Recent Developments in Magnetoelectrics Vaijayanti Palkar

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1 Recent Developments in Magnetoelectrics Vaijayanti Palkar Department of Condensed Matter Physics & Materials Science Tata Institute of Fundamental Research Mumbai , India.

Tata Institute of Fundamental Research Mumbai 400 05,

in) 400 scientists working in various disciplines

2 Tata Institute of Fundamental Research Mumbai , India ( 400 scientists working in various disciplines School of Mathematics School of Natural Sciences School of Technology and Computer Science.

3 Plan of the talk Introduction to magnetoelectrics BiFeO 3 Modified BiFeO 3 systems Summary

4 Magnetoelectrics Coexistence of magnetic & ferroelectric ordering in certain range of temperature It could be a combination of Ferroelectric or Antiferrelectric Ferromagnetic Ferrimagnetic Antiferromagnetic Desired co-existence for device applications: Ferroelectric & Ferro/Ferrimagnetic at RT

5 Terminology in field of ferroelectricity - influenced by analogy with field of ferromagnetism Ferromagnetism - interaction of magnetic dipole moments associated with individual atoms electric dipole moments in ferroelectrics reside in every unit cell of crystal Only crystals having non-centrosymmetric structures exhibit ferroelectricity

6 Ferroelectric crystal is characterized by Presence of spontaneous polarization Direction of polarization could be reversed by means of applied electric field Polar state Non-polar state At T c material transforms to non-polar state Presence of domains

7 Commonly accepted criterion as evidence of ferroelectricity is saturated hysteresis loop Ps Pr Ec High dielectric constant Pyroelectric and piezoelectric nature Presence of remnant polarization

8 Ferroelectrics therefore have high application potential Piezoelectric & pyroelectric sensors, actuators DRAM and Non-volatile memories, MEMS Magnetic materials are also known for variety of application Most important is data storage, MEMS Magnetoelectrics find applications in magnetic as well as ferroelectric devices

9 Moreover, due to coupling between the two order parameters polarization can be brought by either means (electric or magnetic field) additional degree of freedom in device design new applications like multiple state memory elements can be thought of Physics is rich and fascinating Hence Magnetolelectrics is attracting many more scientists these days

10 Oxygen Bismuth Iron Perovskite structure of BiFeO 3 (1960 s) Ferroelectric T c ~ 810 C Antiferromagnetic Ordering (TN) ~ 380 C

11 Difficult to synthesize single phase BiFeO 3 since, many compounds with different Bi & Fe compositions exist temperature stability for BiFeO 3 phase is narrow We have developed novel methods to synthesize single phase BiFeO 3 [1,2] 1. V. R. Palkar et.al. Applied Physics Letters 76, 2764 (2000) 2. Shetty et.al. Pramana, 58, 1027 (2002)

12 1. Oxide Mixing Bi 2 O 3 and Fe 2 O 3 in Stoichiometric proportions precalcination at 610 C/1 hour grinding and calcination at 800 C/1 hour leaching with 5% HNO 3 2. Wet chemical route co-precipitation of Bi & Fe as hydroxy complex single step calcination at 550 C/ 30 min. leaching out impurities using 5% nitric acid

13 However, even single phase material could not exhibit saturated ferroelectric hysteresis loop as difficult to maintain oxygen stoichiometry small variation in oxygen number partially changes Fe 3+ to Fe 2+ Synthesis of thin films by PLD using phase pure target was attempted since, 1. Energetic process 2. Possible to control oxygen pressure

14 Target : Phase pure BiFeO 3 Substrate : Pt/TiO 2 /SiO 2 /Si Substrate temperature : ~ 625 C Target to substrate distance : ~ 5cm Oxygen pressure : 0.3 torr Laser energy incident on target : 40 mj

15 Film Characterization : X-ray diffraction (XRD) Scanning Electron Microscope (SEM) Ferroelectric loop tracer Differential Scanning Calorimeter (DSC) Dielectric Response with temperature SQUID magnetometer

17 SEM picture of BiFeO 3 thin film grown on Pt/TiO 2 /SiO 2 /Si substrate by PLD

18 Saturated Ferroelectric Hysteresis Loop Palkar et.al. Applied Physics Letters 80(9), 1628 (2002)

19 Presence of saturated farroelectric loop but being weakly magnetic, no signal on SQUID magnetomater For desired applications scientists are hunting for materials with coexistence of ferromagnetic & ferroelectric ordering occurs at room temperature Unfortunately the combination is very rare (Hill et.al.) Attempts are therefore made to induce ferromagnetism in BiFeO 3 by doping without disturbing its ferroelectric nature

20 statistical distribution of Fe +3 and ions with +4 valency in the octaheda or creation of lattice defects in BiFeO 3 are likely to cause ferrimagnetism in the system Mn exists in +4 state Compound BiMnO 3 exists, doping with Mn in BiFeO 3 is feasible Hence, Bi 0.9 La 0.1 Fe 1-x Mn x O 3 (0 x 0.5) samples were prepared by wet chemical route and tested V.R. Palkar et.al. J. Appl Phys., 93, 4337 (2003)

21 2000 Intensity (arb. units) , , 211 Mn 0.5 Mn 0.3 Mn , 300 Mn Theta XRD patterns for Bi0.9La0.1Fe1-xMnxO3 (0 x 0.5) samples indicates presence of impurity phase

22 M (emu/gm) 4.0 T=5K Bi 0.9 La 0.1 FeO 3 Bi 0.9 La 0.1 Fe 0.9 Mn 0.1 O 3 Bi 0.9 La 0.1 Fe 0.7 Mn 0.3 O 3 Bi 0.9 La 0.1 Fe 0.5 Mn 0.5 O H (koe) M-H isotherm obtained for Bi 0.9 La 0.1 Fe 1-x MnO 3 samples Using SQUID magnetometer

23 -16 Heat Flow (mw) o C Temperature o C DTA curve for Bi 0.9 La 0.1 Fe 0.5 Mn 0.5 O 3 indicating presence of ferroelectric transition

24 M-H curve indicate increase in magnetization with increase in Mn Concentration However, desired enhancement in magnetism is not achieved Samples are basically antiferromagnetic but have weak ferromagnetism Origin of weak ferromagnetism could be rotation of spins towards field direction in presence of applied magnetic field Mn substitution has not affected ferroelectric Nature

25 Since Mn doping failed to give desired results, it was thought of doping BiFeO 3 with Tb, Gd, Dy etc. at Bi site due to, high magnetic moment Ionic radius of Bi +3 match with Tb, Gd, Dy etc. so replacement is feasible Bi site doping is not likely to disturb crystal symmetry and hence the ferroelectricity V.R. Palkar et.al. PRB (Rapid Comm.) Communicated

26 Results are presented for Bi.825 Tb.075 La.1 FeO 3 bulk samples synthesized by partial co-precipitation route developed in our lab. Bi/La/Tb Fe O Crystal Structure XRD-pattern

27 2.40 Heat Flow (mw) Bi Tb La 0.1 FeO 3 BiFeO Temperature ( o C)

28 Dielectric Constant, K Bi Tb La 0.1 FeO Temperature ( o C)

29 -9 Heat Flow (mw) BiFeO 3 Bi Tb La 0.1 FeO Temperature ( 0 C)

30 P S (µc/cm 2 ) 4 Bi Tb La 0.1 FeO V 10V 9V Applied Electric Field (V/cm)

31 Magnetic Moment (µ B /fu) Bi Tb La 0.1 FeO 3 BiFeO 3 T = Room Temp H (koe)

32 Block Diagram of the Set-Up used for Testing Magnetoelectric Coupling Behavior

33 474 Dielectric Constant, K Bi Tb La 0.1 FeO 3 T = Room Temp H (T)

34 4 3 P S (µc/cm 2 ) 2 1 Bi Tb La 0.1 FeO 3 T = Room Temp H (T)

35 We claim synthesis of bulk material exhibiting co-existence of magnetic & ferroelectric ordering at room temperature for the first time Magnetoelectric coupling between the two has been also proved

36 However, for real device applications, it is desirable to realize thin films of this novel magnetoelectric material on Si based substrate Films of Bi.9-x Tb x La.1 FeO 3 are grown by using Pulsed Laser Deposition technique on Si/SiO 2 /TiO 2 /Pt substrate V. R. Palkar et.al. Applied Phys. Lett. (communicated)

37 113, 211, Target Pt substrate 021, Thin Film Intensity (arb. Units) Theta XRD pattern of target and thin film of Bi.6 Tb. 3 La.1 FeO 3 0

38 X div. = Y div. = µc/cm 2 Volts Saturated ferroelectric hysteresis loop obtained on Bi 0.6 Tb 0.3 La 0.1 FeO 3 thin film grown on Si/SiO 2 /TiO 2 /Pt substrate by PLD. Film thickness is ~3900. Applied voltage is 0.4 V. Saturation polarization (P s ) and remnant polarization (P r ) values are, ~8 µc/cm 2 and ~4.5 µc/cm 2 respectively.

39 300k

40 3 P s (µc/cm 2 ) Magnetic Poling Field

41 SEM picture of as grown thin film of Bi.6 Tb. 3 La.1 FeO 3 SEM picture of Thin film of Bi.6 Tb. 3 La.1 FeO 3 after poling with magnetic field of 5 Tesla

42 Summary Synthesized new bulk compound exhibiting magnetoelectric coupling at room temperature for the first time Properties are improved in thin films Results are highly important due to variety of possible novel applications Discovery could bring revolution in device fabrication of Non-volatile memories

43 Acknowledgement Prof. S. Bhattacharya (Director TIFR) Prof. S.K. Malik Dr. Darshan Kundaliya Dr. Ganesh Kumara K. Mr. A.V. Gurjar Ms. B.A. Chalke Mr. J. John

44 Thank You

Chapter 3 Chapter 4 Chapter 5

Chapter 3 Chapter 4 Chapter 5 Preamble In recent years bismuth-based, layer-structured perovskites such as SrBi 2 Nb 2 O 9 (SBN) and SrBi 2 Ta 2 O 9 (SBT) have been investigated extensively, because of their potential use in ferroelectric