Aggregates of between 2 and 1000 atoms make an excellent laboratory in which to study the static and dynamic properties of finite-systems. Among the most intriguing of those properties are magnetic order, which is extremely sensitive to structure and bonding, and the crossover behaviors that are analogous to bulk phase transitions, particularly the melting and freezing transitions. This individual investigator award supports two experimental projects in such clusters. The first project will explore the evolution of magnetism and magnetic properties with size in clusters of a wide range of materials, including those that are, in the bulk, ferromagnetic, antiferromagnetic, and nonmagnetic. The reduced dimensionalities of these systems and their high surface-to-volume ratios favor enhanced magnetic ordering and unusual couplings between their magnetic and spatial structures. The experiments to be performed will use magnetic deflection to measure effective magnetizations in the clusters and, in concert with theoretical models, to understand the magnetic characteristics of these particles. The second project will examine the time evolution of isolated, thermally excited clusters as they undergo spontaneous thermal isomerization. The experiments will be performed using pump-probe techniques with picosecond and femtosecond laser pulses to follow the structural and energetic evolutions of such clusters. In addition to watching the crossover from the low temperature solid-like phase to a liquid-like phase at high temperatures, these experiments will explore the roles of classical and quantum dynamics in describing the shape changing process. Both experiments involve cutting edge vacuum, electronic, laser, and optical technology and will prepare the graduate students taking part in this research for either academia or industry.
Atomic clusters offer a conceptual bridge between the worlds of individual atoms and those of bulk materials. Consisting of between 2 and 1000 atoms, these tiny particles represent a middle ground in which to study the growth of bulk behaviors out of the simpler behaviors of atoms and molecules. Issues to explore with increasing particle size include both basic properties (e.g. magnetism) and phase transitions (e.g. melting and freezing). As technology pushes toward ever smaller dimensions, a clear understanding of such tiny systems is becoming a practical necessity. This individual investigator award supports two experimental projects that address this middle ground. The first project will explore the evolution of magnetism and magnetic properties with size in clusters of both the traditional magnetic metals (iron, cobalt, nickel) and the more unusual elements (rare earths, chromium, rhodium). With most of their atoms on their surfaces, these clusters are often unusually magnetic and have exotic thermal and structural characteristics. The experiments to be undertaken will measure the magnetic and thermal properties of a wide range of clusters and seek to identify systems with conceptually and/or technologically important magnetic characteristics. The second project will examine the time evolution of clusters that contain enough thermal energy to change their shapes even in the isolation of vacuum. Neither truly solid nor truly liquid, these shape-changing systems offer insight into the complexities of ultra-small systems in thermal environments. The experiments to be performed will use ultrafast laser pulses to watch the clusters change shape in real time and will look for both the familiar classical features of melting and freezing and the less familiar quantum features common in the atomic-scale world. Of particular interest is the surprising ease with which tiny particles rearrange thermally, a factor that inevitably shortens the short life spans of many tiny technological structures. Both efforts will involve cutting edge experimental techniques and will provide the students involved with skills and training that will prove useful in either academia or industry.
