Low Energy Spin Transfer Torque

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

BackgroundAnisotropic 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 (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

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%)

Today’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 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 SPRAMPotential 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 Cited1. 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