Growth Behavior of Co on Al(001) substrate

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. Co Al 3ML Al on Co(001) 2.86 Å 4.05 Å 3ML Co on Al(001) Surface Alloying of Co on Al(001) 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. Nonmagnetic behavior Magnetic behavior B2 - CoAl Magnetic Behavior of CoAl • B2-CoAl & Al-rich CoxAl1-x Nonmagnetic behavior • Co-rich CoxAl1-x Ferro magnetic behavior Al-rich Co-rich

5. Motivation Al deposition Co substrate Al on Co substrate Co: Ferromagnetic CoAl: Nonmagnetic GMR structure 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. z[001] y[010] x[100] Calculation Methods (001) Substrate Conditions • Embedded Atom Method Potentials for Co-Al system • x,y  Periodic Boundary Conditionz  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:1.306 × 10-1 nm/nsec 1440 atoms (144atoms/ML) Substrate dimensions : (12  6  5)a0 a0 : bulk lattice constant

7. Co Thin Film I 1~10ML: 0.1 eV incident E. of Co adatom 10~20ML: 3.0 eV 12 ML 15 ML 18 ML z[001] x[100]

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

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

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

11. Co Thin Film Growth II 1~20 ML: 0.1 eV, Sub. Temp.: 300 K 12 ML 15 ML 18 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.