Possibilities of X-ray and neutron topography for domain and phase coexistence observations

Abstract

The topographic techniques, in which images of single crystals are made using the local variations in intensity of Bragg-diffracted beams of either X-rays or neutrons, have unique possibilities for the observation of magnetic domains and phase coexistence. Their capacities are different and complementary. Neutron topography, because it involves a magnetic contribution to the scattering amplitude, can image directly very diverse kinds of magnetic domains. It is particularly useful in the case of antiferromagnets, where the staggered arrangement of the magnetic moments, the definition of the domains, does not couple to macroscopic probes. It can also reveal phase coexistence. However it is slow and has bad resolution, typically 0.1 mm, or worse in the case of polarized neutrons, because there are few neutrons. X-ray topography (using normal Thomson scattering, not magnetic scattering, which is still a weak effect) senses local variations in lattice distortion. It therefore may reveal magnetic domains indirectly, and will normally show phase coexistence. Especially in the case of synchrotron radiation, it is fast (10⁻³-10⁻¹s range at ESRF) and has quite good resolution (a few μm). It requires good crystals, typically those which feature very strong extinction in standard diffraction work. The principle, implementation, limitations, availability of these topographic methods are discussed, and examples of domain and phase coexistence observations given.

Key Words:

• Magnetic domains
• observation techniques
• phase transitions
• neutron scattering
• synchrotron radiation scattering
• diffraction topographic techniques.