1. Growth Behavior of Co on Al(001) substrate
Sang-Pil Kim1),2), Seung-Cheol Lee1), Kwang-Ryeol Lee1) and Yong-Chae Chung2)
Future Technology Research Division, KIST, Seoul, Korea
Department of Ceramic Engineering, Hanyang University, Seoul, Korea

2. Introduction
1~2nm
Typical structure of spintronic device
The efficiency of spintronic devices (e.g. TMR, GMR ... ) are largely dependent on the electronic structure of the active layers, and electronic structures are closely related to both interface and thin film structures.
Research on interface or thin film structures in hetero-epitaxial growth
FCC/BCC  Ni on Fe(100), Cu on Fe(100), Fe on Al(100) … HCP/FCC, BCC/HCP  Co on Cr(100), Co on Mo(100) …

3. Surface Alloying of Co on Al(001)
3ML Al on Co(001)
3ML Co on Al(001) Co
4.05 Å
2.86 Å Al
Even when the incident energy of the Co atom was 0.1 eV spontaneous intermixing of Co and Al was observed*.
Surface unit cell of Al(001) and B2-CoAl(001) have less than 0.1% lattice mismatch  Highly ordered surface alloying
* S.-P. Kim et al., J. Appl. Phys., 93, 8564 (2003)

4. Magnetic Behavior of CoAl
Nonmagnetic
behavior
Magnetic
B2 - CoAl
B2-CoAl & Al-rich CoxAl1-x  Nonmagnetic behavior
Co-rich CoxAl1-x  Ferro magnetic behavior
Al-rich
Co-rich

5. Motivation Investigate thin film growth behavior of Co films on CoAl
Al deposition
Co substrate
Al on Co substrate
Co: Ferromagnetic
GMR
structure
CoAl: Nonmagnetic
Co: Ferromagnetic
Active intermixing & Nonmagnetic B2-CoAl for Co on Al substrate
Sharp interface & Layer-by-layer growth of Al for Al on Co substrate
Investigate thin film growth behavior of Co films on CoAl

6. Calculation Methods (001) Substrate Conditions
Embedded Atom Method Potentials for Co-Al system
x,y  Periodic Boundary Condition z  Open surface (fixing the bottom-most two layers)
Substrate Temp. : 300K
Incident Energy with normal incidence  0.1 eV, and 3.0 eV
Time step : 1 fs  5000 fs (5 ps) / atom
Deposition rate: × 10-1 nm/nsec
1440 atoms (144atoms/ML)
Substrate dimensions : (12  6  5)a0
x[100]
z[001]
y[010]
a0 : bulk lattice constant

7. Co Thin Film I 1~10ML: 0.1 eV incident E. of Co adatom 10~20ML: 3.0 eV
z[001]
x[100]

8. Size Effect Test Same behavior! (50×6×8)a0  9600 substrate Al atoms
15 ML
18 ML
Same behavior!
20 ML

9. Phase Transition fcc-Co: 3.56 Å vs. Al: 4.05 Å
Sinking due to the lattice mismatch
A little moved to the one side
≈60°
fcc-Co: 3.56 Å
vs.
Al: 4.05 Å
Face centered tetragonal (FCT)
HCP (11-20)
A: HCP, B: FCT, C: B2(BCC)

10. Geometric RelationshipsCo
~ 0.1 %
7.4 % larger than substrate
CoAl
x[100]
z[001] Al

11. Co Thin Film Growth II 1~20 ML: 0.1 eV, Sub. Temp.: 300 K 12 ML 15 ML
z[001]
x[100]

12. Schematic of Strain Energy
Whole system
Strain Energy
Al substrate
Co film
HCP
HCP+FCT
FCT
Co film structure

13. Summary
Growth behavior of Co thin film on Al substrate by Molecular Dynamics simulation.
Co thin film was composed of two different hetero epitaxial grains of HCP and FCT structure.
The fraction of each grain seems to be determined by minimizing the total strain energy of both thin film and substrate.