N. A. Pertsev

Theoretical current-voltage characteristics of ferroelectric tunnel junctions

We present the concept of ferroelectric tunnel junctions (FTJs). These junctions consist of two metal electrodes separated by a nanometer-thick ferroelectric barrier. The current-voltage characteristics of FTJs are analyzed under the assumption that the direct electron tunneling represents the dominant conduction mechanism. First, the influence of converse piezoelectric effect inherent in ferroelectric materials on the tunnel current is described. The calculations show that the lattice strains of piezoelectric origin modify the current-voltage relationship owing to strain-induced changes of the barrier thickness, electron effective mass, and position of the conduction-band edge. Remarkably, the conductance minimum becomes shifted from zero voltage due to the piezoelectric effect, and a strain-related resistive switching takes place after the polarization reversal in a ferroelectric barrier. Second, we analyze the influence of the internal electric field arising due to imperfect screening of polarization charges by electrons in metal electrodes. It is shown that, for asymmetric FTJs, this depolarizing-field effect also leads to a considerable change of the barrier resistance after the polarization reversal. However, the symmetry of the resulting current-voltage loop is different from that characteristic of the strain-related resistive switching. The crossover from one to another type of the hysteretic curve, which accompanies the increase of FTJ asymmetry, is described taking into account both the strain and depolarizing-field effects. It is noted that asymmetric FTJs with dissimilar top and bottom electrodes are preferable for the non-volatile memory applications because of a larger resistance on/off ratio.

Resistive switching in metal–ferroelectric–metal junctions

The aim of this work is to investigate the electron transport through metal–ferroelectric–metal (MFM) junctions with ultrathin barriers in order to determine its dependence on the polarization state of the barrier. To that end, heteroepitaxial Pt/Pb(Zr0.52Ti0.48)O3/SrRuO3 junctions have been fabricated on lattice-matched SrTiO3 substrates. The current–voltage (I–V) characteristics of the MFM junctions involving a few-nanometer-thick Pb(Zr0.52Ti0.48)O3 barriers have been recorded at temperatures between 4.2 K and 300 K. Typical I–V curves exhibit reproducible switching events at well-defined electric fields. The mechanism of charge transport through ultrathin barriers and the origin of the observed resistive switching effect are discussed.

Coercive field of ultrathin Pb(Zr0.52Ti0.48)O3 epitaxial films

The polarization reversal in single-crystalline ferroelectric films has been investigated experimentally and theoretically. The hysteresis loops were measured for Pb(Zr0.52Ti0.48)O3 films with thicknesses ranging from 8 to 250 nm. These films were grown epitaxially on SrRuO3 bottom electrodes deposited on SrTiO3 substrates. The measurements using Pt top electrodes showed that the coercive field Ec increases drastically as the film becomes thinner, reaching values as high as Ec≈1200 kV/cm. To understand this observation, we calculated the thermodynamic coercive field Eth of a ferroelectric film as a function of the misfit strain Sm in an epitaxial system and showed that Eth strongly depends on Sm. However, the coercive field of ultrathin films, when measured at high frequencies, exceeds the calculated thermodynamic limit. Since this is impossible for an intrinsic coercive field Ec, we conclude that measurements give an apparent Ec rather than the intrinsic one. An enormous increase of apparent coercive fie...

Anomalous polarization inversion in ferroelectrics via scanning force microscopy

Local poling of ferroelectrics by the sharp conducting tip of a scanning force microscope (SFM) is studied experimentally and theoretically. The formation of the inverse domain under the SFM tip, where the polarization is oriented in the direction opposite to that of the poling field, is reported for bulk ferroelectrics (single crystals of solid solutions PbZn1/3Nb2/3O3–PbTiO3). This finding confirms earlier results on ferroelectric thick films, thus proving the universality of the anomalous polarization inversion in ferroelectric media. It is shown that the inverse domain grows with the increase of the poling voltage and duration and remains stable for a long time after the removal of electric field. The growth process is described by a dynamic model assuming that the appearance of inverse domains is due to a local internal electric field directed against the poling one. This field is attributed to the space charge formed beneath the SFM tip due to injection of charge carriers and their subsequent drift and trapping. Poling voltage and poling time dependences of the domain size are correctly described by the model. Implications of the anomalous polarization inversion for the domain engineering and dense data storage in ferroelectrics are discussed.

Giant magnetoelectric effect via strain-induced spin reorientation transitions in ferromagnetic films

It is shown theoretically that a giant magnetoelectric susceptibility exceeding

Magnetic tunnel junction on a ferroelectric substrate

{10}^{\ensuremath{-}6}\text{ }\text{s}/\text{m}

Resistive switching via the converse magnetoelectric effect in ferromagnetic multilayers on ferroelectric substrates

may be achieved in the ferromagnetic/ferroelectric epitaxial systems via the magnetization rotation induced by an electric field applied to the substrate. The predicted magnetoelectric anomaly results from the strain-driven spin reorientation transitions in ferromagnetic films, which take place at experimentally accessible misfit strains in

Domain populations in epitaxial ferroelectric thin films: Theoretical calculations and comparison with experiment

{\text{CoFe}}_{2}{\text{O}}_{4}

Grain-Boundary Effect on the Curie-Weiss Law of Ferroelectric Ceramics and Polycrystalline Thin Films: Calculation by the Method of Effective Medium

and Ni films.

DOMAIN-WALL CONTRIBUTION TO THE PIEZOELECTRIC RESPONSE OF EPITAXIAL FERROELECTRIC THIN FILMS

The concept of a magnetic tunnel junction fabricated on a ferroelectric substrate is described theoretically. It is shown that the application of a moderate electric field to a substrate having strong piezoelectric response may induce an in-plane magnetization rotation in a ferromagnetic electrode made of a highly magnetostrictive cubic material with small magnetocrystalline anisotropy. Remarkably, an abrupt change of the junction’s electrical resistance can result from the substrate-induced magnetization reorientation in the free ferromagnetic layer. Hence the described hybrid multiferroic device may be employed as an electric-write nonvolatile magnetic memory cell with nondestructive readout.