Multiferroics for spintronics Manuel Bibes

Transcription

1 Multiferroics for spintronics Manuel Bibes 2 H. Béa 1, M. Gajek 1, X.-H. Zhu 1, S. Fusil 1, G. Herranz 1, M. Basletic 1, B. Warot-Fonrose 3, S. Petit 4, P. Bencok 6, K. Bouzehouane 1, C. Deranlot 1, E. Jacquet 1, A. Barthélémy 1, J. Fontcuberta 5 and A. Fert 1 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) 1

2 Ferroics and multiferroics Ferromagnetic materials Ferroelectric P E Sensors FeRAM Ferroelastic M H Partially filled d/f shells Magnetic Data Storage Spintronics Switchable parameter : M External stimulus : H 2

3 Ferroics and multiferroics Ferroelectric materials P Lack of inversion symmetry Sensors E FeRAM Ferroelastic Ferromagnetic M H Magnetic Data Storage Spintronics Switchable parameter : P External stimulus : E 3

4 Ferroics and multiferroics Multiferroic materials Ferroelectric P Lack of inversion symmetry Sensors E FeRAM Ferroelastic Ferromagnetic From Spaldin and Fiebig, Science 39, 391 (25) Switchable parameter : M, P External stimulus : H, E M H Partially filled d/f shells Magnetic Data Storage Spintronics Multiple-state memories M switching by E P switching by H 4

5 Insulating oxides : a classification La.1 Bi.9 MnO 3 NiFe 2 O 4 CoFe 2 O 4 EuO BiMnO 3 CoCr 2 O 4 BaTiO 3 PZT PbTiO 3 YMnO 3 Tb Mn 2 O 5 TbMnO 3 BiFeO 3 BiCrO 3 Cr 2 O 3 NiO LaMnO 3 PbZrO 3 LaFeO 3 TiO 2 ZnO SrTiO 3 MgO Magnetically polarizable Ferro(i)magnetic Electrically polarizable Ferro(i)electric Magnetoelectric Derived from Eerenstein, Mathur and Scott, Nature 442, 759 (26) 5

6 Multiferroics - examples of magnetoelectric coupling (Gd,Dy,Tb)MnO 3 : spiral magnets Spiral ordering breaks inversion symmetry (improper) ferroelectricity appears (but no finite magnetization) CoCr 2 O 4 : conical magnet Conical ordering breaks inversion symmetry (improper) ferroelectricity appears (with a finite magnetization) Apply a magnetic field : change magnetic state switch polarization on/off Kimura et al, PRB 71, (25) Flip polarization direction by a magnetic field Yamasaki et al, PRL 96, 2724 (26) 6

7 Insulating oxides : a classification La.1 Bi.9 MnO 3 NiFe 2 O 4 CoFe 2 O 4 EuO BiMnO 3 CoCr 2 O 4 BaTiO 3 PZT PbTiO 3 YMnO 3 Tb Mn 2 O 5 TbMnO 3 BiFeO 3 BiCrO 3 Cr 2 O 3 NiO LaMnO 3 PbZrO 3 LaFeO 3 TiO 2 ZnO SrTiO 3 MgO Magnetically polarizable Ferro(i)magnetic Electrically polarizable Ferro(i)electric Magnetoelectric Derived from Eerenstein, Mathur and Scott, Nature 442, 759 (26) 7

8 BiFeO 3 A room-temperature antiferromagnetic ferroelectric 8

9 BiFeO 3 : bulk properties Rhombohedral Perovskite (R3c) a=3.96å, α=89.4 Ederer et Spaldin, Phys. Rev. B, 71, 641 (R) (25) Neaton et al. Phys. Rev. B, 71, (25) Ferroelectric T C =11K P S =6µC/cm² (197 paper, bad crystal) J. R. Teague et al., Solid State Commun., 8, 173 (197) P S =6µC/cm² (27 paper, good crystal) D. Lebeugle, M. Viret et al, PRB in press Condmat/76.44 G-type Antiferromagnetic T N =64K Canted spins weak FM M S =.1µ B /f.u. Incommensurate cycloidal modulation P. Fisher et al., J. Phys. C,13, 1931 (198) Popov et al. in Magnetoelectric Interaction Phenomena in Crystals (NATO Science Series, 24) p

10 BiFeO 3 : thin films properties Wang et al., Science, 299, 1719 (23) : BFO//STO (1) t BFO =2nm : P S =55µC/cm² t BFO =7nm : M S =15emu/cm 3 (~1µ B /fu) t BFO =4nm : M s =5emu/cm 3 (.3µ B /f.u.) Claim of enhanced polarization and magnetization compared to bulk 1

11 Growth of BiFeO 3 thin films Intensity (counts) B B B B B S S B S t S BFO =7nm B B T dep =58 C B B B B B S S S t S BFO =7nm P= mbar B B B 2θ ( ) B: BiFeO 3, S: SrTiO 3, *: γ-fe 2 O 3, : α-fe 2 O 3, :Bi 2 O 3 75 C 58 C 52 C B B B B Pulsed laser deposition on SrTiO 3 (1) Target : Bi 1.15 FeO 3 Growth conditions : T=58 C, P O2 =6.1-3 mbar Bi much more volatile than Fe : low P or high T (Bi evaporates more than Fe) Fe oxides high P or low T (excess Bi) Bi oxides 11

12 Growth of BiFeO 3 thin films T dep (K) Phase diagram P (mbar) Pure BiFeO 3 Presence of FeO x Presence of BiO x Pure BiFeO 3 obtained in a very narrow region around T dep =58 C and P O2 =6.1-3 mbar Appl. Phys. Lett.,87, 7258 (25) 1-4 1, 1,1 1,2 1,3 1 / T dep (K -1 ) 12

13 Growth of BiFeO 3 thin films Parasitic Fe-rich phases Intensity (counts) B (1) S (1) F (21) X-ray diffraction F (4) B (2) S (2) F (42) B : BFO S : STO 2θ ( ) F : γ-fe 2 O 3 B (3) S (3) F (8) B (4) S (4) T=58 C P O2 =1-4 mbar : 12nm P O2 =1-3 mbar : 1nm 6nm 25nm Intensity (counts) nm 25nm P= mbar θ ( ) Quantification of γ-fe 2 O 3 by XRD : not easily detectable P (mbar) T dep (K) Pure BiFeO 3 Presence of FeO x Presence of BiO x 1, 1,1 1,2 1,3 1 / T dep (K -1 ) 13

14 Growth of BiFeO 3 thin films Parasitic Fe-rich phases M (emu.cm -3 ) P O2 =1-3 mbar : 25nm 6nm 1nm P O2 =1-4 mbar 12nm H (koe) SQUID measurements P O2 =6 1-3 mbar 12nm T dep =58 C SQUID, 1K BFO + γ-fe 2 O 3 : up to M s ~15emu/cm 3 γ-fe 2 O 3 ferrimagnetic (M s =43emu/cm 3 ) Pure BFO : M s ~2emu/cm 3 M (1-5 emu) γ-fe 2 O 3 peak area (u.a.) Magnetic moment comes from γ-fe 2 O 3 Phys. Rev. B, 74, 211R (26) Similar conclusions : Eerenstein et al. Science, a (25) Wang et al., Science, 37, 123b (25) BFO weak bulk-like ferromagnetic moment 14

15 (1) BiFeO 3 films Magnetic properties Neutron diffraction In R3c bulk BFO : [3] * H single peak G-type antiferromagnet [11] * H peak with satellites cycloidal modulation Averaging to zero of the linear ME effect from Sosnowska et al., J. Phys. C, 15, 4835 (1982) In pseudo-cubic notation : [3] * H [½ ½ ½]* C [11] * H [-½-½½]* C Intensity (arb. units) (a) BFO(24nm)//STO (1) da ta fit [½ ½ ½ ]* k= 1.55Å (c) q h =q k =q l q h =q k =-q l Intensity (arb. units) 5 data fit simulated peaks (SP) sum of SP [-½ -½ ½]* k=1.2å -1 G-type antiferromagnetic order as in bulk BFO No cycloidal modulation contrary to bulk BFO Linear magnetoelectric effect allowed Phil. Mag. Lett. 87, 165 (27) (Special issue on multiferroic thin films Eds. N.D. Mathur and MB) 15

16 (1) BiFeO 3 films Ferroelectric properties Polarization loops Piezoelectric loops Polarization (µc/cm²) Voltage (V) Piezoresponse force microscopy d33 (pm/v) Vdc(Volts) 6 nm 2 nm BFO films are ferroelectric with a large polarization (~7 µc/cm²) and piezoelectric with a d 33 coefficient of ~2 pm/v Ferroelectricity is preserved down to 2 nm. Jpn. J. of Appl. Phys., 45, L187 (26) 16

17 Examples of devices Principle : voltage-controlled exchange bias V AF-FE Bottom electrode Binek et Doudin, JPCM, 17, L39 (25) Magnetic tunnel junction with multiferroic barrier (FE and AFM) Spin-valve on top of a multiferroic film (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) 17

18 Exchange bias with BiFeO 3 films Au CoFeB 5nm BFO 35nm CoFeB deposited by sputtering H c =42Oe, H e =-62Oe Appl. Phys. Lett.,89, (26) M (µemu) 5-5 CoFeB//SiO 2 (a) H (Oe) M (µemu) H c (Oe) 5-5 H dep Hmeas AGFM T=3K (e) M (µemu) -5 H (Oe) ( ) (Oe) 5 H dep Hmeas H (Oe) Cycle number AGFM T=3K M (µemu) 5-5 H dep Hmeas H (Oe) AGFM T=3K Robust room temperature exchange bias between BFO and CoFeB With NiFe : J. Dho et al., Adv. Mat., 18, 1445 (26) 18

19 Exchange bias with BiFeO 3 films 22,588 3K Au CoFeB Cu CoFeB BFO R (Ohm) 22,586 22, H (Oe) GMR measured on top of BFO at RT Exchange bias has shifted the GMR curve 19

20 Tunnel junctions with BiFeO 3 barriers O 2 growth pressure : mbar.46 mbar CoO 2.5nm Co 12.5nm BFO 5nm LSMO 15nm R (kohm) S~3x3µm² H (koe) (a) 3K 1mV TMR (%) TMR / TMR 3K (arb. units) (b) T (K) Appl. Phys. Lett., 89, (26) T* (exchange bias due to CoO, not to BFO) Positive TMR up to ~3% rather large value for an AFM barrier positive spin polarization at Co/BFO interface TMR vanishes around 2K : Local deoxygenation of LSMO 2

21 Tunnel junctions with BiFeO 3 barriers R (MOhm) 4,5 4, 3,5 3, 2,5 S~5x5nm² 1mV 3K 1 2, H (koe) Positive TMR up to ~1% larger value than without STO polarization at Co/BFO interface still positive thanks to STO, better quality of the upper LSMO interface see Appl. Phys. Lett., 82, 233 (23) TMR (%) TMR (%) 12 1 O 2 growth pressure : mbar.46 mbar.46 mbar CoO 2.5nm Co 12.5nm BFO 5nm STO 1.6nm LSMO15nm Temperature (K) T * 21

22 Tunnel junctions with BiFeO 3 barriers STO TMR (%) 12 1 TMR (%) LAO V DC (V) Bias (mv) Bias dependence of TMR in LSMO/STO/Co and LSMO/LAO/Co junctions Science 286, 57 (1999) APL 87, (25) TMR is positive and decreases symmetrically with bias voltage Behaviour is completely different from the case of LSMO/STO/Co and LSMO/LAO/Co junctions Possible reasons : oxygen vacancies at LSMO/BFO interface, symmetry filtering, etc 22

23 Magnetoelectric switching Domain structure and magnetoelectric switching AF plane Zhao et al, Nat. Materials (26) P along <111> directions : 8 possible variants When an electric field is applied, P can be switched to different directions Only some of them yield a rotation of the AF plane Important to know and control the ferroelectric domain structure 23

24 (1) BiFeO 3 films Ferroelectric properties Domain structure Piezoresponse force microscopy (PFM) (1) films (111) films (1) film.8 miscut (1) film 4 miscut OP OP IP IP IP IP 8 variants are present only 4 variants are present Das et al., APL, 88, (26) Domain structure can be controlled by playing with the substrate miscut angle and/or orientation. 8 variants are present only 4 variants are present Ideally, one type of domain orientation would be required, with the appropriate polarization switching mechanism (19?) 24

25 (La,Bi)MnO 3 A ferromagnetic ferroelectric 25

26 Ferromagnetic tunnel barriers Ferromagnetic tunnel barriers: the spin-filter effect Non Magn. Ferro. Metal Insulator ϕ Δϕ exch Ferro. Half Metal Parallel Config. large I, low R Since the tunnel transmission depends exponentially on the barrier height, a highly-spin polarized current is generated by the barrier. J e Δϕ 2 exch 1/ 2 ( ϕ ± ) Expected TMR : TMR 2PE PB = 1 P P E B Antiparallel Config. low I, large R P B = J J + J J 26

27 Ferromagnetic tunnel barriers Spin-filters J.S. Moodera et al, PRB 42, 8235 (199) P. LeClair et al, APL 8, 625 (22) Au/EuS/Al spin-filter Superconducting Al is used the spin-analyzer Early spin-filtering experiments focused on Eu chalcogenides Large spin-filtering efficiency (>9%) have been measured Limited by low Tc of EuX compounds Au/EuS/Gd spin-filter Ferromagnetic Gd is used as the spin-analyzer Ferromagnetic oxides 27

28 BiMnO 3 Crystal structure Magnetic ordering c z y x Mn +3 d 4 Unusual orbital ordering resulting from Bi 6s -O 2p interaction Moreira dos Santos et al, PRB (22) Ferromagnetic order T C =15K, M s =3.6 µ B O -2 b Bi +3 Electronic structure a Synthesis at high pressure Distorted perovskite structure (Monoclinic symmetry) Polar structure Ferroelectric (?) Son et al, APL 84, 4971 (24) «Magnetodielectric effect» observed in bulk samples Kimura et al, PRB 67, 1841 (23) Ferromagnetic insulator with E g =.5 ev and E g = 2.6 ev Shishidou et al, JPCM 25 Partial substitution of Bi by La : lower synthesis pressure, stabilization of perovskite phase Troyanchuk et al, Low. Temp. Phys. 28, 569 (22). Growth of BiMnO 3 and La.1 Bi.9 MnO 3 thin films 28

29 La 1-x Bi x MnO 3 thin films Growth by PLD KrF laser at 2 Hz Fluence: 2 J/cm². Single phase films only in a very narrow window La-substitution helps to stabilize the perovskite phase See for details : O 2 Pressure: 1-1 mbar. T dep from 575 C to 7 C PRB 75, (27) JAP (25) M (emu/cm 3 ) K Magnetic properties BMO x= ; 3nm LBMO x=.1 ; 3nm H(kOe) H (koe) M (emu/cm 3 ) LBMO 7 nm BMO films have a reduced moment compared to bulk T C close to bulk (15K) Substitution by La further reduces max. moment and slightly decreases T C (as in bulk) Low magnetic moment likely due to Bi vacancies Very thin films are also ferromagnetic M (emu.cm -3 ) T (K) BMO x= ; 3nm LBMO x=.1 ; 3nm 29

30 La.1 Bi.9 MnO 3 thin films Ferroelectric properties LBMO(3nm)/LSMO 1 PFM image after writing stripes at +/-4V 18 Phase (deg) d 33 (pm.v -1 ) V DC (V) V DC (V) 18 LBMO(2nm)/LSMO LBMO films as thin as 2 nm are still ferroelectric at room temperature Nature Materials 6, 196 (27) 3

31 BiMnO 3 -based junctions Tunnel magnetoresistance R (MΩ) R (MΩ) LSMO / STO (1 nm) / BMO (4 nm) / Au Junction #1 3K 3K 8K TMR ~ 5% 2K I (na) K H (koe) 5 V DC (V) BMO LSMO V DC =1mV, T=3K Junction #2 PRB (R) (25) Thick resist Thin resist BMO (4nm) STO (1nm) Au Au LSMO nanojunction Junctions defined by nano-indentation lithography Nanoletters 3, 1599 (23) I(V) Large TMR at low temperature spin-filtering by the BMO layer Polarization induced by BMO : 22% Spin-filter effect vanishes at T < T C 31

32 LBMO-based junctions Tunnel magnetoresistance LSMO / STO (1 nm) / LBMO (4 nm) / Au R (MΩ) I (µa) mv V DC (V) 4K TMR = 9% Polarization induced by LBMO : 35% TMR decreases rapidly with bias Magnons? Defects? 5 2 mv 5 mv H (koe) JAP 99, 8E54 (26) 32

33 LBMO-based junctions Tunnel electroresistance I (µa) G (1-4 Ω -1 ) E (MV.cm -1 ) (a) (b) LSMO / LBMO (2 nm) / Au I (µa) V DC (V) T=4K V DC (V) TER (%) A ~2% electroresistance effect is observed TheshapeoftheG(V) curvesuggestsdirect tunneling Nature Materials 6, 196 (27) V DC (V) What is the origin of the TER? 33

34 Ferroelectric tunnel barriers Tunneling through a ferroelectric tunnel barrier From Tsymbal and Kohlstedt Science 313, 181 (26) Several mechanisms leading to current modulation upon polarization reversal 34

35 Ferroelectric tunnel barriers Tunneling through a ferroelectric tunnel barrier From Tsymbal and Kohlstedt Science 313, 181 (26) Several mechanisms leading to current modulation upon polarization reversal 35

36 Ferroelectric tunnel barriers Tunneling through a ferroelectric tunnel barrier From Tsymbal and Kohlstedt Science 313, 181 (26) Several mechanisms leading to current modulation upon polarization reversal 36

37 Ferroelectric tunnel barriers Tunneling through a ferroelectric tunnel barrier From Tsymbal and Kohlstedt Science 313, 181 (26) Several mechanisms leading to current modulation upon polarization reversal 37

38 Ferroelectric tunnel barriers Tunneling through a ferroelectric tunnel barrier Φ P Φ + Large average barrier height low tunnel current E F LSMO Au E F LSMO Au P Φ - Low average barrier height large tunnel current LSMO Au Hysteretic I(V) curves are expected An electroresistance effect should occur upon polarization reversal See details in Zhuravlev et al, PRL 94, (25) 38

39 LBMO-based junctions Combination of tunnel electroresistance and magnetoresistance 18 1mV 3K 1 LBMO LSMO 1 Au 3 R (kω) After +2V After -2V H (koe) 2 4 R(kΩ) 2 4 After 1.5V After +1.5V After 1.5V After +1.5V H (koe) H (koe) H (koe) H (koe) R (kω) Demonstration of a 4-resistance state system with a multiferroic barrier Effect was observed on several junctions and is reproducible Nature Materials 6, 196 (27) 39

40 LBMO-based junctions Temperature dependence of TER and TMR TMR (%) T CM 1 1 Bulk BMO T CE TER (%) T (K) Temperature dependence of TER and TMR consistent with an origin related to to ferroelectricity and ferromagnetism, respectively 4

41 Conclusions and perspectives BiFeO 3 Single phase films can be grown in a narrow range of P and T They are G-type antiferromagnetic and ferroelectric at RT LSMO/BFO/Co junctions show a large positive TMR BFO induced exchange bias on CoFeB (La,Bi)MnO 3 based junctions La.1 Bi.9 MnO 3 films are ferromagnetic and ferroelectric LBMO junctions show TMR and electroresistance (4 resistance states) We suggest electroresistance effect due to ferroelectric switching in the barrier Control of the magnetic state by electric field? Perspectives : Magnetization manipulation with an electric field at RT New materials, ideally ferromagnetic multiferroics with high T C s, M S and P are required See review on Oxide Spintronics by M. Bibes and A. Barthélémy, IEEE Trans. Electron Devices 54, 13 (27) Financial support by EU Streps Nanotemplates and MaCoMuFi, EU Marie-Curie program, ESF Thiox program, French ANR program, Spanish Ministry for Research and EGIDE 41