Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-18

Magnetic Dead Layer on c(8×4), p(2×2) Iron Silicide and Si(111)-7×7 Surfaces
Mohammad Tawheed Kibria, Tatsushi Nomitsu, Takeshi Nakagawa, and Seigi Mizuno
1

Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering,
Kyushu University, Kasuga-koen, Kasuga, Fukuoka 816-8580, JapanAbstract
We report the magnetic dead layer (MDL) at the interface of the Fe/iron silicides (c(8×4),
p(2×2)) and Fe/Si(111)-7×7 surface by magneto-optical Kerr effect (MOKE). We found that atroom temperature the ferromagnetic order appears at 5.5 ML, 8.2 ML and 10.9 ML forFe/c(8×4) silicide, p(2×2) silicide, and Si(111)-7×7 surface, respectively. From the thickness
dependent magnetization measurement, we decided the MDL for Fe/c(8×4) silicide, Fe/p(2x2)silicide, and Fe/Si(111)-7x7 to be 3 ML, 5 ML and 7.5 ML, respectively. Our results indicatethat the c(8×4) iron silicide surface prevents further silicide formation effectively than thep(2×2) silicide and Si(111)-7×7 surface.

1. Introduction
Magnetic dead layer (MDL) is the layer
which has no magnetic moments atinterfaces.1 MDL has been a concern since no
MDL would make the spin transport
efficient.2 Plenty of research have been
devoted to analyze the magnetic properties ofFe thick films on Si substrates.3,4 So far notenough experiments have not been conducted
for the investigation of the MDL of ironsilicide surfaces.5 A detailed study of theinterfacial magnetism of the ultrathin Fe filmon iron silicide phases, c(8×4) and p(2×2),
and Si(111)- 7×7 surfaces by magneto-optical
Kerr effect (MOKE). The interfacial MDL
for Fe film on iron silicide phases, c(8×4) andp(2×2), and Si(111)- 7×7 surfaces wasestimated by the MOKE. In particular, weshow that the MDL for c(8×4) silicide,p(2×2) silicide, and Si(111)- 7×7 surfaces are
3 ML, 5 ML, and 7.5 ML respectively.

2. Experimental
The experiments were conducted in a
multi-chamber UHV system with a basepressure of 2×10−8 Pa, allowing in situ
transfer between facilitates for surface
analytical technique (LEED) and magnetic
properties analysing technique (MOKE). The
ultrasonic cleaning with ethanol was done
before introducing the Si(111) crystal on anonmagnetic sample holder into the chamber.
The sample was degassed by direct current
heating at 800 K for 5-12 hours. After that,repeated flash heating was done at
approximately 1400 K, a clean reconstructed
Si(111)-7×7 surface was confirmed by Low
Energy Electron Diffraction (LEED).
Two iron silicide phases, c(8×4) andp(2×2), were prepared by the deposition of
1.7 monolayer (ML) Fe on Si(111)-7×7surface and the subsequent annealing at 900
K for c(8×4) and 700 K for p(2×2).6Figure 1. Experimental set-up for Magneto-optic Kerr Effect49

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-18
Fe thin films were prepared on theseiron silicide, (p(2×2) and c(8×4)) andSi(111)- 7×7 surfaces at room temperature(RT). The magnetic properties werecharacterized by in situ of magneto-opticKerr effect (MOKE) in polar and longitudinalgeometries (see Fig. 1). Fe deposition rate
wascalibrated
byQuartz
crystalmicrobalance (0.2 ML/min). The MOKE
experimental process shown in Fig. 2. Each
MOKE measurement was done after thedeposition of 2 ML of Fe on the (p(2×2) andc(8×4)) and Si(111)-7×7 surfaces andcontinued upto 30 ML of Fe film on thesample. The whole experiment was followed
the similar experimental process. Afterwards,
all the MOKE data was analyzed to exploredthe magnetic dead layer of each sample.

further study of the magnetic properties byMOKE.
3.2. Magneto-Optic Kerr Effect
The magnetic properties of the ultrathin
Fe films upto 30 ML on the iron silicides(p(2×2) and c(8×4)) and Si(111)- 7×7 surface
were investigated at RT. The presence of theferromagnetic order of Fe/iron silicidesurface and Fe/Si(111)- 7×7 surface wasstudied by several sets of magnetisation Kerrloops. We observed longitudinal geometry,
i.e., the magnetic field in-plane of the film,
throughout the whole Fe thickness range (seeFig. 3). Below the coverage of 5.5 ML, there
was no indication of a ferromagnetic (FM)order, in the iron silicides and Si(111)-7×7surface. This implies the absence of FM statein iron silicide and Si(111)-7×7 surface atlower Fe coverage. The MOKE result
showed two significant results in p(2×2) andc(8×4) iron silicide surface respectively.
Firstly, the FM order starts to appear at thecoverage of 5.5 ML Fe on c(8×4) silicidesurface. Secondly, the FM order occurs at thecoverage of 8.2ML Fe on p(2×2) silicidesurface. MOKE results did not show any FMorder occurrence in Si(111)-7×7 surface
below the 10ML Fe coverage at RT.
To identify the MDL on iron silicide andSi(111)-7×7 surface, we performed athickness dependent study of Kerrrotation (KR). Figure 1 shows the KR ofFe films grown on iron silicides andSi(111)-7×7. Thickness dependence ofKerr rotation shows that the Fe film onc(8×4) silicide has higher KR than thep(2×2) silicide and Si(111)-7×7 surface.
With increasing the Fe coverage, theKR linearly increases. On c(8x4)silicide, the extrapolated KR is zero at
3 ML, indicating that the magnetic
dead layer (MDL) for c(8×4) is 3 ML. In
the same way, Kerr rotation as afunction of Fe coverage indicates thatthe MDL is 5 ML and 7.5 ML for p(2×2)
silicide and Si(111)- 7×7 surface,respectively.

Figure 2. Flow-chart for experimental process.
3. Results
3.1. Low Energy Electron Diffraction

Fe growth on Si(111) surface wasobserved by LEED. The LEED patterns
indicated three different surface structures,
which are c(8×4) silicide, p(2×2) silicide andSi(111)-7×7 surface respectively. At the Fe
coverage of 1.7ML, the surface is fully
covered with the p(2×2) or c(8×4) silicide
films depending on the annealing
temperature. The p(2×2) phase appeared at
the annealing temperature of 700 K. With
increasing the annealing temperature upto
900 K the p(2×2) structure transformed to thec(8×4) silicide, which is in good agreementwith previous reports.4,6,7 The c(8×4) phase
was oriented with three orientation, each ofthis orientation is rotated by 120°. The c(8×4)
iron silicide phase did not appear at the higher
coverage of θFe~4ML, while the p(2×2)
silicide phase appeared at higher Fe
coverages (θFe>4ML).4 Observing the LEED
patterns help us to identify each surface ofiron silicides and Si(111)-7×7 towards the4. Discussions
The MDL for c(8x4), p(2x2), and Si(111)-50

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-18

Figure 3. Magnetization loops in the longitudinal Kerr effect as a function of Fe coverage on (a)c(8×4) silicide, (b) p(2×2) silicide, and (c) Si(111)-7×7 surface. Fe coverage is indicated in theplots.

7×7 are 3, 5, 7.5 ML, respectively. This is dueto the intermixing of Fe and Si leading to theformation of thin non-magnetic

rather stable.
5. Conclusion
To summarise, we have demonstrated
the interfacial magnetism and a magnetic
dead layer for ultrathin Fe film/iron silicideand Si(111)-7×7 interface at RT by MOKE.
The results show that~5.5 ML Fe/c(8×4)
silicide, 8.2 ML Fe/p(2×2) silicide and 10 ML
Fe/Si(111)-7×7 interface is not FM ordered
respectively. This different behaviour of theFe film on the iron silicide and Si(111)-7×7
surfaces is due to the interfacial intermixing
between Fe and Si atom. The thickness
dependent study of Kerr rotation indicates theMDL for Fe/c(8×4) silicide is 3 ML,Fe/p(2×2) silicide is 5 ML and Fe/Si(111)7×7 is 7.5 ML respectively. The interfacialiron layer does not contribute to theferromagnetism. Our study explores theinterfacial FM nature of the ultrathin Fe film
on iron silicide and Si(111)-7×7 surface.

Figure 4. Magnetic Dead Layer Model.
Middle layer is (a) MDL for c(8×4) and (b)MDL for p(2×2), Top layer is Fe film inboth case. Arrow indicates the FM state of
top layer in both system.

Fe-Si interfacial phases. The additional Fe
deposition over the MDL interfaces produces
the FM order for iron silicide and Si surfaces(see Fig 4.). As in the LEED result, the c(8×4)
silicide on the clean Si(111)-7×7 surface isprepared at a higher temperature, comparedwith the p(2x2) silicide. Thermal stability ishigher than the p(2×2) silicide as reported byM. Krause et al. and S. Hajjar et. al.6,7 Ourresults indicate that the c(8×4) Fe silicidesurface prevents further silicide formation
effectively since the c(8×4) surface preparedat the higher annealing temperature would be

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Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

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