We clarify here certain aspects of the magnetic field (H) – temperature (T) phase diagram of YbMnO3, a hexagonal Rare-Earth manganite oxide in which two multiferroic ordered states – ferroelectricity and antiferromagnetism coexist at low temperature. Single crystals of YbMnO3 were carefully grown from a Floating Zone (FZ) at low speed, then oriented and studied at variable temperature and magnetic field. Magnetization and heat capacity measurement show features corresponding to the antiferromagnetic (AFM) ordering of Mn3+, and the rare earth Yb3+. We find that the ordering temperature of Mn3+ is independent of applied magnetic field up to 5T. However, contrary to previous reports in flux-grown crystals, we do not observe a complete suppression of Yb3+ order above 0.1T. Instead, we find that Yb3+ remains at least up to 1 T, suggesting a revision of our current understanding of the ordering mechanism of the Mn-Yb and Yb –Yb sub-lattices in this hexagonal structure.

Pure ZnO, and Al doped ZnO, 120 -300 nm thin films on glass substrates, were synthesized by inkjet printing technique using zinc and aluminum acetate solution as precursors and a two stage heat treatment process to obtain polycrystalline hexagonal wurtzite structure with the mean grain size of 25 and 30 nm respectively. All films exhibit a transmittance above 85-90% in the visible wavelength range below 700 nm. In the Al doped films the UV absorption spectra show a strong absorption onset below 380nm followed by shoulders centered around 325 nm depending on the film thickness. The electrical conductivity of Al doped ZnO thin films is larger by two orders of magnitude than that for pure ZnO films while the photoconductivity increases by about three orders of magnitude under UV irradiation. The photoresponse of the films with UV irradiation in terms of the rise and decay times in the frequency range from 5 to 500 Hz is also presented and discussed.

The wide band gap semiconductor ZnO is well known for its multifunctionality in the form of ferromagnetism (FM), piezoelectricity, and magneto optics. ZnO has been found to grow with intrinsic oxygen deficiencies which in turn are believed to give ferromagnetism and high conductivity in this material. Doping Zn2+ sites by V5+ ions creates a mixed valency as well as strain in the original ZnO hexagonal structure because of the reduced ionic size of vanadium. The mixed valency creates charge polarity between Zn-O and V-O bonds. This charge polarity and the rotation of the nonlinear V-O bonds with respect to Zn-O bonds under electric field have been shown to produce ferroelectricity. Furthermore, Mn doping of ZnO has also shown enhancement in ferromagnetic properties in ZnO. For this material to be a viable ferromagnetic material the magnetic properties should not be from segregated phases. In the present study we have grown undoped, Mn, and V doped ZnO thin films using pulsed laser deposition (PLD). ZnO target with 2% atomic Mn doping and a target with 0.5% atomic V doping were prepared by solid state reactions and sintering. Films were grown both epitaxially on sapphire substrates and in polycrystalline form on silicon substrates. Magnetization measurements by the PPMS showed M vs. H hysteresis loops with saturation for all ZnO: Mn films. V doped films showed high saturation polarization for film deposited at high pressures. We have also fabricated epitaxial bilayers of ZnO:V/ZnO:Mn on sapphire substrates. Ferroelectric and ferromagnetic properties of these heterostructures are presented.

A new multiferroic composite ceramics with the general formula (x)Ba(Sr)Fe12O19-(1-x)BaTiO3 (x=0.1, 0.5) was synthesized via a simple solid-state reaction technique. Crystal structure analysis performed for both materials revealed the presence of two crystalline phases pertinent to the initial composite components. X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to testify the crystallinity, microstructure, and local magnetoelectric interactions between ferroelectric and ferromagnetic grains. Magnetic measurements revealed that the saturation magnetization is proportional to the volume fraction of ferrite phase. Dielectric studies demonstrated strong frequency relaxation due to space charge polarization and high conductivity loss making macroscopic magnetoelectric measurements difficult. Novel nanoscale magnetoelectric effect observed by AFM is discussed.

Search for novel multi-functional materials, especially multiferroics, which are ferromagnetic above room temperature and at the same time exhibit a ferroelectric behavior much above room temperature, is an active topic of extensive studies today. Ability to address an entity with an external field, laser beam, and also electric potential is a welcome challenge to develop multifunctional devices enabled by nanoscience. While most of the studies to date have been on various forms of Bi- and Ba based Ferrites, rare earth chromites are a new class of materials which appear to show some promise. However in the powder and bulk form these materials are at best canted antiferromagnetics with the magnetic transition temperatures much below room temperature. In this presentation we show that thin films of YbCrO3 deposited by Pulsed Laser Deposition exhibit robust ferromagnetic properties above room temperature. It is indeed a welcome surprise and a challenge to understand the evolution of above room temperature ferromagnetism in such a thin film. The thin films are amorphous in contrast to the powder and bulk forms which are crystalline. The magnetic properties are those of a soft magnet with low coercivity. We present extensive investigations of the magnetic and ferroelectric properties, and spectroscopic studies using XAS techniques to understand the electronic states of the constituent atoms in this novel Chromite. While the amorphous films are ferromagnetic much above room temperature, we show that any observation of ferroelectric property in these films is an artifact of a leaky highly resistive material.

Multifunctional materials such as the single phase compound Pb(Fe0.5Nb0.5)O3 (PFN), where ferroelectric and antiferromagnetic order coexist, are very promising and have great interest from the academic and technological points of view. In this work, coupling of the ferroelectric and magnetic ordering has been observed. For this study, a combination of the small signal response using the impedance spectroscopy technique and the electromechanical resonance method with the large signal response through standard ferroelectric measurement, has been used with and without an applied magnetic field. The measurements to determine the electrical properties of the ceramic were performed as functions of the bias and poling electric fields. A simultaneous analysis of the complex dielectric constant , impedance , electric modulus , and the electromechanical coupling factors is presented. The results are correlated with a previous study of structural, morphological, small signal dielectric frequency-temperature response, and the ferroelectric hysteretic, magnetic and magnetodielectric behaviors. The observed shifts of the resonance and antiresonance frequency values can be associated with change of the domain size favored by the readjustment of the oxygen octahedron when the magnetic field is applied.From P-E hysteresis loops obtained without and with an external applied magnetic field a maximum value of dc magnetoelectric coefficient αME = 4 kV/cm T (400 mV/cm Oe) was obtained.

The goal of this research is to develop a process suitable for producing monolithic conformal substrates with a spatial arrangement of material cells according to a particular design creating novel material systems, useful for many multi- functional electronic and Radio Frequency devices. In this study, MCT ceramics (Mg-Ca-Ti-O systems) and organic binders (polymer solution) are mixed and fabricated as films through a process called tape casting to compromise between high dielectric constant and flexibility. Prior to optimizing the process, several characterization studies are carried out: Commercial spray dried MCT powders (Transtech Inc.) with dielectric constant k=70 and k=20 were analyzed as pressed and produced into tape cast films. Dielectric properties are then measured by an Agilent 16451B material analysis kit and their microscopic behavior is examined by scanning electron microscopy. Results show that flexible composite films show a maximum dielectric constant of ε∼22 unlike their powder pressed form with ε ∼16 but their loss behavior deteriorates when compared with their sintered form and a loss tangent factor of 0.001. The difference is attributed to the air content vs. polymer presence of the material in powder pressed form. Also, these substrates naturally are no longer flexible; hence studies are focused on their tape cast form. The potential of these dielectric shades to serve as candidate constituents for producing monolithic textured polymer-ceramic-composites with controllable loss is studied further. Four properties are of prime importance: tunability of dielectric constants to achieve miniaturization, flexibility via low temperature processing of polymers and loss controllability.

Multiferroic composite thin films consisting of PbNb0.02Zr0.2Ti0.8O3 (PNZT) and La0.7 Sr0.3 MnO3 (LSMO) were deposited on SiO2/Si substrates. SiO2 films were deposited by pulsed electron deposition and LSMO and PNZT films were prepared using chemical solution deposition process using a metal organic deposition route. Individual films and the test structure PNZT/LSMO/ SiO2/Si were characterized using various characterizing techniques. Preliminary results of magnetic field dependent capacitance (magneto-capacitance) on the test structure are reported. A change in capacitance from 18.92 pf to 5.49 pf is observed as frequency changes from 50 KHz to 1 MHz, when no external magnetic field is applied. When a magnetic field of 330 Oe (positive or negative) is applied, the change in magneto-capacitance is appreciable, with a maximum change of 37 % being observed at a frequency of 1 MHz.

Multiferroic materials are recognized today as one of the new emerging technologies with huge potential for both academic research and commercial developments. Multiferroic composites are in particular more attractive for studies due to their enhanced properties, especially at room temperature, in comparison to the single-phase multiferroics. In this paper, we examine some of the theoretical aspects regarding one type of multiferroic composites (laminated structures) and we discuss one of the many possible applications of these exciting structures. We highlight the main advantages composite systems have over single-phase multiferroics and the similarities that exist between them.