A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few 'multiferroic' materials that exhibit both of these properties, but the 'magnetoelectric' coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.

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Multiferroic and magnetoelectric materials

Enhancement of polarization and related properties in heteroepitaxially constrained thin films of the ferroelectromagnet, BiFeO3, is reported. Structure analysis indicates that the crystal structure of film is monoclinic in contrast to bulk, which is rhombohedral. The films display a room-temperature spontaneous polarization (50 to 60 microcoulombs per square centimeter) almost an order of magnitude higher than that of the bulk (6.1 microcoulombs per square centimeter). The observed enhancement is corroborated by first-principles calculations and found to originate from a high sensitivity of the polarization to small changes in lattice parameters. The films also exhibit enhanced thickness-dependent magnetism compared with the bulk. These enhanced and combined functional responses in thin film form present an opportunity to create and implement thin film devices that actively couple the magnetic and ferroelectric order parameters.

/pdf/epitaxial-bifeo3-multiferroic-thin-film-heterostructures-2ns67gte2l.pdf

Epitaxial BiFeO3 multiferroic thin film heterostructures.

Recent research activities on the linear magnetoelectric (ME) effect?induction of magnetization by an electric field or of polarization by a magnetic field?are reviewed. Beginning with a brief summary of the history of the ME effect since its prediction in 1894, the paper focuses on the present revival of the effect. Two major sources for 'large' ME effects are identified. (i) In composite materials the ME effect is generated as a product property of a magnetostrictive and a piezoelectric compound. A linear ME polarization is induced by a weak ac magnetic field oscillating in the presence of a strong dc bias field. The ME effect is large if the ME coefficient coupling the magnetic and electric fields is large. Experiments on sintered granular composites and on laminated layers of the constituents as well as theories on the interaction between the constituents are described. In the vicinity of electromechanical resonances a ME voltage coefficient of up to 90?V?cm?1?Oe?1 is achieved, which exceeds the ME response of single-phase compounds by 3?5 orders of magnitude. Microwave devices, sensors, transducers and heterogeneous read/write devices are among the suggested technical implementations of the composite ME effect. (ii) In multiferroics the internal magnetic and/or electric fields are enhanced by the presence of multiple long-range ordering. The ME effect is strong enough to trigger magnetic or electrical phase transitions. ME effects in multiferroics are thus 'large' if the corresponding contribution to the free energy is large. Clamped ME switching of electrical and magnetic domains, ferroelectric reorientation induced by applied magnetic fields and induction of ferromagnetic ordering in applied electric fields were observed. Mechanisms favouring multiferroicity are summarized, and multiferroics in reduced dimensions are discussed. In addition to composites and multiferroics, novel and exotic manifestations of ME behaviour are investigated. This includes (i) optical second harmonic generation as a tool to study magnetic, electrical and ME properties in one setup and with access to domain structures; (ii) ME effects in colossal magnetoresistive manganites, superconductors and phosphates of the LiMPO4 type; (iii) the concept of the toroidal moment as manifestation of a ME dipole moment; (iv) pronounced ME effects in photonic crystals with a possibility of electromagnetic unidirectionality. The review concludes with a summary and an outlook to the future development of magnetoelectrics research.

https://iopscience.iop.org/article/10.1088/0022-3727/38/8/R01/pdf

Revival of the Magnetoelectric Effect

The piezoelectric properties of relaxor based ferroelectric single crystals, such as Pb(Zn1/3Nb2/3)O3–PbTiO3 and Pb(Mg1/3Nb2/3)O3–PbTiO3 were investigated for electromechanical actuators. In contrast to polycrystalline materials such as Pb(Zr,Ti)O3, morphotropic phase boundary compositions were not essential for high piezoelectric strain. Piezoelectric coefficients (d33’s)>2500 pC/N and subsequent strain levels up to >0.6% with minimal hysteresis were observed. Crystallographically, high strains are achieved for 〈001〉 oriented rhombohedral crystals, although 〈111〉 is the polar direction. Ultrahigh strain levels up to 1.7%, an order of magnitude larger than those available from conventional piezoelectric and electrostrictive ceramics, could be achieved being related to an E-field induced phase transformation. High electromechanical coupling (k33)>90% and low dielectric loss <1%, along with large strain make these crystals promising candidates for high performance solid state actuators.

Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals

BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Physics and Applications of Bismuth Ferrite

The lead barium niobate solid solution system, Pbl-xBaxNb206 (PBN[( l-x)%l), has drawn increasing interest mainly because of its unique properties as a tungsten bronze type ferroelectric relaxor with a morphotropic phase boundary (MPB) separating a tetragonal ferroelectric phase 4mm (with polarization dipole along [OO 11) and an orthorhombic ferroelectric phase m2m (with polarization dipole along [ 1 lo]). Extraordinarily large dielectric, piezoelectric, and pyroelectric properties of morphotropic phase boundary PBN compositions have been reported. A morphotropic phase boundary has been shown to exist in the composition region near 1-x = 0.63l. Optical memory effects associated with a field-induced phase transition were also observed for the nearMPB composition single crystals172. In the present paper we report measurements of linear electrooptic coefficients and the temperature variation of the optical birefringence. Analysis of the above results were made in association with previously measured dielectric and pyroelectric properties. Electrooptic response of the PBN crystals to applied square wave voltage pulses were also studied. Transparent optical quality single crystals of Pb1-~BaxNb206 , where 1-x varies from 0.50 to 0.65, were grown by Czochralski technique. Transverse linear electrooptic coefficients were measured by using both a standard half wave voltage method and a Senarmont compensator method employing lock-in phase detection techniques. Measurements of temperature variation of optical birefringence, An(T), were carried out by monitoring the transmitted intensity between crossed polarizers in the temperature range from room temperature up to -450OC depending on the ferroelectric phase transition temperature of the corresponding sample.

Electrooptic Properties Of Lead Barium Niobate (PBN) Single Crystal

A simple and direct method is proposed which can be used for quantitatively distinguishing the mechanisms of domain reorientation processes in polycrystalline materials. Using this method, the 90 degrees domain reorientation in Pb(Zr/sub x/Ti/sub 1-x/)O/sub 3/ (PZT) ceramic under electric field was examined by X-ray diffraction analysis. It was found that the polarization switching in the PZT ceramic with a composition near the morphotropic phase boundary is predominantly controlled by two successive 90 degrees domain processes rather than by 180 degrees domain wall reversal. Experimental results also indicate that the coercive field of the ceramics is related to the cooperative deformation associated with each grain, which arises from the 90 degrees domain reversal process. >

90 degrees -domain reversal in Pb(Zr/sub x/Ti/sub 1-x/)O/sub 3/ ceramics

A preliminary evaluation of the pyroelectric response of selected compositions from the variable order-disorder [VOD] phase region of this system has been conducted under modest DC biasing conditions in order to assess the performance of these materials for application in small area pyroelectric devices. The figure of merit [FD] was observed to be significantly enhanced over that of the pure Pb(Sc½Ta½)O3 material due to the combined effects of Ti doping, ordering, and the application of a biasing field.

Lead scandium tantalate-lead titanate materials for field-stabilized pyroelectric device applications

By using an optical interferometer and other more conventional methods, the responses of piezoelectric and dielectric coefficients of lead zirconate-titanate (PZT) ceramics to AC electric fields have been directly investigated. The experimental results demonstrate the importance of electromechanical nonlinearities present in the ferroelectric ceramics. A phenomenological theory, based on a model of G Arlt et al. (1987), has been extended to describe the extrinsic contribution to piezoelectric, elastic, and dielectric coefficients. This can be attributed to both the linear and nonlinear vibrations of 90 degrees domain walls in tetragonally distorted ferroelectric ceramics. The proposed theory shows qualitative agreement with the experimental results. >

Electromechanical nonlinearity of ferroelectric ceramics and 90 degrees domain wall motions

S i n g 1 e c r y s t a l s o r ' 0 .9 PD ( Z n l I 3 N b 2 030 . 1 P b T i 0 3 were g r o w n u s i n g t h e f l u x m e t h o d . T h e d i e l e c t r i c p r o p e r t y a l o n g t h e p s e u d o c u b i c [I111 and [ l o 0 1 d i r e c t i o n s was m e a s u r e d as a f u n c t i o n of t e m p e r a t u r e and f r e q u e n c y . T h e e f f e c t s o f p o l i n g a n d D C b i a s f i e l d were i n v e s t i g a t e d . T h e p y r o e l e c t r i c p r o p e r t y a l o n g t h e s e two d i r e c t i o n s was a l s o measured. 1. I n t r o d u c t i o n The Pb(Znl, 3Nb2, )03-PbTi03 (PZN-PT) s y s ten is known t o n a v e a n e a r m o r p h o t r o p i c p h a s e b o u n d a r y b e t w e e n t h e t e t r a g o n a l and r h o m b o h e d r a l p h a s e i n t h e range o f 9 t o 9.5 mole% P b T i 0 3 . C r y s t a l s w i t h more t h a n 9.5% P b T i 0 3 s h o w t e t r a g o n a l s y m m e t r y , w h i l e t h o s e w i t h l e s s t h a n 94, P b T i 0 3 s h o w rhombohedral symmetry t l l . C r y s t a l s w i t h 9% PbTi03 were f o u n d t o h a v e v e r y l a r g e d i e l e c t r i c a n d p i e z o e l e c t r i c c o n s tan ts [ 2 I . I n t h i s p a p e r t h e g r o w t h and c h a r a c t e r i z a t i o n of c r y s t a l s w i t h s t a r t i n g c o m p o s i t i o n s 0.9PZN-O.1PT a r e d i s c u s s e d . T h e d i e l e c t r i c a n d p y r o e l e c t r i c p r o p e r t i e s a l o n g t h e p s e u d o c u b i c 1 1 1 1 1 a n d I 1 0 0 1 d i r e c t i o n s a n d t h e e f f e c t s of p o l i n g a n d DC b i a s f i e l d o n t h e d i e l e c t r i c b e h a v i o r a r e a l s o i n v e s t i g a t e d , 2 . C r y s t a l Growth S i n g 1 e c r y s t a l s 0 f 0. 9 P b ( Z n l 3 N b 2 1 030 . 1 P b T i 0 3 (PZNO.1PT) were g r o w n f r o m m o l t e n PbO f l u x by s l o w c o o l i n g . S t a r t i n g m a t e r i a l s were PbO. ZnO. N b 2 0 5 a n d T i 0 2 i n s t o i c h i o m e t r i c p r o p o r t i o n . A compound t o f l u x r a t i o o f 1 : 2 i n m o l e w a s u s e d . T h e c h a r g e was p l a c e d i n a p l a t i n u m c r u c i b l e a n d h e a t e d t o 1230°C. After a s o a k i n g p e r i o d o f 5 h r s , t h e c h a r g e was s l o w l y c o o l e d a t a r a t e of 3-SuC/hr t o 900°C a n d t h e n t o room t e m p e r a t u r e i n 2 4 h r s . T h e PbO t ' l u x w a s r e m o v e d by d i s s o l v i n g i n h o t a c e t i c a c i d . T h e c r y s t a l s o b t a i n e d a r e m o s t l y i n t h e form o f a n a r rowhead or i r r e g u l a r i n s h a p e w i t h a l i n e a r d i m e n s i o n of 1 t o 3 mm. and h a v e brown c o l o r . Some s m a l l e r c r y s t a l s a l s o s h o w r e c t a n g u l a r f a c e s a n d h a v e l i g h t e r c o l o r . Back r e f l e c t i o n Laue t e c h n i q u e r e v e a l s t h a t t h e p s e u d o c u b i c 1 1 1 1 1 d i r e c t i o n i s a l o n g t h e d i r e c t i o n o f t h e a r rowhead and t h e I1001 d i r e c t i o n is normal t o t h e r e c t a n g u l a r race. X-ray p o w d e r d i f f r a c t i o n p a t t e r n s do n o t i n d e x a s a s i n g 1 e p h a s e . I m p o s i n g a PhOmbOhedPal s y m m e t r y r e s u l t s i n t h e p a r a m e t e r s a = 4.0501 and a = 80"54' x h i c h a c c o u n t for cer tain l i n e s , w h e r e a s i m p o s i n g a t e t r a o n a l s y m m e t r y r e s u l t s i n a = 4 . 0 2 8 1 . c = 4 . 0 8 4 f w h i c h a c c o u n t f o r t h e res t o f t h e l i n e s . T h e s e v a l u e s a g r e e v e r y w e l l w i t h t h e v a l u e s f o r t h e two p h a s e s as shown i n F ig . 1 i n R e f e r e n c e L11, i n d i c a t i n g t h a t b o t h t h e r h o m b o h e d r a l a n d tetragonal p h a s e s c o e x i s t . I n t h e a b o v e a n a l y s i s , a c o m p u t e r program u s i n g A p p l e m a n ' s l e a s t s q u a r e s method f o r c e l l r e f i n e m e n t 131 was used . C o m p o s i t i o n a n a l y s e s b y e l e c t r o n m i c r o p r o b e were p e r f o r m e d on arrowhead-shaped and r e c t a n g u l a r s a m p l e s f r o m two g r o w t h t r i a l s . T h e c o m p o s i t i o n f o r e a c h s a m p l e was o b t a i n e d from a v e r a g i n g t h e v a l u e s a t f i v e l o c a t i o n s , t h e two ar rowhead-shaped s a m p l e s h a v e 0 .113 a n d 0.117 p a r t s o f T i i n t h e o x i d e f o r m w h i l e t h e two r e c t a n g u l a r o n e s h a v e 0.117 a n d 0.129 p a r t s . The o v e r a l l a v e r a g e o f T i i s f o u n d t o b e .119 p a r t s whereas Zn i s l e a s t h a n t h e p r o p o r t i o n i n t h e s t a r t i n g f o r m u l a PbZn 3Nb 6Ti.103. L o s s o f Z n O t h r o u g h e v a p o r a t i o n took ' p l i c e d u r i n g g r o w t h d u e t o t h e h i g h e r i n i t i a l t e m p e r a t u r e . 3 . D i e l e c t r i c P r o p e r t i e s S a m p l e s i n p l a t e l e t form w i t h t h e major f a c e n o r m a l t o t h e p s e u d o c u b i c [lll] o r I1001 d i r e c t i o n were p r e p a r e d . B a c k r e f l e c t i o n Laue , m e t h o d w a s u s e d i n o r i e n t i n g s a m p l e s . The d i e l e c t r i c c o n s t a n t K a n d l o s s a l o n g t h e p s e u d o c u b i c 11111 a n d [ l o 0 1 d i r e c t i o n s were m e a s u r e d a s a f u n c t i o n o f t e m p e r a t u r e a t f o u r f r e q u e n c i e s : 0 .1 , 1, 1 0 a n d 100 KHz a n d f o r u n p o l e d a n d p o l e d s t a t e s . P o l i n g w a s e f f e c t e d by c o o l i n g t h e s a m p l e f r o m 200°C t o 0°C u n d e r a D C f i e l d o f 1 0 K V l c m . F i g s . 1 a n d 2 show t h e d i e l e c t r i c c o n s t a n t and loss as a f u n c t i o n of t e m p e r a t u r e f o r u n p o l e d s a m p l e s i n t h e [1111 and [ l a 0 1 d i r e c t i o n s , r e s p e c t i v e l y . T h e y s h o w t h e b e h a v i o r o f a r e l a x o r t y p e f e r r o e l e c t r i c . N e a r room t e m p e r a t u r e t h e r e i s a d i f f u s e d t r a n s i t i o n f r o m t h e rhombohedra l to t e t r a g o n a l p h a s e , and a t 1 7 5 ° C a t r a n s i t i o n from t h e t e t r a g o n a l t o c u b i c p h a s e w i t h i n c r e a s i n g t e m p e r a t u r e . F i g s . 3 a n d 4 show t h e e f f e c t o f p o l i n g . P o l i n g a l o n g t h e [1111 a n d L1001 d i r e c t i o n s h a s a v e r y d i f f e r e n t e f f e c t o n t h e K v s T c u r v e s . When p o l e d a l o n g 1 1 1 1 1 , K r i s e s s h a r p l y a t t h e r h o m b o h e d r a l t o t e t r a g o n a l (R-T) p h a s e t r a n s i t i o n , r e a c h i n g a v a l u e o f 1 2 , 0 0 0 s l i g h t l y a b o v e t h e 50000, , 0 . 2 0 CT 0 i L 0.15 5

L. E. Cross

Papers

Electrooptic Properties Of Lead Barium Niobate (PBN) Single Crystal

90 degrees -domain reversal in Pb(Zr/sub x/Ti/sub 1-x/)O/sub 3/ ceramics

Lead scandium tantalate-lead titanate materials for field-stabilized pyroelectric device applications

Electromechanical nonlinearity of ferroelectric ceramics and 90 degrees domain wall motions

Growth, Dielectric and Pyroelectric Properties of 0.9Pb(Zn l/3 Nb 2/3 )O 3 -0.1PbTiO 3 Single Crystals