Recent Developments in Magnetoelectrics Vaijayanti Palkar Department of Condensed Matter Physics & Materials Science Tata Institute of Fundamental Research Mumbai 400 005, India.
Tata Institute of Fundamental Research Mumbai 400 05, India (www.tifr.res.in) 400 scientists working in various disciplines School of Mathematics School of Natural Sciences School of Technology and Computer Science.
Plan of the talk Introduction to magnetoelectrics BiFeO 3 Modified BiFeO 3 systems Summary
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
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
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
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
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
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
Oxygen Bismuth Iron Perovskite structure of BiFeO 3 (1960 s) Ferroelectric T c ~ 810 C Antiferromagnetic Ordering (TN) ~ 380 C
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)
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
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
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
Film Characterization : X-ray diffraction (XRD) Scanning Electron Microscope (SEM) Ferroelectric loop tracer Differential Scanning Calorimeter (DSC) Dielectric Response with temperature SQUID magnetometer
SEM picture of BiFeO 3 thin film grown on Pt/TiO 2 /SiO 2 /Si substrate by PLD
Saturated Ferroelectric Hysteresis Loop Palkar et.al. Applied Physics Letters 80(9), 1628 (2002)
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
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)
2000 Intensity (arb. units) 1500 1000 500 101 012 003, 021 202 113, 211 Mn 0.5 Mn 0.3 Mn 0.1 122, 300 Mn 0.0 0 20 25 30 35 40 45 50 55 60 2 Theta XRD patterns for Bi0.9La0.1Fe1-xMnxO3 (0 x 0.5) samples indicates presence of impurity phase
M (emu/gm) 4.0 T=5K 3.5 3.0 2.5 2.0 1.5 1.0 0.5 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 3 0.0 0 10 20 30 40 50 60 H (koe) M-H isotherm obtained for Bi 0.9 La 0.1 Fe 1-x MnO 3 samples Using SQUID magnetometer
-16 Heat Flow (mw) -12-8 -4 834 o C 0 250 350 450 550 650 750 850 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
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
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
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
2.40 Heat Flow (mw) 2.25 2.10 1.95 1.80 Bi 0.825 Tb 0.075 La 0.1 FeO 3 BiFeO 3 1.65 1.50 200 240 300 350 400 Temperature ( o C)
Dielectric Constant, K 540 480 420 Bi 0.825 Tb 0.075 La 0.1 FeO 3 160 240 320 Temperature ( o C)
-9 Heat Flow (mw) -10-11 -12-13.95-14.10 BiFeO 3 Bi 0.825 Tb 0.075 La 0.1 FeO 3-14.25 780 800 820 840 860 Temperature ( 0 C)
P S (µc/cm 2 ) 4 Bi 0.825 Tb 0.075 La 0.1 FeO 3 2 0-2 14V 10V 9V -4-100 -50 0 50 100 Applied Electric Field (V/cm)
Magnetic Moment (µ B /fu) 0.3 0.2 0.1 0.0-0.1-0.2-0.3 Bi 0.825 Tb 0.075 La 0.1 FeO 3 BiFeO 3 T = Room Temp. -60-30 0 30 60 H (koe)
Block Diagram of the Set-Up used for Testing Magnetoelectric Coupling Behavior
474 Dielectric Constant, K 472 470 468 466 464 462 Bi 0.825 Tb 0.075 La 0.1 FeO 3 T = Room Temp. 0.0 0.2 0.4 0.6 0.8 1.0 H (T)
4 3 P S (µc/cm 2 ) 2 1 Bi 0.825 Tb 0.075 La 0.1 FeO 3 T = Room Temp. 0 0 2 4 6 8 10 H (T)
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
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)
113, 211, 122 1000 800 600 400 200 Target 101 102 Pt substrate 021, 202 110 300 Thin Film Intensity (arb. Units) 0 20 30 40 50 60 2 Theta XRD pattern of target and thin film of Bi.6 Tb. 3 La.1 FeO 3 0
X div. = 0.100 Y div. = 2.133 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.
300k
3 P s (µc/cm 2 ) 2 1 0 0 2 4 6 8 10 Magnetic Poling Field
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
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
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
Thank You