Solitary Magnons in the S=5/2 Antiferromagnet CaFe2O4

CaFe2O4 is a S = 5/2 anisotropic antiferromagnet based upon zig-zag chains having two competing magnetic structures, denoted as the A (up arrow up arrow down arrow down arrow) and B (up arrow down arrow up arrow down arrow) 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 result in magnetic antiphase boundaries along c which freeze on the time scale of similar to 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 similar to 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.