Ferroics and Multiferroics Edited by Hardev Singh Virk Wolfgang Kleemann Ferroics and Multiferroics Special topic volume with invited peer reviewed papers only Edited by Hardev Singh Virk and Wolfgang Kleemann Copyright  2012 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of the contents of this publication may be reproduced or transmitted in any form or by any means without the written permission of the publisher. Trans Tech Publications Ltd Kreuzstrasse 10 CH-8635 Durnten-Zurich Switzerland http://www.ttp.net Volumes 189 of Solid State Phenomena ISSN 1662-9787 (Pt. B of Diffusion and Defect Data - Solid State Data (ISSN 0377-6883)) Full text available online at http://www.scientific.net Distributed worldwide by and in the Americas by Trans Tech Publications Ltd Trans Tech Publications Inc. Kreuzstrasse 10 PO Box 699, May Street CH-8635 Durnten-Zurich Enfield, NH 03748 Switzerland USA Phone: +1 (603) 632-7377 Fax: +41 (44) 922 10 33 Fax: +1 (603) 632-5611 e-mail: [email protected] e-mail: [email protected] Editors Note When Thomas Wohlbier, Director of Publications, Trans Tech Publication (TTP) Ltd. offered one of us (HSV) to prepare a special volume of the Solid State Phenomena series, Professor Virk thought of Nanomaterials as a worthy title for this volume. After finding too much traffic in this area of Nanotechnology, he changed his mind and selected ‘Multiferroics’ as the new title of this volume, which has undergone further metamorphosis to ‘Ferroics and Multiferroics’. To handle this onerous task, he proposed to invite another expert colleague (WK) as a co-editor, to which TTP readily agreed. Multiferroics, materials that simultaneously show ferromagnetism and ferroelectricity, and often also ferroelasticity, attract now considerable attention, because of the interesting physics involved and also as they promise important practical applications. Typical multiferroics belong to the group of the perovskite transition metal oxides, and include rare-earth manganites and –ferrites. Several new multiferroic systems have been developed during recent years, with very strong coupling between ferroelectric and magnetic degrees of freedom. The field of multiferroic materials has recently undergone a tremendous development in exploring new systems and understanding their physics. Being extremely attractive to both the scientific and the application community, it demands first of all understanding of the basic ferroic properties such as ferroelectricity and ferromagnetism. The coexistence of different order parameters gives rise to novel coupling mechanisms and requires new measurement techniques. Hence, when entering this field, the traditional advanced knowledge of one type of materials and one kind of measurement techniques has undoubtedly to be expanded, where the traditional communities can/must learn from each other. The present special issue on ‘Ferroics and Multiferroics’ is thought to help bridging this gap while offering new insights into both bare ferroic materials and multiferroics, including molecular and disordered thin films and composite materials. Besides scientific interest in their physical properties, ferroics and multiferroics have potential for applications as actuators, switches, magnetic field sensors or new types of electronic memory devices. This volume includes twelve papers on different subjects selected by the Editors. The first two chapters serve as an introduction to ‘Multiferroic Memory’ and ‘Multiferroicity’ contributed by the well-known group of Ram S. Katyar in USA and Ravi Kumar’s team in India, respectively. Chapter 3 is written by Wolfgang Kleemann on a new class of multiferroics, known as ‘Disordered Multiferroics’. Chapter 4 by Yukio Watanabe gives an exhaustive treatment on ‘Basics of Ferroelectricity and its Origin’. Chapter 5 by Jatinder Yakhmi and Vaishali Bambole is focused on ‘Molecular Spintronics’ covering all aspects of this important new field of research. To keep some balance between theory and experiment, we have invited contributions from well established theorists. Chapters 6-8 are based on theoretical aspects of ferroelectricity, namely, ‘Direct and Inverse Effect’ by Mirza Bichurin’s group, applications of the Landau–Ginzburg theory to ferroelectric lattices by K.H. Chew, and phase transition and the H/D isotope effect by Takayoshi Ishimoto and Masanori Tachikawa. Chapter 9 by Kanhaiya Lal Yadav and Amit Kumar deals with multiferroic composite films. Chapter 10 by Vijay Srivastava and Kanwal Preet Bhatti reviews the progress in the area of ferromagnetic shape memory Heusler alloys and their potential applications in technology. Chapter 11 by Rajshree B. Jotania and Hardev S. Virk gives an overview of hexaferrities in general and Y-type hexaferrities, in particular, with emphasis on their characterization. The authors are looking for a multiferroic signature in Y-type hexaferrities. Finally, Chapter 12 by Parmendra Kumar Bajpai is an updated review of dielectric relaxation phenomenon in ferroelectric relaxor materials, both lead based and lead free types. This volume consists of both review articles and research papers on different themes proposed for it. Each contribution is unique and there is hardly any overlap. Review writing is also an art which lies in the domain of experts. Most of the authors of the review papers have kept in mind the interests of new entrants to the field of ‘Ferroics and Multiferroics’. They have covered the historical development of the field and added their own contribution to it. It is a moral duty of Editors to offer their gratitude to referees of the reviews and other thematic papers accepted for publication. All papers have been peer reviewed by 2-4 referees. The publishers and editors acknowledge their immense contribution to improve the quality of this special volume. Editors will fail in their duty, if they do not acknowledge the unflinching support of Professor Rasa Pirc of the Jozef-Stefan Institute, Ljubljana, Slovenia, as a referee, and Dr. K.H. Chew of the University of Malaya, Kuala Lumpur, Malaysia, for his help at various stages of preparation of this volume. H.S.Virk W. Kleemann Table of Contents Editors Note Multiferroic Memory: A Disruptive Technology or Future Technology? A. Kumar, N. Ortega, S. Dussan, S. Kumari, D. Sanchez, J. Scott and R. Katiyar 1 Combining Magnetism and Ferroelectricity towards Multiferroicity D. Shukla, N.E. Rajeevan and R. Kumar 15 Disordered Multiferroics W. Kleemann 41 Intrinsic Free Electrons/Holes at Polarization Discontinuities and their Implications for Basics of Ferroelectricity and its Origin Y. Watanabe 57 Molecular Spintronics J. Yakhmi and V. Bambole 95 Electromechanical Resonance in Magnetoelectric Composites: Direct and Inverse Effect M.I. Bichurin, V.M. Petrov, R.V. Petrov and S. Priya 129 Recent Applications of Landau-Ginzburg Theory to Ferroelectric Superlattices: A Review K.H. Chew 145 Theoretical Study on the Phase Transition and the H/D Isotope Effect of Squaric Acid T. Ishimoto and M. Tachikawa 169 Fabrication and Study of Hot Pressed Co Zn Fe O -PVDF PbTi Zr O and 0.6 0.4 2 4 0.7 0.3 3 Co Zn Fe O -PVDF-BaTi Zr O Multiferroic Composite Films 0.6 0.4 2 4 0.7 0.3 3 K.L. Yadav and A. Kumar 179 Ferromagnetic Shape Memory Heusler Alloys V. Srivastava and K.P. Bhatti 189 Y-Type Hexaferrites: Structural, Dielectric and Magnetic Properties R.B. Jotania and H.S. Virk 209 Dielectric Relaxation Phenomena in some Lead and Non-Lead Based Ferroelectric Relaxor Materials: Recent Advances P.K. Bajpai 233 © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/SSP.189.1 Multiferroic Memory: A Disruptive Technology or Future Technology? Ashok Kumar1,2,a, Nora Ortega1,b, Sandra Dussan1,c, Shalini Kumari1,d, Dilsom Sanchez1,e, James Scott1,3,f and Ram Katiyar1,g 1Department of Physics and Institute for Functional Nanomaterials, University of Puerto Rico, San Juan, Puerto Rico, USA, PR-00936-8377 2National Physical Laboratory, New Delhi 110012, India 3Department of Physics, University of Cambridge, Cambridge CB2 3EQ, UK [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected] Keywords: Multiferroics, thin and ultrathin films, bilayers and superlattices, FeRAM, MRAM, MERAM, magnonic logics, nonvolatile memory 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, BiFeO , one of the widely investigated materials in this decade. 3 Basic multiferroic features of well known room temperature single phase BiFeO in bulk and thin 3 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. 1. Introduction The advent of silicon-based complementary metal-oxide-semiconductor (CMOS) technology has changed the lifestyle of the modern age. Magnetic and ferroelectric material - based memories have a very small market share of the total Si industry; however, they are some of the major constituents of the memory industry. A very tiny magnetic and/or ferroelectric memory element can be embedded within the microelectronics of computer chips and data processing machines to maintain the non-volatility of memories. This is a competitor for FLASH memory that is orders of magnitude faster and operates at lower voltages, and is hence seen as a future replacement for FLASH. Semiconductor industry scientists are aware of Moore's law [1] that guides the requirements of power and density, doubling the bit-density every 24 months for integrated circuits (IC) for next generation devices. Rapid progress in the microelectronic industry can be maintained only by 2 Ferroics and Multiferroics miniaturization of different kinds of memory elements to cope with the next generation Si technology; however, an additional variable parameter or extra degree of freedom to overcome the quantum effects and excessive power dissipation with decrease in dimensionality (< 10 nm) of the materials would be desirable. Therefore, it is imperative to develop high component density electronic devices with low power consumption with new materials and new phenomena. In this context, spintronics, magneto-electrics, molecular electronics and carbon-based electronics within the general topic of nanotechnology are expected to help the industry follow Moore's predictions/law. In this review article we will focus only on the state of art development of multiferroic research. Bismuth ferrite (BiFeO - BFO) is the only established and fully recognized room temperature 3 multiferroic that has all three ferroic order parameters, i.e. anti/ferromagnetic, ferroelastic and ferroelectric, however, the cross coupling among these ferroic parameters in single phase form is very weak and poorly understood [2, 3]. Most multiferroics to date (such as terbium manganites TbMnO or TbMn O ) switch only nC/cm2 with applied magnetic fields approximately 1000×, too 3 2 5 small for reliable discrimination between “1” and “0” states [4, 5]. To overcome weak magnetoelectric (ME) coupling coefficient, interfacial coupling with the magnetic materials grown over layers of multiferroic materials, exchange bias coupling with magnetic materials in the form of multilayers and superlattices has been utilized. It is expected that multilayer structures of ferroelectric/magnetic and multiferroic/magnetic materials will be the key for future nonvolatile memory elements [6]. Single phase or multilayer forms of such materials can be used as computer memories in which a state "1" is stored as magnetization (M) or polarization (P) as +M (or +P), and "0" as -M (or -P). However, both ferroelectric random access memories (FRAMs) and magnetic random access memories (MRAMs) have disadvantages: Ferroelectrics are easy to write (voltage-driven, very low power, very fast - 250 ps), but hard to read (destructive read operation with reset). Magnetic memories are easy to read but generally slower and use more power to write. Magnetoelectric RAMs (MERAMs) would produce formidable competition for electrically erasable programmable read-only memory - FLASH EEPROMs, particularly in view of the fact that magnetoelectric RAMs (MERAMs) would operate at < 1.0 V, an international target for all microelectronics in the next decade, which FLASH devices are unlikely to meet without cumbersome internal charge pumps [7]. A major world-wide effort is underway to find materials that are simultaneously ferromagnetic and ferroelectric in order to combine the best qualities of both kinds of memories. Clearly a completely new line of thinking is required to overcome the intrinsic limitations of direct linear ME coupling of form polarization (P) × magnetization (M), (PM) in the free energy equation. Cross coupling phenomenon in a novel multiferroic material can be represented by the Venn diagram (Fig. 1). Our group has also carried out extensive efforts to design and discover novel single phase and heterostructure ME multiferroic materials to get a higher degree of ordered parameters and stronger coupling. The resulting material (a series of the solid solution of Pb(Zr ,Ti )O - Pb(Fe Ta )O (0 1-x x 3 1-x x 3 < x < 1); PZT/PFT in single phase form) is ultra-low loss (1% or less), which is extremely important for GHz phase-shifters (for which the important device parameter is insertion loss), and they exhibit both ferromagnetic switching and ferroelectric switching up to about 400 K [8]. Another series of single phase materials (solid solution of Pb(Zr ,Ti )O - Pb(Fe W )O (0 < x < 1-x x 3 1-x x 3 1); PZT/PFW ME materials) were investigated by our group, that show at room temperature three logic states (+1, 0, -1) under small magnetic field (~ 0.50 T) [9, 10]. Heterostructure and superlattices of PZT and La Sr MnO (LSMO) were investigated with and without external 0.7 0.3 3 magnetic field; these also displayed interface-mediated switching of large polarization under application of very small magnetic field (~ 0.3 T) [11, 12]. Prof. Ramesh's group [13] has also shown 180-degree switching of magnetic domains and magnetization for BFO and magnetic alloys multilayers at room temperature; this has particular interest for MERAM memory elements. Giant Hardev Singh Virk and Wolfgang Kleemann 3 interfacial magnetoelectric coupling in ferroelectric tunnel (BaTiO ) junctions with ferromagnetic 3 electrodes (LSMO as bottom and Fe as top electrode), has also shown potential for low-power spin- based memory elements, but unfortunately useful magnitudes were observed only at very low temperature. This review article only offers the limited information from the world of multiferroic research; however, glimpses of the state of art room temperature research can be noted. P M E P,M,S H E,H S E Fig. 1 Venn diagram of different ferroic orders and their cross coupling. Cross coupling represents magnetoelectric phenomena where symbols represent: Polarization (P), Magnetization (M), Strain(S), Electric Field (E), and Magnetic field (H). 2.0 Origin and Compatibility of Single Phase Multiferroics 2.1 Perovskite Based Single Phase Multiferroics. The microscopic origin of the magnetism in insulators lies in the localized electrons which are present in the partially filled d or f orbitals of transitions metal and rare earth metal ions which possess localized spin and hence magnetic moments. Exchange interactions among the localized spins lead to magnetic ordering. Ferroelectric materials have several sources for ferroelectricity; among them the most common is perovskite structure (ABO type) with empty d shells at B-site with a lack of centrosymmetry, which combined 3 with covalent bonding between oxygen ions and B-site metal ions produce very high polarization. In this system ferroelectricity arises from off-center shifts of transition ions; these transition metal ions, i.e. Ti+4, Nb+5, Ta+5, W+6, etc. at B-site form strong covalent bonds with the one or more oxygen ions and hence net polarization. dn and d0 shells of the transition ions form very stable off- center shift in O octahedra which usually produce incompatibility for multiferrocity, with a few 6 exceptions [14]. There is large number of bismuth and lead based multiferroics materials in nature: i.e. BiFeO , BiMnO and Pb(Fe,W)O , Pb(Fe,Ta)O , Pb(Fe,Nb)O , PbVO and their solid 3 3 3 3 3 3 solutions. The lone pairs of the Bi3+ and Pb2+ ions play the major role in the origin of ferroelectricity in this group. Charge ordering and the geometrical frustration also lead to ferroelectricity in the magnetic materials; these groups of materials are basically magnetic in nature. In case of charge ordering, the different valences of the transition metal ions lead to inequivalent sites which may cause ferroelectricity, e.g. in Pr Ca MnO [15] or nickelates, RNiO . YMnO is the classical 0.5 0.5 3 3 3 example of the geometrically frustrated multiferroic in which ferroelectricity is due to the tilting of the practically rigid MnO block. It has high ferroelectric phase transition temperature (914 K) and 5 low magnetic phase transition temperature (76 K) with polarization (P~ 6 µ/cm2) [16]. 2.2 Multiferroicity due to Spiral Spin Orders. Recently discovered magnetic multiferroics are very rich in physics. In these systems multiferroicity exists only at low temperature and the magnetically ordered state. Various types of spin order can have a potential to break the inversion symmetry and produce a spontaneous ferroelectric polarization (P). This is valid for both collinear and non-collinear forms of magnetic order, when they are placed on some specific lattice geometry. For example, the up-up-down-down spin arrangement along the atomically alternating A-B lattice