Fabricated Magnetic Structures -...

Boulder Summer School

LECTURE 3

Magnetic Junction Devices

Fabricated Magnetic Structures

Yuri Suzuki


Lectures on Fabricated Magnetic Structures

• Introduction• Synthesis and fabrication techniques for magnetic

structures• Magnetic behavior in small magnetic structures• Magnetic Junction Devices

– Patterning of Junction Devices– Spin Polarization– Interfaces


Areal Density of Magnetic HDD and DRAM

AR

EA

L200

2Eh.

PR

Z

Ed Grochowski

1970 1980 1990 2000 2010

Year

10 -2

10 -1

10

10 1

10 2

10 3

10 4

10 5

10 6

Are

al D

ensi

ty, M

bit

s/in

ch2

25% = 2X per 3 years

40 2

60 1.5

100 1

25% CGR

60% CGR

40% CGR

1M4M

16M

64M

256M

1G

3380E

Ultrastar 18XP

Ultrastar XP

3390-2

Ultrastar

36ZXb

Ultrastar 146Z10

100% CGR

Travelstar 40GN

Microdrive II

Travelstar 30GN

512M

2G4G

8G

DRAM projections after 2001 are based on industry capacities and constant chip area

Travelstar 80GN


Lithographic Critical Feature Roadmap forGMR Heads and Semiconductor IC

Robert Fontana

Line

wid

th20

02c.

prz 1980 1990 2000 2010

Year of Production

0.01

0.1

1

10

100

Cri

tica

l Fea

ture

(u

met

ers

)

IC FeaturesIC General

IC GateHead Features

P2wGMRw

P2w

GMRw


Fabricating Magnetic Heterostructureswith length scales below 100nm

• lithographic processes to create small structures fromcontinuousmagnetic multilayers– optical lithography, incl. DUV

– e-beam lithography

– Au particles as milling masks

Liesl FolksHGST


Deep UV lithography

• manufacturable, but slow and expensive• not good for materials testing Marie-Claire Cyrille

HGST


e-beam Lithography

Jordan KatineHGST

resist pillars

vias in AlOx

• fast, expensive, very effective


TEM Cross-section ofSpin-transfer Device

alumina

Au

Cu

IrMn 70 ÅCo 50 Å

Cu 40 ÅCo 30 Å

Cu 200 Å

Jordan KatineHGST


Au spheres as milling masksfor creating small pillars

• rapid process for pillar formation:

– attach gold particles (50 - 150 nm) to Au-coated full-film CPPsample

• Lewis et al., JVST B 16 (1998) 2938

– ion mill into MR stack to define pillars

– lift off Au particles

• minimum diameter determined by

layer mill rates and thicknesses

Liesl FolksHGST


Scanning Imaging/Conductance Probe

•imaging of pillars (tapping or contact mode AFM)

•IV curves measured with stationary conducting probe

•tunnel magnetoresistance TMR curves measured with conducting AFM (c-AFM) in applied fields

•require contact + lead resistance < 100 W

•ideally, contact + lead resistance << 100 W

•conductance maps with c-AFM

• Bietsch et al., JVST B 18 1160 (2000)

• Bietsch & Michel, APL 80 3346 (2002)

• Kubota et al., Jpn. JAP 41 L 180 (2002)

• Worledge & Abraham, APL 82 4523 (2003)


AlOx MTJ pillar• good contact (R = 50 W) onexposed PtMn surface

• pillar resistance RJ = 650 WRA = RJ * 0.05 = 32 W mm2

• TMR loop recorded during

30 second field sweep

• c.f. properties measured byconventional means:TMR = 20% and RA = 25 Wmm2

dR / R(10% per div.)

50Field (Oe)

0 100

contact modetopograph conductance map

24 %

Liesl FolksHGST


Au Bead Patterning Technique• Attach Au spheres by decorating CPP magnetic films with colloidal gold particles by immersion into aqueous suspension• Use bifunctional molecular linkers to attach particles to surface• Ar ion milling

• AlOx overcoat and subsequent Au lift-off

Liesl FolksHGST


STO

LSMO (600Å)

Au (1000Å)

Fe3O4

STO

PR

STO

PR PR

STO

SiO2SiO2

SiO2

Junctions are definedby photo lithographyand ion mill.

SiO2 are deposited bylow temperaturesputtering.

• Lift off SiO2

• Expose bottomelectrodes

• Au contact pads are definedby the third mask and afollowing lift off processing.

Fabrication of Magnetic Junctions


Spin Polarized Thin Film Materials

Soulen et al., Science 282, 85 (1998).

Sample

PbSpin polarization as measured by Andreev reflection


Spin Polarization

spin polarized materialsuperconductor spin polarized material

S-I-F junction F-I-F’ junction

superconductor

spin polarized material

Andreev Reflection


Magnetism at Surfaces and Interfaces

• intrinsic nature of ferromagnetism at surfaces and interfaces

Bulk, intermediate length scale (50Å) andsurface (5Å)magnetization are probed bySQUID, magnetic circular dichroism and spinpolarized photo-emission. Park et al. PRL 811953 (1998)

• surface/interface roughness• magnetic domain walls• electrode quality• barrier quality• interface quality


• Meservey-Tedrow junctions(superconductor-insulator-ferromagnet)

CrO2

Al

Parker et al. PRL 88 196601 (02)P=90-94%

• Point contact Andreev reflection Ji et al. PRL 86 5585 (01)

P=96%

Conductance

Voltage

Spin Polarization of CrO2


Spin Polarization of SrRuO3

• Negatively spin-polarized SrRuO3:

D. C. Worledge and T. H. Geballe, PRL 85, 5182 (2000)

T = 0.3K

SrRuO3 50nm

SrTiO3 (100)YBCO 50nm

Al 2nm AlOx

SrTiO31.5nm

H

ÿ A weak negative spinpolarization.

P = -0.095


Magnetic Tunnel Junctions (MTJ)

sP = 1/RP µ N1 N2 + N1 N2

sAP = 1/RAP µ N1 N2 + N1 N2

Tunnel resistance

P =N - N

N + N

Degree of spin polarization

N , N --- densities of majority andminority spin states at Fermi levelJunction

magnetoresistance……

JMR = RAP-RPRAP

JMR = =RAP RP

RAP

2P1P2

1+P1P2

For half metals where P=1, JMR=1 !

Fe3O4/Insulator/Fe3O4

Parallel magnetization

Anti-parallel magnetization

Minority spin band

Majority spin band

JMR

Applied Field


LCMO|NdGaO3|LCMO| epi

Manganite Junctions

Sun et al., APL73, 1008 (1998)Jo et al., PRB61, R14905 (2000)

-3 -2 -1 0 1 2 3

50

100

150 Cool-down A

d.paper.talk.APS.aps98.L100bb1.opj

Single Trace

R (kW

)

H (kOe)

0

100

200

300

Rhigh/Rlow=9.7

Cool-down B

8x2mm2, T=14K

Cool-down A10-Trace Averaged

LSMO|SrTiO3|LSMO| epi


Manganite Junctions

0 100 200 3000

200

400

f.paper.talks.aps99.fig4.opj

Rlow

Rhigh

R (kW

)

T (K)

0.0

1.0

2.0

3.0

Junction size=1x2mm2

DG

(mS)

0.24 0.26 0.28 0.30 0.32

104

105

106

130K

290K

T1/4 (K1/4)

R (W

)

-2 -1 0 1 20.122

0.124

0.126

0.128155K

H (kOe)

0.32

0.36

0.4095KR(

H)

(MW

)

0.26

0.28

0.30

0.3255K

0.30

0.35

0.40

0.45

f.paper.talk.aps.aps99.fig4b.opj

1x2mm2

Ibias=20nA15K

Sun et al., APL 73, 1008 (1998)

Strong decrease of MR upon increasing T.

Boulder Summer SchoolJ. Z. Sun, et al., PRB 61 11244 (2000)

Interface Stability

-4 -2 0 2 446

47

48

49

50

After anneal: +1 mA -1 mA

H (kOe)

-4 -2 0 2 49.0

9.5

10.0

H

T=13.4 K

Before anneal: +1 mA -1 mA

R (

kW)

2

8

• LSMO/STO/Fe junction

• Before/after 220C/15min:

• 5´ increase of RJ.

• Becomes asymmetric

• MR changes sign!

• Junction interface unstable

against moderate heat

treatment.

• Interface FeOx formation!


Manganite Junctions

Cross-sectional TEMof junction 30Å SrTiO3Barrier.

Noh et al. APL 79 234 (01)


CrO2 | Natural barrier | Co trilayers

X. W. Li, Ph. D. Thesis, 1999 (BrownUniv. w. Prof. Gang Xiao and A. Guptaat IBM).

• Observation of an inversemagnetoresistance


Fe3O4/SrTiO3/(La,Sr)MnO3 Junctions

Ghosh et al., APL 73, 689 (1998).


Fe3O4 based Epitaxial Oxide MTJs

Li et al. APL 73 3282 (1998)

van der Zaag et al.JMMM 211 301 (2001)

MR of 1.5% at 150K and <1% at 190K • small coercivity difference• tunnel barrier choicePoly Fe3O4 junctions (Panchula et al. unpublished )• JMR= -44% at low T

Fe3O4 MgO


Magnetic Tunnel Junctions (MTJ)

Minority spin band

Majority spin band

Low resistance

Fe3O4/Insulator/Fe3O4

High resistance





Fe3O4/Insulator/La0.7Sr0.3MnO3

Resistance

Applied Field

Resistance

Applied Field


3.32105

3.486105

3.652105

3.818105

3.984105

-20

-15

-10

-5

0

-4000 -2000 0 2000 4000

MR (%

)

0

-10

Field (kOe)0 2 4

-20-2-4

Magnetoresistance andMagnetics in OxideJunctions

• well defined parallel and antiparallelstates• magnetoresistance as high as 30%

R(kΩ

)

400

380

0-1000 1000Field (Oe)

Mag

neti c

mom

ent

( mem

u )

0

3

Fe3O4 (800Å)

La0.7Sr0.3MnO3 (600Å)

CoCr2O4 (60Å)

SrTiO3 substrate


I-V characteristics of the MTJ

-1000

-500

0

500

1000

0

1

2

3

4

5

6

-500 0 500Bias Voltage [mV]

T=200KC

urre

nt (

mA

)C

onductance [mW

-1]

ß By fitting the I-V data with Simmons’s tunnel equation forintermediate-voltage range (V<j0/e), we obtain the following results:j0 = 1.0eV, d = 20Å


Hopping Transport in Oxide Junctions

0

1

2

3

4

0 50 100 150 200 250 300 350 400

0.042

0.044

0.046

0.048

0.05

0.052

0 7 14 21 28 350 14 28

0.052

0.050

0.048

0.046

0.046

Cond

uct a

nce

( mΩ

-1)

Bias (mV)

Bias (mV)

GS= Gdir+Gres+SGNhop[V]N-2/(N+1)

Cond

uct a

nce

( mΩ

- 1)

• Inelastic hopping through localizedstates in the CoCr2O4 barrier• Hopping through 2 states inthe barrier fits well at low biaseswhile hopping through 2 or 3 statesfits well over a larger bias range

N=2 and 3

N=2

N


Spin Polarization of Fe3O4

• Spin polarized photoemission studies of Fe3O4: P=-40% at 10K Alvarado et al. PRL 34 319 (75)

P=-80% at RT Dedkov et al. PRB 65 64417 (02)

P=-55% at RT Huang et al. JMMM 239 261 (02)

• Fitting high temperature MR(T) to magnon excitations: P=Po(1-aT3/2)

0

5

10

15

20

25

30

35

0 50 100 150 200 250 300

MRSpin Polarization

0

-10

-20

-30

0 100 200 300

MR (%)Spin Polarization (%)

Temperature (K)

T [K]P[

%]

-

-

-

-

-

-

P0 = -39%

P=P0(1-aT3/2)*

10

15

20

25

30

35

100 120 140 160 180 200 220 240 260

Pth=-67% at 0KSrinitiwarawong et al. J.Phys.Cond.Matter 13 7987 (01)


Low temperature peak due to …..• Verwey transition of Fe3O4• Magnetic transition of the CoCr2O4 barrier

High T ->Higher MR Low T->Lower MR

0

5

10

15

20

25

30

35

0 50 100 150 200 250 300

MRSpin Polarization

0

-10

-20

-30

0 100 200 300

MR (%)Spin Polarization (%)

Temperature (K)

Spin Polarization of Fe3O4


Field-effect-transistor with manganite channel

• three terminal device based on manganites

• carrier modulation through PZT gate

T. Wu et al., PRL86, 5998 (2001).


Oxide Devices: Sensors

Stuart Parkin and Jonathan Sun, IBM

Sensor length scaleswith track width

• Sub-100nm scaling of MR headschallenge existing technology:– GMR: too low in resistance to give

enough signal.– MTJ: too resistive. Shot noise hurts

S/N.• Can oxide MR elements fill the

gap???+ Large MR.+ Intermediate resistance.- 300 K performance?- Switching field control?- Noise characteristics in deep sub-

micron region?- Processing compatibility?


Acknowledgements

• Liesl Folks, Hitachi Global Storage Tech.• Jordan Katine, Hitachi Global Storage Tech.• Caroline Ross, MIT• Jonathan Sun, IBM Yorktown Heights• Guohan Hu, Hitachi Global Storage Tech.

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