Solitary magnons in the S=5 over 2 antiferromagnet CaFe2O4

CaFe2O4 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 ~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 ~ 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.