E. Dan Dahlberg

Magnetic Microscopy Center
University of Minnesota

There has been a renaissance in magnetism in the last two decades or so. In the area of micromagnetics (although in the modern context it should be nanomagnetics), major breakthroughs have resulted from the development of new magnetic imaging techniques [1].  A powerful magnetic microscope is the magnetic force microscope (MFM), a variant of the atomic force microscope.  One of the frontiers in magnetism being pushed back is to understand the domain structure and the magnetization reversal in nanometer sized particles.  We have utilized the high resolution MFM (30 nm) we developed [2] to increase our fundamental understanding of magnetism on this length scale.

First, I will present a very elementary introduction to micromagnetics research and a description of MFM with a hands-on demonstration of the basic principle.  I will then discuss the field induced magnetic reversal in stadia shaped particles on the order of hundreds of nanometers wide and about twice that in length.  In general for the small aspect ratio stadia (length to width ratio) the magnetization reverses by the formation of a single vortex and its propagation down the length of a stadium (when the fields are applied perpendicular to the long axis).  The surprising discovery is the importance of virtual particles (vortex-antivortex pairs) creation and annihilation in the magnetic reversal in larger aspect ratio stadia.

1. E. Dan Dahlberg and Jian-Gian Zhu, Physics Today 48, 34 April 1995.

2. George D. Skidmore, Sheryl Foss, and E. Dan Dahlberg, Appl. Phys. Lett.
71, 3293-3295 (1997).

Supported by ONR and the University of Minnesota MRSEC.