Although they exist in almost every magnetic material, you cannot see them: magnetic domains are microscopically small regions of uniform magnetization. Every magnetic material is divided into such domains. Scientists call them “Weiss domains” after physicist Pierre-Ernest Weiss. In 1907, he recognized that the magnetic moments of atoms within a bounded domain are equally aligned.
All pursuit of this theory has so far been limited to two-dimensional images and material surfaces. Accordingly, researchers have only ever been able to see a domain in cross section. Together with colleagues from Doctor Ingo Manke and his group have developed a method by which they can image the full spatial structure of magnetic domains – even deep within materials. To do this, special iron-silicon crystals were produced for which the research group had already developed model representations. Their actual existence has now been proven for the first time. With it, the researchers have solved a decade-old problem in imaging.
Most magnetic materials consist of a complex network of magnetic domains. The researchers’ newly developed method exploits the areas where the domains meet – the so-called domain walls. Within a domain, all magnetic moments are the same, but the magnetic alignment is different from one domain to another. So, at each domain wall, the direction of the magnetic field changes. The researchers exploit these changes for their radiographic method in which they use not light, but neutrons.
Magnetic fields deflect the neutrons slightly from their flight path, just as water diverts light. An object under water cannot be directly perceived because of this phenomenon; the object appears distorted and in a different location. Similarly, the neutrons pass through domain walls along their path through the magnetic material. At these walls, they are diverted into different directions.
This diversion, however, is only a very weak effect. It is typically invisible in a neutron radiogram, since non-diverted rays overshadow it. The researchers therefore employ several diffraction gratings in order to separate the diverted rays. During a measurement, they rotate the sample and shoot rays through it from all directions. From the separated rays, they can calculate all domain shapes and generate an image of the domain network in its entirety.
COMPAMED.de; Source: Helmholtz-Zentrum Berlin für Materialien und Energie