First-Principles study of vibrational and non-collinear magnetic properties of the perovskite to post-perovskite pressure transition of NaMnF3

TL;DR

This first-principles study reveals a reconstructive phase transition in NaMnF3 at 8 GPa, changing its structure and magnetic order, with novel high-spin state retention and significant non-collinear magnetism in the post-perovskite phase.

Contribution

The paper uncovers the pressure-induced phase transition and magnetic reordering in NaMnF3, highlighting the persistence of high-spin states and large spin canting in the post-perovskite phase.

Findings

Phase transition at 8 GPa from Pnma to Cmcm structure.

Change from G-type to C-type antiferromagnetic ordering.

Large ferromagnetic spin canting observed in post-perovskite phase.

Abstract

We performed a first-principles study of the structural, vibrational, electronic and magnetic properties of NaMnF3 under applied isotropic pressure. We found that NaMnF3 undergoes a reconstructive phase transition at 8 GPa from the Pnma distorted perovskite structure toward the Cmcm post-perovskite structure. This is confirmed by a sudden change of the Mn-F-Mn bondings where the crystal goes from corner shared octahedra in the Pnma phase to edge shared octahedra in the Cmcm phase. The magnetic ordering also changes from a G-type antiferromagnetic ordering in the Pnma phase to a C-type antiferromagnetic ordering in the Cmcm phase. Interestingly, we found that the high-spin d-orbital filling is kept at the phase transition which has never been observed in the known magnetic post-perovskite structures. We also found a highly non-collinear magnetic ordering in the Cmcm post-perovskite phase that drives a large ferromagnetic canting of the spins. We discuss the validity of these results with respect to the U and J parameter of the GGA+U exchange correlation functional used in our study and conclude that large spin canting is a promising property of the post-perovskite fluoride compounds.