Inelastic neutron and x-ray scattering from incommensurate magnetic systems

Abstract

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