Kinetics and Energetics of Kinetics and Energetics of Interfacial Mixing in Co-Cu Interfacial Mixing in Co-Cu systemsystem

Seung-Suk Yoo3, Sang-Pil Kim1,2, Seung-Cheol Lee1, Kwang-Ryeol Lee1 and Yong-Chae Chung2

1. Future Technology Research Division, KIST2. Department of Ceramic Engineering, Hanyang University

3. School of Materials Science & Engineering, Seoul National University

KIMM KIMM 상변태 열역학 분과 통합 심포지움상변태 열역학 분과 통합 심포지움

MotivationMotivation

Interfacial Mixing for binary system

Miscible system? Immiscible system?

Surface energy differenceor

Strain energy difference (lattice mismatch)

Co-Al, Co-Ti, Ni-Al, Fe-Al … Co-Cu, Fe-Cu, Fe-Ag, Fe-Au…

Co-Al system : large surface energy difference (Δγ = 60 %)large lattice mismatch (Δa0 = 14 %)

Co-Cu system : Δγ = 28.5 %, Δa0 = 1.8 %

Surface alloy formation for Co-AlSurface alloy formation for Co-Al

In spite of room temp. (300K) and very low incident energy of adatom(0.1eV), spontaneous surface alloy was formed.

In spite of room temp. (300K) and very low incident energy of adatom(0.1eV), spontaneous surface alloy was formed. Local acceleration

Co/Al(001)

Low Activation Barrier

Co on Al (001)*

4ML Co on Al(001)

* N.R. Shivaparan, Surf. Sci. 476 152 (2001)

Intermixing of immiscible systemIntermixing of immiscible system

Burrowed Co nanoparticle on Cu(001) surface

Afte

r a

nne

alin

g

at 7

50K

~ 1

00

0K

C. Zimmermann et al., PRB 64, 085419 (2001). D.A. Stewart et al., PRB 68, 014433 (2003).

Interfacial mixing effects

Computational ProcedureComputational Procedure

300K Initial Temperature

300K Constant Temperature

Fix Position

• 1024 Substrate Atoms, 300K• Step Time : 1.0 fs• Case I : Co/Cu (001)• Case II : Cu/fcc-Co(001) • Incident Energy : 0.1eV, 1.0eV, 3.0eV, 5.0eV

XMD 2.5.32 code : MD program http://www.ims.uconn.edu/centers/simul

y[010]

x[100]

z[001]

(001) Substrate(001) Substrate

EAM Potential for Co-Cu system*EAM Potential for Co-Cu system*

Co CuExpt. Calc. Expt. Calc.

a0 (Å) 2.507 2.5107 3.615 3.615

Ecoh (eV) 4.386 4.408 3.513 3.538

B (Gpa) 180 182.3 140 137.5

γ100 (J/m2) N/A 2.789 2.166 1.987

γ110 N/A 3.051 2.237 2.166

γ111 N/A 2.591 1.953 1.903

γ1000 2.775 2.879 N/A N/A

γ-1010 3.035 3.042 N/A N/A

γ11-20 3.791 3.350 N/A N/A

* X. W. Zhou et al., Acta. Mater., 49, 4005 (2001).

Results for low incident energyResults for low incident energy

0.1eV Co on Cu (001) 0.1eV Co on Cu (001)

Mixing Ratio : 1.56%

TopView

SideView

128 atoms 384 atoms

Mixing Ratio : 0.0 %

0.1eV Cu on Co (001) 0.1eV Cu on Co (001)

128 atoms 384 atoms

Results for 5.0 eVResults for 5.0 eV

Cu on Co (001) Cu on Co (001)

128 atoms 384 atoms

Mixing Ratio : 0.78 %

Co on Cu (001) Co on Cu (001)

128 atoms 384 atoms

Mixing Ratio : 21.1%

TopView

SideView

»»

AnalysisAnalysis

Related Factors for MixingRelated Factors for Mixing

Calculate activation barrier on an interfacial mixing

Check the system energy evolutions

Kinetic Factor

Energetic Factor

Local Acceleration Phenomena*Local Acceleration Phenomena*

Co on Cu(001)

0 100 200 300 400 500

0

2

4

6

8

7.610 eV

5.622 eV

3.583 eV

2.734 eV

Kin

etic

En

erg

y (e

V)

# of MD calculations

Co on Cu(001) 0.1 eV 1.0 eV 3.0 eV 5.0 eV

2.63 eV

* S. –P. Kim et al., J. Korean Phys. Soc., 44, 18 (2004).

0 100 200 300 400

0

2

4

6

8

7.797eV

5.930eV

3.901eV

2.923eV

Kin

etic

En

erg

y (e

V)

# of MD Calculations

Cu on Co(001) 0.1eV 1.0eV 3.0eV 5.0eV

2.89 eV

Cu on Co(001)

Local Local AccelerationAcceleration

ContourContour

(001) (001) Surface Surface

Atomic BehaviorsAtomic Behaviors

Mixed mechanismMixed mechanism

Side view

Top view

Side view

Unmixed mechanismUnmixed mechanism

Kinetic Energy EvolutionsKinetic Energy Evolutions

5400 5600 5800 6000 6200 6400 6600-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

Kin

etic

En

erg

y (e

V/A

tom

)

Step

bottom Deposited Co Atom Cu#1 Cu#2 Cu#3 Cu#4

MixingInduced Collision

Bottom Absorption

K.E. of each atoms around incident atom

1

34 2

5400 5600 5800 6000 6200 6400 6600-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

Kin

etic

En

erg

y (e

V/A

tom

)Step

bottom Co#1 Co#2 Co#3 Co#4 Deposied Cu

NO Bottom Absorption

Bump Up

1

34 2

Mixed caseMixed case Unmixed caseUnmixed case

Total Energy Changing

: - 0.476 eV

Mixing can be happened!

Total Energy Changing

: + 0.419 eV

Mixing is very difficult!

0 5 10 15 20 25 30 352.0

2.5

3.0

3.5

4.0

4.5

5.0

Re

lativ

e E

ne

rgy

(eV

)

Number of Calculation

Exchange Barrier of IntermixingExchange Barrier of Intermixing

0.553 eV

ExchangeMechanism

0 5 10 15 20 25 30 35

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Rel

ativ

e E

nerg

y (e

V)

Number of Calcuation

1.21 eV

Co on Cu(001) Cu on Co(001)

Cont.Cont.

16 Co Atoms Mixed on Cu(001)

Energy Barrier

Energy Reduction

9 Cu Atoms forced Mixing on Co(001)

-4.181

-4.180

-4.179

-4.178

-4.177

-4.176

-4.175

-4.174

Number of Mixed Cu Atom

1 2 3 4 5 6 7 8 9

Tot

al E

nerg

y (e

V/a

tom

)

Energy Increase

Energy Barrier

Energy Barrier for Mixing in Cu/Co(001) is 2.5 times higher than Co/Cu(001)

Effective Induced Collision happened Only in Co/Cu(001) Kinetically Mixing can be happened in Co/Cu(001) Kinetically Mixing hardly be happened in Cu/Co(001)

SummarySummary

1. Through the molecular dynamics approach, quantitative analysis in detail for atomic mixing behaviors were investigated.

2. In spite of immiscible system, interfacial mixing of Co on Cu(001) can be observed.

3. Intermixing barrier of Cu on Co(001) is much higher than that of Co on Cu.1.21 eV >> 0.553 eV

4. In the case of Co on Cu is energetically stable (-0.476 eV), but Cu on Co is unstable. (+0.419 eV)