J. Baruchel

Neutron topography: A review

Abstract Neutron diffraction topography is a fairly recent imaging technique suitable for the investigation of defects and magnetic domains in single crystals. A review is made of its principle, history, and instrumental aspects. The main results obtained in materials science, i.e. the possibility of observing virtual slices in bulk samples and the discovery of a new substructure in Bridgman-grown alloy crystals, and in magnetism, viz. the visualization and exploration of the behavior of spin-density-wave domains in chromium, 180° or time-reversed antiferromagnetic domains in MnF 2 , and chirality domains in helimagnetic Tb, Ho and MnP, are described. The relationship between topography, diffraction theory and the problem of extinction is discussed.

180° Antiferromagnetic domains in MnF2 by neutron topography

Abstract 180° antiferromagnetic (piezomagnetic) domains, observed in zero fiels by polarized neutron topography on single crystal plates on MnF2, are large (millimeters) and bounded by curved walls. Their configuration is determined by the stress condition of the specimen and the magnetic field applied at TN, and does not change with further cooling.

Antiferromagnetic domains in nickel oxide by magnetic neutron laue diffraction

Abstract Two kinds of domains had been observed in antiferromagnetic nickel oxide: T-domains, associated with the four 〈111〉 propagation vectors and S-domains, associated with the three equivalent antiferromagnetic directions within a T-domain. Taking advantage of the direct interaction of neutrons with the arrangement and direction of magnetic moments, the T and S antiferromagnetic domain distributions are investigated in several single crystal samples by magnetic neutron Laue diffraction (MNLD). The T-domain behaviour when a stress is applied is qualitatively investigated. In white beam a direct topographic observation and identification of T-domains are presented. By simultaneously measuring the relative intensities of magnetic Bragg reflections from a sample with preferential domain population it is possible to confirm the direction of sublattice magnetization which had been obtained by an optical method. The S-domain distribution is studied as a function of the magnetic field on the one hand, and as a function of time on the other. This last phenomenon is very similar to the well-known magnetic after-effect which occurs in most ferromagnets. It is concluded that MNLD could be a very valuable technique to investigate many topics of magnetism.

STUDY OF THE MORIN TRANSITION IN NEARLY PERFECT CRYSTALS OF HEMATITE BY DIFFRACTION AND TOPOGRAPHY

Abstract The spin reorientational magnetic Morin transition is investigated on nearly perfect crystals of hematite by combining neutron diffraction, neutron and synchrotron radiation topography, and magnetization measurements. The samples are platelet shaped crystals, with their main surface parallel to the (1 1 1) basal plane. The magnetization measurements show that the main part of the transition occurs over a rather narrow temperature range (about 3 K), whereas this range appears broadened to about 13 K on the neutron diffraction curves. Neutron topographs show the whole crystal image during transition and X-ray topographs show zone contrast when modifying the applied magnetic field. These experimental facts indicate that the interfaces which separate the weakly ferromagnetic and the antiferromagnetic phases are parallel to the main surfaces of the crystals. This is the basis for a very simple model which explains, through severe extinction effects, the apparent contradiction between neutron diffraction and magnetization measurements, and leads to an estimation of the crystal perfection parameters.

Search for commensurate effects in the helimagnetic phase of holmium

Abstract Critical field measurements and the direct determination, through neutron diffraction, of the mean turn angle as a function of temperature in the helimagnetic phase of holmium have revealed small singularities in the region around 95.5 K where ultrasound anomalies have been detected.

Helimagnetic-ferromagnetic phase coexistence in MnP: A synchrotron radiation and neutron diffraction topography investigation

Abstract Neutron and white-beam synchrotron radiation topography have been used to observe the helimagnetic-ferromagnetic phase coexistence in single crystal MnP. The phase transition was approached by varying either the temperature or the magnetic field. The interface between the two phases displays a finely meshed zigzag shape with the ‘needles’ being directed along the easy axis of magnetization and arranged (when no external magnetic field is applied) around an average direction corresponding to a state of minimal elastic deformation. This morphology appears to result from competition between the elastic and magnetostatic terms of the total energy. The relative magnitudes of these terms are different for temperature- and field-induced transitions. The helimagnetic region is, in the field-driven case, expelled to the edges of the sample and forms a type of ‘supplementary domain’ pattern, the interface retaining its finely structured zigzag shape.

Investigation of the effect of an applied electric field on α-LiIO3 crystals by synchrotron radiation topography

Abstract An investigation of the enhancement (or extinction reduction) of some Bragg reflections when an electric field is applied has been carried out on α-LiIO3 using neutron diffraction, electrical conductivity measurements and mainly synchrotron radiation topography. The study has led to new information concerning the way the reduction of extinction occurs. When an electric field is applied, the topographs performed on hkl reflections (l≠0) show fine lines parallel to the c-axis. Thicker ones, similar to those observed optically by previous workers, further develop from an electrode. The behaviour of these lines when the electric field, the temperature or the orientation of the sample is varied, or in the presence of some crystal defects (growth bands or start of cleavage), suggests that the inhomogeneous motion of ions under the action of the applied electric field leads to a distortion of the lattice which in turn induces an important change of extinction conditions in the crystal.

Neutron diffraction investigation of the nuclear and magnetic extinction in MnP

The absolute values of the reflecting powers ρ are measured for the 200 and 2 ± τ, 0, 0 set of magnetic and nuclear reflections in the helimagnetic phase of a good-quality crystal of MnP as a function of its thickness. Severe and very different extinction effects are observed for the magnetic and nuclear reflections (ymagnetic ~ 0.4, ynuclear ~ 0.02 for the largest thickness). This corresponds to the spectacular result that the magnetic reflecting powers ρ± are twice as big as the nuclear one ρN, in spite of the fact that the scattering cross sections |F±|2 are about ten times smaller than the nuclear |FN|2. The nuclear results appear consistent with dynamical theory while the magnetic ones are not. They can be explained by Zachariasens type II secondary extinction model based on the chirality domain pattern. The same measurements were performed in the ferromagnetic phase, yielding yferro ≃ 0.03. A model using the relative sizes of the ferromagnetic and chirality domains is presented.

Coupling between antiferro and ferromagnetic domains in hematite

Abstract We observe a coupling between the weak ferromagnetic (WF) component and the antiferromagnetic (AF) vector by investigating 180° domains in the WF phase of hematite, in keeping with the symmetry of the Dzialoshinski interaction, and a memory effect associated with the coupling between the signs of the major type domains in the WF and AF phases when going through the Morin transition.

Impurity related memory effect of the antiferromagnetic domains in MnF2

Abstract It has been observed for the first time, that the 180 antiferromagnetic domains basically remain unaltered in most of the investigated MnF 2 crystals after warming to T N +Δ T and then cooling again below T N , if Δ T T M . The value of T M was found to be changing from 0 to 50 K depending on the crystal preparation. A strong correlation between this memory effect and the oxygen content of the samples has been established by using both neutrons and luminescence techniques.