I I Vilfan

Bio: I I Vilfan is an academic researcher from University of Ljubljana. The author has contributed to research in topics: Random field & Domain wall (magnetism). The author has an hindex of 2, co-authored 2 publications receiving 196 citations.

Interface pinning and dynamics in random systems.

Abstract: A detailed low-temperature treatment of the domain wall or interface pinning by imperfections in disordered systems with discrete symmetry of the order parameter is presented. Crossover behavior as well as analogies between pinning mechanisms in different systems is analyzed. Pinning may arise from random bonds, when the disordering agents do not break the local symmetry of the order parameter, or from random fields, when the disordering agents do break this symmetry. The interface roughness and response to an external driving force are discussed. The model is explained for dilute magnetic systems in a uniform field where the magnetic domain walls are pinned by random fields and/or random bonds. The results are, however, more general and apply also to interfaces in other systems, e.g., in fluid-fluid interfaces, (anti)ferroelectrics, solitons in incommensurate systems, etc. The interface roughness and pinning pressure (force per unit area) are estimated for weak and strong pinning and their scaling relations to length scale, temperature, frequency, and disorder strength (concentration) are given. The interface contribution to the static and dynamic susceptibility at low temperatures is evaluated. Because of pinning, the low-temperature dynamical susceptibility of disordered ferromagnets in or out of equilibrium carries a [ln(1/\ensuremath{\omega})${]}^{2/\mathrm{\ensuremath{\theta}}}$ frequency dependence in addition to the Debye relaxation behavior. In particular, \ensuremath{\theta}=(d+1)/3 for random-field systems, and \ensuremath{\theta}(d=2)=1/3 and \ensuremath{\theta}(d=3)\ensuremath{\approxeq}0.83 for random-bond systems.

Anomalous relaxation in the random-field ising model and related systems

Abstract: Pour des systemes a anisotropie du champ cristallin faible, la taille du domaine magnetique est gouvernee par un mecanisme de Bruinsma-Aeppi et depend du champ applique. Une anisotropie forte entraine l'ancrage des parois de domaine par les liaisons aleatoires. Les champs aleatoires expliqueraient la relaxation de l'aimantation apres la disparition du champ applique

Stress and frequency dependence of the direct piezoelectric effect in ferroelectric ceramics

Abstract: It is shown that at weak alternating stress the relationship between the piezoelectrically induced charge and applied stress in ferroelectric ceramics has the same form as the Rayleigh law for magnetization versus magnetic field in ferromagnetic materials. Applicability of the Rayleigh law to the piezoelectric effect is demonstrated in detail for lead zirconate titanate (PZT) ceramics. Experimental results indicate that the dominant mechanism responsible for piezoelectric hysteresis and the dependence of the piezoelectric coefficient on the applied ac stress is the pinning of non-180 degrees domain walls. The dependence of the piezoelectric coefficient on the frequency of the driving stress is examined and is shown to be due to the frequency dispersion of both reversible and irreversible components of domain-wall displacement. Analysis of the stress dependence of the piezoelectric phase angle reveals piezoelectric hysteresis contributions that are not necessarily due to Rayleigh-type displacement of domain walls. Piezoelectric properties of a modified lead titanate composition that exhibits non-Rayleigh type behavior are examined and compared with the properties of PZT ceramics. (C) 1997 American Institute of Physics.

Hysteresis in Piezoelectric and Ferroelectric Materials

Abstract: Keywords: hysteresis ; ferroelectric ; piezoelectric Reference LC-CHAPTER-2006-001 Record created on 2006-08-03, modified on 2017-05-10

Direct imaging of the spatial and energy distribution of nucleation centres in ferroelectric materials.

Abstract: Macroscopic ferroelectric polarization switching, similar to other first-order phase transitions, is controlled by nucleation centres. Despite 50 years of extensive theoretical and experimental effort, the microstructural origins of the Landauer paradox, that is, the experimentally observed low values of coercive fields in ferroelectrics corresponding to implausibly large nucleation activation energies, are still a mystery. Here, we develop an approach to visualize the nucleation centres controlling polarization switching processes with nanometre resolution, determine their spatial and energy distribution and correlate them to local microstructure. The random-bond and random-field components of the disorder potential are extracted from positive and negative nucleation biases. Observation of enhanced nucleation activity at the 90 composite function domain wall boundaries and intersections combined with phase-field modelling identifies them as a class of nucleation centres that control switching in structural-defect-free materials.

Evidence of domain wall contribution to the dielectric permittivity in PZT thin films at sub-switching fields

Abstract: Through the use of relations analogous to that of the Rayleigh law, it is demonstrated that the ac electric field dependence of the permittivity of ferroelectric thin films can be described. It is further shown that both reversible and irreversible components of the permittivity decrease linearly with the logarithm of the frequency of the ac field. The results demonstrate that the models describing the interaction of domain walls and randomly distributed pinning centers in magnetic materials can be extended to the displacement of domain walls in ferroelectric thin films.

The relaxor enigma — charge disorder and random fields in ferroelectrics

Abstract: Substitutional charge disorder giving rise to quenched electric random-fields (RFs) is probably at the origin of the peculiar behavior of relaxor ferroelectrics, which are primarily characterized by their strong frequency dispersion of the dielectric response and by an apparent lack of macroscopic symmetry breaking at the phase transition. Spatial fluctuations of the RFs correlate the dipolar fluctuations and give rise to polar nanoregions in the paraelectric regime as has been evidenced by piezoresponse force microscopy (PFM) at the nanoscale. The dimension of the order parameter decides upon whether the ferroelectric phase transition is destroyed (e.g. in cubic PbMg1/3Nb2/3O3, PMN) or modified towards RF Ising model behavior (e.g. in tetragonal Sr1−x BaxNb2O6, SBN, x ≈ 0.4). Frustrated interaction between the polar nanoregions in cubic relaxors gives rise to cluster glass states as evidenced by strong pressure dependence, typical dipolar slowing-down and theoretically treated within a spherical random bond-RF model. On the other hand, freezing into a domain state takes place in uniaxial relaxors. While at T c non-classical critical behavior with critical exponents ρ ≈ 1.8, β ≈ 0.1 and α ≈ 0 is encountered in accordance with the RF Ising model, below T c ≈ 350 K RF pinning of the walls of frozen-in nanodomains gives rise to non-Debye dielectric response. It is relaxation- and creep-like at radio and very low frequencies, respectively.