Unusual phase transition into frustrated antipolar phase in ferromagnetic EuAl12O19


Abstract: Hexagonal EuAl12O19 is a quasi-two-dimensional ferromagnet below 1.3 K. Pyroelectric current measurements revealed a weak ferroelectric polarization below TC = 49 K. The existence of a ferroelectric phase transition is supported by an anomaly in specific heat and thermal expansion. However, the temperature dependence of permittivity does not show a peak at TC, but only a change of slope. This could argue in favor of an improper or pseudo-proper ferroelectric phase transition. However, single crystal synchrotron diffraction studies revealed no structural change at TC and second harmonic generation measurements also showed no signal down to 5 K. This indicates that EuAl12O19 remains macroscopically centrosymmetric (space group P63/mmc) down to low temperatures. We propose to explain the observed behavior by frustrated antiferroelectricity or frustrated antipolar correlations below TC. An external electric field induces a weak polarization visible in the pyrocurrent, but without the field the sample remains centrosymmetric. Dynamical frustration of antipolar order makes it impossible to see the long-range structural change in XRD and explains the observed strong relaxor ferroelectric-like dielectric dispersion below TC. Similar frustrated antiferroelectricity was theoretically predicted in the isostructural BaFe12O19 below 4 K [Wang and Xiang, PRX 4, 011035 (2014)], but it was not experimentally observed due to the occurrence of quantum paraelectricity. However, the theory from Wang and Xiang predicts that Al cations are much more polar than Fe and this is the reason, why the antipolar correlations begin to build in EuAl12O19 already at 49 K.

Finally, we show that EuAl12O19 exhibits freezing of dielectric relaxation at zero temperature, which would suggest that this system is the electrical analogue of the classical spin liquid known in frustrated magnetic systems.

The seminar will be chaired by Hynek Němec, Department of Dielectrics.