Abstract
We show that frustrated quasidoublets without time-reversal symmetry can host highly unconventional magnetic structures with continuously distributed order parameters even in a single-phase crystal. Our study comprises a comprehensive thermodynamic and neutron diffraction investigation on the single crystal of , which entails non-Kramers ions arranged on a geometrically perfect triangular lattice. The crystal electric field randomness caused by the site-mixing disorder of the nonmagnetic and ions merges two lowest-lying crystal electric field singlets of into a ground-state quasidoublet. Well below , a small fraction of the antiferromagnetically coupled Ising quasidoublets with small inner gaps condense into two-dimensional up-up-down magnetic structures with continuously distributed order parameters, and give rise to the columnar magnetic neutron reflections below , with highly anisotropic correlation lengths, in the triangular plane and between the planes. The remaining fraction of the ions remain nonmagnetic at 0 T and become uniformly polarized by the applied longitudinal field at low temperatures. We argue that the similar model can be generally applied to other compounds of non-Kramers rare-earth ions with correlated ground-state quasidoublets.
7 More- Received 11 July 2019
- Revised 31 October 2019
DOI:https://doi.org/10.1103/PhysRevX.10.011007
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
Geometrical frustration describes a collection of spins on some symmetric lattice where not all of the interactions can simultaneously minimize energy. This kind of system cannot achieve conventional low-temperature ordering, where neighboring spins are oriented parallel to one another (ferromagnets) or in opposing directions (antiferromagnets). Specifically, an ideal “triangular-lattice Ising antiferromagnet” (where spins can be oriented only along a special direction, up or down) remains completely disordered even down to absolute zero, retaining a large residual entropy that violates the third law of thermodynamics. However, to date, the relevant real materials are still rare. Recently, researchers synthesized the triangular-lattice Ising antiferromagnet . Here, we report on a thorough single-crystal investigation of the low-temperature magnetism of .
Its structural sibling was proposed as a quantum spin-liquid candidate. Here, the ion has an odd number of the electrons, and thus the crystal electric field (CEF) always produces the lowest-lying doublet (the effective spin- moment), according to Kramers theorem. However, in the situation is different: The ion has an even number of the electrons, and CEF singlets are expected. Our present work indicates that the inner energy gap between the two lowest-lying singlets is small, and the site-mixing disorder of the nonmagnetic and ions with different valences distributes that inner gap throughout the crystal. These two lowest-lying CEF singlets are characterized as a “ground-state quasidoublet.”
We observe a very unconventional and complex magnetic order in at low temperatures. A similar geometrically frustrated model can be generally applied to other non-Kramers rare-earth magnets with similar disorder-induced ground-state quasidoublets.