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Description
The interplay of ferromagnetic exchange, Dzyaloshinsky-Moriya interaction and crystal potential results in the complex phase diagram of the chiral magnet MnSi ($T_C$ ≈ 30 K). In Mn$_{1-x}$Fe$_x$Si, long-range magnetic order is suppressed and helimagnetic correlations vanish at $x$ ≈ 0.2 along with a redistribution of d states at the Fermi energy.
Here, we present a study of the lattice dynamical properties of Mn$_{1-x}$Fe$_x$Si with 0 ≤ $x$ ≤ 0.22. Employing time-of-flight neutron spectroscopy and high energy resolution inelastic x-ray scattering, we investigate the doping dependence of phonon energies, $E_{phon}$, and line widths, $\Gamma_{phon}$ ($\Gamma_{phon} \propto$ 1/life time). In contrast to the general trend of slightly increasing energies with doping because of the reduced lattice constant, we find a significant softening and broadening of a phonon mode propagating along the [111] direction, which is also the direction of the magnetic ordering wave vector in MnSi. Ab-initio lattice dynamical calculations based on density-function theory predict an increasingly strong electron-phonon coupling for this particular mode linked to changes of the Fermi surface geometry upon doping.
