Spin-waves in antiferromagnetic single crystal LiFePO$_4$

TL;DR

This study investigates the spin-wave behavior in single crystal LiFePO₄ using neutron scattering, revealing anisotropic interactions and a slightly rotated magnetic ground state, with theoretical modeling supporting the experimental findings.

Contribution

First detailed measurement of spin-wave dispersions in single crystal LiFePO₄, including theoretical modeling of anisotropic magnetic interactions and ground state orientation.

Findings

Anisotropic spin-wave dispersions along a* and b* directions.

Theoretical modeling with linear spin-wave theory including J₁, J₂, J⊥, and D.

Ground state moments are slightly rotated from the (010) axis.

Abstract

Spin-wave dispersions in the antiferromagnetic state of single crystal LiFePO$_4$ were determined by inelastic neutron scattering measurements. The dispersion curves measured from the (010) reflection along both {\it a}$^\ast$ and {\it b}$^\ast$ reciprocal-space directions reflect the anisotropic coupling of the layered Fe$^{2+}$ (S = 2) spin-system. The spin-wave dispersion curves were theoretically modeled using linear spin-wave theory by including in the spin-Hamiltonian in-plane nearest- and next-nearest-neighbor interactions ({\it J}$_1$ and {\it J}$_2$), inter-plane nearest-neighbor interactions ({\it J}$_\bot$) and a single-ion anisotropy ({\it D}). A weak (010) magnetic peak was observed in elastic neutron scattering studies of the same crystal indicating that the ground state of the staggered iron moments is not along (010) direction, as previously reported from polycrystalline samples studies, but slightly rotated away from this axis.