This award supports theoretical research and education in the properties of nanostructures which have the potential to function in future electronic devices which are based on electron spin dynamics instead of traditional charge dynamics. Researchers will investigate the mechanisms of spin transfer and spin transport in molecular magnetic nanostructures, in static and dynamic cases, where the spin-orbit interaction is significant. Research builds on the known property that spin degrees of freedom in materials couples to orbital motions of electrons and the observation that this plays an important role in molecular nanostructures where one observes current-induced magnetization reversal and anomalies in the Kondo effect. Research will identify how the spin-orbit interaction in nanostructures can be controlled by changing geometry, adsorbed molecules, chemical composition, or applied electric field.
The nanostructures under study are nanoscale magnetic molecules (diameter of a few nm), particularly single molecule magnets with substantial spin-orbit coupling, (i) adsorbed onto nonmagnetic or magnetic metal surfaces, and (ii) bridged between nonmagnetic or magnetic electrodes. The specific objectives of the research are (a) to quantify the spin-orbit interaction for the molecular nanostructures using a first-principles method, and (b) to investigate the effect of the spin-orbit interaction on the spin transfer and spin transport through the molecular nanostructures, using both model Hamiltonians and realistic atomic-scale simulations based on a first-principles method and nonequilibrium Green?s function method. Especially when the magnetic molecules are adsorbed on a ferromagnetic surface or electrodes, large spin transfer is expected due to a strong coupling between the molecules and the surface or electrodes. The proposed realistic simulations will lead to results of almost immediate relevance to experiment (such as an amount of spin transfer for given nanostructure) and to theory (for example, the coupling between the molecules and the electrodes), as well as will provide guidance to theory and experiment.
This research will be conducted with the participation of graduate students, who will be trained in modern methods of theory and computer simulation as applicable to nanoscale systems, preparing them for careers in science and nanotechnology. The PI will connect the research activities with the educational activities at her institution, thereby making connections between the course material and real-world applications and up-to-date research to allow the students to acquire perspectives and skills in tackling scientific problems. Finally the research will be performed in collaboration with scientists from the US and abroad, strengthening and enriching the US ties to the international community.
NON-TECHNICAL SUMMARY: This award supports theoretical research and education in the properties of nanostructures which have the potential to function in future electronic devices which are based on electron magnetic characteristics associated with the spin of the electron instead of traditional charge dynamics which is the basis of normal electrical current. Researchers will investigate the mechanisms of spin transfer and spin transport in ultra small scale molecular magnetic nanostructures. Research builds on the known property that electron spin couples to motion of electrons and the observation that this plays an important role in molecular nanostructures where one observes current-induced magnetic phenomena. Research will identify how the spin-motion interaction in nanostructures can be controlled by changing geometry, adsorbed molecules, chemical composition, or applied electric field.
This research will be conducted with the participation of graduate students, who will be trained in modern methods of theory and computer simulation as applicable to nanoscale systems, preparing them for careers in science and nanotechnology. The PI will connect the research activities with the educational activities at her institution, thereby making connections between the course material and real-world applications and up-to-date research to allow the students to acquire perspectives and skills in tackling scientific problems. Finally the research will be performed in collaboration with scientists from the US and abroad, strengthening and enriching the US ties to the international community.
In the past year (Sept.1, 2012 - Aug.31, 2013), we focused on two things: (1) To understand mechanisms of asymmetric charge transfer between an individual nanoscale magnetic molecule based on twelve manganese atoms and a substrate, (2) To understand unique features in differential conductance of nanoscale magnetic molecule based on four iron atoms bridged between gold electrodes. The first project was in collaboration with scanning tunneling microscopy experimental groups in China (Prof. Wang's group in Southwest University and Prof. Xue in Tsinghua University). The experimental data suggested that individual manganese-based magnetic molecules (Mn12) can be deposited onto a semi-metallic bismuth substrate. Based on this experimental input, using first-principles-based large-scale high-performance computer simulations, we identified an optimized geometry of the Mn12 molecule on a bismuth substrate and calculated electronic and magnetic properties. We found that the charge transfer occurred via one particular Mn site which is closest to the substrate, leading to reduction of the energy gap between occupied and unoccupied molecular levels, and reduction of the magnetic anisotropy barrier by 30% compared to the corresponding barrier for an isolated Mn12 molecule. Our reduced energy gap and local density of states at several molecular orbitals near the Fermi level are in good agreement with the experimental data. This work was published in Polyhedron and in ACS Nano. Our findings show an effect of environmental factors such as interactions with a substrate on electronic and magnetic properties of a nanoscale magnetic molecule. Additionally, our results provide insight into a way that charge can be asymmetrically transferred without physically or chemically breaking magnetic cores in the Mn12 molecule when individual magnetic molecules are deposited on various substrates. The second project was in collaboration with an experimental group led by Prof. van der Zant (Delft, Netherlands) who were able to place a nanoscale magnetic molecule between gold electrodes and to perform transport measurements of the molecule. Inspired by the experimental capability of measurements of differential conductance of such a small molecule in contact to gold electrodes, we calculated differential conductance of a nanoscale magnetic molecule as a function of an external magnetic field, using model Hamiltonian and parameters obtained from first-principles-based computer simulations. We found that the differential conductance shows unique features at low temperatures. The computer simulations of the electronic and magnetic properties of the magnetic molecule were performed by the PI's postdoc (XiaoLiang Zhong), the PI's graduate student (Yoh Yamamoto), and the PI's undergraduate student (Michael Warnock). The paper of this work is in preparation. Our findings in this work show a possibility of observation of unique quantum features of a nanoscale magnetic molecule at the single-molecule level and of control of its magnetic and transport properties using environmental factors. The PI also participated in the summer school of "Theoretical & computational modeling of magnetically ordered molecules and electronic nano-transport of spins" (ComoMoment-2013, at Como in Italy, August 24-30, 2013) to give lectures and a tutorial to the students and postdocs in the field. In this summer school, the PI presented her work in a pedagogical fashion to educate the students and postdocs. In summary, the PI focused on understanding charge transfer and electron transport properties of a nanoscale magnetic molecule, using large-scale computer simulations and model Hamiltonian. Both of the topics were inspired by experimental realization of the systems of interest. The work of the charge transfer was carried out in collaboration with the experimental group, and the other work was performed with the PI's group members (a postdoc and students). Our findings open avenue to build nanostructures in a more controllable fashion and to take advantage of environmental factors on the nanostructures.
