Stripes, antiphase boundaries, and solitary magnons in Harmunite

CCMMP Seminars
Chris Stock
Martin Dove
April 19th, 2016 at 14:00
GO Jones Room 610

Harmunite is the mineral form of CaFe2O4 which is a S=5/2 anisotropic antiferromagnetic based upon zig-zag chains having two competing magnetic structures that differ by the c-axis stacking of ferromagnetic stripes.  We apply neutron scattering to demonstrate that the competing order parameters results in magnetic antiphase boundaries which freeze on the nanosecond timescale 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 of a few unit cells in size.  We discuss these in the context of solitary magnons predicted to exist in anisotropic systems and previously observed through optically driving large amplitude spin waves.  The magnetic anisotropy affords both competing magnetic orders as well as localization of spin excitations in a classical magnet.