10. Magnetoelectric Switching

Transcription

1 Beyond CMOS computing 10. Magnetoelectric Switching Dmitri Nikonov 1

2 Outline Magnetoelectric effect to improve spintronic switching Review of experiments on magnetoelectric switching: Magnetostrictive, Multiferroic, Surface anisotropy. Simulations to understand the effect and its use for fast switching 2

3 Spintronics in Need of Improvement current electrode free FM layer tunneling barrier fixed FM layer electrode STTRAM switched by spin transfer torque t stt em v g P I I B s nm ~ 0.4 c ns Switching time, optimistic E IV t ~ 50 A 0.1V 0.4ns 2 fj stt dd stt Switching Energy, optimistic Joule heat = bottleneck for spin torque energy 3

4 Energy, fj Benchmarks for Beyond-CMOS Devices 10 2 INVFO4 Constant Energy*Delay Inplane spin torque 10 1 Electronics Perp spin torque 10 0 GpnJ 10-1 CMOS HP 10-2 HetJTFET HomJTFET CMOS LP Delay, ps How to reduce delay and energy for spintronics?! 4

5 Pathways to Improve Magnetization Switching Spin torque, in-plane M Spin torque, perp. M 1. Spin Hall effect 2. Magnetoelectric effect, Focus of this lecture Researching new physics to improve spintronics 5

6 Magnetoelectric (ME) Effect Theory F electric magnetic 2 2 E / 2 H / 2 coupling EH 0 0 Energy D displacement 0 E H induction B 0 H E dd d dh direct Maxwell s equations db c de converse Should be equal. In typical materials, direct is 2-4x larger. α 2 μμ 0 εε 0 = εμ/c 2 units of s/m can be normalized to c=speed of light Magnetic and electric can be coupled, how to implement? 6

7 Coupling of Computational Variables Electronics E,I d χ E P α Straintronics σ S ε l M χ M Magnetoelasticity H,Is Spintronics 7

8 Physics of Magnetization Switching Electronics E,I d χ E P α σ S ε l M χ M Spin Torque H,Is Straintronics Spintronics Direct or mediated coupling of magnetic and electrical 8

9 Three Types of ME Effect Magnetostrictive (ME1) Multiferroic (ME2) Bk Strain E Surface anisotropy (ME3) E MgO Fe Various mechanisms under study 9

10 Shape Anisotropy = Demagnetization Origin = interaction between magnetic moments in different parts of the same nanomagnet. Uncompensated magnetic poles at the surface. Ferromagnet, e.g. CoFe or NiFe(permalloy) B dem =demagnetization field M=magnetization z y x M Energy = -M*B dem M M B dem B dem B dem Small B dem Small energy Easy axis Medium B dem Medium energy Large B dem Large energy Hard axis Shape and material anisotropy rule switching 10

11 Material Anisotropy B k =anisotropy field M=magnetization B k M B k M E B k KuV 2K M u s 2 1 mz Due to lattice distortion or stress, one preferred axis of magnetization with lower energy. Both directions along axis possible. Examples: FePt in L01 crystal structure, TbCoFe, CoNi or CoPt multilayers Shape and material anisotropy rule switching 11

12 Magnetostrictive Effect (ME1) Strain E B k FM PE 1. Piezoelectric (PE) material in contact with FM. Examples: oxides like PZT, PMN-PT, see below. 2. Voltage changes polarization of PE and induces stress. 3. Stress changes magnetization of FM. 4. Shows in change of hysteresis from linear to square E-filed creates stress, changes anisotropy 12

13 Magnetostrictive Switching (ME1) M H Magnetic field in-plane. Hysteresis: square = M along H, thin = M perp to H. T. Wu et al., Appl. Phys. Lett. 98, (2011), UCLA P. Shabadi et al., Proc. of Nanoarch, 107 (2011), UCR Hysteresis curve similar to magnetoresistance. 90 deg switch PMN-PT = [Pb(Mg 1/3 Nb 2/3 )O 3 ] (1 x) [PbTiO 3 ] x 13

14 Multiferroic BFO (ME2) Bi Fe O BiFeO 3 Ferroelectric (FE) below T C = 1100 K Fe atoms shift to corner of the cube in E- field L.W. Martin et al., J. Phys.: Condens. Matter 20, (2008), UC Berkeley 14

15 Multiferroic BFO (ME2) Bi Fe Fe Fe Fe O BiFeO 3 Ferroelectric (FE) below T C = 1100 K Fe atoms shift to corner of the cube in E- field Antiferromagnetic (AFM) below T N = 640 K Spins on Fe interchange in direction FE direction is orthogonal to AFM L.W. Martin et al., J. Phys.: Condens. Matter 20, (2008), UC Berkeley Coupling of electric and magnetic above room temperature Fe Fe Fe 15

16 Multiferroic : Domains Regular domains are formed (depending on the substrate and growth conditions) FE polarization (and thus spin polarization) changes at domain walls 109 domain wall 71 domain wall 180 domain wall Domain walls and their direction crucial in multiferroics 16

17 144nm Multiferroic Structure (ME2) Pt/Ti poling electrodes Pt/CoFe Pt/Ti leads BiFeO 3 DyScO 3 FM = CoFe on top of multiferroic BFO Voltage applied between two side electrodes Domains in BFO. Magnetization tied to polarization. Detected by anisotropic magnetoresistance current in plane. J. T. Heron et al., Phys. Rev. Lett. 107, (2011), UC Berkeley, Intel participation and funding 17

18 Multiferroic 180deg Switching (ME2) E=- 130k V/cm E=+ 130k V/cm 1. Switching FE polarization in each domain by 90deg 2. Resulting change of net polarization by 180deg 3. Magnetization in CoFe follows 4. Shows by change of magnetoresistance vs. angle of external magnetic field J. T. Heron et al., Phys. Rev. Lett. 107, (2011), UC Berkeley and Intel 18

19 Surface Anisotropy Switching (ME3) Modulation of surface anisotropy with applied bias. Origin = electric field at interfaces. Up to 90 deg switch. T. Maruyama et al., Nature Nano 4, 158 (2009), Osaka U. 19

20 Voltage Control of Coercivity(ME3) Starts with switching of magnetic anisotropy => coercive field changes, hysteresis loop shifts J. G. Alzate et al., IEDM (2012), UCLA 20

21 Surface Anisotropy 180deg Switching (ME3) Can switch by 180 degrees by playing with the hysteresis. Need external bias field Hbias. Heff =magnetoelectric field. J. G. Alzate et al., IEDM (2012), UCLA 21

22 Switching Characteristics (ME3) Relatively fast switching demonstrated for the first time (few ns) Combining ME switching with spin transfer torque (STT) lets get rid of the external bias field. J. G. Alzate et al., IEDM (2012), UCLA 22

23 Atomistic Modeling of Surface Anisotropy (ME3) E MgO Fe Energy U b Energy E F U b Momentum Fixed Layer MgO Free Layer Momentum NEGF quantum transport equations (tunneling, spin scattering) self-consistently coupled with Poisson equation and LLG equation (magnetic dynamic). Roksana Golizadeh-Mojarad (DTS) 23

24 Spin Density in Surface Anisotropy (ME3) E s = 0.0V/nm E s = -1.5V/nm MgO Fe MgO Fe inplane M x, nm x, nm Negative E field: - Shifts band edges for various spin directions - Transforms out-of-plane to in-plane magnet! Roksana Golizadeh-Mojarad (DTS) 24

25 Experiments on ME Switching Type Source E H alpha Structure MV/m Oe s/m su T. Maruyama et al., Nature Nano 4, 158 (2009) MgO/Fe ms X. He et al., Nat Mater. 9, 579 (2009) (Pd/Co)mult/Cr2O3 ms Y. Chen, APL 97, (2010) E-07 FeCoV/PMN-PT ms T. Brintlinger et al., Nano Lett. 10, 1219 (2010) FeGa/BTO ms T. Lahtinen et al., IEEE TM 47, 3768 (2011) CoFe/BTO ms N. Tiercelin et al., APL 99, (2011) (TbCo2/FeCo)mult/PZT mf J. T. Heron et al., PRL 107, (2011) CoFe/BFO ms T. Wu et al., APL 98, (2011) Ni/PMN-PT ms P. Shabadi et al., Nanoarch 107 (2011) Ni/PMN-PT ms T. Fitchorov et al., JAP 110, (2011) E-09 FeGa/PMN-PT su Y. Shiota et al., Nat. Mater. 11, 39 (2012) CoFe/MgO/Fe su W.-G. Wang et al., Nat. Mater. 11, 64 (2012) CoFeB/MgO/CoFeB su J. Zhu et al., PRL 108, (2012) CoFeB/MgO/CoFeB su J. G. Alzate et al., IEDM (2012) CoFeB/MgO/CoFeB 25

26 B, Tesla Comparison of ME switching experiments Best corner 100/c Wu Chen 1/c Fitchorov Tiercelin Heron Lahtinen He Brintlinger Shabadi 0.01/c Shiota Maruyama Alzate Zhu Wang Constant ME coefficient, E, MV/m Magnetostrictive (ME1) Multiferroic (ME2) Surface anisotropy (ME3) Magnetic and electric field varies with mechanism 26

27 Comparison of switching mechanisms Mechanism Pro Con Magnetostrictive Lowest E-field More robust mechanism Multiferroic Directional magnetoelectric field 180deg switching Surface anisotropy No additional materials 180deg switching with H-field or spin torque May not scale to thinner layers Change of anisotropy, not a directional field Exotic materials, difficult to fabricate Highest E-field Race is still one for the best switching mechanism 27

28 Magnetic Switching Improvement electrode piezoelectric free FM layer electrode Fewer layers. No spin polarized current involved. No need for tunneling oxide, MgO voltage t mag Switching time by rotation of magnetization 2 B bottleneck. What is effective field exactly? eff E Q V me me dd Switching energy small = charging a capacitor ME switching = time defined by anisotropy 28

29 Spin projections Static field switching 1 B me ~[0 sin(0.5) cos(0.5)] 0.5 m x m y 1 m z 0 m z Time [ns] -1 1 m y m x 1 Size = 30nm*30nm*6nm Anisotropy B k = 0.6T alpha = 0.1, B me =0.2T, t sw = 250ps B me Relaxation in a magnetic field is still slow 29

30 Precessional Switching B dem =demagnetization field M=magnetization B me =magnetoelectric field B dem B dem B me B me B dem B me gives a kick to M out of plane,creates large B dem B dem causes fast precession of M Remove B me, M falls into a stable state Caution: need a precise duration of the pulse, or flips back! Use demagnetization to switch without expense of energy 30

31 Spin projections Precessional in-plane ME switching B me m x m y 1 m z 0 m z Time [ns] -1 1 m y m x 1 B me Size = 60nm*20nm*3nm Anisotropy B k = 0T alpha = 0.01, B me =0.4T by voltage pulse, t pulse =26ps Precessional switching is much faster 31

32 Spin projections Precessional out-of-plane ME switching B me m x m y 1 m z 0 m z Time [ns] -1 1 m y m x 1 Size = 30nm*15nm*15nm Anisotropy B k = 0.5T alpha = 0.01, B me =0.4T, t pulse =30ps B me Precessional switching is much faster 32

33 B, Tesla Future direction for switching - precessional Best corner 100/c Wu Chen 1/c Fitchorov Tiercelin Heron Lahtinen He Brintlinger Shabadi 0.01/c Shiota Maruyama Alzate Zhu Wang E, MV/m Magnetostrictive (ME1) Multiferroic (ME2) surface anisotropy (ME3) B me ~0.05T needed for precessional switching. By stronger field, e.g. decreasing thickness. Not proven. Higher field required for precessional switching 33

34 Energy, fj Benchmarks for Beyond-CMOS Devices Electronics INVFO4 Inplane spin torque Perp spin torque Constant Energy*Delay 10 0 GpnJ Spin Hall CMOS HP HetJTFET Precess ME Static ME CMOS LP HomJTFET Delay, ps Spin torque spintronics slower and higher energy 34

35 Summary Magnetoelectric switching is necessary to make spintronic devices competitive Magnetoelectric effect is under active investigation in universities, three mechanisms demonstrated: Magnetostrictive, Multiferroic, Surface anisotropy. Use of precession switching provides for fast, low energy write of magnetization Need materials with higher magnetoelectric field. 35