The International Conference On Metallurgical Coatings And Thin Films ICMCTF 2005 CMSELCMSEL Hanyang Univ. Co/CoAl/Co Trilayer Fabrication Using Spontaneous Intermixing of Co and Al: Molecular Dynamics Simulation Sang-Pil Kim 1), Seung-Cheol Lee 2), Kwang-Ryeol Lee 2) Deok-Soo Kim 3) and Yong-Chae Chung 1) 1)Department of Ceramic Engineering, Hanyang University, Seoul, Korea 2)Future Technology Research Division, KIST, Seoul, Korea 3)Department of Industrial Engineering, Hanyang University, Seoul, Korea

CMSELCMSEL Hanyang Univ. ICMCTF 2005 TMR J. Appl. Phys., 93, 467 (2003) NATURE Mater., 3, 862 (2004) NATURE Mater., 3, 868 (2004) Magnetic RAM (MRAM) Properties of MRAM are largely dependent on the atomic structure of Interface and Nonmagnetic layer Amorphous Al 2 O 3 Single Crystal MgO(001)

CMSELCMSEL Hanyang Univ. ICMCTF 2005 J. Appl. Phys., 93, 8564 (2003) Surface Alloying of Co-Al Amorphous mixing Highly ordered mixing Surface unit cell of Al(001) and B2-CoAl(001) has less than a 0.1% lattice mismatch  Highly ordered intermixing Even when the incident energy of the Co atom was 0.1 eV, spontaneous intermixing of Co and Al was formed. Co 4.05 Å 2.86 Å Al

CMSELCMSEL Hanyang Univ. ICMCTF 2005 MRS Proceeding 777, T (2003) *J. Korean Phys. Soc., 45, 1210 (2004) Thin Film Growth Behavior 3ML Al on Co(001) 3ML Co on Al(001) Atomic configurationsLayer density No mixing & Sharp Interface Mixing & Interface alloying  Largely different thin film growth behavior was reported  It could be explained by the kinetic energy barrier for intermixing*

CMSELCMSEL Hanyang Univ. ICMCTF 2005 Stable intermetallic compound  B2(CsCl) structure Co-Al system B2 - CoAl Spin-Up Spin-Down FCC - Al HCP - Co B2 - CoAl  The perfectly ordered B2-CoAl does not show any magnetic behavior Nonmagnetic MetalMagnetic MetalNonmagnetic Metal Ab-initio calculations

CMSELCMSEL Hanyang Univ. ICMCTF 2005 Co (FM) layer CoAl (NM) layer Final Goal Co on Al(001) Spontaneous intermixing of Co on Al On Al(001), formation of highly ordered B2-CoAl Al on Co & Co on CoAl  No mixing Co  Ferromagnetic material Al & B2-CoAl  Nonmagnetic materials Al on Co(001) Co on B2(Al top)

CMSELCMSEL Hanyang Univ. ICMCTF 2005 Co(1120) HCP(1120) vs. BCC(100) J. Crystal Growth, 139, 363 (1994)Appl. Surf. Sci., 113/114, 165 (1997)  Epitaxial growth of Co(1120) films on Mo(100) or Cr(100) has been reported.  Correlation between HCP(1120) and cubic(100) orientation was successfully explained in Acta Mater., 53, 1073 (2005)  Epitaxial growth of Co(1120) films on Mo(100) or Cr(100) has been reported.  Correlation between HCP(1120) and cubic(100) orientation was successfully explained in Acta Mater., 53, 1073 (2005) HCP-Co(1120)/BCC-Mo(100)HCP-Co(1120)/BCC-Cr(100)

CMSELCMSEL Hanyang Univ. ICMCTF 2005 Geometric Relationships  Fabrication of well-ordering Co(1120)/CoAl(100)/Co(1120) StructureDirectionLength (Å) HCP Co [1100] 4.34 [0001] 4.07 B2 CoAl [110] 4.04 FCC Al [100] 4.05

CMSELCMSEL Hanyang Univ. ICMCTF 2005 Calculation Methods Co(1120) substrate: 3840 atoms (192 atoms/ML) Interatomic potential  Co-Al embedded atom method (EAM) potential* x,y  Periodic Boundary Condition z  Open surface (fixing the bottom-most two layers) Sub. Temp. : 300K Incident Energy (K i )  0.1 eV for Al-depo., 3.0 eV and 5.0 eV for Co-depo. The positions of adatoms  Randomly selected in xy plane at the distance of 30Å Time step : 1 femto-second (fs) Deposition rate:  nm/nsec MD program: XMD code ( * J. Mater. Res., 12, 2559 (1997)

CMSELCMSEL Hanyang Univ. ICMCTF ML Al on Co(1120) Al on Co(1120) 5ML Al on Co(1120) No mixing Lattice distortion Co(1120) substrate Al adatoms with the incident energy of 0.1 eV on Co(1120) substrate Well separated interface between Al and Co substrate Al film was a little distorted due to the lattice mismatch  a Al =4.05 Å, a Co = 4.07 Å [0001], 4.34 Å [1100]

CMSELCMSEL Hanyang Univ. ICMCTF 2005 Atomic Configuration 20 MLs of Co atoms were subsequently deposited with a relatively high incident energy of 3.0 eV and 5.0 eV on the FCC Al(100) layers. Well ordered Al(100) films apparently disappeared  Most of Al atoms seemed to be mixed with deposited Co atoms. Similar crystallographic symmetry Strong affinity between Co and Al  Enhanced the epitaxial growth of well-intermixed alloys on the Co(1120) surface

CMSELCMSEL Hanyang Univ. ICMCTF 2005 Layer Coverage Layer coverage fraction of Co and Al atoms along the [1120] direction. Deposit with 3.0 eV incident energy resulted in more pronounced alternating mixed layers than the case of 5.0 eV. Sharp interface could be observed between CoAl mixed layer and Co(1120) film nm 0.88 nm Displacement of Al atoms from their original positions in the course of deposition process.

CMSELCMSEL Hanyang Univ. ICMCTF 2005 Local Structure Distribution BCC or B2 atoms: surrounded by eight nearest neighbor atoms  59.5% for 3.0 eV, 40.1% atoms for 5.0 eV identified as BCC atoms Various compositions of Co x Al 1-x structures were formed 3.0 eV incident energy: bimodal distribution  Co atoms penetrated and mixed less actively with Al atoms 5.0 eV: major two peaks Co x Al 1-x (x=0.5 & 0.812) B2-CoAl  well-intermixed, Co x Al 1-x (x=0.812)  80% zone Information of the exact local geometric structures at the mixed region

CMSELCMSEL Hanyang Univ. ICMCTF 2005 Structural Configuration Co(1120)/CoAl/Co(1120) multilayers Structural configurations after filtering process Classified into HCP, BCC, Other (not belonging to BCC or HCP) Mixed region composed of BCC atoms could be clearly observed  Sharp interfaces of Co(1120)/Co x Al 1-x, and Co x Al 1-x /Co(1120) BCC atom HCP atom Other

CMSELCMSEL Hanyang Univ. ICMCTF 2005 Conclusions Based on the molecular dynamics, the multilayer system of Co(1120)/Co x Al 1-x /Co(1120) was proposed and performed the quantitative atomic and structural analysis. FCC-Al(100) films grew on Co(1120) without intermixing  Large difference of activation energy barrier for Co/Al and Al/Co Subsequent deposition of Co atoms with 3.0 eV and 5.0 eV on 3ML Al(100) thin films  Co x Al 1-x structures of various compositions were found to be formed in the mixed region