PhD Markus Schiebl

Dielectric Spectroscopy at the Spin-Driven Ferroelectric Phase Transition in Chiral Multiferroic DyMnO3

Abstract

In magnetoelectric multiferroics the onset of ferroelectricity is coupled to the onset of a magnetic structure that breaks the inversion symmetry. The dynamics of such coupled ordering transition can be probed via the dielectric response of the system. Generally, the question which type of ferroelectric transition (order-disorder or displacive) applies for multiferroics is important and DyMnO3 belongs to the most prominent members of this interesting material class. In the ferroelectric state, the Mn3+- spins form a cycloidal magnetic structure whereas in the paraelectric state, a collinear sinusoidal modulated spin structure is proposed to exist. However, the exact role in the formation of multiferroicity of the so-called collinear sinusoidalmagnetic state, preceding the multiferroic state under cooling, is not yet fully clarified.

Detailed dielectric studies near the spin-driven ferroelectric phase transition reveals the indication of an order-disorder type ferroelectric transition with a double well potential. This potential reflects a dynamical switching between magnetic cycloids of the opposite chirality in the vicinity of the ferroelectric phase transition boundary by applying an electric field. Several parameters of the model correlate well with physical properties of DyMnO3. Thus, the characteristic energies of magnetic ordering and the value of the static electric polarization are in agreement with known values. Most importantly, the experimental data and the simple model suggest to explain the paraelectric sinusoidal phase in rare-earth manganate as a dynamical equilibrium of cycloids with opposite chiralities. In addition to the dielectric results, this hypothesis resolve several experimental constraints which contradicted the concept of static sinusoidally modulated magnetic phase.