1. Kinetics and Energetics of Interfacial Mixing in Co-Cu system
KIMM 상변태 열역학 분과 통합 심포지움
Kinetics and Energetics of Interfacial Mixing in Co-Cu system
Seung-Suk Yoo3, Sang-Pil Kim1,2, Seung-Cheol Lee1, Kwang-Ryeol Lee1 and Yong-Chae Chung2
1. Future Technology Research Division, KIST
2. Department of Ceramic Engineering, Hanyang University
3. School of Materials Science & Engineering, Seoul National University

2. Motivation Interfacial Mixing for binary system Miscible system?
Immiscible system?
 Co-Al, Co-Ti, Ni-Al, Fe-Al …
 Co-Cu, Fe-Cu, Fe-Ag, Fe-Au…
Surface energy difference or
Strain energy difference (lattice mismatch)
Co-Al system : large surface energy difference (Δγ = 60 %) large lattice mismatch (Δa0 = 14 %)
Co-Cu system : Δγ = 28.5 %, Δa0 = 1.8 %

3. Surface alloy formation for Co-Al
* N.R. Shivaparan, Surf. Sci (2001)
Co on Al (001)*
4ML Co on Al(001)
Low Activation Barrier
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)

4. Intermixing of immiscible system
C. Zimmermann et al., PRB 64, (2001).
D.A. Stewart et al., PRB 68, (2003).
After annealing
at 750K ~ 1000K
 Burrowed Co nanoparticle on Cu(001) surface
 Interfacial mixing effects

5. Computational Procedure
(001) Substrate
300K Initial Temperature
300K Constant Temperature
Fix Position
y[010]
x[100]
z[001]
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 code : MD program 

6. EAM Potential for Co-Cu system*
Expt.
Calc.
a0 (Å)
2.507
2.5107
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
3.051
2.237
γ111
2.591
1.953
1.903
γ1000
2.775
2.879
γ-1010
3.035
3.042
γ11-20
3.791
3.350
* X. W. Zhou et al., Acta. Mater., 49, 4005 (2001).

7. Results for low incident energy
0.1eV Co on Cu (001)
0.1eV Cu on Co (001)
Top
View
128 atoms
384 atoms
128 atoms
384 atoms
Side
View
 Mixing Ratio : 1.56%
 Mixing Ratio : 0.0 %

8. » Results for 5.0 eV Co on Cu (001) Cu on Co (001) Top View Side View
128 atoms
384 atoms
128 atoms
384 atoms
Side
View
 Mixing Ratio : 21.1%
 Mixing Ratio : 0.78 %

9. Related Factors for Mixing
Analysis
Related Factors for Mixing
Kinetic Factor
Calculate activation barrier on an interfacial mixing
Energetic Factor
Check the system energy evolutions

10. Local Acceleration Phenomena*
Co on Cu(001)
Cu on Co(001)
2.63 eV
2.89 eV
Local Acceleration Contour
(001) Surface
* S. –P. Kim et al., J. Korean Phys. Soc., 44, 18 (2004).

11. Atomic Behaviors Mixed mechanism Side view Top view Unmixed mechanism

12. Kinetic Energy Evolutions
 K.E. of each atoms around incident atom
Mixed case
Unmixed case
Bottom Absorption
NO Bottom Absorption
Induced Collision
Bump Up
Mixing 1 1 4 2 4 2 3 3

13. Exchange Barrier of Intermixing
Mechanism
Co on Cu(001)
Cu on Co(001)
0.553 eV
1.21 eV
Total Energy Changing
: eV
 Mixing can be happened!
Total Energy Changing
: eV
 Mixing is very difficult!

14. 9 Cu Atoms forced Mixing on Co(001)
Cont.
16 Co Atoms Mixed on Cu(001)
9 Cu Atoms forced Mixing on Co(001)
Energy Increase
Energy Barrier
Energy Barrier
Energy Reduction
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)

15. Summary
Through the molecular dynamics approach, quantitative analysis in detail for atomic mixing behaviors were investigated.
In spite of immiscible system, interfacial mixing of Co on Cu(001) can be observed.
Intermixing barrier of Cu on Co(001) is much higher than that of Co on Cu. 1.21 eV >> eV
In the case of Co on Cu is energetically stable ( eV), but Cu on Co is unstable. ( eV)