Molecular Dynamics Simulation to understand the Interfacial Structure of Thin Films Sang-Pil Kim and Kwang-Ryeol Lee Computational Science Center Korea Institute of Science and Technology Good morning, my name is sang-pil kim from KIST. Today, I will talk about my research results, entitled “Molecular dynamics simulation to understand the interfacial structure of thin films”.

Devices with Thin Multilayers Spin valve for magnetic devices As the scale of devices goes down to an atomic level, controlling and understanding at the interface is crucial for the performance of the devices. Especially this effect can significantly affect the magnetic devices such as GMR or TMR. In this present work, we investigated the deposition behavior in Co-Al system by using MD simulation. And we focused on interface and surface structure evolution during deposition. Major materials issue is the interfacial structure in atomic scale

Calculation Procedure Adatom (normal incident  0.1 eV) 300K Initial Temperature 300K Constant Temperature Fixed Atom Position Co-Al EAM potential x,y-axis : Periodic Boundary Condition z-axis : Open Surface MD time step : 1.0 fs R. Pasianot et al, Phys. Rev. B45, 12704 (1992). A. F. Voter et al , MRS Proc. 82, 175 (1987). C. Vailhe et al, J. Mater. Res. 12, 2559 (1997).

Deposition in Co-Al system Deposition energy = 0.1 eV This is the deposition animation of Al on Co and its opposite case. The deposition energy of each adatom was set 0.1 eV which is similar with the thermal evaporation energy. As you can see, in the case of Al on Co, Al atom is only landed and vibrated at the position, but in the case of Co on Al surface, Co atom is mixed with Al surface atoms at the moment of approaching the surface. Al on Co (001) Co on Al (001)

Conventional Concepts This behavior could not be explained by conventional concept of deposition behaviors. Because the conventional concept regarded substrate as a static media. According to this concept, we could imagine only these kinds of behaviors like this. Ultra thin film Substrate is not a static media but an active media!

Asymmetry in Interfacial Mixing 3ML Al on Co (001) 8ML Co on Al (001) However, as you can see, when many atoms were deposited sequentially up to several mono layers, the difference between two cases becomes obviously. Interface structure in the case of Co on Al(001) turned out to be a B2 structure which is very stable inter-metallic compound between Co and Al system.

Energy Barrier for Intermixing Incorporation energy barrier calculation by using DFT Surface intermixing could not be happened such a low incident energy of adatom(0.1eV) Interestingly, these results were obtained in the 0.1eV of incident energy. From the first principle calculation, the energy barrier for the intermixing was much higher than incident energy. C. Kim et al., J Korean Phys. Soc. (2004).

Local Acceleration Effects As the depositing atom approaches to the substrate, the kinetic energy of adatom increases very rapidly at the vicinity of the substrate surface.  “Local Acceleration Effect*” About this, We suggested the local acceleration effect and confirmed that this effect enable adatom to mix in such a low incident energy. From monitering the kinetic energy of adatom, as you can see, as the depositing atom approaches to the surface, the kinetic energy of adatom increased very rapidly at the vicinity of the substrate surface. In the case of Co on Al, about 4 eV regardless of initial energy was calculated.

Local Acceleration Deposition energy = 0.1 eV Al on Co (001) Let’s see this movies again. Different from the previous viewing, we could observe the local acceleration of adatoms at the moment of approaching the surface. Al on Co (001) Co on Al (001)

Contour of Local Acceleration Co on Al (001) Al on Co (001) This acceleration is highly dependent on the local geometry. Therefore, the maximum value was shown in the hollow site, the minimum was shown on top site. Comparing this acceleration with energy barrier, we could confirm that the local acceleration played an important role in forming asymmetric intermixing in such low deposition energy. Energy barrier for atomic penetration Without relaxation With relaxation Median value Co on Al(001) 4.78 eV 0.56 eV 2.67 eV Al on Co(001) 5.95 eV 1.66 eV 3.81 eV

Experimental Evidences Coaxial Impact Collision ion Scattering Spectroscopy (CAICISS) Al on Co (0001) Co on Al (001) This prediction by using MD simulation was confirmed from the experimental work. From measuring this spectroscopy, only in the case of deposition Co on Al, surface structure was changed. This is the experimental evidence of asymmetric intermixing behavior. Only in the case of deposition Co on Al sub., surface structure was changed.  Experimental evidence of asymmetric intermixing behavior

Magnetic Property Calculation Nonmagnetic behavior Magnetic behavior B2 - CoAl Ab-initio calculations Spin-Up Spin-Down FCC - Al B2 - CoAl HCP - Co We calculated the magnetic properties of various compositions in Co-Al system by using ab-initio calculation. From this calculation, B2-CoAl does not show any magnetic behavior only show the magnetism in Co-rich case. Nonmagnetic Metal Nonmagnetic Metal Magnetic Metal  The perfectly ordered B2-CoAl does not show any magnetic behavior

Experimental Evidences Asymmetric Magnetic Behaviors in Co-Al system Co/Co(3nm)/Cu/Si subs.  Reference sample Cu/Al(3nm)/Co(3nm)/Cu/Si subs.  Al/Co case Cu/Co(3nm)/Al(84nm)/Si subs.  Co/Al case This prediction was also confirmed by MOKE measurement of these three cases. As a result, in the case of Co on Al case, the magnetism was only decreased, I could estimate the mixing thickness from calculate degree of the decreasing. As a result, the thickness was quantitatively agreement with the MD prediction. Only in the Co/Al case, magnetism was reduced by interfacial mixing. Hysteresis loop cannot be observed when the thickness of Co was smaller than 1 nm or 3 atomic layers, which agrees well with the MD results

Conclusions The asymmetry of the surface intermixing during thin film growth can be understood by comparing the local acceleration effect with the energy barrier for mixing. CAICISS & MOKE results were clearly shown the asymmetric intermixing as predicted in MD simulations. Magnetism of B2-CoxAl1-x alloy was exhibited only for the Co concentration higher than 50 atomic %. Let me conclude this work, among the thin film growth behaviors, we could observe unusual intermixing behavior in Co-Al system. From this work, we could explain this asymmetric behavior in an atomic point of view by using classical MD simulation method. And we could suggest this asymmetric intermixing was mainly due to the different local acceleration effect and different energy barrier for intermixing. And our understandings are confirmed by various experimental measurements. Thank you for your attention.