Solitary magnons in the $S={5\over2}$ antiferromagnet CaFe$_{2}$O$_{4}$

TL;DR

This study investigates the magnetic excitations in CaFe$_{2}$O$_{4}$, revealing localized solitary magnons and phase boundaries due to competing magnetic orders, using neutron scattering techniques.

Contribution

It provides experimental evidence of quantized, localized spin wave excitations in a classical anisotropic antiferromagnet with competing magnetic phases.

Findings

Observation of magnetic antiphase boundaries freezing at 200 K.

Detection of nine localized spin wave excitations.

Identification of solitary magnons in an $S=5/2$ system.

Abstract

CaFe$_{2}$O$_{4}$ is a $S={5\over 2}$ anisotropic antiferromagnet based upon zig-zag chains having two competing magnetic structures, denoted as the A ($\uparrow \uparrow \downarrow \downarrow$) and B ($\uparrow \downarrow \uparrow \downarrow$) phases, which differ by the $c$-axis stacking of ferromagnetic stripes. We apply neutron scattering to demonstrate that the competing A and B phase order parameters results in magnetic antiphase boundaries along $c$ which freeze on the timescale of $\sim$ 1 ns at the onset of magnetic order at 200 K. Using high resolution neutron spectroscopy, we find quantized spin wave levels and measure 9 such excitations localized in regions $\sim$ 1-2 $c$-axis lattice constants in size. We discuss these in the context of solitary magnons predicted to exist in anisotropic systems. The magnetic anisotropy affords both competing A+B orders as well as localization of spin excitations in a classical magnet.