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Microscope maps miniscule magnetism.

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. If one regards electrons as particles, polarization means that their spins are all oriented in the same direction; if one regards electrons as waves, polarization means that the waves all vibrate in the same direction. Whatever picture one uses, the polarization is related to the direction of magnetization in the part of the surface the given secondary electrons come from.

The new instrument, developed by John Unguris, Daniel Pierce and Robert Celotta of the NBS Center for Radiation Research and Gary Hembree of the NBS Center for Manufacturing Engineering, combines a SEM with a spin polarimeter to get the usual picture of the surface topography and at the same time a map of the magnetic domains. It is not the first attempt to do this, but it claims to be more practical. The work will be described in the September JOURNAL OF MICROSCOPY.

The first attempt to combine a SEM with a spin detector used a device called a Mott spin analyzer and an electron beam 10 microns in diamter. The Mott analyzer takes up about a cubic meter of space and operates at 100,000 volts, requiring special high-voltage protection. Unguris and colleagues developed an analyzer about the size of a human first that operates at about 150 volts. It analyzes the electron spins by observing how the electrons scatter from a polycrystalline gold film. The way they scatter depends on their spin. This group also uses a narrower electron beam, one 10 nanometers in diameter.

With the device, they have mapped the surfaces of single crystals of iron mixed with 3 percent silicon. The illustration above shows magnetic domains in a section of the crystal surface 50 by 50 microns. The difference in polarization between light and dark domains is approximately 60 percent. The best resolution of detail so far achieved with the device is 50 nanometers, a limit imposed not by electron characteristics but by vibration of the stage on which the sample rested.
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Title Annotation:scanning electron microscope - SEM
Author:Thomsen, Dietrick E.
Publication:Science News
Date:Aug 24, 1985
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