TECHNICAL: Fundamental investigation of atomic-sized magnetic point contacts made of transition metals (such as Co and Ni) is a fertile ground for exploring new phenomena that are either entirely new or quantum analogs of effects observed in larger ensembles. The intellectual excitement lies in building a new knowledge base for the nascent field of nano- and atomic-scale spintronics. Currently, no direct understanding exists between the spin-polarized quantized conductance/magneto-conductance and underlying contact diameter/geometry. This gap is due to the previous unavailability of stable contacts. This barrier has been overcome from PI's previous studies, paving the way for such systematic studies. Specifically, this project is focused on gaining fundamental understanding of 'structure-property' relationship in atomic-sized quantum conductors, namely, relationship between contact diameter/geometry, multivalent state, and magnetic structure versus spin-polarized quantized conductance, magnetoresistance, and their temperature-dependence, using Co and Ni. The spin-polarized quantized physical properties will be directly correlated with the underlying contact geometry/structure using high-resolution transmission electron microscopy with in-situ quantized conductance measurements. Spin-polarized electron transmission across atomic-sized magnetic conductors can open new vistas for conceiving novel electronic devices that are far more intricate, dense, fast, and robust. The same force that produces magnetism can be harnessed to create 'valves' or 'gates' across an atom to regulate charge transport, in effect, making such an atomic-scale device a microcosm of a modern day electronic circuitry.
The atomic-sized spintronics devices have potential applications in data storage. The same quantum exchange force also causes shifts in electronic levels, which in turn can alter electronegativity and excitation states - key factors to a fundamental understanding of elemental chemisorption, catalysis, and enzymatic reactions in biology. Education and outreach efforts will involve hands-on training for graduate, undergraduate, and high-school students in PI's laboratories. Outreach activities will include PI's participation in the SUNY-wide Louis Stokes Alliance for Minority Program for minority undergraduate students. A three to five-week summer apprentice program for minorities and women students from Buffalo-area inner city high schools will be offered. PI will develop a lecture/hands-on experimental module called "Discreteness of matter" using gold atomic-sized conductors, targeting both high-school students and undergraduate students. Module will demonstrate how conductance eventually becomes discrete (quantized) at atomic scale. Few experiments in science exist where quantum effects can be so readily demonstrated at room temperature, and with ease. The module would also allow ratio of two of the most fundamental constants (electron charge and Planck constant) to be measured directly - the unit of quantized conductance, and will be highlighted.
Realization of atomic-sized electronics and magnetic devices requires a fundamental understanding and regulation of their electron transmission behavior, magnetism, conditions under which conductance quantization prevails, and their mechanical stability against disruptive forces of entropic thermal fluctuations. The present study provides a comprehensive understanding of these aspects. Outcomes: Through this grant, we have succeeded in mapping the evolutionary trace of deformation behavior, magnetism, and electron transport across atomic-sized samples, beginning with a bridge made of just a single atom. These results have revealed a remarkable enhancement in strength as the size of the sample approaches the Fermi wavelength of electrons, and in the limit of a single-atom bridge the modulus and strength is several times higher than that for bulk metals. Remarkably, our experiments on the size-dependence of modulus in magnetic samples also exhibit clear and exciting evidence for a large modulus defect (the so-called delta-E effect) that increases with decrease in sample size. Consistent with this, the load-deformation curves are found to be non-linear for ferromagnetic samples, in contrast to linear behavior for non-magnetic materials. In terms of Broader Impacts, several metrology techniques were developed, one of which has now been licensed to Precision Scientific Instruments, Inc. in Buffalo, NY for commercialization. Personnel engaged in this project directly participated in their development. They include involvement of students in (1) Development of differential thermal coefficient technique to study the mechanistic structure of quantum contacts. (2) Development of two dual STM/AFM systems to study the behavior of contacts under strain. (3) Development of a technique to study domain wall velocity in nanoconstrictions, and (4) Development of an electrochemical method to make magnetic nanowires with constrictions to study propagation of domain wall velocities across them. These efforts have resulted in two patents, one pending and one granted (7425826).
