Multifunctional materials: the case of multiferroics. Can they revolutionize some ...€¦ · ·...

1

Multifunctional materials: the case of multiferroics. Can they revolutionize

some modern electronic components?

Michel Viret, Dorothée Colson, Delphine LebeugleDSM/IRAMIS/SPEC, CEA Saclay, France

Alexandra MouginLPS, University of Orsay, France

Delphine Lebeugle Séminaire ICMAB, 26 Octobre 2007

NNNNSSSS_+

Magnetization and electric polarization

Magnetic dipole → Magnetization Electric dipole → Polarization

MMMM PPPP

Magnetic fields and electric fields in nature :

3Ferroelectric and ferromagnetic materials

Ferroelectricity (T<TCE):Ps can be switched by Eappl

P

EcE

++

++

+Ps

-Ps

Ferromagnetism (T<TCM) :

MS can be reversed by Happ

M

+MS

HcH

- MS

M

+MS

HcH

- MS

+ _EEEE

---- PPPP

+P+P+P+P

_ +

NNNNSSSSBBBB

+M+M+M+M

----MMMM NNNN SSSS

4

Ferroelectric

Magnetic :

Ferro and ferrimagnetic (rare) AF spiral structures (the most common)

Magnetoelectric coupling

Introduction to multiferroic materials

Daniel Khomskii, Physics 2, 20 (2009)

Multiferroics:at the same time FE and FM

Rare materials

5

Review : First compounds ~1950

Few multiferroic materials (~60)

Perovskite-type structures ex : YMnO 3, BiFeO 3 …

Interests : Studying coupling between the ≠ variables

ex : coupling between ferroelectric and magnetic order parameters H influences PS

E influences M

Fundamental physics:

- understanding the origin of the coupling

- studying physics of magnetic oxides + electric fields

Technological applications:

- bifunctional materials : spintronics + ferroelectric technology

Possibility of switching P S by H (several examples) or M by E (rare)


6

_+

_ +

BBBB

+ P+ P+ P+ P

---- PPPP

NNNNSSSS

N S

EEEE

+ M+ M+ M+ M

---- MMMM

Magneto-electric coupling

E →→→→ M, H →→→→ P


7

Maxwell’s equations in vacuum linking E and B:


cjeB→

→

mjmq E

→→

spinjmq

mq−

ES

→ A spin current generates an electric field

8

In the solid state

Electric fields generated by:

1) Spin currents associated with charge currents

‘Anomalous Hall Effect’ in ferromagnetic metals

2) Spin currents without charge currents

Can occur in insulators!


9

V

d-orbitals d-orbitals

p-orbitals

P

sj

1e2e

12er

js = spin current

Katsura-Balatsky-Nagaosa PRL07

PS due to the magnetic structure

sjeP→→→

∝ 12 x


→ In non-colinear magnetic structures, a polarisation can exist

Problem: P is normally tiny!But: P and M are linked!

In a solid with non-colinear magnetism (generalised Dzyaloshinski-Moriya interactions):

Typically oxydes with distorted crystallographic cells : EDM = Dij . (Si x Sj)O2-

Fe3+Fe3+

Pi→

10

Ferroelectricity in spiral magnets

PS α q ij ×××× (Si x Sj) qij : unit vector connecting 2 sitesSi,j : local spins

→ Analogous to Dzialoshinskii-Moriya (DM) interaction

EDM = 0 if PS Q and PS e Q : propagation vectore : spin rotation axis

Spiral magnetic structure : (cycloidal or helicoidal)

P≠0 → Improper ferroelectricity

Mostovoy, PRL, 96, 067601 (2006)

Sergienko et al., PRB, 73, 094434 (2006)

Sinusoidal magnetic structure :

P=0 → No ferroelectricity


Technological point of view

Spintronics

Giant MagnetoResistance (GMR) →→→→ 2008 Nobel Prize : Albert Fert (Thalès)

http://www.research.ibm.com/research/demos/gmr/cybe rdemo1.htm

Bit 0

Bit 1

Discharged capacitor ↔ Bit 0Charge capacitor ↔ Bit 1

RAM (Random Access Memory) + Volatile

= SDRAM (Synchronous Dynamic Random Access Memory)

Technological point of view

Volatile capacitive memory :

0

0

0 0

1 1

1

1

0

0 0

1 0 10

1 0

10

10

0

0

10↔

working memory : volatile memory + perpetual discharge

RAM (Random Access Memory) + Non-volatile

= MRAM (Magnetic Random Access Memory)

Technological point of view : the future !!

: non-volatile memory + working memory : energy loss + writing defaults

Conclusion : we have to address the bits really individually and without any dissipation→→→→ Addressing with an electric field

14

Technological applications

Technological advances : - transducers, sensors

- spintronics (spin-filter, spin-transistor)

- information storage technology (FeRAM and MRAM)

→ encoding information in a single multiferroic bit : 4 states memory :

1 (+P,+ M)

2 (+P,- M)

3 (-P,+ M)

4 (-P,- M)


Gajek et al., Nature Materials, 6, 296 (2007)

→ data written electrically (low energy) and read magnetically

V

MNMMF

Au

→→→→ Low R→→→→ High RE

15

Switching PS with magnetic field in spiral magnets

P(H) in TbMn2O5 at 3K and 28K

Hur et al., Nature, 429, 392 (2004)

Low temperature

Weak effect

Existing materials

Switching P with H demonstrated

What about switching M with E?

16

(010)

[111]

The case of BiFeO3 : ferroelectric, ferroelelastic and magnetic at 300K

Ferroelectric properties (TC ~ 1090 K )

Cubic perovskite structure → pseudo-cubic : rhombohedral distortion PS due to Bi3+ and Fe3+ displacements along [111]

α = 89.47°

PS [111]

54 pm (Bi 3+)

13 pm (Fe3+)

Large atomic displacements →→→→ large P S

Kubel et al., Acta Cryst. B, 46 , 698 (1990)

17

λ = 64 nm

[10-1]

[111]

PS

e

q

Magnetic properties (TN ~ 640K)

Modulated antiferromagnetic structure

Cycloidal arrangement of the Fe3+ magnetic moments

PS ~ (e ×××× q) q : propagation vectore : spin rotation axis

The case of BiFeO3 : ferroelectric, ferroelelastic and magnetic at 300K

Sosnowska et al., J. of Phys. C, 15 , 4835 (1982)

18

Peritectic melting point → flux method

Composition : 0.8Bi2O3/0.2Fe2O3

2 impurities : Bi2Fe4O9 and Bi25FeO39

Importantly : Tcryst. < TCurie

Single crystal of BiFeO31.4 x 1.6 x 0.04 mm3 (SEM)

500 µm500 µm

Synthesis

600

40 60 80Fe2O3 Bi2O3

700

800

900

1000

20

Liquide (L)

Bi 2

Fe 4

O9

α α α α B

iFeO

3

Bi 4

0Fe 2

O63

L + Bi 2Fe4O9

L + αααα BiFeO 3

αααα BiFeO 3+

Bi 40Fe2O63

L + ββββ BiFeO 3

β β β β B

iFeO

3

% molaire

Tem

péra

ture

(°C

)

1 ferroelectric domain crystal

c

d

2 ferroelectric domains crystal

Polarized light optical microscopy

Ferroelectric studies

_ +---- PPPP

_++ P+ P+ P+ P

_++ P+ P+ P+ P

20

Polarization loop on BiFeO3 single crystals

-200

-100

0

100

200

-200 -100 0 100 200

Tension (V)

Cou

rant

(nA

)PS [111]

Eapp

54°

PS [111]Eapp

54°

Lebeugle et al., PRB, 76, 024116 (2007)

* Neaton, PRB, 71, 014113 (2005)

Charge current versus applied voltage :

High resistivity ρ(300K,100V) ~ 6.1010 Ω.cm

Very large PS~ 100 µC/cm² (BaTiO3 Ps ~ 25 µC/cm²) as theoretically predicted*

Ec ~ 12 kV/cm, Emax ~ 40 kV/cm

First P(E) at 300K on BiFeO3 single crystals

-75

-50

-25

0

25

50

75

-50 -40 -30 -20 -10 0 10 20 30 40 50

E (kV/cm)

P[0

10] (

µµ µµC/c

m²)∫∝ I.dtP

T = 300K

Ferroelectric properties

22

In neutron diffraction : 1 magnetic spiral = 2 satellites in reciprocal space

The crystals have become FE bi-domain and the magnetic structure has changed

Neutron diffraction and magnetoelectric study

λ = 64 nm

[10-1]

[111]

PS

e

q

λ = 64 nm

[10-1]

[111]

PS

e

q

4-circles neutron diffraction Super 6-T2 diffractometer / ORPHEE reactor / LLB, France

0.988

0.994

1

1.006

1.012

-0.012 -0.006 0 0.006 0.012

(ξ,0,−ξ)ξ,0,−ξ)ξ,0,−ξ)ξ,0,−ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

q1 = [δδδδ 0- δδδδ]

q2 = [0δδδδ- δδδδ]

a*c*

b*

q3 = [- δδδδ δδδδ 0]

0.988

0.994

1

1.006

1.012

-0.012 -0.006 0 0.006 0.012

(ξ,0,−ξ)ξ,0,−ξ)ξ,0,−ξ)ξ,0,−ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

0.988

0.994

1

1.006

1.012

-0.012 -0.006 0 0.006 0.012

(ξ,0,−ξ)ξ,0,−ξ)ξ,0,−ξ)ξ,0,−ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)


q2 = [0δδδδ- δδδδ]

a*c*

b*


Pic magnétique : (1/2, -1/2, 1/2) Pic magnétique : (1/2, -1/2, 1/2)

0.988

0.994

1

1.006

1.012

-0.012 -0.006 0 0.006 0.012

(ξ,0,−ξ)ξ,0,−ξ)ξ,0,−ξ)ξ,0,−ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

71°


q2 = [0 δδδδ- δδδδ]

a*c*

b*



0°

0.988

0.994

1

1.006

1.012

-0.012 -0.006 0 0.006 0.012

(ξ,0,−ξ)ξ,0,−ξ)ξ,0,−ξ)ξ,0,−ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

(ξ,0

,ξ)

71°


q2 = [0 δδδδ- δδδδ]

a*c*

b*



0°

EEEE

2 satellites in reciprocal spacein the virgin state

4 satellites in reciprocal spaceAfter application of E

23

Domain I

P [111]q1

Rotation plane : (-12-1) = P[111] × q1


24

Domain II

P [1-11]

q1

Rotation plane : (121) = P’[1-11] × q1


25

P [111]

P [1-11]

q1

E

Lebeugle et al., PRL100, 227602 (2008).


Evidence for the coupling between M and P at 300K by neutron diffraction on BiFeO3 single crystals.

26

In order to address a net M moment : FM layer on BFO crystal

BFO (AFM and FE at 300K ) + FM layer →→→→ compensated system

BFO// NiFe layer (10nm) // Protection layer of Au (4 nm)

[101]

[10-1]

(010)

[101]

[10-1]

(010)

Crystal of BFO 1x1x0.04 mm3

BiFeO crystal

Au (4nm)

3

NiFe (10nm)

Au (4nm)

Exchange coupling

27

Noguès et al., JMMM, 192, 203 (1999)

FM and AFM layers in contact :

• Enhancement of Hc

→→→→ Exchange coupling at interface.

Cooling under Hcool through TN :

→→→→ Exchange bias at interface :

• Shift or « bias » of hysteresis loop

• Unidirectional anisotropy

• Heb(θ) sinusoidal• Hc(θ) and Heb(θ) max along Hcool

I)

II)

III)

cool N

1

2

Hcool through TNcool N

1

2

Hcool through TN

Hc = f(θ)

Hc = f(θ)

Heb = f(θ)Heb

12

34

3 4

Unidirectional exchange coupling : exchange bias

28

• Roughness and formation of domain walls to the interface

• Requirement of uncompensated spins :

N = L² / a² L² : area of the AFM domain

a : cell parameter

Heb α - Jex/L

Small domain size : bias is higher (multidomain state is favorable)

In summary :

2 kinds of spins are involved :

1) Pinned uncompensated spins with strong anisotropy : Bias

2) Spins with weak anisotropy free to rotate : No bias but enhanced HC

Angular dependances of Hc and Heb evidence the nature of involved

spins and the strength of AFM anisotropy / EC.

Malozemoff model

Why Hebexp ≠ 0 in compensated systems ?

29

500 µm500 µm

Appearance of easy and hard axes at 90°from each ot her

Crystal in the virgin state

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

-15 -10 -5 0 5 10 15

H (mT)

M (a.u.)

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

-15 -10 -5 0 5 10 15

H (m T)

M (a.u.)

Hard axis

Easy axis

0

4

8

120

30

60

90

120

150180

210

240

270

300

330

0

4

8

121.2

0.8

0.4

0.0

0.4

0.8

1.2

Hsw

itch (

mT

)

Hswitch

Mrem

α (°)

Mre

m/M

s

30

H

H

++

++

++

++

++

++

--

--

--

--

- -- -

++

++

++

++

+ ++ +

(a) BFO

(b) Py

(c) Py

q

Uniaxial exchange due to a rippling pattern induced in the permalloy

31

E = 25 kV/cm

-30 -20 -10 0 10 20 30

-0.010

-0.005

0.000

0.005

0.010

Domains 1+2 Domain 1

α = 65°

Ker

r ro

tatio

n (°)

H (mT)

500 µm500 µm

Different regions where the ansotropy axes are at 9 0°between each other

After poling

0.0

0.4

0.8

1.20

30

60

90

120

150180

210

240

270

300

330

0.0

0.4

0.8

1.2

Mre

m/M

s

Domain 1 Domain 2

αααα (°)

0

5

10

15

030

60

90

120

150180

210

240

270

300

330

0

5

10

15

αααα (°)

Hsw

itch (

mT

)

Domain 1 Domain 2

32

Electric contrast

Magnetic contrast during H sweeps

The magnetic anisotropy is linked to the polarisati on domains

Exchange coupling

33

Mag2FE2

After several poling cycles, the exchange domains d o not correspond exactly to the polarisation domains

→→→→ Consistent with an exchange caused by the cycloids

34

Thin films:

Strain effects suppress the cycloid→ generates a global magnetic moment→ Can induce a linear Magneto-electic effect: αijMixPj→ Better for the magneto-electric coupling??

Perspectives

SrTiO3 (Substrate)BiFeO3 (AFM + FE)CoFeB (FM)Au (Protection)

SrTiO3 (Substrate)BiFeO3 (AFM + FE)CoFeB (FM)Au (Protection)

Laser

HFC

longitudinal magnetic hysteresis cycle (MOKE) of a CoFeB layer deposited on BFO/STO.

-150 -100 -50 0 50 100 150

-0.010

-0.005

0.000

0.005

0.010

Rot

atio

n K

err

(°)

H (Oe)

Hech

Magnetic exchange at the interface:

Exchange Bias

35Conclusion

Room-temperature multiferroics are a promising route to design magnetic/electric memories

Fully compensated crystals not ideal?Thin film?Substitution?

→ Make the AF a little magnetic…

→ Perspectives using Exchange Bias to make a device addressable by H and E fields.

36Addendum

Photovoltaic effect in BiFeO3T. Choi, S. Lee,* Y. J. Choi, V. Kiryukhin, S.-W. CheongSCIENCE VOL 324 3 APRIL 2009

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