Accurate simulations of magnetic excitations in the neutron simulation package McStas

TL;DR

Accurate simulations of magnetic excitations in McStas introduces SpinWave_BCO, a linear spin wave theory–based component for simulating inelastic neutron scattering from ferromagnetic and two-sublattice antiferromagnetic excitations on a body-centered orthorhombic lattice. The authors derive the relevant dispersions and scattering intensities, implement them in McStas, and validate the results against theory and SpinW with MnF$_2$, including an altermagnetic dispersion extension. The results show excellent agreement for both dispersion and absolute cross sections, establishing a solid foundation for realistic magnon simulations in neutron instrument modeling and design. The work paves the way for broader lattice geometries, more complex spin orders, and polarization-aware implementations in future McStas developments.

Abstract

A new component for the accurate simulation of neutron scattering from magnetic excitations has been developed for the neutron ray-tracing software McStas. The component SpinWave_BCO simulates inelastic neutron scattering from ferro-, antiferro-, and altermagnetic excitations in a body-centered orthorhombic crystal structure, where the dispersion relation and scattered neutron intensities are derived using linear spin wave theory. Data from a simulated Triple-Axis Spectrometer with an extremely high resolution have been verified by direct comparison with theory and by comparison to data simulated using the package SpinW. The component serves as a proof-of-concept for the implementation of a more general linear spin wave component in McStas.

Figures (8)

• Figure 1: Magnetic structure and exchange parameters for the ferromagnetic (left) and antiferromagnetic (right) system. The plots are generated using SpinW Toth:2015.
• Figure 2: Simulated data for the ferromagnetic system. (left) Simulated dispersion compared with theory. Red points show the fitted peak positions. The solid white line shows the dispersion fitted to the red points and the dashed magenta line shows the dispersion calculated from theory. (right) Integrated differential cross section compared with theory.
• Figure 3: Examples of constant- q scans for the antiferromagnetic case. Left is along $(10l)$ and right is along $(h01)$. The solid lines show the fitted functions used to find the intensity peak centers and the dashed lines show the peak centers from theory.
• Figure 4: Map over scanned directions in reciprocal space. Labels correspond to those seen in figure \ref{['fig:intensity_plots']}.
• Figure 5: Simulated intensity data measured along four directions in $\mathbf{q}$-space. Points show fits to intensity peaks. The solid white line is the dispersion fitted to these points and the dashed magenta line shows the dispersion calculated from eq. (\ref{['eq:AFdispersion']}).