Phonon Mechanism of the Magnetostructural Phase Transition in MnAs

Density functional theory was used to study structural and dynamical changes related to the magnetostructural phase transition in MnAs. The soft mode inducing the transition from the high-symmetry hexagonal to the low-symmetry orthorhombic phase was revealed. A giant coupling between the soft mode and magnetic moments was found and its crucial role in the magnetostructural transition was established. The estimated phonon contribution to the total entropy change has the opposite sign to the magnetic entropy change.

Figure 1

Phonon dispersion relations for hexagonal phase calculated under several pressures. For values of optimized volume and soft-mode frequencies, see Table .

Figure 2

Phonon spectra in the hexagonal phase calculated with fully relaxed 16-atoms supercell for a few fixed values of magnetic moment on Mn atoms at 0 kb pressure.

Figure 3

The Mn magnetic moment and the total energy of the 16-atoms supercell as a function of the Mn atoms displacement in the soft mode at the $M$ point of the Brillouin zone of hexagonal phase.

Figure 4

Calculated phonon entropy for the hexagonal and orthorhombic structures calculated at $p=0$ (upper curve) and $p=20\mathrm{kb}$ (lower curve). In the inset, we plot the change of phonon entropy at $T_c$.