers. Graduate students who participated in this project have advanced to a faculty position (Yan Li, Utah Physics Department) and prestigious postdoctoral positions (Wei Han, IBM Almaden; Kathy McCreary, NRC fellowship at NRL; Adrian Swartz, Stanford). The project involved six undergraduate students. The three students who have graduated are all in graduate school in Physics. The project also sponsored five high school students to summer internships (see image 5) and supported the involvement of five teachers. For the high school internships, Kawakami developed an online application system, which helped to broaden the applicant base for the program. This will continue to help promote diversity in science and engineering.
