Growth Behavior of Co on Growth Behavior of Co on Al(001) substrateAl(001) substrate

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

1) Future Technology Research Division, KIST, Seoul, Korea

2) Department of Ceramic Engineering, Hanyang University, Seoul, Korea

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IntroductionIntroduction

• 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) …

1~2nm

Typical structure of spintronic device

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Surface Alloying of Co on Al(001)Surface Alloying of Co on Al(001)

Surface unit cell of Al(001) and B2-CoAl(001) have less than 0.1% lattice mismatch Highly ordered surface alloying

Even when the incident energy of the Co atom was 0.1 eV spontaneous intermixing of Co and Al was observed*.

Co

4.05 Å

2.86 Å

Al

3ML Co on Al(001)

* S.-P. Kim et al., J. Appl. Phys., 93, 8564 (2003)

3ML Al on Co(001)

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Magnetic Behavior of CoAlMagnetic Behavior of CoAl

B2 - CoAlB2 - CoAl

B2-CoAl & Al-rich CoxAl1-x Nonmagnetic behavior

Co-rich CoxAl1-x

Ferro magnetic behavior

Nonmagneticbehavior

Magneticbehavior

Al-rich Co-rich

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MotivationMotivation

Co: Ferromagnetic

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

Al deposition

Co substrate Al on Co substrate

GMRstructure

Investigate thin film growth behavior of Co films on CoAl

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Calculation MethodsCalculation Methods

(001) Substrate(001) Substrate

1440 atoms (144atoms/ML)

Substrate dimensions : (12 6 5)a0

a0 : bulk lattice constant

• 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: 1.306 × 10-1 nm/nsec

ConditionsConditions

x[100]

z[001]y[010]

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Co Thin Film ICo Thin Film I1~10ML: 0.1 eV incident E. of Co adatom10~20ML: 3.0 eV

12 ML

15 ML

18 ML

x[100]

z[001]

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Size Effect TestSize Effect Test

15 ML

18 ML

20 ML

(50×6×8)a0 9600 substrate Al atoms

Same behavior!

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Phase TransitionPhase Transition

A: HCP, B: FCT, C: B2(BCC)

Sinking due to the lattice mismatch A little moved to the one side

Face centered tetragonal (FCT) HCP (11-20)

≈60°

fcc-Co: 3.56 Åvs.

Al: 4.05 Å

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Geometric RelationshipsGeometric Relationships

Al

CoAl

Co

7.4 % larger than substrate

~ 0.1 %

x[100]

z[001]

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Co Thin Film Growth IICo Thin Film Growth II

12 ML

15 ML

18 ML

1~20 ML: 0.1 eV, Sub. Temp.: 300 K

x[100]

z[001]

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Schematic of Strain EnergySchematic of Strain Energy

Str

ain

Ene

rgy

HCP FCT

Al substrate

Co film

HCP+FCT

Whole system

Co film structure

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Growth behavior of Co thin film on Al substrate by

Molecular Dynamics simulation.

Co thin film was composed of two different hetero e

pitaxial grains of HCP and FCT structure.

The fraction of each grain seems to be determined b

y minimizing the total strain energy of both thin film

and substrate.

SummarySummary