2010 Distinguished Lecturers
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Biomagnetics is an interdisciplinary field where magnetics, biology and medicine overlap. It has a long history since 1600, when William Gilbert published his book De Magnete. Recent advances in biomagnetics have enabled us not only to detect extremely weak magnetic fields from the human brain, but also to control cell orientation and cell growth by extremely high magnetic fields. Pulsed magnetic fields are used for transcranial magnetic stimulation (TMS) of the human brain, and both high frequency magnetic fields and magnetic nanoparticles have promising therapeutic applications for treatments of cancers and brain diseases such as Alzheimer’s and Parkinson's. On the imaging front, magnetic resonance imaging (MRI) is now a powerful tool for basic and clinical medicine. New methods of MRI based on the imaging of impedance of the human body, called impedance MRI, and the imaging of neuronal current activities in the human brain, called current MRI, are also being developed.
The phenomenon of exchange bias has remained something of a mystery since it was discovered in core-shell particles in 1956 [1]. Over the subsequent years many different models have been proposed to explain this effect, most of which agree with some experimental data that can be found in the literature. No single theory, however, has been able to explain the data consistently for different systems.
Tremendous progress has been made recently in the development of magnetic Heusler compounds specifically designed as materials for spintronic applications [1]. While problems in the field of spintronics remain, the use of half-metallic Heusler compounds provides a prospect for novel solutions.
One of the current frontiers in magnetism is to understand the domain structure and the magnetization reversal in nanometer-sized particles. Explorations at these length scales have been aided by the development of new magnetic imaging techniques [1], one of which is the magnetic force microscope (MFM), a variant of the atomic force microscope. We have utilized the high resolution MFM (30 nm) we developed [2] to increase our fundamental understanding of magnetism on this length scale. I will discuss the field-induced magnetic reversal in particles on the order of hundreds of nanometers in width and about twice that in length. In general, for the small aspect (length to width) ratio, the magnetization reverses by the formation of a single vortex and its propagation down the length of a particle (when the fields are applied perpendicular to the long axis). There are some surprises when the aspect ratio of the particles increases.