The degree of ordering in solids has a very important impact on their physical properties and the ensuing applications. For example, magnetic atoms frequently form so-called antiferromagnetic order, where the magnetic moments (spins) of the nearest neighbors in the crystal lattice are oriented in mutually opposite (antiparallel) directions, since this allows lowering the overall energy.
A preferred antiparallel orientation of spins in a triangular lattice leads to the so-called frustration of the system. Indeed, the third atom in the triangle “does not know” how to direct its spin (as shown in Fig. 1a); moreover, the role of the “third” will be played alternately by any of the concerned atoms. This interaction leads to a dynamical disorder of the spins in the system, and it has been known for quite some time that the so-called spin liquid can be formed under suitable circumstances. This disordered state behaves analogically to a classical liquid in many aspects, but upon a decrease in temperature, the system does not freeze, but it stays in a dynamic liquid state down to the absolute zero temperature.
Researchers from the Department of Dielectrics at FZU together with their colleagues from the Faculty of Mathematics and Physics of Charles University have succeeded for the first time in observing an electrical analogue of the spin liquid in EuAl12O19.
This crystal exhibits a complex behavior: one can observe both magnetic responses due to interactions among the spins carried by europium atoms and electrical properties owing to a specific ordering of aluminum and oxygen ions. The crystal lattice contains bipyramides AlO5 sitting in a triangular structure (Fig. 1b). Oxygen anions form a cage, in which the aluminum cation may move to some extent. The electrical response of the material is then related to the displacement of Al cations with respect to the center of the negatively charged cage, forming an electric dipole.
The dynamics of these dipoles are very important for the response of the crystal. At room temperature, Al ions oscillate at terahertz frequencies and their motion is independent of the neighboring bipyramids; upon cooling, the motion progressively slows down towards GHz and MHz frequencies. A qualitative change occurs at the temperature of 49 K: the neighboring dipoles start to influence each other, and a strong correlation is established in the triangle, leading to the birth of the dipolar liquid.
The motion of dipoles continuously slows down upon cooling and freezes at the absolute zero temperature. The described behavior is reflected in the measured dielectric spectra – see Fig. 2. Formation of the frustrated antipolar phase is documented by a wide ensemble of our results. Besides the mentioned dielectric spectra, it is namely the synchrotron x-ray diffraction, measurements of the specific heat, acoustic and optic vibrations of the crystal lattice, the heat expansion, and the electric polarization of the material; it was also confirmed by first-principle calculations.
The discovery of dipolar liquid in EuAl12O19 will certainly motivate other scientific groups to search for such unusual behavior in other compounds. Indeed, under certain conditions, a quantum dipolar liquid can be formed, which might be useful for applications in quantum computers due to a long-range quantum entanglement of the dipoles.
G. Bastien et al. Advanced Materials 36, 2410282 (2024)