Prof. E. Dan Dahlberg (University of Minnesota)

"Aging, Rejuvenation, and Memory Effects in a Single Crystal Spin Glass"

Abstract: Spin glasses are a prototype of complex systems for which the dynamics can be studied experimentally only out of equilibrium. We have systematically studied the phenomena of aging, rejuvenation, and memory, common to many glassy systems. We used ac susceptibility measurements made on a single crystal of CuMn with a 7.91% concentration of manganese and a Tg of 41 K. Aging occurs when the temperature is held fixed, leading to a relaxation or decrease of the susceptibility. Rejuvenation is occurs when the sample has been aged and then upon lowering the temperature “forgets” that it was aged and returns to a reference cooling curve (hence it gets “younger”). In the memory effect, after temperature cycling, the heating curve seems to “remember” its previous cooling history. In this talk, we discuss these effects with a focus on rejuvenation and our attempts to understand its underlying mechanism with a phenomenological model. The ac frequency was 1 Hz and the ac field was 5 Oe. The temperatures investigated were from 37 K to 8.5 K. Our data show that both the rejuvenation and memory results appear to favor a temperature dependent hierarchical model.

Short bio: Dr. E. Dan Dahlberg is a College of Science and Engineering Distinguished Professor and Professor of Physics at the University of Minnesota. His current research interests include the fundamental statistical mechanics of the dynamics of mesoscale particles and the dynamics of spin glasses; he is the Director and PI of the Magnetic Microscopy Center (MMC) at the University. He received his PhD from UCLA under the direction of Professor R. L. Orbach following receipt of a BS and MA from the University of Texas at Arlington under the guidance of Professor Truman Black.