X-ray and neutron topographical studies of magnetic materials

Abstract

Neutron and synchrotron topography have proved to be complementary tools for the investigation of magnetic domains of all kinds (ferro-, ferri- and antiferromagnetic) and of magnetic phase coexistence in single crystals. These imaging techniques, which map the special variations of the reflecting power, allow the simultaneous observation of the crystalline (dislocations, twins, precipitates,…) and ‘magnetic’ defects. The origin of the magnetic related contrast in the case of X-rays resides mainly in the variation of magnetostrictive distortion between neighbouring domains. For neutrons contrast mainly arises from changes in the domains or phase structure factors. X-ray topographic experiments lead to a better understanding of the strains created by the presence of magnetic inhomogeneities. Neutron topography is the only technique that is capable of visualizing ‘exotic’ magnetic domains like 180° antiferromagnetic domains (MnF 2, α-Fe 2O 3,…) or chirality domains (right hand/left hand helix) in helimagnets. These investigations show the relationship between domain size and sample purity, domain shape and magnetic history of the crystal, and highlight several striking ‘memory effects’. Synchrotron topography, in the ‘real time’ or the ‘stroboscopic’ versions, allows the observation of the shapes and movements of the domains and interfaces as a function of the temperature or the applied field, as well as their interaction with crystalline defects. Several examples are presented to illustrate the capabilities of these imaging techniques.