c. alexander, w. h. butler, w. d. doyle, h. fujiwara, j. w ... · pdf filecenter for materials...

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A NSF Materials Research Science and Engineering Center

MRAMs

C. Alexander, W. H. Butler, W. D. Doyle, H. Fujiwara,J. W. Harrell, G. J. Mankey, R. Schad, P. B. Visscher, (G)

MINT Review Meeting, 11/5/2003



Outline

• Motivation and introductory remark• Issues to be addressed• Topics chosen from preliminary work• Summary



Motivation

• Although it is at the stage that the first product of MRAM will soon be announced, there are still critical problems to be solved:– reproducibility of switching,– scalability for future high density devices.

• Our group has a wide range of experience to address those problems: – materials physics and synthesis (Mankey, (G)), – thin film magnetics including exchange-coupled multilayers (Doyle, Fujiwara), – high speed magnetization switching/resonance (Alexander, Doyle, Visscher) – thermal agitation effect (Harrell), – transport phenomena (Butler, Schad), – magnetic and/or transport measurements (Alexander, Doyle, Mankey, Schad,

(G), Fujiwara), – computer simulation and visualization (Visscher).



Write process and operation margin

Word current

Digit current

Selected bit

Coincident-current schemee.a.

MTJ

Schematic cross-section of an MRAM cell

Word line

Digit line

Pinned layer

Easy axisMemory layer

o

Hh.a.

Hd

Hw

He.aM

Operation marginNiFe yoke

Critical field for switching and operation margin



x

1

0.5

0

-0.5

-1

y 10.50-0.5-1

Decrease of marginHh.a.

x

-0.5

y0-

Ideal margin

Dispersion effect

Thermal effectWord disturbance

Digit disturbance

MO

He.a



Issues to be addressedGeneral basic issues on MRAM:

• Dominant causes for irreproducible switching andswitching field distribution (SFD).

• Thermal agitation effect on switching• Size effect on noise in magnetic tunneling elements

Optimum design and experiment on memory layers withwide operation margin:

• Conventional coincident (easy/hard axis field) pulse operation scheme

• Sequential (+/− 45º field) pulse operation scheme (recently proposed by Motorola)



Topics to be presented

• Quasi-static and dynamic measurements of SFD• Conventional coincident-pulse operation MRAM:

– Proposed multilayer free-layer system with an increased operation margin

• Sequential-pulse operation MRAM:– Analytic/numeric parameter optimization—margin maximization– Energy landscape to help understand the switching mechanism– LLG (Landau-Lifshitz-Gilbert) simulation of switching



Dynamic Measurements of SFD• FMR (Ferromagnetic Resonance):

− ∆H0 (extrapolation of ∆H to zero frequency) measures anisotropy distribution.

• Stripline measurements (nanosecondpulses):– measure remanent coercivity by MOKE – Glass substrate – Dependence on field direction

We have demonstrated that our FMR and MOKE-strip line systems have sufficient sensitivity on NiFe samples 2 x 10 micronsx 50 nm.

Glass Sub.

Film

Copper

Dielectric

Laser

Cross section of CPW

-30 -20 -10 0

-1

0

1

Mr /

Mr 0

Field (Oe)

100 ns 25 ns 5 ns 2 ns

(300 x 300 µm2)



Composite free-layer for conventional coincident pulse-driven MRAM

#2

Critical switching field curves

AF layer

Soft F layer

#1

Coupling layer

210-1-2

2

1

0

-1

-2

Hdigit

Hword

Hk

HkMargin

ab

c

210-1-2

2

1

0

-1

-2

Hdigit

Hword

100 Oe

HkMargin

ab

c

(0.4 x 0.2µm2

x 2nm)

-100 Oe

Two-layer MRAM cell



Sequential-pulse driven MRAM(Analytic/numeric parameter optimization--

margin maximization)

Sequential application of Hw and Hd

t

t

Hw

Hd

o

o

Synthetic AF memory cell

M1

M2

AF coupling Hw“0”

“1”Hw

HdHd

e.a.

h.a.

Corresponding magnetizationconfigurations

(a) (b)

(c)

(d)

(e)



Analytic/numeric optimization - margin maximization

90º60º

30º

0º

120º

150º

180º

(continued)

hd

hw

o

Operation field margin in heasy, hhard domain

hhard

heasy

Equal angle curves in heasy, hhard domain

Taking in to account of thermal agitation effect, we tried to maximize operation margin (V = 0.3 × 0.15 µm2 × 2 nm, Hmax= 150 Oe).



Energy contours for SAF MRAM

hword

hword+hdigit

hdigit

m

M1

M2

Blue vector m: static equilibrium in the presence of the word field.

h drags m up the slope, along the trajectory shown in blue.

Contours of constant static energy

Later, when hword+hdigitis applied, m moves so h is again perpendicular to the contour.

(total magnetization)

HI



HA (y)

Worl

d Line

DigitLine

M2M1

EA (x)

Total magnetization trajectories for sequential pulse-field application

mx

my

mx

my

my

mx mx

Switching No Switching

my

T=0

T=R.T.t

t

Hw

Hdo

oτrise = τ fall = 2 ns



Summary• General basic issues:

• Investigate dominant causes for irreproducible switching through − measurements of static and dynamic SFD,− observation of nucleation sites for switching by SEMPA,− simulation of thermal agitation effect on switching.

• Study size effect on noise in magnetic tunneling elements.

• Design and experiment on memory layers for both Conventional Coincident-Pulse driven scheme and Sequential-Pulse driven scheme.

• Proposed a new memory layer composed of F-AF coupled layers for theconventional scheme.

• Established analytic/numeric optimization method for both types of MRAMs.

• Demonstrated effectiveness of dynamic simulation and visualization for both designing and understanding.



CPW (coplanar wave guide)-MOKE system for the study of fast switching of patterned films

Lock in Ampl. Computer

PulseGenerator

Photodiode

Polarizer

Analyser

He-Ne laser

Spatial filter

Objective LensElectromagnet

CPW

CCD



Time series plots I

T=0 T=300KRise Time = 0

hJ=3.1



Time series plots II

T=0Rise Time = 2 ns

T=300K

hJ=3.1



Minimum Switching Field

Minimum Switching Field is the minimum value of the Word and the Digit field required to switch the sample.

Fields are normalized to Hk.

Little effect of the rise time.



Dynamic simulation and visualization

Fig. 5. t (ns)φ

(deg

rees

)

angle of M from easy direction

hard-axis pulse

Roller-coaster visualization



Quasi-static Measurements of SFD

MOKE/VSM• Mr-H angular dependence(SFD/matrial

parameters)•T/sweep rate dependence(∆E forswitching)

Seed layer

Mag. layer

Cap. layer

104 elements

• Clarify factors controlling and /or having possible correlation with the SFD.

• Conceivable causes for SFD :Geometrical factors:

macroscopic (aspect ratio/thickness)microscopic defects (grains/grain-

boundaries)Material inhomogeneities

• Focus on microscopic factors.

• Measurement: MOKE / VSM / SQUIDSEMPA (ORNL / Max Planck Inst.)

Observe reversal nucleation.Specify inhomogeneities.

(poly/single-X’tal/amorphous)

SEMPA(rsl:15 nm, with H )

c. alexander, w. h. butler, w. d. doyle, h. fujiwara, j. w ... · pdf filecenter for materials...

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