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Phys Rev Lett. 2016 Jul 1;117(1):017201. doi: 10.1103/PhysRevLett.117.017201. Epub 2016 Jun 28.

Solitary Magnons in the S=5/2 Antiferromagnet CaFe_{2}O_{4}.

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School of Physics and Astronomy and Centre for Science at Extreme Conditions, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom.
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA.
Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, 136 Frelinghuysen Road, Piscataway, New Jersey 08854, USA.
School of Physical Sciences, University of Kent, Canterbury CT2 7NH, United Kingdom.
ISIS Facility, Rutherford Appleton Labs, Chilton, Didcot OX11 0QX, United Kingdom.
Institute Laue-Langevin, 6 rue Jules Horowitz, Boite Postale 156, 38042 Grenoble Cedex 9, France.
School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, United Kingdom.
Laboratory for Neutron Scattering, Paul Scherrer Institut, CH-5232 Villigen, Switzerland.
Jülich Centre for Neuton Science JCNS, Forschungszentrum Jülich GmbH, Outstation at MLZ, Lichtenbergstraße 1, D-85747 Garching, Germany.
NIST Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, USA.
Department of Materials Science, University of Maryland, College Park, Maryland 20742, USA.


CaFe_{2}O_{4} is a S=5/2 anisotropic antiferromagnet based upon zig-zag chains having two competing magnetic structures, denoted as the A (↑↑↓↓) and B (↑↓↑↓) 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 ∼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.

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