Inelastic Neutron and X-ray Scattering from Incommensurate Magnetic Systems

TL;DR

This paper explores magnetic and lattice excitations in incommensurate systems MnSi and Cr using neutron and X-ray scattering, revealing detailed insights into their magnetic states, phonon anomalies, and potential for future studies under extreme conditions.

Contribution

It provides new experimental insights into the excitations of incommensurate magnetic systems, including the nature of Stoner excitations and phonon anomalies, and discusses future research possibilities.

Findings

Stoner excitations are spin flip excitations in MnSi.

Kohn anomalies occur at incommensurate positions in Cr.

Anomalous phonon softening caused by electron-phonon coupling.

Abstract

Neutrons and X-rays are powerful probes for studying magnetic and lattice excitations in strongly correlated materials over very wide ranges of momentum and energy transfers. In the focus of the present work are the incommensurate magnetic systems MnSi and Cr. Under application of a magnetic field, helically ordered MnSi transforms into a weak itinerant ferromagnet. Using polarized neutrons we demonstrate that the Stoner excitations are spin flip excitations. The amplitude (longitudinal) fluctuations associated with the magnon modes are already strong far away from T_C. Interestingly, even the non spin flip excitations associated with the Stoner modes are observable. In Cr, we have observed Kohn anomalies in the phonon spectrum at those incommensurate positions in reciprocal space, where the spin density wave is observed. The corresponding phonon and magnon modes are not coupled. In addition, an anomalous softening of a transverse phonon branch along the N-H zone boundary line is observed that is caused by strong electron phonon coupling. High resolution neutron scattering indicate that the low energy Fincher-Burke excitations may rather correspond to localized modes in momentum and energy and not to propagating collective modes. Finally, we demonstrate that in the near future it may become feasible to investigate excitations in very small samples thus allowing to measure the dynamics of strongly correlated materials under extreme conditions and in the vicinity of quantum phase transitions.