18 research outputs found
Crystal truncation rods in kinematical and dynamical x-ray diffraction theories
Crystal truncation rods calculated in the kinematical approximation are shown
to quantitatively agree with the sum of the diffracted waves obtained in the
two-beam dynamical calculations for different reflections along the rod. The
choice and the number of these reflections are specified. The agreement extends
down to at least of the peak intensity. For lower intensities,
the accuracy of dynamical calculations is limited by truncation of the electron
density at a mathematically planar surface, arising from the Fourier series
expansion of the crystal polarizability
The influence of antiferromagnetic spin cantings on the magnetic helix pitch in cubic helimagnets
In cubic helimagnets MnSi and Cu2OSeO3 with their nearly isotropic magnetic
properties, the magnetic structure undergoes helical deformation, which is
almost completely determined by the helicoid wavenumber k = D / J, where
magnetization field stiffness J is associated with isotropic spin exchange, and
D is a pseudoscalar value characterizing the antisymmetric
Dzyaloshinskii-Moriya (DM) interaction. While the wavenumber can be measured
directly in a diffraction experiment, the values of J and D can be calculated
from the constants of pair spin interactions, which enter as parameters into
the Heisenberg energy. However, the available analytical expression for D,
which is of the first order in the spin-orbit coupling (SOC), has significant
problems with accuracy. Here we show that hardly observable distortions of the
magnetic structure, namely the antiferromagnetic spin cantings, can
significantly change the constant D in the next approximation in SOC, thus
affecting the wavenumber of magnetic helicoids. The obtained analytical
expressions agree with the results of numerical simulation of magnetic helices
in Cu2OSeO3 to within a few percent.Comment: 12 pages, 3 figure
Handling of Multiple‐Wave Effects in the Measurement of Forbidden X‐Ray Reflections in TeO 2
Multiple-wave X-ray reflections usually aggravate the measurement of Bragg reflections, especially of weak “forbidden” reflections. Accurate analysis of multiple-wave peaks usually allows to avoid this. However, multiple-wave reflections can also provide information about crystal structure, since crystal cell parameters determine the positions of multi-wave peaks. The forbidden reflections 002 and 100 in paratellurite are measured and an approach based on semi-kinematical X-ray scattering used to handle the multiple-wave interferences is shown here