Table Of ContentFerroics
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.
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Volumes 189 of
Solid State Phenomena
ISSN 1662-9787
(Pt. B of Diffusion and Defect Data - Solid State Data (ISSN 0377-6883))
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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
