Electrical Characterisation of Non-Stoichiometric Bismuth Ferrite Bi1±0

Minimising Conductivity Leakage in Multiferroic BiFeO3 by Changing Bi/Fe Ratio

MAT356 Literature Survey & ProjectChristopher GauntSupervisor: Professor Anthony R. West

Bismuth Ferrite (BiFeO3 or BF) as a Multiferroic Material

Multiferroic materials, such as BF, have a wide range of device applications e.g. multi-state, non-volatile, memory storage media.

Figure 2: Proposed MERAM device using a layer of FE-AFM ceramic (green) Adapted from ii

i. W. Lane, Engineering functionality in the multiferroic BiFeO3 – controlling chemistry to enable advanced applications, The Royal Society of Chemistry (2010) [39] 10813–10826.ii. M. Bibes & A. Barthélémy, Multiferroics: Towards a magnetoelectric memory, Nature Materials (2008) [7] 425 – 426.

Figure 1: Polarisation scheme. Adapted from i

Resistance when magnetisations are parallel

Resistance when magnetisations are anti-parallel

BF

Origin of Multiferroic Properties in Bismuth Ferrite

Figure 3: BiFeO3 R3c space group unit cell. Adapted from i

• Ferroelectric properties are driven by the 6s2 lone pair in Bi3+

• G-type Anti-ferromagnetic behaviour driven by anti-parallel spin compensation in canted FeO6 octahedra.

Ferromagnet Anti-ferromagnet

Figure 4: Types of ferromagnetism

i. C. Ederer, and N. A. Spaldin, Weak ferromagnetism and magnetoelectric coupling in bismuth ferrite, Phys. Rev. B, 2005, 71, 060401

A Materials Challenge: Phase Purity & Leakage Conductivity

Research is being hindered:

• Difficulties synthesising phase pure ceramic BF,

• High leakage conductivity,

The main objective of this study was to:

1. Determine if leakage conductivity can be reduced by changing Bi/Fe ratio.

The secondary objectives of the study were to

2. Determine the dominant cause of leakage conductivity

3. Determine if nitrates can be used to reduce the amount of secondary phases

Origin of Secondary PhasesBF phase is typically found to co-exist with:

• Bi25FeO39 (Sillenite)

• Bi2Fe4O9 (Bi2). The potential causes found during the study include:

• Decomposition at low peritectic decomposition temperature

• Evaporation of Bi and/or Bi2O3

• Inadequate temperature/reaction time

• Impurities affecting solubility of Secondary phases.

Figure 5: Phase Diagram of the Pseudo-Binary Bi2O3-Fe2O3 system in the temperature range 600

- 1000 °C. Adapted from i

Bi rich phase Stoich. phase Fe rich phase

Peritectic Phase Transition

i. A. Maítre, M. François, and J.C. Gachon, Basic and Applied Research: Section I Experimental Study of the Bi2O3-Fe2O3 Pseudo-Binary System Journal of Phase Equilibria and Diffusion, 2004, 25 [1], 59-67.

Potential Solutions through different Processing techniques

Figure 6: Proposed Sequence of Mechanosynthesis. Adapted from i

• Rapid liquid phase sintering

• Mechanosynthesis

• These are the only two ways known to obtain phase pure BF

i. A. Perejon, N. Murafa, P. E. Sanchez-Jimenez, J. M. Criado, J. Subrt, M. J. Dianez, and L. A. Perez-Maqueda, Direct mechanosynthesis of pure BiFeO3 perovskite nanoparticles: reaction mechanism, J. Mater. Chem. C, 2013, 1, 3551–3562.

Potential Sources of Leakage Conductivity• Oxygen Loss is thought to be a possible cause due to the

requirement reduction of Fe3+ to Fe2+ (n-type)

• Volatilisation of Bi3+ and/or Bi2O3 at high temperatures would require formation of O, Bi and Fe vacancies (ionic)

• Oxidation of Fe3+ to Fe4+ has also been suggested due to the creation of Vo in each of the pre-proposed charge compensation mechanisms (p-type)

x

22 2 4

O OO O g V e

2 32 3 [Volatilisation] 2 3

Bi O Bi OBi O Bi O V V

2 32 3 [Volatilisation] 2 3

Bi O Fe OBi O Bi O V V

x

22 2 4

O OV O g O h

Sample Prep

• 3 samples of BF were prepared of different Bi content (± 0.05 moles and stoichiometric)

• The reagents Bi2O3 and Fe(NO3)3∙9H2O were used.

Sample Ref Composition Amount

of Bi2O3/gAmount of

Fe(NO3)3∙9H2O/g

BF1 Bi1.05FeO3 1.911 3.089BF2 Bi1+0FeO3 1.829 3.171BF3 Bi0.95FeO3 1.794 3.206

Table 1: Mass of reagents to be measured out and actual measured mass quantities

Experimental Procedure

750 °C for 18 hours 800 °C for 4 hours

Room temp to 300 °C.

Multiple heating and drying runs

Samples pressed into pellets and sintered

XRD was carried prior to and after

sintering

Volume and Density measurements were

taken on the Accupyc 1340

Impedance Spectroscopy carried

out using Au electrodes

Impedance with InGa and DC Bias on BF1

and BF3

Impedance on samples quenched in

Liquid N

XRD Phase Analysis

Samples Compound Phase

Space Group

Lattice Parameters / Å Volume of Unit Cell / Åa b c

BF1

BiFeO3

R3c (161) 5.578 “ 13.87 373.63

BF2 R3m (160) 5.574 “ 6.934 186.55

BF3 “ “ “ “ “

BF1 & BF2

Bi25FeO40

I23 (197) 10.18 “ “ 1054.64

BF3 I23 (197) 10.18 “ “ 1056.22

BF2 & BF3

Bi2Fe4O9 Pbam (55) 7.965 8.44 5.994 402.94

Table 2: Physical Characteristics of Main and Secondary Phase detected by XRD phase analysis in the samples

Impedance Measurements for BF1

Figure 7: Impedance Spectroscopy Plots (Top) Impedance Plot and (Bottom) C’ vs frequency for (Left) Slow Cooled and (Right) Quenched BF1

0 1e6 2e6 3e6 4e6 5e6

-5e6

-4e6

-3e6

-2e6

-1e6

0

Z'

Z''

Impedance

25C81C124C234C293C

0 30000

-15000

0

Z'

Z'' Impedance

25C81C124C234C293C

102 103 104 105 106 10710-12

10-11

10-10

10-9

Frequency (Hz)

C'

C' & C"

76C126C179C231C274C

0 2e6 4e6 6e6 8e6 1e7

-1e7

-8e6

-6e6

-4e6

-2e6

0

Z'

Z''

Impedance76C126C179C231C274C

0 250000 500000 750000 1000000

-500000

-250000

0

Z'

Z''

Impedance

76C126C179C231C274C

102 103 104 105 106 10710-12

10-11

10-10

10-9

Frequency (Hz)

C'

C' & C"

25C81C124C234C293C

0 50000 100000 150000 200000

-200000

-150000

-100000

-50000

0

Z'

Z''

Impedance

26C76C124C174C228C

0 5000 10000 15000

-7500

-2500

Z'

Z''

Impedance

26C76C124C174C228C



102 103 104 105 106 10710-12

10-11

10-10

10-9

Frequency (Hz)

C'

C' & C"

26C125C223C278C323C

0 5.0e5 1.0e6 1.5e6 2.0e6 2.5e6

-2.5e6

-2.0e6

-1.5e6

-1.0e6

-5.0e5

0

Z'

Z''

Impedance26C125C223C278C323C

0 50000 100000 150000 200000

-100000

-50000

0

Z'

Z''

Impedance

26C125C223C278C323C

102 103 104 105 106 10710-12

10-11

10-10

10-9

Frequency (Hz)

C'

C' & C"

26C76C124C174C228C

0 1e6 2e6 3e6 4e6 5e6

-5e6

-4e6

-3e6

-2e6

-1e6

0

Z'

Z''

Impedance

25C81C125C237C320C

0 25000 50000 75000 100000

-50000

-25000

0

Z'

Z''

Impedance

25C81C125C237C320C



102 103 104 105 106 10710-12

10-11

10-10

10-9

Frequency (Hz)

C'

C' & C"

25C81C125C237C320C

0 10000 20000 30000 40000

-40000

-30000

-20000

-10000

0

Z'

Z''

Impedance

36C76C124C170C239C

0 1000 2000 3000 4000

-2000

-1000

0 Z'

Z''

Impedance

36C76C124C170C239C

102 103 104 105 106 10710-11

10-10

10-9

10-8

Frequency (Hz)

C'

C' & C"

36C76C124C170C239C

• Can be interpreted through use of Equation 1

• Where σ is conductivity, Ea is activation energy, k is Boltzmann’s constant, T is temperature and n is the number of charge carriers

Arrhenius Plots of Conductivity

a10

E 1000log n2.3k T

(1)

Figure 10: BF1 Figure 11: BF2

Figure 12: BF3

Comparison with existing literature

Figure 13: Comparison of reported conductivity data for BiFeO3 with samples obtained during the investigation. Adapted from i

i. A. Perejon, N. Maso, A. R. West, P. E. Sanchez-Jimenez, R. Poyato, J. M. Criado, and L. A. Perez-Maqueda, Electrical Properties of Stoichiometric BiFeO3 Prepared by Mechanosynthesis with Either Conventional or Spark Plasma Sintering, J. Am. Ceram. Soc., 2013, 96 [4] 1220-1227.

Conclusions

• Increasing Bi to Fe ratio may be effective in reducing conductivity

• Shown a change of conductivity mechanism is present

• Grain boundaries dominate resistivity.

• Unable to comment on effectiveness of reagents used.

Any questions?

References1. J. Wang, J. B. Neaton, H. Zheng, V. Nagarajan, B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D. G. Schlom, U. V. Waghmare, N.

A. Spaldin, K. M. Rabe, M. Wuttig, R. Ramesh, Epitaxial BiFeO3 Multiferroic Thin Film Heterostructures, Science, 2003, 299, 1719.

2. K. Y. Yun, M. Noda, M. Okuyama, H. Saeki, H. Tabata, K. Saito, Structural and multiferroic properties of BiFeO3 thin films at room temperature, J. Appl. Phys., 2004, 96, 6, 3399-3405.

3. Y-K. Jun, W-T. Moon, C-M. Chang, H-S. Kim, H. S. Ryu, J. W. Kim, K. H. Kim, S.-H. Hong, Effects of Nb-doping on electric and magnetic properties in multi-ferroic BiFeO3 ceramics, Solid State Commun., 2005, 135, 133-137.

4. N. A. Hill, Why Are There so Few Magnetic Ferroelectrics?, J. Phys. Chem. B, 2000, 104 (29) 6694-6709.5. G.L. Yuan, S.W. Or, Y.P. Wang, Z.G. Liu, J.M. Liu, Preparation and multi-properties of insulated single-phase BiFeO3 ceramics,

Solid State Commun., 2006, 138, 76-81.6. I. Szafraniak, M. Połomska, B. Hilczer, A. Pietraszko, L. Kepinski, Characterization of BiFeO3 nanopowder obtained by

mechanochemical synthesis, J. Eur. Ceram. Soc., 2007, 27, 4399-4402.7. C. Ederer, and N. A. Spaldin, Weak ferromagnetism and magnetoelectric coupling in bismuth ferrite, Phys. Rev. B, 2005, 71,

060401(R). 8. V.A. Khomchenko, D.A. Kiselev, M. Kopcewicz, M. Maglione, V.V. Shvartsman, P. Borisov, W. Kleemann, A.M.L. Lopes, Y.G.

Pogorelov, J.P. Araujo, R.M. Rubinger, N.A. Sobolev, J.M. Vieira, A.L. Kholkin, Doping strategies for increased performance in BiFeO3, Journal of Magnetism and Magnetic Materials, 2009, 321 (11) 1689-1808.

9. A. A. Porporati, K. Tsuji, M. Valant, A.-K. Axelsson, and G. Pezzotti, Raman tensor elements for multiferroic BiFeO3 with rhombohedral R3c symmetry, J. Raman Spectrosc., 2010, 41, 84-87.

10. K. L. Da Silva, D. Menzel, A. Feldhoff, C. Kubel, M. Bruns, A. Paesano, Jr., A. Duvel, M. Wilkening, M. Ghafari, H. Hahn, F. J. Litterst, P. Heitjans,K. D. Becker, and V. Sepelak, Mechanosynthesized BiFeO3 Nanoparticles with Highly Reactive Surface and Enhanced Magnetization, J. Phys. Chem., 2011, 115 7209-7217.

11. W-T. Chen, A. J. Williams, L. Ortega-San-Martin, M. Li, D.C. Sinclair, W. Zhou, and J. P. Attfield, Robust Antiferromagnetism and Structural Disorder in BixCa1-xFeO3 Perovskites, Chem. Mater., 2009, 21, 2085-2093.

References12. G. L. Cao, C. Ma, Z. Chen, H. X. Yang, H. F. Tian and J. Q. Li, Oxygen-vacancy ordering in multiferroic Bi1−xCaxFeO3−x/2 (0.1 <

x <0.5), EPL., 2013, 102, 27002.13. A. Perejon, N. Murafa, P. E. Sanchez-Jimenez, J. M. Criado, J. Subrt, M. J. Dianez, and L. A. Perez-Maqueda, Direct

mechanosynthesis of pure BiFeO3 perovskite nanoparticles: reaction mechanism, J. Mater. Chem. C, 2013, 1, 3551–3562.14. P. Fischer, M. PoIomska, I. Sosnowska, and M. Szymanski, Temperature dependence of the crystal and magnetic structures of BiFeO,

J. Phys. C: Solid St. Phys., B, 1980.15. A. Perejon, N. Maso, A. R. West, P. E. Sanchez-Jimenez, R. Poyato, J. M. Criado, and L. A. Perez-Maqueda, Electrical Properties of

Stoichiometric BiFeO3 Prepared by Mechanosynthesis with Either Conventional or Spark Plasma Sintering, J. Am. Ceram. Soc., 2013, 96 [4] 1220-1227.

16. J. R. Teague, R. Gerson, and W.J. James, Dielectric hysteresis in single crystal BiFeO3, Solid State Commun., 1970, 8, 1073-1074.17. J. M. Park, M. Sohgawa, T. Kanashima, and M. Okuyama, Preparation of Epitaxial BiFeO3 Thin Films on La-SrTiO3 Substrate by

Using Magnetic-field-assisted Pulsed Laser Deposition, Journal of the Korean Physical Society, 2013, 62 [7] 1041-1045.18. N. Maso, and A. R. West, Electrical Properties of Ca-Doped BiFeO3 Ceramics: From p-Type Semiconduction to Oxide-Ion

Conduction, Chem. Mater., 2012, 24, 2127-2132.19. S. M. Selbach, T. Tybell, M-A. Einarsrud, T. Grande, Phase transitions, electrical conductivity, and chemical stability of BiFeO3 at

high temperatures, J. Solid State Chem., 2010, 183, 1205–1208.20. M. Valant, A-K. Axelsson, and N. Alford, Peculiarities of a Solid-State Synthesis of Multiferroic Polycrystalline BiFeO3, Chem.

Mater. 2007, 19, 5431-5436.21. G. Catalan, J. F. Scott, Physics and Applications of Bismuth Ferrite, Adv. Mater., 2009, 21, 2463-2485.22. A. Maítre, M. François, and J.C. Gachon, Basic and Applied Research: Section I Experimental Study of the Bi2O3-Fe2O3 Pseudo-

Binary System Journal of Phase Equilibria and Diffusion, 2004, 25 [1], 59-67.23. M. I. Morozov, N. A. Lomanova, and V. V. Gusarov, Specific Features of BiFeO3 Formation in a Mixture of Bismuth(III) and

Iron(III) Oxides, Russ. J. Gen. Chem., 2003, 73 [11], 1676 -1680.24. J. T. S. Irvine, D. C. Sinclair, and A. R. West, Electroceramics : Characterization by Impedance Spectroscopy, Adv. Mater., 1990, 2

[3], 132-138.

MERAM device operation• Bits are stored by the magnetization direction of the bottom ferromagnetic layer (blue),

• Bits are read by the resistance of the magnetic trilayer (Rp when the magnetizations of the two ferromagnetic layers are parallel),

• Bits are written by applying a voltage across the multiferroic ferroelectric antiferromagnetic layer,

• Magnetization of the bottom ferromagnetic layer is coupled to the spins in the multiferroic (small white arrows),

• Strong magnetoelectric coupling reverses the ferroelectric polarization in the multiferroic,

• This changes the magnetic configuration in the trilayer from parallel to antiparallel, and the resistance from Rp to antiparallel (Rap),

• A hysteretic dependence of the device resistance with voltage is achieved (blue curve).

BF1 XRD Trace

BF2 XRD Trace

BF3 XRD Trace

Pycnometry• Stoichiometric sample BF2 was shown to have a

density of 8.0314 ± 0.0162 g/cm2.

• Within the range of densities observed for the commonly occurring secondary phases Bi2Fe4O9 and Bi25FeO39.

• Fairly close to the theoretical density of phase pure BF.

Z’’ & M’’ Impedance Plots

Impedance Measurements for BF samples

Sample Slow CoolσB /µS cm-1

σGB /nS cm-1

CB / pF cm-1

CGB / pF cm-1

Bulk Ea/ eV

GB Ea/ eV

BF1 10.6 23.6 4.72 117 0.284 0.586BF2 10.7 0.927 5.32 68.1 0.304 0.728BF3 10.0 1.28 3.40 62.2 0.267 0.705Sample Quench

σB /µS cm-1

σGB /nS cm-1

CB / pF cm-1

CGB / pF cm-1

Bulk Ea/ eV

GB Ea/ eV

BF1 9.21 26.9 3.85 68.8 0.407 1.105BF2 84.5 477 7.92 693 0.188 0.526BF3 55.5 2010 7.02 624 0.320 0.565

Table 3: Approximate Electronic Property measurements obtained by extrapolation of Room Temperature Impedance Spectroscopy data

Interpreting Capacitance

Capacitance / F Cause10-12 Bulk

10-11 Minor, second phase

10-11 – 10-8 Grain boundary

10-10 - 10 -9 Bulk ferroelectric

10-9 – 10 -7 Surface Layer

10-7 – 10 -5 Sample-Electrode interface

10-4 Electrochemical reactions

Capacitance values and their possible interpretations i

i. J. T. S. Irvine, D. C. Sinclair, and A. R. West, Electroceramics : Characterization by Impedance Spectroscopy, Adv. Mater., 1990, 2 [3], 132-138.

Electrical Characterisation of Non-Stoichiometric Bismuth Ferrite Bi1±0

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