Low Energy Spin Transfer T orque RAM (STT -RAM / SPRAM) Zach Foresta April 23, 2009
Overview Background ◦ A brief history ◦ GMR and why it occurs ◦ TMR structure What is spin transfer? A novel device A future for SPRAM
Background Timeline Events leading to the discovery of Spin Transfer Torque RAM Michel Julliere discovers TMR Lord Kelvin observes at very low temperatures 1975 anisotropic magnetic resistance (AMR) 1857 Sir Neville Mott develops Terunobu Miyazaki discovers model for the “anomalous” room temperature TMR 1995 electrical resistivities of the ferromagnetic transition- Slonczewski and Berger metals 1936 propose concept of "spin transfer" 1996 First prototype 2MBit SPRAM device 2007 1857 1877 1897 1917 1937 1957 1977 1997 2017 Predicted TMR ratios of 1000% using MgO barrier layer 2001 IBM uses GMR devices in read heads for hard drives Fert and Grünberg discover GMR 1988
Background Anisotropic Magneto-resistance (AMR) 1857- Lord Kelvin discovers AMR using Fe and Ni “…I found that iron, when subjected to magnetic force, acquires an increase of resistance to the conduction of electricity across, the lines of magnetization…the electric conductivity of nickel is similarly influenced by magnetism, but to a greater degree…” -Lord Kelvin, Proceedings of the Royal Society of London, Vol. 8, 1857, pp. 546550
Background Giant Magneto-resistance (GMR) The concept for “spin transfer” evolved from the physical principles of GMR Fathers of GMR Albert Fert Peter Grünberg
Background Giant Magneto-resistance (GMR) Spin valve type GMR device ◦ Anti-ferromagnetic (AF) ◦ Pinned Layer- ferromagnetic (FM) ◦ Nonmagnetic (NM) ◦ Free Layer- ferromagnetic (FM)
Background Giant Magneto-resistance (GMR) External magnetic field switches the orientation of the free layer between parallel (P) and anti-parallel (AP) Resistance in the P-state is lower than the AP-state (R AP > R P ) Good at differentiating between “1” or “0” using changes in resistance
Background Giant Magneto-resistance (GMR) Practical Application IBM HD read heads
Background GMR why it occurs? 1922 – Stern-Gerlach Experiment Electrons have intrinsic angular momentum which depend on their quantized spin number (m s = ±1/2)
Background GMR why it occurs? Electrical Resistance is caused by electron scattering Probability of scattering depends on the number of available quantum states for the electron to scatter into, which depends strongly on the relative direction of the electron's spin and the magnetic field inside the FM layer.
Background GMR why it occurs? GMR effect in the P and AP states P-state experiences less scattering ◦ Up spin electrons have no states to scatter into.
Background Tunneling Magneto-resistance (TMR) 1975 – Discovered by Michel Julliere ◦ At 4.2 K Julliere observed resistance changes on the order of 14% ◦ His work was disregarded as impractical until room temperature TMR was achieved in 1995 by Terunobu Miyazaki
Background TMR structure SPRAM devices implements a TMR structure ◦ Main differences between TMR and GMR GMR has a middle NM layer whereas TMR uses an insulator TMR uses current induced magnetic fields to switch from AP to P states and vice versa TMR achieves resistance ratio results on the order 500% (GMR ~ 50%) T oday’s HD readers are predominantly using TMR structures
Background TMR structure Cobalt ferromagnetic layers Magnesium Oxide insulating barrier layer
Background TMR structure Why Co? ◦ Cobalt is missing 3 electron in the 3d level 1936 - Sir Neville Mott’s model states that the 3d level electrons act as scatterers near the fermi level. The density of states at the fermi energy is mostly spin down meaning Co has more quantum states for scattering.
Background TMR structure Why MgO? ◦ Magnesium oxide has a cubic crystalline structure which aids in conservation of electron momentum transfer ◦ 2001 – predicted TMR ratio of 1000% using this insulating layer.
What is Spin Transfer? 1996 – Slonczewski and Berger propose the idea of spin transfer ◦ “When a current of polarized electrons enters a ferro-magnet, there is generally a transfer of angular momentum between propagating electrons and the magnetization of the film.”
What is Spin Transfer? In short – instead of changing the magnetic field externally it is done by polarized currents. The result – Similar to GMR/TMR a resistance change is seen from AP to P states.
A Novel Device A group developed a 1.8 V 2Mb SPRAM chip ◦ Using NiFe(2nm)/ CoFe(1nm)/MgO(1nm)/CoFe(1nm) TMR memory cell
A Novel Device Components of one Bit ◦ Bit Line (BL) – where state is read ◦ Source Line (SL) – current source ◦ Word Line (WL) – current state regulator ◦ TMR device – memory cell
A Novel Device Writing “0”s and “1”s
Novel Device Major Pitfalls ◦ Switching Current Direction In order to write “0” or “1” current must be either into or out of SL ◦ Reading Bits without accidental writing Reading is similar to writing There exists a critical current density Jc - for parallelizing and Jc + for anti-parallelizing.
Novel Device Current Switching ◦ Use a flip flop gate depending on inputs of SALT and SALB Example SALT = High (H) + SALB = Low (L) = parallelizing (BL to SL)
Novel Device Parallelizing Anti-Parallelizing
Novel Device Accidental writing while reading (accidental spin reversal) Disturbance – a measure of likelihood of spin reversal. ◦ Parallelizing direction has larger disturbance when reading AP-state, therefore lower chance of spin reversal.
Novel Device Testing this hypothesis by varying the TMR ratio For P direction – As TMR ratio increases the possibility of spin reversal decreases. For AP direction – does not depend on TMR ratio.
Novel Device Finished Product
Novel Device Proven 10 year life cycle ◦ No degradation of R AP / R P after 1 billion write cycles Fast read/write Non-volatile Instant-ON capable Low energy
Future of SPRAM How it compares against MRAM ◦ Scalability > MRAM ◦ Lower Energy ◦ More complex control scheme
Future of SPRAM Potential for higher speeds Research conducted by Sarah Gerretsen, University of California ◦ TMR device of Co (20nm) /MgO (5nm) /Co (5nm) with a switching current of 1.96mA resulted in a 0.104ns P to AP state switching time
Work Cited J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); 195, L261 (1999). 1. L. Berger, Phys. Rev. B 54, 9353 (1996); J. Appl. Phys. 81, 4880 (1997); Phys. Rev. B 59, 11465 (1999); J. Appl. Phys. 89, 2. 5521 (2001); “Interaction of electrons with spinwaves in the bulk and in multilayers cond - mat”/0203314. M.D. Stiles & A. Zangwill. “Anatomy of Spin - Transfer Torque.” May 2002 3. Alain Schuhl, Daniel Lacour, C. R. Physique 6 (2005) 945 – 955 4. P. M. Levy 2008. “An Idiosyncratic History of Giant Magnetoresistance”. NSDL Classic Articles in Context. Issue 2, 5. December 2008. <http:// wiki.nsdl.org/index.php/PALE:ClassicArticles/GMR> Butler, W.H., Zhang, X.G., Schulthess, T.C. & MacLaren, J.M., 2001. “Spin -dependent Tunneling Conductance of 6. Fe|MgO|Fe Sandwiches.” Phys. Rev. B 63, 054416 and Mathon, J. & A. Umerski, A., 2001. “Theory of Tunneling Magnetoresistance of an Epitaxial Fe/MgO/Fe(001) Junction.” Phys. Rev. 63, 220403(R) S. Maekawa & T. Shinjo.“Spin Dependent Transport in Magnetic Nanostructures.” (Eds.) London: Taylor and Francis 7. (2002) pg. 81 Kawahara, T., Takemura, R., Miura, K., Hayakawa, J., Ikeda, S., Lee, Y.M., Sasaki, R., Goto,Y., Ito,K., Meguro, T., 8. Matsukura, F., Takahashi, H., Matsuoka, H. & Ohno, H. . “2 Mb SPRAM (SPin -Transfer Torque RAM) With Bit-by-Bit Bi-Directional Current Write and Parallelizing- Direction Current Read.” IEEE Journal of Solid -State Circuits, vol. 43, NO. 1, January 2008 pg. 109-120. Sarah Gerretsen. “Spin Transfer Torque in Ferromagnetic Materials.” Department of Physics and Astronomy, 9. University of California, Los Angles, Ca, 90095 J. C. Sankey, Y.- T. Cui, R. A. Buhrman, D. C. Ralph, J. Z. Sun, J. C. Slonczewski. “Measurement of the Spin -Transfer- 10. Torque Vector in Magnetic Tunnel Junctions.” Nature Physics 4, 67 - 71 (2008) V. K. Dugaev, J. Barnas. “Classical description of current -induced spin- transfer torque in multilayer structures.” J. Appl. 11. Phys. 97, 023902 (2005) Evgeny Y. Tsymbal. "Magnetic Tunnel Junction." <http:// 12. physics.unl.edu/~tsymbal/tsymbal_files/TMR/sdt_files/page0001.html>.
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