Low Energy Spin Transfer Torque RAM (STT-RAM / SPRAM) Zach Foresta April 23, 2009

Transcription

1 Low Energy Spin Transfer Torque RAM (STT-RAM / SPRAM) Zach Foresta April 23, 2009

2 Overview Background A brief history GMR and why it occurs TMR structure What is spin transfer? A novel device A future for SPRAM

3 Background Timeline Events leading to the discovery of Spin Transfer Torque RAM Lord Kelvin observes anisotropic magnetic resistance (AMR) 1857 Sir Neville Mott develops model for the anomalous electrical resistivities of the ferromagnetic transitionmetals 1936 Michel Julliere discovers TMR at very low temperatures 1975 Terunobu Miyazaki discovers room temperature TMR 1995 Slonczewski and Berger propose concept of "spin transfer" 1996 First prototype 2MBit SPRAM device 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

4 Background Anisotropic Magneto-resistance (AMR) 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

5 Background Giant Magneto-resistance (GMR) The concept for spin transfer evolved from the physical principles of GMR Fathers of GMR Albert Fert Peter Grünberg

6 Background Giant Magneto-resistance (GMR) Spin valve type GMR device Anti-ferromagnetic (AF) Pinned Layer- ferromagnetic (FM) Nonmagnetic (NM) Free Layer- ferromagnetic (FM)

7 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

8 Background Giant Magneto-resistance (GMR) Practical Application IBM HD read heads

9 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)

10 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.

11 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.

12 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

13 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%) Today s HD readers are predominantly using TMR structures

14 Background TMR structure Cobalt ferromagnetic layers Magnesium Oxide insulating barrier layer

15 Background TMR structure Why Co? Cobalt is missing 3 electron in the 3d level 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.

16 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.

17 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.

18 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.

19 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

20 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

21 A Novel Device Writing 0 s and 1 s

22 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.

23 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)

24 Novel Device Parallelizing Anti-Parallelizing

25 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.

26 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.

27 Novel Device Finished Product

28 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

29 Future of SPRAM How it compares against MRAM Scalability > MRAM Lower Energy More complex control scheme

30 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

31 Work Cited 1. J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); 195, L261 (1999). 2. L. Berger, Phys. Rev. B 54, 9353 (1996); J. Appl. Phys. 81, 4880 (1997); Phys. Rev. B 59, (1999); J. Appl. Phys. 89, 5521 (2001); Interaction of electrons with spinwaves in the bulk and in multilayers cond-mat / M.D. Stiles & A. Zangwill. Anatomy of Spin-Transfer Torque. May Alain Schuhl, Daniel Lacour, C. R. Physique 6 (2005) P. M. Levy An Idiosyncratic History of Giant Magnetoresistance. NSDL Classic Articles in Context. Issue 2, December < wiki.nsdl.org/index.php/pale:classicarticles/gmr> 6. Butler, W.H., Zhang, X.G., Schulthess, T.C. & MacLaren, J.M., Spin-dependent Tunneling Conductance of Fe MgO Fe Sandwiches. Phys. Rev. B 63, and Mathon, J. & A. Umerski, A., Theory of Tunneling Magnetoresistance of an Epitaxial Fe/MgO/Fe(001) Junction. Phys. Rev. 63, (R) 7. S. Maekawa & T. Shinjo. Spin Dependent Transport in Magnetic Nanostructures. (Eds.) London: Taylor and Francis (2002) pg Kawahara, T., Takemura, R., Miura, K., Hayakawa, J., Ikeda, S., Lee, Y.M., Sasaki, R., Goto,Y., Ito,K., Meguro, T., 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 Sarah Gerretsen. Spin Transfer Torque in Ferromagnetic Materials. Department of Physics and Astronomy, University of California, Los Angles, Ca, J. C. Sankey, Y.-T. Cui, R. A. Buhrman, D. C. Ralph, J. Z. Sun, J. C. Slonczewski. Measurement of the Spin-Transfer- Torque Vector in Magnetic Tunnel Junctions. Nature Physics 4, (2008) 11. V. K. Dugaev, J. Barnas. Classical description of current-induced spin-transfer torque in multilayer structures. J. Appl. Phys. 97, (2005) 12. Evgeny Y. Tsymbal. "Magnetic Tunnel Junction." < physics.unl.edu/~tsymbal/tsymbal_files/tmr/sdt_files/page0001.html>.

32 Questions