Frustration model and spin excitations in the helimagnet FeP

TL;DR

This study investigates the complex magnetic frustration and spin excitations in FeP, revealing anisotropic magnon dispersions and quantifying exchange interactions using neutron scattering and spin-wave theory.

Contribution

It provides the first detailed spin-excitation spectra of FeP and constructs an effective Hamiltonian to explain its magnetic frustration mechanism.

Findings

Magnons are gapped with a ~5 meV energy.

Magnon modes show strongly anisotropic, quasi-one-dimensional dispersion.

Weak two-dimensional antiferromagnetic interactions drive frustration.

Abstract

The metallic compound FeP belongs to the class of materials that feature a complex noncollinear spin order driven by magnetic frustration. While its double-helix magnetic structure with a period $\lambda_{\text{s}} \approx 5c$, where $c$ is the lattice constant, was previously well determined, the relevant spin-spin interactions that lead to that ground state remain unknown. By performing extensive inelastic neutron scattering measurements, we obtained the spin-excitation spectra in a large part of the momentum-energy space. The spectra show that the magnons are gapped with a gap energy of $\sim$5 meV. Despite the 3D crystal structure, the magnon modes display strongly anisotropic dispersions, revealing a quasi-one-dimensional character of the magnetic interactions in FeP. The physics of the material, however, is not determined by the dominating exchange, which is ferromagnetic. Instead, the weaker two-dimensional antiferromagnetic interactions between the rigid ferromagnetic spin chains drive the magnetic frustration. Using linear spin-wave theory, we were able to construct an effective Heisenberg Hamiltonian with an anisotropy term capable of reproducing the observed spectra. This enabled us to quantify the exchange interactions in FeP and determine the mechanism of its magnetic frustration.