Low Energy Spin Transfer T orque RAM (STT -RAM / SPRAM) Zach - - PowerPoint PPT Presentation

Low Energy Spin Transfer T

• rque

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

Lord Kelvin observes anisotropic magnetic resistance (AMR) 1857 Sir Neville Mott develops model for the “anomalous” electrical resistivities of the ferromagnetic transition- metals 1936 Michel Julliere discovers TMR at very low temperatures 1975 Fert and Grünberg discover GMR 1988 IBM uses GMR devices in read heads for hard drives Terunobu Miyazaki discovers room temperature TMR 1995 Slonczewski and Berger propose concept of "spin transfer" 1996 Predicted TMR ratios of 1000% using MgO barrier layer 2001 First prototype 2MBit SPRAM device 2007

1857 1877 1897 1917 1937 1957 1977 1997 2017

Events leading to the discovery of Spin Transfer Torque RAM

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

• rientation of the free layer between

parallel (P) and anti-parallel (AP)

 Resistance in the P-state is lower than the

AP-state (RAP> RP)

 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 (ms = ±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

• f 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
• n 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

• day’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

• f 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

• f 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 RAP / RP 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

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, 11465 (1999); J. Appl. Phys. 89,

5521 (2001); “Interaction of electrons with spinwaves in the bulk and in multilayers cond-mat”/0203314.

3.

M.D. Stiles & A. Zangwill. “Anatomy of Spin-Transfer Torque.” May 2002

4.

Alain Schuhl, Daniel Lacour, C. R. Physique 6 (2005) 945–955

5.

• P. M. Levy 2008. “An Idiosyncratic History of Giant Magnetoresistance”. NSDL Classic Articles in Context. Issue 2,

December 2008. <http:// wiki.nsdl.org/index.php/PALE:ClassicArticles/GMR>

6.

Butler, W.H., Zhang, X.G., Schulthess, T.C. & MacLaren, J.M., 2001. “Spin-dependent Tunneling Conductance of 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)

7.

• S. Maekawa & T. Shinjo.“Spin Dependent Transport in Magnetic Nanostructures.” (Eds.) London: Taylor and Francis

(2002) pg. 81

8.

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. 109-120.

9.

Sarah Gerretsen. “Spin Transfer Torque in Ferromagnetic Materials.” Department of Physics and Astronomy, University of California, Los Angles, Ca, 90095

10.

• 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, 67 - 71 (2008)

11.

• V. K. Dugaev, J. Barnas. “Classical description of current-induced spin-transfer torque in multilayer structures.” J. Appl.
• Phys. 97, 023902 (2005)

12.

Evgeny Y. Tsymbal. "Magnetic Tunnel Junction." <http:// physics.unl.edu/~tsymbal/tsymbal_files/TMR/sdt_files/page0001.html>.

Questions