Magnetic interactions in PdCrO$_2$ and their effects on its magnetic structure

TL;DR

This study uses neutron scattering to analyze the magnetic interactions in PdCrO$_2$, revealing that the crystal symmetry influences the magnetic structure and identifying exchange interactions and gaps consistent with a coplanar magnetic order.

Contribution

It provides the first comprehensive neutron scattering analysis of PdCrO$_2$, clarifying the magnetic structure and the role of symmetric anisotropic exchange interactions.

Findings

Crystalline symmetry remains $Rar{3}m$ below $T_N$

Two low-energy gaps explained by dipolar-type exchange

Magnon dispersion modeled with exchange up to sixth neighbors

Abstract

We report a neutron scattering study of the metallic triangular lattice antiferromagnet PdCrO$_2$. Powder neutron diffraction measurements confirm that the crystalline space group symmetry remains $R\bar{3}m$ below $T_N$. This implies that magnetic interactions consistent with the crystal symmetry do not stabilise the non-coplanar magnetic structure which was one of two structures previously proposed on the basis of single crystal neutron diffraction measurements. Inelastic neutron scattering measurements find two gaps at low energies which can be explained as arising from a dipolar-type exchange interaction. This symmetric anisotropic interaction also stabilises a magnetic structure very similar to the coplanar magnetic structure which was also suggested by the single crystal diffraction study. The higher energy magnon dispersion can be modelled by linear spin wave theory with exchange interactions up to sixth nearest-neighbors, but discrepancies remain which hint at additional effects unexplained by the linear theory.