Solid State Phenomena Vol. 189

Abstract: The term "Multiferroic" is coined for a material possessing at least two ferroic orders in the same or composite phase (ferromagnetic, ferroelectric, ferroelastic); if the first two ferroic orders are linearly coupled together it is known as a magnetoelectric (ME) multiferroic. Two kinds of ME multiferroic memory devices are under extensive research based on the philosophy of "switching of polarization by magnetic fields and magnetization by electric fields." Successful switching of ferroic orders will provide an extra degree of freedom to create more logic states. The "switching of polarization by magnetic fields" is useful for magnetic field sensors and for memory elements if, for example, polarization switching is via a very small magnetic field from a coil underneath an integrated circuit. The electric control of magnetization is suitable for nondestructive low-power, high-density magnetically read and electrically written memory elements. If the system possesses additional features, such as propagating magnon (spin wave) excitations at room temperature, additional functional applications may be possible. Magnon-based logic (magnonic) systems have been initiated by various scientists, and prototype devices show potential for future complementary metal oxide semiconductor (CMOS) technology. Discovery of high polarization, magnetization, piezoelectric, spin waves (magnon), magneto-electric, photovoltaic, exchange bias coupling, etc. make bismuth ferrite, BiFeO₃, one of the widely investigated materials in this decade. Basic multiferroic features of well known room temperature single phase BiFeO₃ in bulk and thin films have been discussed. Functional magnetoelectric (ME) properties of some lead-based solid solution perovskite multiferroics are presented and these systems also have a bright future. The prospects and the limitations of the ME-based random access memory (MERAM) are explained in the context of recent discoveries and state of the art research.

Abstract: The attempts to combine both the magnetic and ferroelectric properties in one material started in 1960s predominantly by the group of Smolenskii and Schmid [1. Dzyaloshinskii first presented the theory for multiferroicity in Cr₂O₃, which was soon experimentally confirmed by Astrov [5,. Further work on multiferroics was done by the group of Smolenskii in St. Petersburg (then Leningrad) [7, but the term multiferroic was first used by H. Schmid in 1994 [. These efforts have resulted in many fundamental observations and opened up an entirely new field of study. Schmid [ defined the multiferroics as single phase materials which simultaneously possess two or more primary ferroic properties. The term multiferroic has been expanded to include materials which exhibit any type of long range magnetic ordering, spontaneous electric polarization, and/or ferroelasticity. In the past decade, several hundreds of papers related to multiferroic materials and magnetoelectric effect have been published every year, making this topic one of the hottest areas in condensed matter physics from fundamental science as well as applications viewpoints. This article sheds light on recent progress about the developments of new multiferroics by combining unconventional magnetism and ferroelectricity with an emphasis on Bi based multiferroic materials. Specifically results of Ti doped BiMn₂O₅ and Bi doped Co₂MnO₄ multiferroics are discussed.

Abstract: Disordered multiferroic materials (type-III multiferroics) escape the conventional schematics of type-I and type-II multiferroics, where two types of ferroic long-range order are expected to coexist under different interdependences and promise to attain a maximized bilinear (α or EH) magnetoelectric effect under special symmetry conditions. Nevertheless sizable higher order ME response occurs also in disordered systems such as in the simultaneous dipolar and spin glasses (multiglass) Sr_0.98Mn_0.02TiO₃ and K_0.94Mn_0.03TaO₃, the quantum paraelectric antiferromagnet EuTiO₃, the spin glass and relaxor ferroelectric PbFe_0.5Nb_0.5O₃, and the antiferroelectric antiferromagnetic dipole glass CuCr_1-xIn_xP₂S₆. They have in common to show large quadratic magneto-capacitance effects, Δε H², which are related to dominating third-order E²H² terms in their free energies and do not require special symmetry conditions. The polarization controlled exchange coupling can achieve giant fluctuation-enhanced values in the vicinity of critical magnetic fields as observed, e.g., in EuTiO₃. Exceptionally, even the first-order EH-type magnetoelectric effect is observed whenever metastable homogeneous order parameters are induced by field cooling as in EuTiO₃, or in the spin glass phase of the relaxor multiferroic Pb (Fe_0.5Nb_0.5)O₃ at T < T_g = 10.6 K.

Abstract: This article reviews the theories and experiments on the macroscopic/nanoscopic scales, which indicate that nearly free electrons/holes appear at polarization discontinuities as a result of not only extrinsic mechanisms such as imperfections but also intrinsic mechanisms. We examine the consistency of these theories and experiments with conventional ones. Such electrons/holes lead to various novel properties of ferroelectrics and provide new insight into ferroelectricity, including fundamental issues such as the origin of ferroelectricity. This origin appears naturally compatible with the existence of multiferroicity.

Abstract: The emergence of spintronics (spin-based electronics), which exploits electronic charge as well as the spin degree of freedom to store/process data has already seen some of its fundamental results turned into actual devices during the last decade. Information encoded in spins persists even when the device is switched off; it can be manipulated with and without using magnetic fields and can be written using little energy. Eventually, spintronics aims at spin control of electrical properties (I-V characteristics), contrary to the common process of controlling the magnetization (spins) via application of electrical field. In the meantime, another revolution in electronics appears to be unfolding, with the evolution of Molecular Spintronics which aims at manipulating spins and charges in electronic devices containing one or more molecules, because a long spin lifetime is expected from the very small spin-orbit coupling in organic semiconductors. This futuristic area is fascinating because it promises the integration of memory and logic functions,

Abstract: Magnetoelectric (ME) coupling in the composites is mediated by the mechanical stress and one would expect orders of magnitude stronger coupling when the frequency of the ac field is tuned to acoustic mode frequencies in the sample than at non-resonance frequencies. A model is presented for the increase in ME coupling in magnetostrictive-piezoelectric bilayers for the longitudinal, radial, and bending modes in the electromechanical resonance region. We solved the equation of medium motion taking into account the magnetostatic and elastostatic equations, constitutive equations, Hooke's law, and boundary conditions. We estimated the ME voltage coefficient for direct ME effect and ME susceptibility for inverse ME coupling. The frequency dependence of the ME voltage coefficient and ME susceptibility reveals a resonance character in the electromechanical resonance region. Then we considered ME interaction in the magneto-acoustic resonance region at the coincidence of electromechanical and magnetic resonance. Variation in the piezomagnetic coefficient with static magnetic field for magnetic layer results in a dependence of ME voltage on applied bias magnetic field. As an example, we considered specific cases of cobalt ferrite or yttrium-ferrum garnet - lead zirconate titanate and nickel/permendur - lead zirconate titanate bilayers. Estimated values of ME voltage coefficient versus frequency profiles are in agreement with data.

Abstract: This article briefly reviews recent developments of Landau-Ginzburg theory to ferroelectric phase transitions in superlattices. An overview of the contributions of Landau-type theory to study ferroelectric superlattices is given. Recent findings from first-principles calculations and experiments on intermixing, local polarization coupling and polar discontinuity at interfaces that are not address in these contributions are highlighted. This is followed by a review of recent developments of Landau-Ginzburg theory that addresses these emergent phenomena at interfaces, which is the focus of this review article. The Landau-Ginzburg approach to ferroelectric superlattices with spatial distribution of polarization is outlined. It describes the formation of intermixed layer with properties different from those of both layers. These intermixed layers are mutually coupled through the local polarization at interfaces. Polarization continuity or continuity at interfaces is determined by the nature of the intermixed layer formed at the interface region. Recent results obtained in investigating superlattices comprised primarily of ferroelectric and paraelectric materials are discussed. The results include modulated polarizations, phase transitions, dielectric susceptibilities and switching behaviors.

Abstract: We investigated the phase transition and the isotope effect in squaric acid (H₂C₄O₄, abbreviated H₂SQ), a hydrogen-bonded dielectric material. Using first-principles calculation, we found that Jahn-Teller distortion of the unit structure (C₄H₄O₄) was the major driving force for the phase transition in the H₂SQ crystal. In order to elucidate the isotope effect on the phase transition in deuterated squaric acid (D₂SQ), we employed the multi-component molecular orbital (MC_MO) method, which directly takes into account the quantum effects of protons and deuterons. Using this model, we successfully predicted the difference between the phase transition temperature of H₂SQ and that of D₂SQ to be 192K, which is in reasonable agreement with the experimental value of 145 K. We found that the isotope effect in the H₂SQ/D₂SQ system was based more on shrinking distribution of the deuteron wave rather than that of the proton wave. When the MC_MO method was coupled with adequate cluster models, first-principles calculations were effective to determining the origin of the phase transition and the H/D isotope effect in hydrogen-bonded dielectric materials.

Abstract: Multiferroic composite films of (i) Co_0.6Zn_0.4Fe₂O₄(CZFO)-PbTi_0.7Zr_0.3O₃(PZT)-poly (vinylidene-fluoride)(PVDF) and (ii) Co_0.6Zn_0.4Fe₂O₄-BaTi_0.7Zr_0.3O₃(BZT)-PVDF were prepared by hot press method for magneto-dielectric studies. Different multiferroic composite films were named as CPT-1 (CZFO:PZT; 3:1) CPT-2 (CZFO:PZT; 3:2), CPT-3(CZFO:PZT; 3:3), CBT-1 (CZFO:BZT; 3:1), CBT-2 (CZFO:BZT; 3:2) and CBT-3 (CZFO:BZT 3:3). The entire composites were made with 70% ceramic and 30% wt. PVDF polymer. Line scanning by Scanning electron microscope (SEM) and Atomic force microscopy (AFM) images shows a homogeneous distribution of constituents in the composite film. It is observed that the dielectric permittivity (ε´) follows the MaxwellWagner model. Remnant polarization (P_r) and magnetocapacitance (MC) were found to vary with an applied magnetic field at room temperature. The absolute value of the magnetocapacitance (MC) was found higher for CBT-2 (MC ~ 0.79%) than for CBT-3 (MC ~ 0.57%) but lower than for CPT-3 (MC ~ 1.2%). A linear fit of the MC with M² yields the magnetoelectric quadratic coupling constant |γ| ~ 4.96 × 10^-6 for CBT-1, which is around 150 times lower than for CPT-1 (|γ| ~ 7.92 × 10^-4).