Shalini Kumari

Dielectric anomalies due to grain boundary conduction in chemically substituted BiFeO3

We describe systematic studies on Nd and Mn co-doped BiFeO3, i.e., (Bi0.95Nd0.05)(Fe0.97Mn0.03)O3 (BNFM) polycrystalline electroceramics. Raman spectra and X-ray diffraction patterns revealed the formation of rhombohedral crystal structure at room temperature, and ruled out structural changes in BiFeO3 (BFO) after low percentage chemical substitution. Strong dielectric dispersion and a sharp anomaly around 620 K observed near the Neel temperature ( TN ∼ 643 K of BFO) support strong magneto-dielectric coupling, verified by the exothermic peak in differential thermal data. Impedance spectroscopy disclosed the appearance of grain boundary contributions in the dielectric data in the region, and their disappearance just near the Neel temperature suggests magnetically active grain boundaries. The resistive grain boundary components of the BNFM are mainly responsible for magneto-dielectric coupling. Capacitive grain boundaries are not observed in the modulus spectra and the dielectric behavior deviates from the ...

Ferroelectric and photovoltaic properties of transition metal doped Pb(Zr0.14Ti0.56Ni0.30)O3-δ thin films

We report nearly single phase Pb(Zr0.14Ti0.56Ni0.30)O3-δ (PZTNi30) ferroelectric having large remanent polarization (15–30 μC/cm2), 0.3–0.4 V open circuit voltage (VOC), reduced band gap (direct 3.4 eV, and indirect 2.9 eV), large ON and OFF photo current ratio, and the fast decay time. Reasonably good photo current density (1–5 μA/cm2) was obtained without gate bias voltage which significantly increased with large bias field. Ferroelectric polarization dictates the polarity of VOC and direction of short circuit current (ISC), a step forward towards the realization of noncentrosymmetric ferroelectric material sensitive to visible light.

Effect of thickness on dielectric, ferroelectric, and optical properties of Ni substituted Pb(Zr0.2Ti0.8)O3 thin films

We report thickness dependent dielectric, ferroelectric, and optical properties of Ni substituted Pb(Zr0.2Ti0.8)O3 thin films. The Pb(Zr0.2Ti0.8)0.70Ni0.30O3−δ (PZTNi30) thin films for various thicknesses, ranging from 5 nm to 400 nm, were fabricated by pulsed laser deposition technique. Giant dielectric dispersion, low dielectric loss, large dielectric constant ∼1000–1500 from 100 Hz to 100 kHz, and diffused dielectric anomaly near 570–630 K were observed in PZTNi30 thin films. These films show well saturated ferroelectric hysteresis, with large remanent polarization. It also illustrated excellent optical transparency which decreased from 82 to 72% with increasing film thickness from 5 nm to 400 nm for the probe wavelengths ranging from 200 to 1100 nm. A decrease in direct bandgap (Eg) values from 4 eV to 3.4 eV and indirect-Eg values from 3.5 eV to 2.9 eV were observed for PZTNi30 thin films with increase in film thickness from 5 nm to 400 nm, respectively. The direct and indirect bandgaps were discusse...

Reconstructing phase diagrams from local measurements via Gaussian processes: mapping the temperature-composition space to confidence

We show the ability to map the phase diagram of a relaxor-ferroelectric system as a function of temperature and composition through local hysteresis curve acquisition, with the voltage spectroscopy data being used as a proxy for the (unknown) microscopic state or thermodynamic parameters of materials. Given the discrete nature of the measurement points, we use Gaussian processes to reconstruct hysteresis loops in temperature and voltage space, and compare the results with the raw data and bulk dielectric spectroscopy measurements. The results indicate that the surface transition temperature is similar for all but one composition with respect to the bulk. Through clustering algorithms, we recreate the main features of the bulk diagram, and provide statistical confidence estimates for the reconstructed phase transition temperatures. We validate the method by using Gaussian processes to predict hysteresis loops for a given temperature for a composition unseen by the algorithm, and compare with measurements. These techniques can be used to map phase diagrams from functional materials in an automated fashion, and provide a method for uncertainty quantification and model selection.Piezoelectrics: Seeing the full picture with machine learningThe functional response of piezoelectrics for arbitrary compositions and conditions cannot be fully obtained using local probes. Although the ferroelectric to paraelectric phase transition temperature can be measured, this is only for a limited number of data points. Now, a team led by Rama Vasudevan at Oak Ridge National Laboratory and colleagues in Puerto Rico, the United States, and India demonstrate that machine learning can be used to obtain the full ferroelectric phase diagram, with functional response. From measurements on four samples of a material, they use a machine-learning technique called Gaussian processes to reconstruct piezoelectric properties for continuously varying voltages and temperatures, which with more commonly used approaches accurately predicts ferroelectric phase diagrams. This could be used to generate piezoelectric phase diagrams, highlighting material compositions with optimal properties that can then be targeted by synthesis.

Insights into the magnetic dead layer in La0.7Sr0.3MnO3 thin films from temperature, magnetic field and thickness dependence of their magnetization

Experimental investigations of the magnetic dead layer in 7.6 nm thick film of La0.7Sr0.3MnO3 (LSMO) are reported. The dc magnetization (M) measurements for a sample cooled to T = 5 K in applied field H = 0 reveal the presence of negative remanent magnetization (NRM) in the M vs. H (magnetic field) measurements as well as in the M vs. T measurements in H = 50 Oe and 100 Oe. The M vs. T data in ZFC (zero-field-cooled) and FC (field-cooled) protocols are used to determine the blocking temperature TB in different H. Isothermal hysteresis loops at different T are used to determine the temperature dependence of saturation magnetization (MS), remanence (MR) and coercivity HC. The MS vs. T data are fit to the Bloch law, MS (T) = M0 (1 – BT 3/2), showing a good fit for T < 100 K and yielding the nearest-neighbor exchange constant J/kB ≅ 18 K. The variations of TB vs. H and HC vs. T are well described by the model often used for randomly oriented magnetic nanoparticles with magnetic domain diameter ≈ 9 nm present ...

Experimental verification of the ab initio phase transition sequence in SrZrO3 and comparisons with SrHfO3 and SrSnO3

We present detailed Raman studies of SrZrO3 (SZO) that show three anomalies in Raman modes: One has a small jump in frequency ω, one has its intensity vanish, and a third has a sharp change in temperature derivative dω(T)/dT from flat below T = 600 K to a Curie–Weiss dependence above 600 K with extrapolation to zero frequency at the known transition temperature T = 970 K, thereby proving the latter to be displacive. In addition, the P4mm ferroelectric phase predicted at high stresses has preliminary support from polarization-voltage experiments. The inference of a new transition in the temperature region 600–650 K is in disagreement with neutron studies. Comparisons are given for family member SrSnO3 and SrHfO3, and we discuss the different conclusions of Kennedy and Knight. We show that a known transition in SrHfO3 is also displacive with a well-behaved soft mode.Dielectrics: into a new phaseAn unexpected atomic arrangement in a material useful for electronics is identified by researchers in India, the UK, and the USA. Ashok Kumar from the CSIR-National Physical Laboratory and co-workers observe a phase change in strontium zirconate at 650 K. Strontium zirconate is an electrical insulator that has great potential in the semiconductor-technology because of its large dielectric constant. As this so-called perovskite is heated, its atomic structure changes at specific temperatures. But the sequence of these phase changes is still a matter of debate. Kumar and colleagues study the structure of strontium zirconate by measuring the optical emission associated with atomic vibrations—a method known as Raman spectroscopy and dielectric spectroscopy. They identify three anomalous features in these results that infer the presence of a new phase transition.

Studies on dielectric, optical, magnetic, magnetic domain structure, and resistance switching characteristics of highly c-axis oriented NZFO thin films

With the rapid development of new device miniaturization technology, there is invigorated interest in magnetic nanostructures for potential application in novel multifunctional devices. In continuation to our search for a suitable magnetic material having Curie temperature (Tc) well above room temperature for multifunctional applications, we have studied the dielectric, optical, magnetic, and resistance switching characteristics of Ni0.65Zn0.35Fe2O4 (NZFO) thin films. The observation of only (004) reflection in the X-ray diffraction patterns confirms the c-axis orientation and high quality growth of NZFO thin films. The presence of mixed valences of Fe2+/Fe3+ cations is probed by X-ray photon spectroscopy, which supports the cationic ordering-mediated large dielectric response. Our investigations reveal NZFO to be an indirect band gap material (∼1.8 eV) with a direct gap at ∼2.55 eV. These nanostructures exhibit high saturation magnetization and a low coercive field with a ferrimagnetic–paramagnetic phase transition of ∼713 K. Magnetic force microscopy studies revealed the stripe-like domain structure of the investigated thin films. In addition, these thin films exhibit reliable and repeatable unipolar resistive switching characteristics. The observed high dielectric permittivity with low loss tangent, large magnetization with soft magnetic behavior, striped magnetic domain structure and reliable resistance switching in NZFO thin films above room temperature suggest potential application in memory, spintronics, and multifunctional devices.

Ferroelectric, ferromagnetic, and multiferroic heterostructures for possible applications as tunnel junctions

Abstract In this chapter a detailed discussion on multilayers architectures is provided to understand their transport properties, tunneling current, interface behavior at atomic scale, and compatibility with silicon and state of the art technology. The chapter deals with the fabrication process and functional properties of multilayer and superlattice structure on various substrates for possible applications as multistates nonvolatile random access memory and logic devices. It starts with an introduction and then explains the concept of spintronics and multistates logics and finally correlates the microstructure with device properties. Various device architectures such as metal–insulator–metal, metal-ferroelectric–metal, metal-ferromagnetic/multiferroic-metal, metal (ferromagnetic)–multiferroic-metal (anti/ferromagnetic), silicon-ferroelectric/multiferroic-metal, ferromagnetic–multiferroic-metal, etc. heterostructures are discussed and explained their suitability as multistates logics and memory devices, possible drawbacks and potential to win the international market. This chapter covers the state of art development of ferroelectric/multiferroic tunnel junctions and possible future spin-devices. A tunneling electroresistance for the realization of intrinsic ferroelectric polarization has been discussed.