In the present work we report on magnetization reversal process, anisotropy and domain structures in ultrathin Au/Co(0001)/Au films deposited on vicinal Si(111) substrates. The measurements were performed using a magneto-optical Kerr effect based magnetometer, a polarizing optical microscope and a ferromagnetic resonance spectrometer. Co thickness induced spin-reorientation from out-of-plane into in-plane magnetization was studied. Changes of in-plane magnetic anisotropy symmetry were deduced from shapes of magneto-optical hysteresis loops and from analysis of angular dependences of the resonance field. The experimental data have been discussed taking into account both uniaxial out-of-plane anisotropy and step-induced uniaxial in-plane anisotropy. A preferential orientation of domain walls in 3ML thick Co films was observed. The finding is explained by the step-induced magnetic anisotropy

A Fleurence

A Maziewski

A Stupakiewicz

B Bartenlian

P Beauvillain

P Gogol

R Gieniusz

R Mégy

T Maroutian

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Materials Science-Poland, Vol. 26, No. 2, 2008 
Magnetic properties of ultrathin Co(0001) 
films on vicinal Si(111) substrate 
A. STUPAKIEWICZ1,2∗, A. FLEURENCE2, R. GIENIUSZ1, A. MAZIEWSKI1,  
T. MAROUTIAN2, P. GOGOL2, B. BARTENLIAN2, R. MÉGY2, P. BEAUVILLAIN2 
1Institute of Experimental Physics, University of Białystok, ul. Lipowa 41, 15-424 Białystok, Poland 
2Institut d’Electronique Fondamentale, Université Paris XI, CNRS, UMR 8622, F-91405 Orsay, France 
In the present work we report on magnetization reversal process, anisotropy and domain structures in 
ultrathin Au/Co(0001)/Au films deposited on vicinal Si(111) substrates. The measurements were per-
formed using a magneto-optical Kerr effect based magnetometer, a polarizing optical microscope and 
a ferromagnetic resonance spectrometer. Co thickness induced spin-reorientation from out-of-plane into 
in-plane magnetization was studied. Changes of in-plane magnetic anisotropy symmetry were deduced 
from shapes of magneto-optical hysteresis loops and from analysis of angular dependences of the reson-
ance field. The experimental data have been discussed taking into account both uniaxial out-of-plane 
anisotropy and step-induced uniaxial in-plane anisotropy. A preferential orientation of domain walls in 
3ML thick Co films was observed. The finding is explained by the step-induced magnetic anisotropy. 
Key words: magnetic anisotropy; ultrathin films; cobalt; domain structure 
1. Introduction 
Ultrathin Au/Co/Au structures have been intensively studied due to their strong 
perpendicular anisotropy [1] making them ideal candidates for application in magnetic 
memory devices. For magnetic ultrathin films, parameters such as growth mode [2], 
interface roughness [3], substrate nature and orientation play a key role in their struc-
tural and magnetic properties such as crystal phase, magnetic anisotropy energy, Curie 
temperature, spin-reorientation transition (SRT), etc. Magnetic films grown on vicinal 
surfaces, where density of monoatomic height steps and their orientation are tunable 
with the miscut angle and miscut direction, exhibit a strong influence on their magnet-
ic properties [4]. Steps on a vicinal Pt surface strongly influence both magnetic aniso-
tropy and the magnetic moment of Co atoms rows [5]. In addition to the magnetocrys-
 __________  
∗Corresponding author: and@uwb.edu.pl 
A. STUPAKIEWICZ et al. 296
talline anisotropy, a uniaxial in-plane anisotropy is induced by a stepped surface. For 
systems such as Fe/Ag(001) and Co/Cu(001), the induced easy axis is aligned with the 
step edges [6] whereas in the case of Fe grown on stepped W(001) and stepped 
Pd(001) it is perpendicular to the step edges [7]. 
In our work, we focused on the magnetic properties of a Au/Co/Au structure 
grown on a vicinal Si(111) surface. 
2. Sample preparation 
A vicinal Si(111) substrate 2º misoriented with respect to the [1 12] direction was 
prepared under UHV conditions by flashing with direct current heating up to 1250 °C 
during a few seconds. The temperature was checked by a thermocouple up to 550 °C and 
by an infrared pyrometer for higher temperatures. After substrate processing, the silicon 
surface is constituted of single- and triple-layers high steps (Fig. 1). Such a Si(111) surface 
with 7×7 reconstructed terraces was examined [8]. A schematic representation of the vi-
cinal surface and basic crystallographic orientations are shown in Fig. 1.  
  
Fig. 1. In-situ STM image (a) and schematic representation of vicinal Si(111) surface (b) 
The following structures were deposited by the molecular beam epitaxy on the vi-
cinal Si(111) substrate: a Cu buffer layer 4 monolayers (ML) thick, deposited at  
100 ºC (i); other layers were deposited at room temperature: 30 ML thick Au(111) 
underlayer (ii), d = 3, 5, 7 and 15 ML thick Co layers (iii) and 30 ML thick Au cover 
layer (iv). Cobalt deposited on the Au(111) surface is expected to form the hcp 
Co(0001) crystallographic phase [1]. 
3. Results and discussion 
Magnetic properties of the samples at room temperature were studied using mag-
neto-optical Kerr effect (MOKE) using a magnetometer with the laser light of  
λ = 640 nm. Magnetization reversal processes enabled one to determine the Kerr rota-
tion and ellipticity in both polar (P-MOKE) and longitudinal (L-MOKE) configura-
Ultrathin Co(0001) films on vicinal Si(111) substrate 297
tions with perpendicular Η⊥ and in-plane ΗII magnetic field applied. The magnetic 
anisotropy was studied using a ferromagnetic resonance (FMR) X-band spectrometer. 
The external magnetic field was applied to the sample along directions defined by 
polar θΗ and azimuthal ϕΗ  angles measured, respectively, from the film normal and 
substrates miscut directions in the sample plane (Fig. 1). The measured resonance field 
(Hr) is related to the magnetic anisotropy constants and enables determination of the 
easy magnetization axes (minima in Hr(θΗ,ϕΗ)). 
 
Fig. 2. Hysteresis loops measured by P-MOKE for 3, 5, 7, 15 ML Co film thickness. 
Inset: dependence of normalized remnant magnetization on the Co thickness; experimental data (dots)  
and solid line calculated assuming anisotropy constants (Eq. (1)) defined from the resonance field Hr 
Figure 2 shows P-MOKE hysteresis loops and normalized remanent magnetization MR 
= M(H = 0)/Ms (inset) for various Co film thicknesses. A canted magnetization state could be 
deduced from the magnetization curve recorded for 7 ML Co films. In general, the reo-
rientation could be tuned by overlayer and/or underlayer structures [2, 9]. The SRT under-
goes for about 9 ML thick Co film, in gold envelope, deposited on a flat substrate. Thus 
the morphology of our vicinal substrate influences decrease of the SRT thickness. 
Figure 3 shows L-MOKE hysteresis loops for the 15 ML Co thick film with the 
magnetization mainly in the sample plane. The influence of the step-edges of the vi-
cinal surface on the magnetic anisotropy was deduced from azimuthal dependence of 
the normalized in-plane ellipticity remanence (Min) (Fig. 3, inset a)). The [1 10]  direc-
tion appears clearly as an easy axis with a square L-MOKE hysteresis loop when the 
field is applied in this direction and the loop with a negligible hysteresis for the field 
applied in the perpendicular direction. The azimuthal dependence of the coercive field 
A. STUPAKIEWICZ et al. 298
Hc(ϕΗ ) in the sample plane was studied by L-MOKE for a 15 ML thick Co film (Fig. 
3, inset b)). The plot gives additional evidence for the hard axis orientation along the 
direction perpendicular to the step edges (0º and 180º). 
 
Fig. 3. Hysteresis loops measured by L-MOKE for 15 ML Co film thickness and the angles   
ϕΗ  = 0 (open circles) and 90º (full circles). Insets show angular dependences measured  
by L-MOKE of: a) normalized in-plane ellipticity remanence, b) coercivity field 
The angular dependences of the resonance field Hr(θΗ,ϕΗ) for 15 ML thick Co film are 
plotted in Fig. 4. The magnetic anisotropy symmetry can be deduced from these depen-
dences. Figure 4 shows that the easy magnetization axis is close to the [1 10]  direction. 
 
Fig. 4. FMR dependence Hr(θΗ;ϕΗ) for 15 ML thick Co sample. Solid lines fitted 
 using the anisotropy constants as follows: Ku1 = 0.81 MJ/m3, Kvic = –0.009 MJ/m3 
Ultrathin Co(0001) films on vicinal Si(111) substrate 299
The following expression of the magnetic anisotropy was used for the FMR curves 
fitting 
 ( ) ( )( )2 2 2 2 2mis 0 vic2 1, , 1 sin sin sin2sA V s
KE d K M K
d
θ ϕ μ θ θ ϕ⎛ ⎞= + − × − +⎜ ⎟
⎝ ⎠
m v  (1) 
where Kv and Ks are the volume and surface anisotropy constants, respectively, Ms is 
the saturation magnetization; m is the normalized magnetization vector and vmis is the 
normalized vector of the vicinity direction [10], Kvic is the step-induced uniaxial aniso-
tropy constant. The fitted Hr(θΗ ,ϕΗ) curves (with magnetic anisotropy constants Ku1  
= (Kv + 2Ks/d) = 0.81 MJ/m3, Kvic = –0.009 MJ/m3) are shown in Fig. 4 as solid lines. 
The solid line in Fig. 2 was calculated using Kv = 0.45 MJ/m3 and Ks = 0.54 mJ/m2 
anisotropy constants. The uniaxial out-of-plane anisotropy constant is in agreement 
with that expected of the hcp Co phase [2, 11]. Analysis of the magnetization curve 
recorded for ΗII applied along the hard axis (Fig. 3) gives a similar uniaxial in-plane 
anisotropy field Hvic = 2Kvic/Ms = 0.013 T. 
Fig. 5. Remnant domain structure 
for 3 ML thick Co sample  
P-MOKE microscopy is a powerful tool to study magnetization reversal processes in 
magnetic film with a perpendicular anisotropy [12]. Figure 5 shows the remanent do-
main structure image recorded for a 3 ML thick Co film. The preference of domain wall 
orientation along the [1 10]  direction is well visible. The preference could be explained 
by a step-induced in-plane magnetic anisotropy determined from both FMR and  
L-MOKE measurements. A similar preference of domain wall orientation structure was 
also observed in ultrathin Co film deposited on a vicinal sapphire substrate [13]. 
4. Conclusion 
Au/Co/Au structures were grown on a vicinal Si(111) surface with various thick-
nesses of Co layers. The symmetry of the magnetic anisotropy observed by both 
MOKE and FMR is connected with growth of hcp Co(0001) film. The out-of-plane 
and step-induced uniaxial in-plane magnetic anisotropies were studied in a 15 ML 
A. STUPAKIEWICZ et al. 300
thick Co film. The anisotropies were found to strongly influence the domain wall 
propagation along the step edges of the vicinal surface. 
Acknowledgements 
This work was supported by the Marie Curie Fellowships for “Transfer of Knowledge” (‘NANO-
MAG-LAB” N 2004-003177) and the ACI Nanoscience of the French government. 
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Received 28 April 2007 
Revised 16 February 2008 
 


Magnetic properties of ultrathin Co(0001) films on vicinal Si(111) substrate

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Magnetic properties of ultrathin Co(0001) films on vicinal Si(111) substrate

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