Spin-phonon coupling across a magnetic quantum critical point in Mn1-xFexSi

• Applicant: Dr. Frank Weber
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2019 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 419331252

This proposal addresses the presence and implications of spin-phonon coupling in Mn1-xFexSi where helimagnetic order is suppressed by increasing Fe concentration to a quantum critical point at x = 0.17. The parent compound at x = 0, MnSi, is a seminal compound for competing magnetic interactions resulting in a complex phase diagram with novel crystalline orders. On the other end at x = 1, FeSi is a narrow-gap and non-magnetic insulator at low temperatures but exhibits temperature-activated paramagnetism at elevated temperatures puzzling the scientific community for nearly 50 years. The noncentrosymmetric crystal structure present for the whole doping series results in a finite spin-orbit interaction providing a natural coupling between magnetic moments and the crystal lattice. Recent investigations of lattice dynamical properties – including our own work – revealed a close but unexpected link between magnetic and lattice degrees of freedom in FeSi: (1) Electronic states mediating conventional electron-phonon coupling are only activated in the presence of strong magnetic fluctuations. (2) Furthermore, phonons entailing strongly varying Fe-Fe distances are damped via dynamic coupling to the temperature-induced magnetic moments, highlighting FeSi as a material with direct spin-phonon coupling and multiple interaction paths. We propose a work program to investigate the energy, momentum and compositional dependence of spin-phonon coupling in Mn1-xFexSi via phonon spectroscopy. Based on the results for FeSi and preliminary data on MnSi, we will focus on the evolution of longitudinal phonons at the R point, i.e., the zone boundary along the [111] direction in momentum space, which exhibit strongly varying Mn/Fe – Mn/Fe distances. Ab-initio lattice-dynamical calculations predict spectacular phonon renormalization at the R point with increasing Fe concentration but closely linked to a magnetically ordered ground state. The suppression of magnetic order at x = 0.17 may have a critical impact on this behavior and could result in a strong lattice dynamical response to the alleged magnetic quantum critical point. Experimentally, we already have 10 well-characterized samples with doping levels 0.03 ≤ x ≤ 0.32. We will employ Raman scattering to obtain a full x dependence of zone center optical phonons and then investigate the most interesting samples via momentum and energy resolved high resolution inelastic x-ray scattering and inelastic neutron spectroscopy in order to measure the evolution of the above discussed phonons at the R point.Our investigation will scrutinize the so far overlooked interaction between spin and lattice degrees of freedom in a seminal material family with potential applications in spintronic devices. Here, understanding the ways in which such materials respond to extrinsic parameters such as doping is a key challenge for developing and functionalizing new materials.

• DFG Programme: Research Grants