September 19, 2007
3pm in Howey Physics Lecture Room 5

 

Ruslan Prozorov, Ames Laboratory

 

"Topological Hysteresis in Superconductors and Ferromagnets"

Usually magnetic hysteresis is caused by the imperfections and defects in the crystal structure resulting in pinning of magnetic flux in superconductors or domain walls in ferromagnets. The alternative cause of pinning is shape and surface - related energy barriers (e.g., geometric and Bean-Livingston) that result in a spatially nonuniform free energy and corresponding metastable states of the system. I will describe a different type of magnetic hysteresis that is only observed in clean, pinning-free samples. Most importantly, this hysteresis cannot be annealed or removed by any sample improvement. We call this phenomenon “ topological hysteresis “ to indicate that the difference in the topologies of the intermediate state in type-I superconductors or ferromagnetic domains in soft ferromagnets can lead to a measurable hysteretic response of magnetization. I will discuss failures of the textbooks (Landau) treatment of the intermediate state as well as topological time-reversal symmetry breaking that lead to some very unusual magnetic patterns. The general conclusions of my talk can be relevant for various fields of physics, chemistry, mathematics and cosmology where pattern formations in complex nonlinear systems take place.