Correlation between magnetism and lattice dynamics for cubic FeGe under pressure

TL;DR

This study investigates how magnetism interacts with lattice dynamics in ferromagnetic cubic FeGe under pressure using a spin-scaling exchange-correlation approach to align the predicted critical pressure $p_c$ with experiment. By combining DFT with phonon calculations and electron-phonon coupling analyses, the work shows that reducing the magnetic moment via ssxc mitigates the R-point phonon anomaly and associated linewidths, with these features disappearing as pressure rises toward and beyond $p_c$. The momentum dependence of electron-phonon matrix elements, rather than Fermi-surface nesting, drives the observed linewidth behavior, linking the magnetism to lattice dynamics through e-ph coupling. These results reveal a distinct mechanism in FeGe for magnetism–lattice dynamics coupling, offering a pathway to tune magnetic and vibrational properties under pressure and contrasting with Mn$_{1-x}$Fe$_x$Si where nesting dominates.

Abstract

This first-principles study investigates the structural, electronic, lattice dynamical properties, and electron-phonon coupling in ferromagnetic cubic B20 FeGe under applied pressure. The implemented spin-scaling exchange-correlation (ssxc) approach allowed to modify the magnetic moment and ferromagnetic phase energetics using a single scaling parameter, thereby yielding an adjustment of the critical pressure ($p_c$) to its experimental value. The ssxc scheme resulted in a subtle energy shift of the electronic bands in the spin-up channel, and reduced the magnetic moment, bringing it closer to the experimentally reported value. Application of the ssxc approach to phonon dispersion and electron-phonon interaction resulted in a slight mitigation of the pronounced softening and large linewidths of the lowest-frequency acoustic branch close to the $R$-point, typically observed with standard DFT calculations. With increasing pressure, phonon anomaly and linewidths diminish significantly and practically disappear at $p_c$ and beyond. This trend parallels the pressure dependence of the magnetic moment. A comparative analysis of the electronic joint density of states with the phonon linewidths revealed that the momentum dependence of linewidths around the $R$-point closely follow the momentum dependence of the electron-phonon matrix elements. This indicates that the correlation between magnetic moment and linewidths under applied pressure originates from the electron-phonon matrix elements, presenting a distinct scenario compared to other B20 family members, where nesting plays a more dominating role.

Figures (9)

• Figure 1: Evolution of (a) the enthalpy difference (between NM and FM phases) as a function of pressure, where a negative value indicates a more stable magnetic phase, and (b) magnetic moment for the FM ssp and ssxc schemes. The vertical lines indicate the critical pressure ($p_c$): $28.7$ GPa for ssxc with $s=0.979$, and $35.3$ GPa for ssp.
• Figure 2: Determined critical pressure ($p_c$) and magnetic moment as a function of the ssxc parameter $s$. The dotted horizontal line indicates the experimental $p_c$ value barla.
• Figure 3: Electronic band structure and density of states of FM FeGe ($p=0$ GPa), for both spin-channels, comparing ssp and ssxc (with $s=0.979$) schemes. The related first Brillouin of the simple cubic structure zone is presented as an inset.
• Figure 4: Spin-polarized density of states (DOS) for FM FeGe, obtained with the ssxc scheme (using $s=0.979$) without applied pressure, showing the contribution by atom (in the unit cell).
• Figure 5: Spin-polarized DOS of two different applied pressure values, $0$ GPa and $35$ GPa, calculated under the ssxc scheme with $s=0.979$.