Spin-phonon coupling across a magnetic quantum critical point in Mn1-xFexSi
Final Report Abstract
The family of transition-metal mono-silicides Mn1-xFexSi crystallizing in the cubic B20 structure exhibits a variety of complex phenomena, which are of interest to both basic and applied science. While the focus has been on the intriguing magnetic properties of MnSi and its sister compounds, the scientific aim of the project “spin-phonon coupling in the vicinity of a magnetic quantum critical point” was to study the lattice dynamical properties in the doping series Mn1- xFexSi up to doping values x where the magnetic transition is suppressed to zero temperature featuring a magnetic quantum critical point. The project idea was based on the established magnetic phase diagram of Mn1-xFexSi, previous phonon spectroscopy in isostructural FeSi, and lattice dynamical calculations predicting increasingly strong electron-phonon coupling properties with Fe doping for 0 ≤ x ≤ 0.2, thus investigating the so far less considered interaction between magnetic and electronic with lattice degrees of freedom. During the project we performed phonon spectroscopy via inelastic neutron and x-ray scattering as well as Raman measurements in the doping range up to x = 0.22. Indeed, we found an anomalous evolution of a phonon with doping in qualitative agreement with our first-principles calculations. However, the effect was much weaker due to an unrealistic description of the electronic band structure in the calculations. Model calculations adjusted for the experimentally observed ordered magnetic moment are quantitatively in much better agreement still showing the anomalous effect. Yet, a direct connection of magnetic and lattice degrees of freedom could not be shown. For instance, magnetic field dependent measurements did not provide evidence for such a scenario. Rather, the increasing phonon renormalization with doping is electron-phonon coupling in nature and due to the evolution of the electronic band structure providing an enhanced nesting feature at the phonon wave vector for doped samples. Whether the increased electron-phonon coupling has a (negative) impact on the magnetic ordering temperature of Mn1-xFexSi is debated and further model calculations are needed.
Publications
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Combined inelastic neutron scattering and ab initio lattice dynamics study of FeSi. Physical Review B, 105(13).
Khan, Nazir; Krannich, Sven; Boll, Dominic; Heid, Rolf; Lamago, Daniel; Ivanov, A.; Voneshen, David & Weber, Frank
