Charudatta Phatak

Enhancement of local piezoresponse in polymer ferroelectrics via nanoscale control of microstructure.

Polymer ferroelectrics are flexible and lightweight electromechanical materials that are widely studied due to their potential application as sensors, actuators, and energy harvesters. However, one of the biggest challenges is their low piezoelectric coefficient. Here, we report a mechanical annealing effect based on local pressure induced by a nanoscale tip that enhances the local piezoresponse. This process can control the nanoscale material properties over a microscale area at room temperature. We attribute this improvement to the formation and growth of β-phase extended chain crystals via sliding diffusion and crystal alignment along the scan axis under high mechanical stress. We believe that this technique can be useful for local enhancement of piezoresponse in ferroelectric polymer thin films.

Visualization of the Magnetic Structure of Sculpted Three-Dimensional Cobalt Nanospirals

In this work, we report on the direct visualization of magnetic structure in sculpted three-dimensional cobalt (Co) nanospirals with a wire diameter of 20 nm and outer spiral diameter of 115 nm and on the magnetic interactions between the nanospirals, using aberration-corrected Lorentz transmission electron microscopy. By analyzing the magnetic domains in three dimensions at the nanoscale, we show that magnetic domain formation in the Co nanospirals is a result of the shape anisotropy dominating over the magnetocrystalline anisotropy of the system. We also show that the strong dipolar magnetic interactions between adjacent closely packed nanospirals leads to their magnetization directions adopting alternating directions to minimize the total magnetostatic energy of the system. Deviations from such magnetization structure can only be explained by analyzing the complex three-dimensional structure of the nanospirals. These nanostructures possess an inherent chirality due to their growth conditions and are of significant importance as nanoscale building blocks in magneto-optical devices.

Nanoscale Skyrmions in a Nonchiral Metallic Multiferroic: Ni2MnGa

Magnetic skyrmions belong to a set of topologically nontrivial spin textures at the nanoscale that have received increased attention due to their emergent behavior and novel potential spintronic applications. Discovering materials systems that can host skyrmions at room temperature in the absence of external magnetic field is of crucial importance not only from a fundamental aspect, but also from a technological point of view. So far, the observations of skyrmions in bulk metallic ferromagnets have been limited to low temperatures and to materials that exhibit strong chiral interactions. Here we show the formation of nanoscale skyrmions in a nonchiral multiferroic material, which is ferromagnetic and ferroelastic, Ni2MnGa at room temperature without the presence of external magnetic fields. By using Lorentz transmission electron microscopy in combination with micromagnetic simulations, we elucidate their formation, behavior, and stability under applied magnetic fields at room temperature. The formation of skyrmions in a multiferroic material with no broken inversion symmetry presents new exciting opportunities for the exploration of the fundamental physics of topologically nontrivial spin textures.

Interface-controlled high dielectric constant Al2O3/TiOx nanolaminates with low loss and low leakage current density for new generation nanodevices

We report on the fundamentals for the synthesis of Al2O3/TiOx nanolaminates (NLs) with an Al2O3 interfacial layer at the electrode/nanolaminate interface, resulting in exceptionally high dielectric constant (k > 550 up to 0.1 MHz), very low losses (tan δ ≤ 0.04 up to 10 kHz), and leakage current density (≤10−8 A/cm2 at 1.0 V). The high k is attributed to the Maxwell-Wagner relaxation between semiconducting TiOx and insulating Al2O3 nanolayers, while low losses and leakage current densities are due to blockage of charged carriers transport through the Al2O3 interfacial layer. Additionally, a high-capacitance capacitor based on the Al2O3/TiOx NL structure is demonstrated on 16 μm deep Si trenches, which can be used to enable the next generation of nanoscale energy storage and memory devices.

Tailoring dielectric relaxation in ultra-thin high-dielectric constant nanolaminates for nanoelectronics

The work reported here demonstrates the feasibility of controlling the dielectric properties—high dielectric constant (k) and substantially extended relaxation frequency—of thin film nanolaminates (NLs) consisting of alternating TiOx and Al2O3 sublayers with various sublayer thicknesses grown by atomic layer deposition. For 150 nm thick TiOx/Al2O3 NLs with sub-nanometer thick sublayers, few Angstrom change in sublayer thickness dramatically increases relaxation cut-off frequency by more than 3 orders of magnitude with high dielectric constant (k > 800). This unusual phenomenon is discussed in the framework of two-phase Maxwell-Wagner relaxation.

X-ray Irradiation Induced Reversible Resistance Change in Pt/TiO2/Pt Cells

The interaction between X-rays and matter is an intriguing topic for both fundamental science and possible applications. In particular, synchrotron-based brilliant X-ray beams have been used as a powerful diagnostic tool to unveil nanoscale phenomena in functional materials. However, it has not been widely investigated how functional materials respond to the brilliant X-rays. Here, we report the X-ray-induced reversible resistance change in 40-nm-thick TiO2 films sandwiched by Pt top and bottom electrodes, and propose the physical mechanism behind the emergent phenomenon. Our findings indicate that there exists a photovoltaic-like effect, which modulates the resistance reversibly by a few orders of magnitude, depending on the intensity of impinging X-rays. We found that this effect, combined with the X-ray irradiation induced phase transition confirmed by transmission electron microscopy, triggers a nonvolatile reversible resistance change. Understanding X-ray-controlled reversible resistance changes can provide possibilities to control initial resistance states of functional materials, which could be useful for future information and energy storage devices.

Magnetic interactions and reversal of artificial square spin ices

Artificial spin ices are nanoscale geometrically engineered systems that mimic the behavior of bulk spin ices at room temperature. We describe the nanoscale magnetic interactions in a square spin ice lattice by an experimentally verified model that accounts for the correct shape of the magnetic islands. Magnetic force microscopy measurements on lithographically fabricated lattices are compared to Monte Carlo simulations of the reversal process of two lattices with different lattice spacings. Lattice node statistics and correlations show significant differences in the reversal mechanism for lattices with different spacings. The effect of structural variations is also compared for the two lattice reversals.

Separation of electrostatic and magnetic phase shifts using a modified transport-of-intensity equation

We introduce a new approach for the separation of the electrostatic and magnetic components of the electron wave phase shift, based on the transport-of-intensity equation (TIE) formalism. We derive two separate TIE-like equations, one for each of the phase shift components. We use experimental results on FeCoB and Permalloy patterned islands to illustrate how the magnetic and electrostatic longitudinal derivatives can be computed. The main advantage of this new approach is the fact that the differences in the power spectra of the two phase components (electrostatic phase shifts often have significant power in the higher frequencies) can be accommodated by the selection of two different Tikhonov regularization parameters for the two phase reconstructions. The extra computational demands of the method are more than compensated by the improved phase reconstruction results.

Bipolar resistance switching in Pt/CuOx/Pt via local electrochemical reduction

The local changes in copper oxidation state and the corresponding resistance changes in Pt/CuOx/Pt nanoscale heterostructures have been investigated using x-ray nanoprobe spectro-microscopy and current-voltage characterization. After gentle electroforming, during which the current-voltage behavior remains non-linear, the low resistance state was reached, and we observed regions of 160 nm width that show an increase in Cu K-alpha fluorescence intensity, indicative of partial reduction of the CuOx. Analysis of the current voltage curves showed that the dominant conduction mechanism is Schottky emission and that the resistance state is correlated with the Schottky barrier height. We propose that the reversible resistivity change in these Pt/CuOx/Pt heterostructures occurs through local electrochemical reduction leading to change of the Schottky barrier height at the interface between Pt and the reduced CuOx layers and to change of the CuOx resistivity within laterally confined portions of the CuOx layer. The...

Creation of artificial skyrmions and antiskyrmions by anisotropy engineering

Topologically non-trivial spin textures form a fundamental paradigm in solid-state physics and present unique opportunities to explore exciting phenomena such as the topological Hall effect. One such texture is a skyrmion, in which the spins can be mapped to point in all directions wrapping around a sphere. Understanding the formation of these spin textures, and their energetic stability, is crucial in order to control their behavior. In this work, we report on controlling the perpendicular anisotropy of continuous Co/Pt multilayer films with ion irradiation to form unique spin configurations of artificial skyrmions and antiskyrmions that are stabilized by their demagnetization energy. We elucidate their behavior using aberration-corrected Lorentz transmission electron microscopy. We also discuss the energetic stability of these structures studied through in-situ magnetizing experiments performed at room temperature, combined with micromagnetic simulations that successfully reproduce the spin textures and behavior. This research offers new opportunities towards creation of artificial skyrmion or antiskyrmion lattices that can be used to investigate not only fundamental properties of their interaction with electron currents but also technological applications such as artificial magnonic crystals.