Dynamics of Mesoscopic Systems Developed with a Tunable Individual Particle Model System

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Nontechnical abstract The key piece of many advanced technologies, such as quantum computing elements, are of a size for which we do not have useful models to describe the relevant physics; the size is commonly referred to as meso, that is between nano and macro. At the smallest length scales, such as atoms, we have models to calculate how the electrons transition between energy levels and how long they can stay in excited or high energy states. However, as we consider more complex systems, such as quantum bits, polymer or plastics dynamics, and biological processes, the added complexity has limited our understanding of the dynamics of the system or how the system transitions from one state to another or how long it can stay in an excited state. The Principal Investigator has developed a model system consisting of magnetic nano/mesoscale particles as small as 200 atoms on a side to develop this important physics. In this model system, the magnetic state, the orientation of the North and South poles (NS) can be determined by the electrical properties of the particles measured by four wires attached to each particle. The particles are so small that the NS orientation fluctuates from thermal noise. Understanding how the NS directions change with time, influenced by the thermal energy or temperature, is directly related to the outstanding physics questions at this length scale. In addition, the nano/mesoscale particles are an ideal model system for other outstanding physics questions related to our understanding of mesoscale physics. The research is conducted by both graduate students and undergraduates as part of their technical training in economically important advanced technologies and physics. These trained students provide the workforce needed by America’s high technology companies. Technical abstract The research explores the dynamics of mesoscale systems and other mesoscale phenomena using a unique model system developed by the Principal Investigator’s research group. The dynamics focus is an exploration of the Arrhenius law that has been untested for systems with complex reversal processes and the other focus is an investigation of stochastic resonance (SR). Both of these long standing physics questions are within reach with the technology developed by the Principal Investigator (PI). This consists of the manufacture of individual magnetic particles as small as 50nm with four nonmagnetic leads attached for four terminal resistance measurements of the anisotropic magnetoresistance. Previous research by the PI has measured random telegraph noise (RTN) in individual nano/mesoscale magnetic particles and how individual RTN oscillators can combine to produce 1/f noise. For the Arrhenius studies, the average RTN dwell times for each of the two states is measured as a function of both the energy barrier separating the states and temperature. The energy barrier is controlled by the application of a dc magnetic field. The data are compared to predictive models. For the SR research a single dot exhibiting RTN is subjected to a small ac magnetic field. The ac field is not sufficient to drive the magnetization through the energy landscape giving rise to the RTN. The thermal noise, however, can enable the transition between states, which is at the heart of SR. Exploring the response of the magnetization as functions of ac field magnitude, temperature, particle energy barriers tests the models of SR. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.