Speaker
Description
The ground-state of the $S =1/2$ Ising 2D Triangular Lattice AntiFerroMagnet (ITLAFM) is the go-to example of a frustrated magnet, and was calculated by Wannier over $70$ years ago [1]. In the absence of strong quantum fluctuations, this ground-state is called a classical spin liquid with a large number of energy-degenerate spin configurations that share the minimum energy. It is expected to have spin-dynamics governed by Arrhenius behavior, with spinons required to transform between each topologically protected configuration [2]. Despite interest in investigating this type of system experimentally, model magnetic materials are hard to come by, with magnetic moments typically deviating from the Ising limit or displaying significant quantum fluctuations.
The rare-earth hexaaluminates with the magnetoplumbite structure are a promising family for the investigation of ITLAFM physics, and I will present our investigations of the compound EuAl$_{12}$O$_{19}$. Despite the well separated triangular layers of magnetic ions, the Eu$^{2+}$ magnetic interactions drive the material to ferromagnetism with a $T_C = 1.3$ K [3].
Instead, we find the sought-after classic-liquid physics of the ITLAFM ground-state not within its magnetic lattice, but within a lattice of antiferroelectrically coupled dipoles. The material contains a triangular lattice of dynamically disordered, but antiferroelectrically correlated, charge displacive dipoles built from Al$^{3+}$ ions sitting off-centre within their bipyramid oxygen cages. Electric dipoles have an advantage over spins that they can be intrinsically Ising, and we label this model instead the Ising Triangular Lattice AntiFerroElectric (ILAFE). I will present our recently published structural, spectroscopic, and thermodynamic measurements on EuAl$_{12}$O$_{19}$, comparing the observed properties to those expected for the ITLAFM [4].
References
[1] G. H. Wannier, “Antiferromagnetism. The Triangular Ising Net,” Physical Review, vol. 79, no. 2. American Physical Society (APS), pp. 357–364, Jul. 15, 1950. https://doi.org/10.1103/physrev.79.357
[2] Z. Zhou et al., “Quantum dynamics of topological strings in a frustrated Ising antiferromagnet,” npj Quantum Materials, vol. 7, no. 1. Springer Science and Business Media LLC, Jun. 08, 2022. https://doi.org/10.1038/s41535-022-00465-3
[3] G. Bastien et al.,"Quasi-two-dimensional ferromagnetism in the triangular magnet EuAl12O19" Physical Review B, vol. 110, no. 9. American Physical Society (APS), Sep. 24, 2024. https://doi.org/10.1103/physrevb.110.094436
[4] G. Bastien et al., “A Frustrated Antipolar Phase Analogous to Classical Spin Liquids,” Advanced Materials, vol. 36, no. 50. Wiley, Oct. 23, 2024. https://doi.org/10.1002/adma.202410282
