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1 IDEMA DISKCON Asia-Pacific 2009 Spin Torque MRAM with Perpendicular Magnetisation: A Scalable Path for Ultra-high Density Non-volatile Memory Dr. Randall Law Data Storage Institute Agency for Science Technology and Research (A*STAR) Singapore 13 March 2009, 12:00 noon 1/12 1/18 1/22 Wouldn t it be great if power trips do not destroy our unsaved data? PCs can start up instantly? our memory sticks could store a lot more and write data at a much higher rate? MRAM has the potential to satisfy these demands! 2/22

2 Overview A short revision on magnetism What is giant magnetoresistance (GMR) & MRAM? What is spin torque transfer, and why is it better? Advantages of perpendicular magnetisation Device optimisation and multi-level storage Device structure for write current reduction Concluding remarks 3/22 Ferromagnetism arises from electron spin Alignment of majority (spin up) un-paired electrons Fe 3d 4s S M N 4/22

3 What is giant magnetoresistance (GMR)? Discovered in 1988, awarded the 2007 Nobel Prize in Physics Giant change in electrical resistance upon application of a magnetic field Magnetic Layer 1 Non-magnetic spacer Magnetic Layer 2 Little scattering = Low Resistance Current flow 5/22 What is giant magnetoresistance (GMR)? Discovered in 1988, awarded the 2007 Nobel Prize in Physics Giant change in electrical resistance upon application of a magnetic field Magnetic Layer 1 Non-magnetic spacer Magnetic Layer 2 More scattering = High Resistance Current flow 6/22

4 Magnetoresistance arises from spin dependent scattering (or tunneling) Current-in-plane (CIP) configuration Magnetic layers Current Flow Good for rapid prototyping & measurements Spacer layer High resistance state Low resistance state Current-perpendicular-to-plane (CPP) configuration Current Flow Insulator layer Spin-dependent electron scattering Spin-dependent electron tunneling Necessary for spin torque switching Current Perpendicular to Plane (CPP) Magnetic Tunnel Junction (MTJ) Larger resistance change, higher signal 7/22 Advantages of MRAM High density and switching speed Memory cells can be as small as 5 nm (vs. 22 nm for Flash) < 30 ns writing speed (vs. a few μs for Flash) MRAM: (5 nm) Flash 22 nm Can be a universal memory Unlimited endurance (>10 15 rewrites, vs. about only 10 6 for Flash) Random access with no refresh (non-volatile) Non-destructive read 8/22

5 Magnetic-field switched MRAM cannot scale towards small sizes Bit line current Bit selection transistors Word line current High density Smaller storage elements Thinner current lines = Smaller magnetic fields Higher writing current required Thinner current lines get destroyed 9/22 Field-switched (or Toggle ) MRAM is not scalable S. Ikeda et al., IEEE Trans. Elec. Dev. 54, 991 (2007) CMOS transistor current decreases with gate width: -Assume 100 μa per 100 nm gate width Current required for magnetic field writing increases as junction size decreases (black line) 10/22

6 GMR and Spin torque transfer: Two sides of the same coin (Newton s 3 rd Law) Electron scattering = Force experienced by the electrons Magnetic layer experiences an equal and opposite force More electrons More force on magnetic layer Softer (free) magnetic layer changes its magnetisation 11/22 Spin torque MRAM (ST-MRAM) is scalable as write current decreases with size CMOS current scales with gate width (linear dimension) ST is based on current density Scales with cross-sectional area of MRAM device ST-MRAM writing current scales with junction size faster than CMOS current (sharper slope) 100 nm 50 nm S. Ikeda et al., IEEE Trans. Elec. Dev. 54, 991 (2007) 12/22

7 Elements with perpendicular magnetisation allows higher storage densities In-plane anisotropy (Vortex states) Perpendicular anisotropy (Uniform magnetisation) Improved: 1) Cell stability 2) Magnetisation uniformity è Higher storage density Spin torque switched (STS) spin valves with perpendicular anisotropy Spin torque switching of perpendicular magnetic layers is more efficient than in-plane layers. S. Mangin, et al. Nat. Mater. 5, 201 (2006) 13/22 Custom-designed UHV Sputter Deposition System & Characterisation Tools in DSI Four-point probe For film CIP-GMR Base pressure < Torr STS circuit platform GHz Spin torque probe station 14/22

8 Film structure and behaviour of perpendicular spin valves Single Spin-valve + Hard Layer Soft Layer Seed layers Layer thicknesses Thinnest layers = 1 to 3 atomic layers Thickest layer = 5 nm Resistance (W) Magnetic field (Oe) 15/22 Different Co-Fe alloy compositions allow the switching fields to be tuned Co vs. Co 90 Fe 10 : Same critical Pd thickness Better fcc (111) orientation of Co gives higher anisotropy Co 65 Fe 35 : Higher critical Pd thickness Higher M s and bcc oriented Soft layer Co 90 Fe 10 /Pd Hard layer Co/Pd 16/22

9 Adding a 3 rd magnetic layer creates a dual spin valve with more resistance levels 15.2% GMR æ æ æ 17/22 Independent behaviour of GMR contribution of each interface Lower spacer GMR = 6% Upper spacer GMR = 7% Total GMR = 13% (7% + 6%) Simple addition of GMR at two interfaces R. Law et al., IEEE Trans. Mag, Intermag 2008 Proceedings 18/22

10 A 3 rd resistance level allows some search applications to be a lot faster = Symmetrical interfacial GMR contributions A Don t care state increases the speed of some search applications by ignoring irrelevant data. 19/22 4 resistance levels allows the storage of 2 bits of information = 2 Asymmetrical interfacial GMR contributions 2 data bits per cell = Doubling of storage density R. Law et al., J. Appl. Phys. (2009), Under review 20/22

11 Enhancement of spin torque efficiency with in-plane spin polariser Reduction of ST-switching time ST-switching curve: 100 ns pulses Hard Layer Soft Layer In-plane spin polariser R. Sbiaa et al., J. Appl. Phys. 105, (2009) R. Law et al., Appl. Phys. Lett. 94, (2009) 21/22 Conclusions The effect of giant magnetoresistance (GMR) can be used in magnetic memory (MRAM), which outperforms existing memory technologies. MRAM can be switched by electrical currents using spin torque transfer, which makes it scalable for smaller sizes (higher densities). Perpendicular magnetic materials have better performance than in-plane ones, and we have discussed the optimisation of such devices. Multi-level storage allows storage densities to be increased beyond conventional size limitations. We have shown spin torque switching and reduction of write current using new MRAM device structures. 22/22

12 Acknowledgements Prof. Chong Tow Chong, DSI Exec. Director Dr. Thomas Liew, DSI Dep. Exec. Director (Research) Dr. Rachid Sbiaa Dr. Tan Ei-Leen Colleagues and collaborators in DSI s Spintronics, Media and Interface Division 23/22