University of Minnesota
School of Physics & Astronomy

Spotlight

Sticky Magnetism

Te-Yu Chen, Mike Erickson, and Andrew Galkiewicz and Professor Paul Crowell
Te-Yu Chen, Mike Erickson, and Andrew Galkiewicz and Professor Paul Crowell
Alex Schumann
                                                       

Graduate students Te-Yu Chen, Mike Erickson, and Andrew Galkiewicz in Professor Paul Crowell’s group are looking at what happens when magnetism gets sticky.

Although you might not know it by looking at a compass needle, magnets do not always point uniformly “north” or “south”. Real magnetic materials consist of many regions, known as domains, each of which may point in a different direction. When a material becomes magnetized, the walls separating domains need to move. For better or worse, however, domain walls tend to get “stuck” on defects, limiting the speed of processes such as magnetization reversal. This phenomenon, known as “pinning,” sets practical limits on the performance of many magnetic devices, but it has been very difficult to study quantitatively. Domain walls are extended objects, but the defects responsible for pinning can be much smaller, and they are therefore extremely difficult to probe directly.

Graduate students Te-Yu Chen, Mike Erickson, and Andrew Galkiewicz in Paul Crowell’s group are addressing this problem by looking at the simplest type of domain structure possible: a vortex. Like its fluid analog, a magnetic vortex consists of a region of circulation surrounding a central core. The core, however, is only about 100 atoms wide. Chen and Galkiewicz have developed a means to take “movies” of the motion of these vortices inside disks that are of the order of 1 micron in diameter. The “action” in these movies occurs on time scales less than one billionth of a second. By analyzing the motion of the vortex core, Chen has developed a model to determine the strength and range of the defects that lead to pinning. Erickson, working with Professor Chris Leighton (Dept. of Chemical Engineering and Materials Science) has been able to control the material properties of the disks, modifying the strength of the pinning. As a result, a detailed quantitative picture of a very sticky phenomenon is emerging.

More information at http://groups.physics.umn.edu/fastspin/