Martin Lilienblum

Scaling Behaviour and Beyond Equilibrium in the Hexagonal Manganites

We show that the improper ferroelectric phase transition in the multiferroic hexagonal manganites displays the same symmetry-breaking characteristics as those proposed in early-universe theories. We present an analysis of the Kibble-Zurek theory of topological defect formation applied to the hexagonal manganites, discuss the conditions determining the range of cooling rates in which KibbleZurek behavior is expected, and show that recent literature data are consistent with our predictions. We explore experimentally for the first time to our knowledge the cross-over out of the Kibble-Zurek regime and find a surprising “anti-Kibble-Zurek” behavior.

Observation of persistent centrosymmetricity in the hexagonal manganite family

The controversy regarding the ferroelectric behavior of hexagonal InMnO3 is resolved by using a combination of x-ray diffraction (XRD), piezoresponse force microscopy (PFM), second harmonic generation (SHG), and density functional theory (DFT). While XRD data show a symmetry-lowering unit-cell tripling, which is also found in the multiferroic hexagonal manganites of P6(3)cm symmetry, PFM and SHG do not detect ferroelectricity at ambient or low temperature, in striking contrast to the behavior in the multiferroic counterparts. We propose instead a centrosymmetric P (3) over barc phase as the ground-state structure. Our DFT calculations reveal that the relative energy of the ferroelectric and nonferroelectric structures is determined by a competition between electrostatics and oxygen-rare-earth covalency, with an absence of covalency favoring the ferroelectric phase.

Manipulation of ferroelectric vortex domains in hexagonal manganites

The modification of ferroelectric vortex domain patterns in hexagonal manganites (here exemplified by YMnO3 and HoMnO3) owing to chemical treatment, thermal annealing, and local electric-field poling is investigated by piezoresponse force microscopy. Chemical treatment transfers the domain pattern into a topographical pattern by domain selective etching. Thermal annealing alters the domain pattern without any sign of temperature memory effects. Local electric fields affect the domain structure with possible signs of electric memory effects. These observations are important for future investigations of the microscopic mechanisms and macroscopic parameters defining the formation of ferroelectric domains in this unusual multiferroic.

Large-area regular nanodomain patterning in He-irradiated lithium niobate crystals

Large-area ferroelectric nanodomain patterns, which are desirable for nonlinear optical applications, were generated in previously He-implanted lithium niobate crystals by applying voltage pulses to the tip of a scanning force microscope. The individual nanodomains were found to be of uniform size, which depended only on the inter-domain spacing and the pulse amplitude. We explain this behavior by the electrostatic repulsion of poling-induced buried charges between adjacent domains. The domain patterns were imaged by piezoresponse force microscopy and investigated by domain-selective etching in conjunction with focused ion beam etching followed by scanning electron microscopy imaging. In order to optimize the He-irradiation parameters for easy and reliable nanodomain patterning a series of samples subjected to various irradiation fluences and energies was prepared. The different samples were characterized by investigating nanodomains generated with a wide range of pulse parameters (amplitude and duration). In addition, these experiments clarified the physical mechanism behind the facile poling measured in He-irradiated lithium niobate crystals: the damage caused by the energy loss that takes place via electronic excitations appears to act to stabilize the domains, whereas the nuclear-collision damage degrades the crystal quality, and thus impedes reliable nanodomain generation.

Lateral signals in piezoresponse force microscopy at domain boundaries of ferroelectric crystals

In piezoresponse force microscopy a lateral signal at the domain boundaries is occasionally observed. In recent years, a couple of experiments have been reported and varying explanations for the origin of this lateral signal have been proposed. Additionally, elaborated theoretical modeling for this particular issue has been carried out. Here we present experimental data obtained on different crystallographic cuts of

Conductivity Contrast and Tunneling Charge Transport in the Vortexlike Ferroelectric Domain Patterns of Multiferroic Hexagonal YMnO3

\rm LiNbO_3

Determination of the effective coercive field of ferroelectrics by piezoresponse force microscopy

,

Anomalous domain inversion in LiNbO3 single crystals investigated by scanning probe microscopy

\rm BaTiO_3

Low-voltage nanodomain writing in He-implanted lithium niobate crystals

, and

Functional ferroic heterostructures with tunable integral symmetry

\rm KTiOPO_4