Multiferroics for Spintronics

Brought to you by PITP (www.pitp.phas.ubc.ca) Multiferroics for spintronics

Manuel Bibes2

H. Béa1, M. Gajek1, X.-H. Zhu1, S. Fusil1, G. Herranz1, M. Basletic1, B. Warot-Fonrose3, S. Petit4, P. Bencok6, K. Bouzehouane1, C. Deranlot1, E. Jacquet1, A. Barthélémy1, J. Fontcuberta5 and A. Fert1

1 Unité Mixte de Physique CNRS / Thales, Orsay (FRANCE) 2 Institut d’Electronique Fondamentale, CNRS, Université Paris-Sud, Orsay (FRANCE) 3 CEMES, CNRS, Toulouse (FRANCE) 4 Laboratoire Léon Brillouin, CEA, Saclay (FRANCE) 5 ICMAB, CSIC, Campus de la UAB, Bellaterra (SPAIN) 6 European Synchrotron Radiation Facility, Grenoble (FRANCE)

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Ferromagnetic materials

Ferroelectric P Sensors E FeRAM

Partially filled d/f shells M Magnetic Data Storage H Ferroelastic Spintronics

Switchable parameter : M External stimulus : H

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Ferroelectric materials

P Lack of inversion symmetry Sensors E FeRAM

M Magnetic Data Storage H Ferroelastic Ferromagnetic Spintronics

Switchable parameter : P External stimulus : E

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Multiferroic materials

Ferroelectric P Lack of inversion symmetry Sensors E FeRAM

Partially filled d/f shells M Magnetic Data Storage H Ferroelastic Ferromagnetic Spintronics From Spaldin and Fiebig, Science 309, 391 (2005) ~ Multiple-state memories Switchable parameter : M, P ~ M switching by E External stimulus : H, E ~ P switching by H

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Insulating oxides : a classification

La0.1Bi0.9MnO3

NiFe2O4 BaTiO3 CoFe2O4 PZT EuO PbTiO3 BiMnO3 YMnO CoCr2O4 3

Tb Mn2O5

TbMnO3

BiFeO3

BiCrO3 Cr2O3 NiO PbZrO LaMnO3 3

LaFeO3

SrTiO3 MgO

TiO2 ZnO

Magnetically polarizable Electrically polarizable Magnetoelectric Ferro(i)magnetic Ferro(i)electric

Derived from Eerenstein, Mathur and Scott, Nature 442, 759 (2006)

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(Gd,Dy,Tb)MnO3 : spiral magnets CoCr2O4 : conical magnet Spiral ordering breaks inversion symmetry Conical ordering breaks inversion symmetry Î (improper) ferroelectricity appears Î (improper) ferroelectricity appears (but no finite magnetization) (with a finite magnetization)

Apply a magnetic field : change magnetic state Î switch polarization on/off

Kimura et al, PRB 71, 224425 (2005) Flip polarization direction by a magnetic field Yamasaki et al, PRL 96, 207204 (2006)

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Insulating oxides : a classification

La0.1Bi0.9MnO3

NiFe2O4 BaTiO3 CoFe2O4 PZT EuO PbTiO3 BiMnO3 YMnO CoCr2O4 3

Tb Mn2O5

TbMnO3

BiFeO3

BiCrO3 Cr2O3 NiO PbZrO LaMnO3 3

LaFeO3

SrTiO3 MgO

TiO2 ZnO

Magnetically polarizable Electrically polarizable Magnetoelectric Ferro(i)magnetic Ferro(i)electric

Derived from Eerenstein, Mathur and Scott, Nature 442, 759 (2006)

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BiFeO3

A room-temperature antiferromagnetic ferroelectric

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Rhombohedral Perovskite (R3c) a=3.96Å, α=89.4°

Ederer et Spaldin, Phys. Rev. B, 71, 060401 (R) (2005)

Neaton et al. Phys. Rev. B, 71, 014113 (2005) G-type Antiferromagnetic

Ferroelectric TN=640K Canted spins weak FM TC=1100K → M =0.01µ /f.u. PS=6µC/cm² (1970 paper, bad crystal) S B J. R. Teague et al., Solid State Commun., 8, 1073 (1970) Incommensurate cycloidal modulation

P. Fisher et al., J. Phys. C,13, 1931 (1980) PS=60µC/cm² (2007 paper, good crystal) D. Lebeugle, M. Viret et al, PRB in press Popov et al. in Magnetoelectric Interaction Pheno- Condmat/0706.0404 mena in Crystals (NATO Science Series, 2004) p. 277

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Wang et al., Science, 299, 1719 (2003) : BFO//STO (001)

3 t =200nm : P =55µC/cm² tBFO=70nm : MS=150emu/cm (~1µB/fu) BFO S 3 tBFO=400nm : Ms=5emu/cm (0.03µB/f.u.)

Claim of enhanced polarization and magnetization compared to bulk

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8 S S SS 10 B B tBFO=70nm B 6 B Pulsed laser deposition on SrTiO (001) 10 3 B 10-1 Target : Bi1.15FeO3

B 104 B B B B 6.10-3 Growth conditions : T=580°C, -3 B P =6.10 mbar 2 B O2 10 10-4

Tdep=580°C 8 Bi much more volatile than Fe : 10 SS St =70nm S BFO • low P or high T (Bi evaporates more than { { 6 Fe) 10 { B { 750°C ⇒ Fe oxides Intensity (counts) B

B • high P or low T (excess Bi) 4 B 10 ⇒Bi oxides B 580°C B 2 B B 10 520°C P=1.2 10-2 mbar 20 40 60 80 100 2θ (°)

B: BiFeO3, S: SrTiO3, *: γ-Fe2O3, |: α-Fe2O3, :Bi2O3

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Phase diagram T (K) dep 1000 900 800

10-1 Pr ese P nce ure of Pure BiFeO obtained in a very BiFe BiO 3 -2 O x narrow region around T =580°C 3 dep 10 and P =6.10-3mbar P O2 rese nc e of Appl. Phys. Lett.,87, 072508 (2005) FeO -3 x

P (mbar) 10

10-4 1,0 1,1 1,2 1,3 1000 / T (K-1) dep

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T (K) dep X-ray diffraction 1000 900 800 10-1 Pre sen P ce 8 ure of B

)

) B

)

) ) iF iO

1 e

3 10 4 T=580°C -2 O x

0 ) 3

0 10

(

P S

S S re 7 ( s

e n ) ce 10 B o

3 ) f F

0 e

) O

)

0 -3

)

4 x

(

0 P (mbar)

2 10

(

6 0

B -4

(

( 10 P =10 mbar :

B O

F 2

5 120nm -4 10 10

-3 P =10 mbar : 1,0 1,1 1,2 1,3 O 4 2 -1 10 1000 / T (K ) 100nm dep 120 3 10 60nm 100 120nm

Intensity (counts) Intensity 25nm 2 -4 2 1080 P=10-3mbar 10 25nm 60 20 40 60 80 100

40 B : BFO 2θ (°) S : STO Intensity (counts) Intensity (counts) F : γ-Fe2O3 1 10 20

92 93 94 94 96 95 96 98 97 100 98 102 99 22θθ (°)(°) Quantification of γ-Fe2O3 by XRD : not easily detectable

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SQUID measurements

150 ) 50 -3 P =10-3mbar : O2 75 25nm 40 60nm 100nm emu)

-5 30

0 P =6 10-3mbar O2 120nm 20 -75 P =10-4mbar M (10 M (emu.cm O2 10 120nm Tdep=580°C -150 SQUID, 10K 0 -6 -4 -2 0 2 4 6 01234 γ-Fe O peak area (u.a.) H (kOe) 2 3 3 BFO + γ-Fe2O3 : up to Ms~150emu/cm Magnetic moment comes from γ-Fe O 3 2 3 γ-Fe2O3 ferrimagnetic (Ms=430emu/cm ) 3 Pure BFO : Ms~2emu/cm

Phys. Rev. B, 74, 020101R (2006) BFO weak bulk-like ferromagnetic Similar conclusions : moment Eerenstein et al. Science, 307 1203a (2005) Wang et al., Science, 307, 1203b (2005)

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Neutron diffraction In R3c bulk BFO : * [003] H single peak → G-type antiferromagnet * [101] H peak with satellites → cycloidal modulation ⇒ Averaging to zero of the linear ME effect

* * In pseudo-cubic notation : [003] H → [½ ½ ½] C * [101] H → [-½-½½]*C from Sosnowska et al., J. Phys. C, 15, 4835 (1982)

BFO(240nm)//STO (001)

¾ G-type antiferromagnetic data 300 da ta fit order as in bulk BFO (a) 150 (c) fit simulated 250 peaks (SP) sum of SP ¾ No cycloidal modulation

200 100 contrary to bulk BFO

150 ¾ Linear magnetoelectric effect

100 50

IntensityIntensity (arb. (arb.units)units) allowed Intensity (arb. units) units) (arb. (arb. Intensity Intensity 50 Phil. Mag. Lett. 87, 165 (2007) 0 0 [½ ½ ½ ]* k=1.55Å-1 [-½ -½ ½]* k=1.2Å-1 (Special issue on multiferroic thin films 0.48 0.49 0.50 0.51 0.52 48 -0.52 -0.51 -0.50 -0.49 -0.48 Eds. N.D. Mathur and MB) qh=qk=-ql q h=qk=q l

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Polarization loops Piezoelectric loops

100 20 80 15 60 10 40 5 20 0 0 -5 -20 -40 -10 d33 (pm/V) d33 -60 -15

Polarization (µC/cm²) -80 -20 -100 -25 -10-8-6-4-20246810 -30 Voltage (V) -6 -4 -2 0 2 4 6 Vdc(Volts) Piezoresponse force microscopy

60 nm 2 nm ¾ BFO films are ferroelectric with a large polarization (~70 µC/cm²) and piezoelectric

with a d33 coefficient of ~20 pm/V ¾ Ferroelectricity is preserved down to 2 nm.

Jpn. J. of Appl. Phys., 45, L187 (2006)

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Principle : voltage-controlled exchange bias

AF-FE

Bottom electrode

Binek et Doudin, JPCM, 17, L39 (2005) Spin-valve on top of a multiferroic film (FE and AFM) Magnetic tunnel junction with multiferroic barrier (FE and AFM) Electric field control of the junction resistance state

Prerequisites : 1. observe robust exchange bias at room-temperature 2. spin-dependent tunneling through multiferroic barrier 3. switch exchange bias direction by E-field (magnetoelectric coupling)

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CoFeB//SiO2 Au CoFeB deposited by sputtering 50 (a) H =42Oe, H =-62Oe CoFeB 5nm c e 0 Appl. Phys. Lett.,89, 242114 (2006) BFO 35nm M (µemu) M (µemu) -50

-300 -200 -100 0 100 200 300 H (Oe)

50 Hdep 50 Hdep 50 Hdep ⊗ Hmeas Hmeas Hmeas

0 0 0 M (µemu) M M (µemu) M AGFM M (µemu) -50 -50 AGFM -50 AGFM T=300K T=300K T=300K -300 -200 -100 0 100 200 300 -300 -200 -100 0 100 200 300 -300 -200 -100 0 100 200 30 0 H (Oe) H (Oe) () (Oe) H (Oe) 90 (e) Robust room temperature 80 20 exchange bias between BFO and | (Oe) | (Oe) | (Oe) | (Oe) cccc

|H|H|H|H 10 CoFeB 0 01234567891011 Cycle number With NiFe : J. Dho et al., Adv. Mat., 18, 1445 (2006)

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22,588 300K Au CoFeB 22,586 Cu

CoFeB R (Ohm)

BFO 22,584

-150 -100 -50 0 50 100 150 H (Oe)

¾ GMR measured on top of BFO at RT ¾ Exchange bias has shifted the GMR curve

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800 1.2 (a) 20 1.0 760 3K O2 growth CoO 2.5nm 15 pressure : Co 12.5nm 10mV 0.8 6.10-3 mbar BFO 5nm 720 10 (arb. (arb. units)units) 0.6

0.46 mbar LSMO 15nm 3K3K R (kOhm)R (kOhm) TMR TMR (%) (%) 0.4 5 680 T* 0.2 TMRTMR / / TMRTMR 0 S~30x30µm² (b) 640 0.0 -3-2-10123 0 50 100 150 200 250 H (kOe) T (K) Appl. Phys. Lett., 89, 242114 (2006)

(exchange bias due to CoO, not to BFO)

Positive TMR up to ~30% ¾ rather large value for an AFM barrier TMR vanishes around 200K : ¾ positive spin polarization at Co/BFO interface Local deoxygenation of LSMO

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100 4,5 S~50x50nm² 10mV 80 4,0 3K O2 growth CoO 2.5nm pressure : Co 12.5nm 60 -3 3,5 6.10 mbar BFO 5nm 40 0.46 mbar STO 1.6nm 3,0 0.46 mbar LSMO15nm R (MOhm) 20 TMR (%)

2,5 0 120

2,0 100 -20 -2 -1 0 1 2 80 60

H (kOe) * 40 T Positive TMR up to ~100% (%) TMR ¾ larger value than without STO 20 ¾ polarization at Co/BFO interface still positive 0 ¾ thanks to STO, better quality of the upper LSMO 0 50 100 150 200 250 300 Temperature (K) interface see Appl. Phys. Lett., 82, 233 (2003)

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16 -50

-40 STO 14 -30

12 -20

TMR (%) -10 LAO 10 0 TMR (%) 8 10 -2 -1 0 1 2 V (V) 6 DC

-1000 -500 0 500 1000 Bias dependence of TMR in Bias (mV) LSMO/STO/Co and LSMO/LAO/Co junctions Science 286, 507 (1999) APL 87, 212501 (2005)

¾ TMR is positive and decreases symmetrically with bias voltage ¾ Behaviour is completely different from the case of LSMO/STO/Co and LSMO/LAO/Co junctions ¾ Possible reasons : oxygen vacancies at LSMO/BFO interface, symmetry filtering, etc

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Domain structure and magnetoelectric switching

AF plane

Zhao et al, Nat. Materials (2006)

P along <111> directions : When an electric field is applied, P can be switched 8 possible variants to different directions Only some of them yield a rotation of the AF plane

Important to know and control the ferroelectric domain structure

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Domain structure

Piezoresponse force microscopy (PFM) (001) films (111) films (001) film (001) film OP OP 0.8° miscut 4° miscut IP IP

IP IP

8 variants are only 4 variants present are present Das et al., APL, 88, 242904 (2006) Domain structure can be controlled by playing with the substrate miscut angle 8 variants are only 4 variants and/or orientation. present are present Ideally, one type of domain orientation would be required, with the appropriate polarization switching mechanism (109° ?) Centre National de Multiferroics for spintronics Manuel Bibesla Recherche Quantum Nanoscience with Spins, Asilomar 3-6 June, 2007 24 Scientifique Brought to you by PITP (www.pitp.phas.ubc.ca)

(La,Bi)MnO3

A ferromagnetic ferroelectric

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Ferromagnetic tunnel barriers: the spin-filter effect

Non Magn. Ferro. Ferro. Metal Insulator Half Metal Since the tunnel transmission ϕ depends exponentially on the barrier 0 Parallel Config. height, a highly-spin polarized Δϕexch current is generated by the barrier.

large I, low R Δϕ (ϕ ±− exch ) 2/1 0 2 ↑↓ ∝ eJ

Expected TMR :

2 PP TMR = BE 1− PP BE − JJ Antiparallel Config. ↓↑ PB = + JJ ↓↑ low I, large R

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Spin-filters

J.S. Moodera et al, PRB 42, 8235 (1990) P. LeClair et al, APL 80, 625 (2002)

Au/EuS/Al spin-filter Superconducting Al is used the spin-analyzer

~ Early spin-filtering experiments focused on Eu chalcogenides Au/EuS/Gd spin-filter ~ Large spin-filtering efficiency (>90%) Ferromagnetic Gd is used as the spin-analyzer have been measured ~ Limited by low Tc of EuX compounds Î Ferromagnetic oxides

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Crystal structure Magnetic ordering

c ~ Unusual orbital ordering resulting from

Bi6s-O2p interaction Moreira dos Santos et al, PRB 66 064425 (2002) Mn +3 d4 Î Ferromagnetic order z ~ T =105K, M =3.6 µ y x C s B O -2

b Bi +3 Electronic structure

~ Synthesis at high pressure ~ Distorted perovskite structure Ferromagnetic insulator with

(Monoclinic symmetry) Eg↑ = 0.5 eV and Eg↓ = 2.6 eV ~ Polar structure Î Ferroelectric (?) Son et al, APL 84, 4971 (2004) ~ « Magnetodielectric effect » observed in bulk samples Kimura et al, PRB 67, 180401 (2003) Shishidou et al, JPCM 2005

~ Partial substitution of Bi by La : lower synthesis pressure, stabilization of perovskite phase Troyanchuk et al, Low. Temp. Phys. 28, 569 (2002).

Î Growth of BiMnO3 and La0.1Bi0.9MnO3 thin films

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Growth by PLD Magnetic properties

~ KrF laser at 2 Hz ~ O Pressure: 10-1 mbar. 2 300 BMO x=0 ; 30nm ~ Fluence: 2 J/cm². ~ Tdep from 575°C to 700°C LBMO x=0.1 ; 30nm 200 10K

) 100 Single phase films only in a very narrow window 3

La-substitution helps to stabilize the perovskite phase 0

300 -100 LBMO

) 150 See for details : PRB 75, 174417 (2007) 3 7 nm

M (emu/cm -200 JAP 97 103909 (2005) 0 -300 -150 M (emu/cm -300 -10-8-6-4-20 2 4-10 6 -5 810 0 5 10 H(kOe) H (kOe)

180 BMO x=0 ; 30nm LBMO x=0.1 ; 30nm ~ BMO films have a reduced moment compared to bulk 160 ~ T close to bulk (105K) 140

C ) ~ Substitution by La further reduces max. moment and -3 120 slightly decreases T (as in bulk) 100 C ~ Low magnetic moment likely due to Bi vacancies 80 ~ Very thin films are also ferromagnetic 60

M (emu.cm 40 20 0 0 50 100 150 200 T (K)

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Ferroelectric properties

LBMO(30nm)/LSMO 100 PFM image after 180° 50 writing stripes at +/-4V 0

) 2 -1 1

-50 0 -1 (pm.V Phase (deg)

33 -2 -100 d -4 -2 0 2 4 0° V (V) DC -4 -2 0 2 4 6 V (V) DC

180° LBMO(2nm)/LSMO

0°

LBMO films as thin as 2 nm are still ferroelectric at room temperature Nature Materials 6, 196 (2007)

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Tunnel magnetoresistance

nanojunction LSMO / STO (1 nm) / BMO (4 nm) / Au AuAu

20 Thick resist Junction #1 50 Thin resist I(V) 0 BMO (4nm) I (nA)

) -50 STO (1nm) LSMO Ω -0.4 0.0 0.4

15 V (V) DC BMO Junctions defined by nano-indentation lithography R (M TMR ~ 50% TMR LSMO Nanoletters 3, 1599 (2003)

10 VDC=10mV, T=3K 1.4 Junction #2 1.2 3K 1 ) 8K ~ Large TMR at low temperature

Ω 0.8

Î spin-filtering by the BMO layer 0.6 30K 20K ~ Polarization induced by BMO : 22% R (M ~ Spin-filter effect vanishes at T < TC 0.4 60K -6 -4 -2 0 2 4 6 H (kOe) PRB 72 020406(R) (2005)

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Tunnel magnetoresistance

LSMO / STO (1 nm) / LBMO (4 nm) / Au

200

0.2 4K 0.1

0.0

-0.1

I (µA)

150 -0.2 -0.5 0.0 0.5 ~ Polarization induced by LBMO : 35% V (V) ~ TMR decreases rapidly with bias DC TMR = 90% ) Magnons ? Defects ? Ω

10 mV 100

R (M

20 mV 50

50 mV

-6 -4 -2 0 2 4 H (kOe)

JAP 99, 08E504 (2006)

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Tunnel electroresistance

LSMO / LBMO (2 nm) / Au 25 E (MV.cm-1) -3-2-10123 20 40 (a) 15 20 10 0 I (µA)I (µA) TER (%) TER -20 5

600 -40 400 0 2.0 -2 -1 0 1 2

I (µA)I (µA) 200

0 V (V) 1.5 0.0 0.5 1.0 1.5 2.0 DC ))

-1-1 V (V) DC

ΩΩ ~ A ~20% electroresistance effect is observed -4-4 1.0 ~ TheshapeoftheG(V) curvesuggestsdirect tunneling 0.5 G (10 G (10 (b) T=4K 0.0 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 What is the origin of the TER ? V (V) DC Nature Materials 6, 196 (2007)

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