Spintronics in applications: Hard drives, MRAM, and spin torque oscillators. Olle Heinonen Materials Science Division Argonne Na5onal Laboratory

Transcription

1 Spintronics in applications: Hard drives, MRAM, and spin torque oscillators Olle Heinonen Materials Science Division Argonne Na5onal Laboratory

2 Acknowledgements I have benefited from collaborations and discussions with numerous colleagues, in particular Bill Butler (Alabama MINT) Xiaoguang Zhang (ORNL) Mark Kief, Kristin Duxstad, Eric Linville, Konstantin Nikolaev, Xilin Peng, Dave Schouweiler, Kaizhong Gao, Haiwen Xi (Seagate) Pranaba Muduli, Johan Åkerman (University of Gothenburg) Janusz Nowak (IBM), Sining Mao (WD), David Larson (Imago), Amanda Petford-Long (ANL), Alfredo Cerezo (Oxford), and many others 2 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

3 Outline Drivers for spintronics Spintronics in Hard Disc Drives (HDDs) Thin-film Anisotropic MagnetoResistance (AMR) Giant MagnetoResistance (GMR) Tunneling MagnetoResistance (TMR) Outlook Magnetic Random Access Memories (MRAMs) Spin torque oscillators Outlook 3 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

4 Drivers for spintronics Basic physics magnetotransport, spin-dependent scattering, magnetic heterostructures (Gruenberg, Fert) Physics funding agencies sometimes novelty-driven so you have to come up with a novelty that will solve the world s problems Semiconductor scaling running out of steam, but still hard to beat Now NAND flash at 19 nm node(!!!!!!), 2 bits/cell, 64 Gbit NAND, 3 bits/cell planned (SanDisk 2011) Scaling limit drives new functionalities, 3D architectures Semiconductor power consumption 4 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

5 Drivers for spintronics power consumption 5 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

6 Drivers for spintronics power consumption Create power- efficient electronics. Magne5c spintronics can create devices that do not consume power in off- state Today: slow & large footprint Future: normally off electronics (Sam Bader) 6 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

7 Areal Density progression in HDDs Perpendicular Recording Longitudinal Recording GMR and MTJ ( Kryder (Mark Kief, Mark 7 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

8 Spintronics in HDDs Basic problem: sense relatively weak (~1000 Oe), spatially localized (~10 nm) fields with high SNR and at high frequencies (~1 GHz) Basic drivers: Scaling to smaller features, larger areal densities (Gbit/in 2 ) while maintaining SNR Power consumption Basic idea in thin film heads: use magnetoresistance to sense stray fields from recorded bits in mediaè spin- and charge-currents in sub-micron thin film structures. 8 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

9 Bit size and reader technologies Areal Density vs. Magne5c Bit Sizes 10 Gb/in 2 20 Gb/in 2 40 Gb/in 2 32 ktpi x 345 kbpi (794 nm x 74 nm) 45 ktpi x 445 kbpi (564 nm x 57 nm) 75 ktpi x 530 kbpi (339 nm x 48 nm) 100 Gb/in 2 200Gb/in ktpi x 600 kbpi (152 nm x 39 nm) 200 ktpi x 1,000 kbpi (127 nm x 25 nm) 1 Terabit/in 2 1,000 ktpi x 1,000 kbpi (25.4 nm x 25.4 nm) High MR ra5o translates to High Signal to Noise ra5o Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 9

10 Thin film GMR and AMR heads (From Jimmy Zhu) Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 10

11 AMR head advantages Enabled the use thin film deposi5on (pla5ng, spueering) and paeerning technologies è rapid scaling (reduc5on of feature sizes) SNR independent of disc velocity (in contrast with induc5ve heads) More complex process technologies Mul5- layer structures Larger materials set (sof magnets, hard magnets, insulators, conductors) Low AMR ra5o ~1% limits SNR with reduced reader size Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 11

12 GMR read heads Much larger magnetoresistance (signal) then in AMR heads higher SNR while shrinking feature size More complicated reader structure, more aeen5on to interface phenomena More advanced deposi5on (spueering) and paeerning techniques Sof Magne5c Shield Ac5ve Sensing Region Current Lead Current Lead Al203 Gap Al203 Gap Permanent magnet Permanent magnet Sof Magne5c Shield Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 12

13 Bulk and interface scattering in CoFe/Cu multilayers GMR is generated by Cu CoFe Bulk spin-asymmetric scattering Interface spin-asymmetric scattering Majority spin suffers few scattering events Minority spin suffers many scattering events 13 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

14 GMR-effect: band-match and transmission properties at CoFe-Cu interface CoFe and Cu in (111) texture have a very good band-match in the majority channel, and a poor band-match in the minority channel Majority spins are transmitted relatively easily across the 2D Fermi surface; minority spins are largely reflected This leads to a spin-asymmetric interface resistance which contributes (in addition to spin-asymmetric scattering within the CoFe layers) to the GMR resistance Transmission probabilities for majority and minority electrons incident from CoFe to Cu. Blue to white colors indicate increasing transmission probablility W.H. Butler, O. Heinonen and X.-G. Zhang, in Physics of Ultra-High Density Magnetic Recording, J. (( 1999 van Ek, M. L. Plumer, and D. Weller (eds) (Springer 14 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

15 CIP spin valves The potential use of the GMR effect in sensors and read-heads was immediately recognized However, multilayer devices were not practical for applications: Low sensitivity Thick structures (relative to shield-to-shield spacing in read-heads) Hard to bias for linear response The GMR Spin Valve [B. Dieny et al., Phys. Rev. B43, 1297 (1991)] fixed this: High sensitivity Thin structure Biased for linear response Compatible with thin-film deposition and patterning techniques scalable 15 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

16 GMR SAF Spin Valve A thin Ru layer sandwiched between PL and RL couples them very strongly antiferromagnetically. SAF does not respond to external field and exerts small stray field on FL easier to bias RL PL 16 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

17 CIP SAF spin valve Unexpected bonus for transport and GMR: Reflection of majority spin electrons incident on Ru from CoFe is beneficial to GMR! Ru on CoFe smooths out the surface (reduces doming ). Transmission probabilities for majority and minority electrons incident on Ru from CoFe. Majority spins tend to be reflected, and minority transmitted. W.H. Butler, O. Heinonen and X.-G. Zhang, in Physics of Ultra-High Density Magnetic Recording, J. van Ek, M. L. (( 1999 Plumer, and D. Weller (eds) (Springer 17 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

18 Current density and GMR properties Better interface control, improved Cu, specular cap, and the magic of Ru improve magnetotransport properties Cu Reference layer Free layer Pinned layer GMR signal 18 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

19 CIP spin valves end-of-life Highest achievable GMR was around 20% Signal voltage limited by maximum current Joule heating. Signal voltage is As the device size decreases, heat is dissipated less efficiently from the reader, so the bias current has to be reduced. Main limitations to continued scaling for SVs were increased heat dissipation unfavorable SNR scaling insulating layers between SV and shields structures prevented small shield-to-shield spacing 19 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

20 From GMR spin valves to MTJs GMR MTJ Free layer Cu Reference layer Free layer! Barrier! Reference layer! Device resistance depends on spindependent scattering at interfaces and bulk films Device resistance depends on spin-dependent tunneling through the barrier layer Bandstructure effects in Co(Fe)- MgO-Co(Fe) can lead to very large magnetoresistance W.H. Butler, X.-G. Zhang, T.C. Schulthess, and J.M. MacLauren, Phys Rev B 63, (2001) 20 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

21 From GMR spin valves to MTJs Similar stack structures, but Metallic spacer replaced with insulating barrier Metallic shields used as contact better thermal management Must insulate permanent magnets from stack (atomic layer deposition) Signal amplitude: 21 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

22 Tunneling readers barrier materials Want: High tunneling magnetoresistance (TMR) ( 2 Low resistance-area (RA) product (~1 Ω-µm Reliable able to withstand bias voltage during lifetime of reader Initial MTJs were based on alumina barriers. Drawbacks: Kinetics of alumina formation makes it hard to get a thin (low ( pinholes ) RA) barrier free of defects Metallic Al deposited on CoFe electrode forms an alloy with Co. 22 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland

23 Early product TMR head (~150 Gbit/in 2 ) Free sensing layer Thin insulating barrier < 1 nm thickness Antiferromagnet for pinning the fixed layer Abueed junc5on layout with hard bias Reader width ~ nm and Shield spacing ~ 80nm Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 23

24 bcc FeCo also has only one band at the Fermi energy, also a Δ 1 band that that decays relatively slowly in the MgO. There is no minority Δ 1 band. Parallel Alignment of FeCo Moments An5- Parallel Alignment of FeCo Moments Tunneling density of states on each atomic layer at k = 0 for FeCo/MgO/FeCo tunnel junction. Top panel, parallel spin alignment, bottom panel, antiparallel spin alignment Zhang and Butler, PRB 70, (2004) Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 24

25 MTJ in HDDs present status and end-of-life Currently in produc5on RA 0.5 Ω(μm) 2 and TMR 100% Reader dimensions < 50 nm x 50 nm Difficult to make narrower (for higher track density) and thinner (for higher linear density) while maintaining stability, amplitude (SNR), and reliability Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 25

26 Spin torque MRAM MRAM = Magnetic Random Access Memory Use a basic mangetic tunneling junction to store information. Free layer Tunneling barrier Pinned layer Low-resistance state High-resistance state Free layer and Pinned layer parallel low-resistance state. Free layer and pinned layer anti-parallel high-resistance state Use shape anisotropy to make the two magnetization configurations bi-stable. Switching? Old scheme uses complicated wires to use current-induced magnetic fields to switch free layer (Freescale). Problem: difficult to scale down (magnetic cross-talk and bit selection) Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland Seagate 2009 Page 26

27 Spin torque A spin- polarized current exerts a torque on the magne5za5on in a thin magne5c film 1 2 Electron flow direc5on Current direc5on Back- scaeered electrons an5- parallel to the magne5za5on in 2 exert a torque on 1 Transmieed electrons parallel to the magne5za5on direc5on in 1 exert a torque on Electron flow direc5on Current direc5on Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 27

28 Perpendicular-anisotropy (PA) spin-torque RAM In in- plane STRAM the direc5on of the Free layer magne5za5on is determined by the shape Free layer Reference layer High- energy Low- energy direc5ons The thermal stability is determined by the energy difference between high- and low- energy direc5ons and depends on the shape. This make the thermal stability sensi5ve to process varia5ons. Difficult to make small ellipses (~1:2 aspect ra5o) small axis 65 nm or smaller with 193 nm stepper. In PA STRAM, the direc5on of the Free layer magne5za5on is determined by intrinsic magne5c anisotropy. This offers advantages over in- plane STRAM: The thermal stability is insensi5ve to shape and process varia5ons Can easily make circles down to ~65 nm diameter with 193 nm stepper The switching current is reduced Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland Page 28

29 In-plane elliptical spin torque magnetic random access memory structure SEM image of nominally 90 nm x 180 nm STRAM note edge roughness, or wobbliness Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland Page 29

30 Perpendicular-anisotropy spin-torque MRAM Advantages: Decouple thermal stability (energy barrier) from processing condi5ons Reduced cri5cal current density for switching but reduced write current can lead to read disturbances Remaining issues: Materials science: must integrate magne5c layers with perpendicular anisotropy while maintaining high- TMR CoFe/MgO/CoFe interface with (001) texture. Thin (very thin, ~1 nm) CoFeB has perpendicular anisotropy (Ohno) Must maintain low cri5cal current density reliability as well as size of selec5on transistor Recent developments perpendicular spin- torque MRAM (Tohoku U NEC): Sato et al, APL 2011 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 30

31 Japanese Journal of Applied Physics 50 (2011) DOI: /JJAP REGULAR PAPER Design and Fabrication of a One-Transistor/One-Resistor Nonvolatile Binary Content-Addressable Memory Using Perpendicular Magnetic Tunnel Junction Devices with a Fine-Grained Power-Gating Scheme Shoun Matsunaga, Masanori Natsui 1, Shoji Ikeda 2, Katsuya Miura 2;3, Tetsuo Endoh 4, Hideo Ohno 2, and Takahiro Hanyu 1 Center for Spintronics Integrated Systems (CSIS), Tohoku University, Sendai , Japan 1 Laboratory for Brainware Systems, Research Institute of Electrical Communication, Tohoku University, Sendai , Japan 2 Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, Sendai , Japan 1T- 1R 64 bit x 128 word using 140 nm CMOS processing Perpendicular- anisotropy 3 Hitachi Advanced Research Laboratory, Kokubunji, Tokyo , Japan CoFeB/MgO/CoFeB 4 Center for Interdisciplinary Research, Tohoku University, Sendai , Japan Received February 24, 2011; accepted March 24, 2011; published online June 20, 2011 A perpendicular magnetic tunnel junction (P-MTJ)-based one-transistor/one-resistor (1T 1R) binary content-addressable memory (CAM) is proposed for a high-density nonvolatile CAM. The proposed CAM cell performs an equality-search operation between an input bit and the corresponding stored bit by detecting the difference of a cell resistance, where the cell resistance is determined by the series connection of one metal oxide semiconductor (MOS) transistor and one P-MTJ device. This circuit structure makes it possible to implement a compact nonvolatile CAM cell circuit with 1.25 m 2 of a cell size in a 0.14 m complementary MOS (CMOS)/P-MTJ process. Moreover, the equality-search operation in a bit-serial fashion is used for great reduction of the activity rate in the proposed CAM cell array, since most of the mismatched words in the CAM are detected by just several higher bits of comparison results in the word circuits. By applying a bit-level fine-grained power gating scheme, a fabricated 64-bit 128-word nonvolatile CAM achieves high density with maintaining low search energy under 3.1% of activity rate in the cell array. # 2011 The Japan Society of Applied Physics 1. Introduction A content-addressable memory (CAM) is a powerful datasearching hardware with a parallel data processing capability. It can be used for a number of applications such as parallel image processors, data compression hardware, and central processing unit (CPU) caches. 1 6) However, a conventional complementary metal oxide semiconductor (CMOS)-based CAM tends to suffer from an area penalty since it must consist of a normal static random access memory (SRAM) cell (six transistors) to perform data storage function and additional logic circuit (three transistors at least) to perform equality-search operation. 7) Moreover, standby power dissipation due to leakage current in a Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland CMOS-based cell circuit is increasingly dominating its power dissipation in recent nanometer-scaled technology. 8 12) One possible solution to realize a high-density and lowstandby-power CAM is to use metal oxide semiconductor (MOS)/nonvolatile-device-hybrid logic-in-memory circuiadvantage to achieve ultra-low power consumption in very large scale integrated circuits (VLSIs). 25) In this paper, we propose a nonvolatile bit-serial binary CAM using P-MTJ-based fine-grained power-gating scheme, and demonstrate its operating mechanism with a fabricated 64-bit 128-word CAM chip in a 0.14 m CMOS/P-MTJ process. Since a P-MTJ device is used as not only a nonvolatile storage element but also a logic-operation element, one-transistor/one-resistor (1T 1R)-style compact CAM cell circuit is implemented. Moreover, a combination of bit-level equality-search scheme and a fine-grained power-gating scheme which is achieved by the nonvolatile storage capability of MTJ devices decreases the cell activity rate to 3.1%, which further reduces power dissipation of the circuit. As the result, an ultra-low-power bit-serial CAM which eliminates most of the wasted standby power can be obtained. 2. MOS/MTJ-Hybrid 1T 1R Binary CAM Cell with Three-State Combined Resistance 31

32 Spin-torque oscillators New physics coupling DC currents with magne5za5on dynamics Spin torque oscillators very high Q- values in nanoscale GHz oscillators Nazarov, Nikolaev, Gao, Cho, and Song MgO MTJ (MMM- 2007) Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 32

33 Magnetization dynamics Landau-Lifshitz-Gilbert equation d ˆ m dt [ H ] eff " # e % = "# e m ˆ $! m ˆ $ 1+ % 2 [ ( H )] eff m ˆ $!!! H eff Precession (conserva5ve torque) ˆ m = Dissipa5on (non- conserva5ve torque)! M! M α = dimensionless damping constant! m ˆ! Magne5za5on precesses around the effec5ve field. The dissipa5ve torque brings the magne5za5on parallel to the effec5ve field.! Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 33

34 Add Slonczewski spin torque :! " = # e a [ J m ˆ FL $ ( m ˆ FL $ m ˆ RL )],! H eff ˆ m RL! a J " #jp M S t a J =effec5ve field due to spin torque j= current density P=spin polariza5on M S =magne5za5on density t= FL thickness! ˆ m Depending on the sign of the current, the spin torque term adds dissipa5on, or pumps energy into the system è can have undamped oscilla5on! Large spin torque can make magne5za5on switch (Ka5ne et al, PRL 2000)! Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 34

35 STOs Metallic nano- contacts or MgO MTJs MTJs provide large signal, but worse tunability Couple oscillators for larger output Sub- micron sized GHz oscillator driven by dc currents(!) with frequencies up to tens of GHz; readily integrated with Si- CMOS using back- end- of- line processing. Issues Synchroniza5on of many oscillators Frequency range for MgO STOs Amplitude for nano- contact STOs Ul5mate coherence limit: Temperature- driven decoherence Mode- hopping driven decoherence Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland 35

36 Back-ups 36 Olle Heinonen 6th Nordic Magne5c Confrence Oct. 4-5, 2011, Pori, Finland