Microscope Maps Miniscule Magnetism

Article excerpt

Electron microscopes are a practical application of the principle that the waves associated with matter really do matter. Electron waves are very much shorter than light waves, so using electrons as probes instead of light reveals finer details, usually about the atomic and molecular structure of objects, than light can expose. Scanning electron microscopes (SEM) delineate the structure of a specimen's surface; transverse electron microscopes send electrons through the sample to find out about its interior. Now a group working at the National Bureau of Standards (NBS) in Gaithersburg, Md., has combined a polarization sensor and a SEM to produce an instrument that both delineates the surface structure of a sample and maps out its magnetic domains.

Magnetic domains are small sections of a metal, for example, in which the inherent magnetism all lines up the same way. Each atom has an inherent magnetism, produced mostly by the spins of its outer, or valence, electrons. In a given domain the magnetic fields produced by the atoms all line up the same way. In a nonmagnetized sample the magnetic fields of the different domains point in random directions, yielding generally no field overall. In a ferromagnetic sample the domains all point the same way and yield a net overall magnetism. (I an antiferromagnetic sample the domains alternate pointing in opposite directions, making an orderly pattern but producing no net magnetism.) A knowledge of the locations, sizes and orientations of magnetic domains is important for the production and understanding of all manner of magnetic devices, particularly magnetic recorders and magnetic memories.

A SEM works by shooting a beam of electrons at the surface of the sample. Striking the surface, these electrons knock out "secondary" electrons, and the information gained from the secondary electrons is used to draw a picture of the surface structure. If the surface is magnetized, the secondary electrons carry information about the magnetization in their polarization. …