arXiv subjects

Marin Alexe

Publications and source records attributed to Marin Alexe.

15 recordsLinked to original sources

arXiv2025

Rapid heat assisted polarization reversal in ferroelectric thin films

We demonstrate that switching of ferroelectric thin-films sandwiched between metallic electrodes can be controlled by laser-assisted heating, reminiscent of heat-assisted magnetic recording. We employ electrical switching cycles that quantify the electrically switchable remanent polarization $P_\mathrm{r}$ and show that 300\,ns voltage pulses alone change the polarization by less than $\Delta P P_\mathrm{r}$ of the average polarization. The transient average temperature modeled by the heat equation can rationalize the polarization change observed for different relative timing $\Delta t$ of the laser pulse, if it arrives before the electrical pulse.

cond-mat.mtrl-sci↗

arXiv2025

Template masks for 4D-STEM

We present a new analysis method for atomic resolution four-dimensional scanning transmission electron microscopy (4D-STEM, in which a diffraction pattern is collected at each point of a raster scan of a focused electron beam across the specimen). In 4D-STEM, each measured intensity has a dual character, forming a pixel in a diffraction pattern and, equally, forming a pixel in a STEM image. Applying a mask to the data to obtain a "virtual" bright field or dark field image is widely used and understood. However, there is a complementary procedure, in which an image (template) is applied to the data to obtain a mask. This mask shows the correlation between the data and the template and, when applied to atomic resolution 4D-STEM data produces an image optimised for the template. This allows, for example, imaging of specific atom columns and is a significant improvement over user-defined masks such as virtual annular bright field imaging. We demonstrate the capability of the approach, separately imaging Li and O atom columns in LiFePO4 and O, Pb and Ti across a domain wall in PbTiO3.These template masks provide a computationally straightforward and general method to probe 4D-STEM data. They are particularly effective for specimens of moderate thickness where multiple scattering produces strong and specific correlations in diffraction patterns.

physics.ins-det↗

arXiv2025

Pyramidal charged domain walls in ferroelectric BiFeO$_3$

Domain structures play a crucial role in the electric, mechanical and other properties of ferroelectric materials. In this study, we uncover the physical origins of the enigmatic zigzag domain structure in the prototypical multiferroic material BiFeO$_3$. Using phase-field simulations within the Landau-Ginzburg-Devonshire framework, we demonstrate that spatially-homogeneous defect charges result in domain structures that closely resemble those observed experimentally. The acquired understanding of the underlying physics of pyramidal-domain formation may enable the engineering of new materials with self-assembled domain structures exhibiting defined domain periodicity at the nanometre scale, opening avenues for advanced applications.

cond-mat.mtrl-sci↗

arXiv2023

Spatially resolving superconductivity in type-II superconductors

Superconductivity is identified by the emergence of a macroscopic zero-resistance state, typically inferred from a vanishing four-probe voltage at finite current. That inference assumes spatially uniform conduction-e.g., at least one continuous superconducting path between the current leads and voltage electrodes that sample a finite potential gradient-and can fail if the drive current bypasses the electrodes or if narrow filaments short the current contacts. Here we introduce a methodology to test these assumptions in superconductors, by using spatially resolved measurements of local variations in dc using cryogenic conductive atomic-force microscopy (cAFM). Using Fe(Se,Te) as a model system, we find that despite bulk measurements consistent with a homogeneous superconducting state, the material exhibits a heterogeneous conducting landscape: micrometre-scale superconducting regions coexist with relatively insulating areas. We further show that cAFM resolves conductance fluctuations at 20 K (> TC) that vary between repeated scans, consistent with expectations for short-lived, pre-formed Cooper pairs in the BCS-BEC crossover regime. These results establish cAFM as a practical tool to validate assumptions underlying four-probe transport and underscore the need for direct spatial probes in materials whose macroscopic response can conceal nanoscale inhomogeneity. Accurate identification of macroscopic properties is critical for materials classes like superconductors that are defined by their macroscopic properties.

cond-mat.supr-con↗

arXiv2017

Direct Probing of Polarization Charges at the Nanoscale

Ferroelectric materials possess spontaneous polarization that can be used for multiple applications. Owing to a long term development for reducing the sizes of devices, the preparation of ferroelectric materials and devices are entering nanometer-scale regime. Accordingly, to evaluate the ferroelectricity, there is a need to investigate the polarization charge at the nanoscale. Nonetheless, it has been generally accepted that the detection of polarization charges using a conventional conductive atomic force microscopy (CAFM) without a top electrode is not feasible because the nanometer-scale radius of an atomic force microscopy (AFM) tip yields a very low signal-to-noise ratio. However, the detection is unrelated to the radius of an AFM tip, and in fact, a matter of the switched area. In this work, we demonstrate the direct probing of the polarization charge at the nanoscale using the Positive-Up-Negative-Down method based on the conventional CAFM approach without additional corrections or circuits to reduce the parasitic capacitance. We successfully probed the polarization charge densities of 73.7 and 119.0 uC/cm2 in ferroelectric nanocapacitors and thin films, respectively. The obtained results show the feasibility of the evaluation of the polarization charge at the nanoscale and provide a new guideline for evaluating the ferroelectricity at the nanoscale.

cond-mat.mtrl-sci↗

arXiv2015

Artefacts in geometric phase analysis of compound materials

The geometric phase analysis (GPA) algorithm is known as a robust and straightforward technique that can be used to measure lattice strains in high resolution transmission electron microscope (TEM) images. It is also attractive for analysis of aberration-corrected scanning TEM (ac-STEM) images that resolve every atom column, since it uses Fourier transforms and does not require real-space peak detection and assignment to appropriate sublattices. Here it is demonstrated that in ac-STEM images of compound materials (i.e. with more than one atom per unit cell) an additional phase is present in the Fourier transform. If the structure changes from one area to another in the image (e.g. across an interface), the change in this additional phase will appear as a strain in conventional GPA, even if there is no lattice strain. Strategies to avoid this pitfall are outlined.

cond-mat.mtrl-sci↗

arXiv2015

Dual gate control of bulk transport and magnetism in the spin-orbit insulator Sr2IrO4

The 5d iridates have been the subject of much recent attention due to the predictions of a large array of novel electronic phases driven by twisting strong spin-orbit coupling and Hubbard correlation. As a prototype, the single layered perovskite Sr2IrO4 was first revealed to host a Jeff=1/2 Mott insulating state. In this material, the approximate energy scale of a variety of interactions, involving spin-orbit coupling, magnetic exchange interaction, and the Mott gap, allows close coupling among the corresponding physical excitations, opening the possibility of cross control of the physical properties. Here, we experimentally demonstrate the effective gate control of both the transport and magnetism in a Sr2IrO4-based field effect transistor using an ionic liquid dielectric. This approach could go beyond the surface-limited field effect seen in conventional transistors, reflecting the unique aspect of the Jeff=1/2 state. The simultaneous modulation of conduction and magnetism confirms the proposed intimate coupling of charge, orbital, and spin degrees of freedom in this oxide. These phenomena are probably related to an enhanced deviation from the ideal Jeff=1/2 state due to the gate-promoted conduction. The present work would have important implications in modelling the unusual physics enabled by strong spin-orbit coupling, and provides a new route to explore those emergent quantum phases in iridates.

cond-mat.mtrl-sci↗

arXiv2014

Crossover of conduction mechanism in Sr2IrO4 epitaxial thin films

High quality epitaxial Sr2IrO4 thin films with various thicknesses (9-300 nm) have been grown on SrTiO3 (001) substrates, and their electric transport properties have been investigated. All samples showed the expected insulating behavior with a strong resistivity dependence on film thickness, that can be as large as three orders of magnitude at low temperature. A close examination of the transport data revealed interesting crossover behaviors for the conduction mechanism upon variation of thickness and temperature. While Mott variable range hopping (VRH) dominated the transport for films thinner than 85 nm, high temperature thermal activation behavior was observed for films with large thickness, which was followed by a crossover from Mott to Efros-Shklovskii (ES) VRH in the low temperature range. This low temperature crossover from Mott to ES VRH indicates the presence of a Coulomb gap (~3 meV). Our results demonstrate the competing and tunable conduction in Sr2IrO4 thin films, which in turn would be helpful for understanding the insulating nature related to strong spin-orbit-coupling of the 5d iridates.

cond-mat.mtrl-sci↗

arXiv2014

Multiferroic Iron Oxide Thin Films at Room-Temperature

In spite of being highly relevant for the development of a new generation of information storage devices, not many single-phase materials displaying magnetic and ferroelectric orders above room temperature are known. Moreover, these uncommon materials typically display insignificant values of the remanent moment in one of the ferroic orders or are complex multicomponent oxides which will be very challenging to integrate in devices. Here we report on the strategy to stabilize the metastable epsilon-Fe2O3 in thin film form, and we show that besides its already known ferrimagnetic nature, the films are also ferroelectric at 300 K with a remanent polarization of 1 microC/cm2. The film polarization shows long retention times and can be switched under small applied voltages. These characteristics make of epsilon-Fe2O3 the first single-ion transition-metal oxide which is ferro(ferri)magnetic and ferroelectric at room temperature. The simple composition of this new multiferroic oxide and the discovery of a robust path for its thin film growth may boost the exploitation of epsilon-Fe2O3 in novel devices.

cond-mat.mtrl-sci↗

arXiv2011

Cross talk by extensive domain wall motion in arrays of ferroelectric nanocapacitors

We report on extensive domain wall motion in ferroelectric nanocapacitor arrays investigated by piezoresponse force microscopy. Under a much longer or higher bias voltage pulse, compared to typical switching pulse conditions, domain walls start to propagate into the neighbouring capacitors initiating a significant cross-talk. The propagation paths and the propagated area into the neighbouring capacitors were always the same under repeated runs. The experimental and the simulated results show that the observed cross-talk is related to the capacitor parameters combined with local defects. The results can be helpful to test the reliability of nanoscale ferroelectric memory devices.

cond-mat.mtrl-sci↗

arXiv2010

Electroresistance effects in ferroelectric tunnel barriers

Electron transport through fully depleted ferroelectric tunnel barriers sandwiched between two metal electrodes and its dependence on ferroelectric polarization direction are investigated. The model assumes a polarization direction dependent ferroelectric barrier. The transport mechanisms, including direct tunneling, Fowler-Nordheim tunneling and thermionic injection, are considered in the calculation of the electroresistance as a function of ferroelectric barrier properties, given by the properties of the ferroelectric, the barrier thickness, and the metal properties, and in turn of the polarization direction. Large electroresistance is favored in thicker films for all three transport mechanisms but on the expense of current density. However, switching between two transport mechanisms, i.e., direct tunneling and Fowler-Nordheim tunneling, by polarization switching yields a large electroresistance. Furthermore, the most versatile playground in optimizing the device performance was found to be the electrode properties, especially screening length and band offset with the ferroelectric.

cond-mat.mtrl-sci↗

arXiv2009

Epitaxial thin films of the multiferroic double perovskite Bi2FeCrO6 grown on (100)-oriented SrTiO3 substrates: Growth, characterization, and optimization

The influence of the deposition pressure PO2 and substrate temperature TS during the growth of Bi2FeCrO6 thin films grown by pulsed laser deposition has been investigated. It is found that the high volatility of Bi makes the deposition very difficult and that the growth of pure Bi2FeCrO6 thin films on SrTiO3 substrates is possible only in a narrow deposition parameter window. We find that the pure Bi2FeCrO6 phase is formed within a narrow window around an oxygen pressure PO2 =1.210-2 mbar and around a substrate temperature TS=680 degC. At lower temperature or higher pressure, Bi7.38Cr0.62O12+x_also called (b*Bi2O3)and Bi2Fe4O9 /Bi2(Fe,Cr)4O9+x phases are detected, while at lower pressure or higher temperature a (Fe,Cr)3O4 phase forms. Some of these secondary phases are not well known and have not been previously studied. We previously reported Fe/Cr cation ordering as the probable origin of the tenfold improvement in magnetization at saturation of our Bi2FeCrO6 film, compared to BiFeO3. Here, we address the effect of the degree of cationic ordering on the magnetic properties of the Bi2FeCrO6 single phase. Polarization measurements at room temperature reveal that our Bi2FeCrO6 films have excellent ferroelectric properties with ferroelectric hysteresis loops exhibiting a remanent polarization as high as 55-60 miroC/cm2 along the pseudocubic (001) direction.

cond-mat.mtrl-sci↗

arXiv2008

Impact of misfit strain on the properties of tetragonal Pb(Zr,Ti)O3 thin film heterostructures

Heterostructures consisting of PbZr0.2Ti0.8O3 and PbZr0.4Ti0.6O3 films grown on a SrTiO3 (100) substrate with a SrRuO3 bottom electrode were prepared by pulsed laser deposition. Using the additional interface provided by the ferroelectric bilayer structure and changing the sequence of the layers, the dislocation content and domain patterns were varied. The resulting microstructure was investigated by transmission electron microscopy. Macroscopic ferroelectric measurements have shown a large impact of the formation of dislocations and the a/c domain structure on the ferroelectric polarization and dielectric constant. A thermodynamic analysis using the LANDAU-GINZBURG-DEVONSHIRE approach that takes into account the ratio of the thicknesses of the two ferroelectric layers and electrostatic coupling is used to describe the experimental data.

cond-mat.mtrl-sci↗

arXiv2008

Finite element method calculations of ZnO nanowires for nanogenerators

The bending of a nonconducting piezoelectric ZnO nanowire is simulated by finite element method calculations. The top part is bent by a lateral force, which could be applied by an atomic force microscope (AFM) tip. The generated electrical potential is 0.3 V. This relatively high signal is, however, difficult to measure, due to the low capacitance of the ZnO nanowire (4x10^{-5} pF) as compared to the capacitance of most preamplifiers (5 pF). A further problem arises from the semiconducting properties of experimentally fabricated ZnO nanowires which causes the disappearance of the voltage signal within picoseconds.

cond-mat.mtrl-sci↗

arXiv2006

Intrinsic ferroelectric properties of strained tetragonal PbZr0.2Ti0.8O3 obtained on layer-by-layer grown, defect-free single crystalline films

PbZrxTi1-xO3 (PZT) is one of the technologically most important ferroelectric materials. Bulk single-domain single crystals of PZT have never been synthesized for a significant compositional range across the solid-solution phase diagram. This leaves the fundamental properties of PZT under debate. Synthesis of defect-free single crystalline films enables us to unambiguously determine the intrinsic quantities that describe ferroelectric materials, such as spontaneous polarization and dielectric constant. Defect-free single crystalline, strained PbZr0.2Ti0.8O3 thin films were grown by pulsed-laser deposition (PLD) onto vicinal SrTiO3 (001) single crystal substrates. Single crystalline SrRuO3 films fabricated by PLD were employed as bottom electrode. The PZT films have square shape hysteresis loops and remnant polarization values of up to Pr= 110 micro C/cm2, which is considerably higher than the theoretical value predicted for (unstrained) bulk single crystalline PZT of the investigated composition. The films have a dielectric constant eps33= 90 and a piezoelectric coefficient d33= 50 pm/V.

cond-mat.mtrl-sci↗