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arXiv · 1606.04243

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

C. Stock·E. E. Rodriguez·N. Lee·M. A. Green·F. Demmel·R. A. Ewings·P. Fouquet·M. Laver·Ch. Niedermayer·Y. Su·K. Nemkovski·J. A. Rodriguez-Rivera·S. -W. Cheong
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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.

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C. Stock, E. E. Rodriguez, N. Lee, M. A. Green, F. Demmel, R. A. Ewings, P. Fouquet, M. Laver, Ch. Niedermayer, Y. Su, K. Nemkovski, J. A. Rodriguez-Rivera, S. -W. Cheong. 2016-06-14. Solitary magnons in the $S={5\over2}$ antiferromagnet CaFe$_{2}$O$_{4}$. https://doi.org/10.1103/physrevlett.117.017201

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