Abstract
Low-energy spin excitations in any long-range ordered magnetic system in the absence of magnetocrystalline anisotropy are gapless Goldstone modes emanating from the ordering wave vectors. In helimagnets, these modes hybridize into the so-called helimagnon excitations. Here we employ neutron spectroscopy supported by theoretical calculations to investigate the magnetic excitation spectrum of the isotropic Heisenberg helimagnet with a cubic spinel structure, in which spin- magnetic ions are arranged in a geometrically frustrated pyrochlore sublattice. Apart from the conventional Goldstone mode emanating from the ordering vector, low-energy magnetic excitations in the single-domain proper-screw spiral phase show soft helimagnon modes with a small energy gap of , emerging from two orthogonal wave vectors and where no magnetic Bragg peaks are present. We term them pseudo-Goldstone magnons, as they appear gapless within linear spin-wave theory and only acquire a finite gap due to higher-order quantum-fluctuation corrections. Our results are likely universal for a broad class of symmetric helimagnets, opening up a new way of studying weak magnon-magnon interactions with accessible spectroscopic methods.
- Received 12 May 2017
DOI:https://doi.org/10.1103/PhysRevX.7.041049
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
In a conventional magnet, the magnetic moments of individual atoms are aligned along a single direction, like compass needles that all point to the north. However, in materials known as helimagnets, this direction rotates gradually, resulting in a helix twisted along a spontaneously chosen direction. When a neutron hits this arrangement, it triggers a wave along the helix called a magnon, similar to wind waves in a cornfield. According to the Goldstone theorem, exciting a magnon along the direction of the helix costs a vanishingly small amount of energy. Magnons can therefore be spontaneously excited, for instance, by temperature fluctuations. Much higher energies are thought to be required for creating magnons in the orthogonal direction, so these should not contribute to the thermodynamic properties of the material. However, we have experimentally demonstrated the existence of a special kind of low-energy magnon, termed a pseudo-Goldstone mode, that can propagate perpendicular to the helix.
The existence of such modes is predicted by the linear spin-wave theory, which neglects interactions between the magnons. Yet, as a result of this approximation, this theory fails to estimate the minimal energy necessary for the pseudo-Goldstone mode to be excited. In our neutron spectroscopy experiment, these modes acquire a finite energy gap that makes them distinctly different from the true Goldstone magnons, which are gapless.
We expect that our result will be applicable to a very broad class of magnetic materials and might help us to understand or predict their thermodynamic behavior at low temperatures. This could lead to practical applications in the emergent field of spintronics—electronics that makes use of the spin of electrons in addition to their charge.
