Magnetic properties of ultrathin Co films on Si (111)

Hai Xua,*, Alfred C.H. Huana, Andrew T.S. Weea, D.M. Tongb

aDepartment of Physics, National University of Singapore, 10 Kent Ridge Crescent, Singapore, Singapore 119260bDepartment of Physics, Shandong Normal University, Jinan 250014, People’s Republic of China

Received 17 December 2002; accepted 9 April 2003 by H. Akai

Abstract

Ultrathin cobalt films on clean (7 £ 7) and Au covered Si (111) substrates were prepared by molecular beam epitaxy. The

structure was studied by using scanning tunnelling microscopy and low energy electron diffraction. Magnetic properties were

determined with the magneto-optic Kerr effect. It was found that Co nucleates in grains that prefer to grow along the bunched

step edges of the Si substrate ([112] direction), which induces a strong in-plane uniaxial anisotropy. By introducing Au buffer

layers, the magnetic characteristics were improved by preventing the silicide reaction between Si and Co. Moreover, the

tendency for step decoration disappears gradually results in the in-plane uniaxial anisotropy reduction.

q 2003 Elsevier Science Ltd. All rights reserved.

PACS: 75.70.Ak; 68.35.Bs; 81.15.Hi

Keywords: A. Magnetic thin film; B. Molecular beam epitaxy; C. Surface structure

1. Introduction

In the past decades, ultrathin magnetic metallic struc-

tures have become important in material science. A great

amount of research has been devoted to the study of

magnetic surfaces and interfaces as well as step induced

anisotropies in ferromagnetic ultrathin films [1–5,9]. A

strong perpendicular anisotropy of cobalt thin films on

different substrates such as Pd (111), Au (111), Ag (111), Cu

(111) has been observed, and a drastic increase of anisotropy

during deposition of a non-magnetic metallic overlayer has

been reported [3,6]. Recently, the growth of magnetic

materials on semiconductors has opened new avenues for

novel magnetic thin film devices [7,8]. On Si substrates,

however, the reaction of deposited 3d transition metals with

silicon substrate hinders the development of magnetic

structures in the ultrathin range [10–12]. In order to

understand the implications of surface morphology, reaction

layers, and substrate effects on the magnetic properties, we

systematically studied the surface morphology of ultrathin

Co films deposited on clean Si (111)-(7 £ 7) surface by

using scanning tunnelling microscope (STM) and low

energy electron diffraction (LEED). The magnetic behav-

iour was measured by using the magneto-optical Kerr effect

(MOKE). In order to control the silicide reaction between

the Co film and the Si substrate, the role of an Au buffer

layer has been discussed.

2. Experimental

All experiments including molecular beam epitaxy

(MBE) were performed in an ultra-high vacuum (UHV)

chamber with a background pressure of 5 £ 10211 mbar.

The UHV system was equipped with Auger electron

spectroscopy (AES), LEED, STM and MOKE. The Si

substrates were introduced via a load-lock and first degassed

overnight, then, flashing to 1500 K further cleaned the

substrates by resistive heating for several seconds. After

flashing, the Si substrates exhibited sharp (7 £ 7) LEED

patterns and (7 £ 7) atomic images were also obtained by

0038-1098/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0038-1098(03)00307-7

Solid State Communications 126 (2003) 659–664

www.elsevier.com/locate/ssc

* Corresponding author. Address: Department of Physics,

National University of Singapore, 10 Kent Ridge Crescent,

Singapore, Singapore 119260. Tel.: þ65-687-446-50; fax: þ65-

677-961-26.

E-mail address: phyxuh@nus.edu.sg (H. Xu).

Fig. 1. STM topography images of Co/Si (111) films. The scan size is 100 nm. (a) Clean 7 £ 7 Si surface, (b) 1ML Co film, (c) 4MLs Co film, (d)

10MLs Co film, (e) 14 MLs Co. Insets of (a)–(c) show the corresponding LEED patterns (60 eV).

H. Xu et al. / Solid State Communications 126 (2003) 659–664660

STM. By controlling the flashing current, substrates with

different step structures were prepared. No contaminations,

e.g. O, C, were found by AES on the clean Si surfaces. Co

and Au were deposited by MBE at different substrate

temperatures. The purity of the Co and the Au source is

99.999%. During the deposition, the system pressure

remained always below 1:8 £ 10210 mbar. Deposition rates

were calibrated by medium energy electron diffraction

(MEED) during growth of Co and Au on W (110). Here, we

define one monolayer of Co and Au as

7:8 £ 1014 atoms/cm2, which is the atom density of the Si

(111) surface.

3. Results and discussions

Fig. 1 shows a series of typical STM images and

LEED patterns of Co deposited on Si (111) at room

temperature (RT). Fig. 1(a) depicts a clean Si (111)-

(7 £ 7) surface. Most of the steps are bunched and

locally run along [112] directions. During deposition of

the first ML of Co, the (7 £ 7) LEED pattern gradually

disappears and a threefold symmetry LEED pattern with

first order spots and a high background is established.

This indicates a locally pseudomorphic fcc growth of the

film. A high resolution STM image shown in Fig. 1(b),

reveals a uniform distribution of small grains (approx.

2 nm) on the terraces and at the step edges. At 4MLs, a

clear threefold symmetry LEED pattern with sharp first

order spots is obtained as well (see Fig. 1(c)). This Leed

pattern has been explained as a characteristic of epitaxial

CoSi2 [10–13], and it becomes stronger after annealing

due to the formation of complete CoSi2 silicide. Above

the 4ML, this threefold symmetry pattern cannot be

obtained any longer, which reflects the disappearance of

long-range order by the conversion of growth to a

polycrystalline Co-rich phase. Above the 10ML, the

irregular polycrystalline Co grains show a loose array

(see Fig. 1(d)) on the terrace. At the step edges, however,

the grains seem to be denser and larger, which shows a

preferred growth along the step orientation. i.e. the [112]

direction. This case becomes clearer with increasing Co

thickness (see Fig. 1(e)). The variation of remanent Kerr

ellipticity and the coercivity dependence on the thickness

of the Co films are shown in Fig. 2(a) and (b). In films

deposited and measured at RT, the first longitudinal loop

occurs at 8.0ML followed by an increasing Kerr signal

with increasing thickness of the Co film. It is obvious

that the initial silicide formation affects the formation of

a ferromagnetic film and delays the onset of magnetisa-

tion. Beyond 10ML, the longitudinal Kerr intensity shows

a linear increase. A linear extrapolation of the MOKE

ellipticity data above the 10MLs to zero signal passes

through 5MLs. It indicates that this initial 5MLs

thickness is non-ferromagnetic layers. This is an

expectable result since some experiments have pointed

out that a non-magnetic epitaxial CoSi2 is formed up to

3–4ML [17,13]. This is also in agreement with the

disappearance of LEED patterns above 5ML as shown in

Fig. 1. When the temperature of the substrate is lowered

to 150 K during growth, the first longitudinal Kerr loop

is observed at 5.0ML when measuring at 150 K, which is

attributed to the reduced silicide formation. From Kerr

signal vs. films thickness curves, it is clear that the Kerr

signal of Co films deposited at LT is significantly larger

than that deposited at RT even for thick films. This

might be attributed to the lower measuring temperature

alone, since the reduction of Kerr intensity due to a

Curie temperature close to the measuring temperature,

affects the remanence of thin films only. Straight-line

extrapolation of the low temperature ellipticity to zero

signal passes through a Co thickness of 3.5ML. This

indicates that the first 3.5MLs are magnetically inactive.

The reduction of magnetically dead layers is due to the

reduced intermixing between Co and Si at low tempera-

tures. In Fig. 2(b), the coercivity shows a gradual

increase for the samples grown at RT. However, when

Fig. 2. Thickness dependence of the remanent Kerr signal (a) and

coercivity Hc (b) of Co/Si (111) at different substrate temperatures.

Open circles and filled squares represent films deposited and

measured at LT and at RT, respectively.

H. Xu et al. / Solid State Communications 126 (2003) 659–664 661

the growth temperature is reduced to 150 K, we see a

rapid increase instead. It indicates that the growth at LT

with reduced mobility of the Co atoms might hinder

domain wall motion. Hence, the pinning of magnetic

domain walls is stronger for LT films.

It has been reported that, the bunched steps structure of

Si surface will disappear and step density increase after

performing an annealing at a higher temperature [19,20].

Fig. 3 shows STM images of Si substrates treated with

different flashing current. It showed a clear different step

density with variation of the treated temperature. In order to

highlight step edges, differentiated image mode is given. In

Fig. 4(a) and (b), it is the corresponding magnetic

measurements of the Co films deposited at RT on the

substrate respective ly shown in Fig. 3. At the 10ML, the

Kerr signal of the Co films on the high step density substrate

is lower than that on bunched step substrate (see Fig. 4(a)). It

is suggested that increasing the step density supplies much

more regions for silicide reaction of Co along the step edges,

so that more amount of CoSi2 is formed in the initial growth

stage and the magnetic properties are deteriorated. Further-

more, from measurements of the magnetisation loop as a

function of the in-plane angle, a strong uniaxial anisotropy

with the easy axis along [112] paralleling to the step

orientation was found for all thickness (see Fig. 4(b)), which

is similar to earlier results [18]. Along the easy direction, a

square loop is observed while along the hard axis the sample

cannot be saturated with the available field and only a minor

loop is seen. The development of uniaxial magnetic

anisotropy is thought to arise from the preferred growth of

the film along the step edges. In this case, it is suggested that

if the step density of the substrate is increased, the

corresponding uniaxial magnetic anisotropy is enhanced.

Fig. 4(b) shows the 20MLs Co film is easier to be saturated

with a slight high remanent Kerr signal and smaller

Fig. 3. Differentiated STM images of Si (111) prepared with

different thermally pre-treated conditions: (a) 6 A flash current, (b)

9 A flash current. The scan size is 200 nm.

Fig. 4. Longitudinal loops for different step-densities of Si

substrates. (a) 10MLs Co film (b) 20MLs Co film.

H. Xu et al. / Solid State Communications 126 (2003) 659–664662

coercivity on a larger density of single steps in easy

direction comparing to that on the bunched step structure.

Moreover, along the hard axis, i.e. [110] direction, the minor

loop is reduced to reversible rotation processes resulting in

almost a straight line.

In order to reduce the silicide reaction between Co and Si

substrate at RT, an Au buffer is deposited on the Si substrate.

To form a flat buffer layer, the Au layers were first grown at

150 K and subsequently annealed to RT. In Fig. 5, typical

STM topography images with different Au thickness are

given. At an Au layer thickness of 3MLs (see Fig. 5(a)),

STM image shows smooth growth with irregularly shaped

islands of 1ML height, i.e. only two layers are exposed on

the terraces. With increasing Au coverage, the step edges of

the substrate gradually disappear as shown in Fig. 5(b).

After the deposition of Co on the Au surface layer, Co grains

were evenly distributed on the terraces, and no preferred

growth at the step edges is observed as shown in Fig. 5(c)

and (d). Co grew on thick Au buffer layers of 4.5MLs, which

exhibit the same tendency as in the case of 3MLs Au layers.

The thickness dependencies of the remanent Kerr ellipticity

and coercivity for Co/Au/Si (111) are shown in Fig. 6. On

introducing an Au buffer layer, the step induced uniaxial

anisotropy is reduced, as can be seen in the inset of Fig. 6(a),

in agreement with the changed growth mode. The onset of

magnetisation is found at the 7ML for a 3MLs Au layers,

which is lower than that 8ML for the sample without Au

buffer. Comparing to Fig. 2(a), the straight-line extrapol-

ation from the data above the 10ML indicates that the

thickness of the magnetically dead layer is slightly changed,

only the first 4.6MLs do not contribute to the magnetisation.

This dead layer reduces to 2.2ML for 4.5MLs Au layer

thickness, it is obviously better than the result without Au

layers. Moreover, Co films grown on a buffer layer of

4.5MLs Au thickness produce smaller coercivities and

hysteresis loops with even larger Kerr ellipticities except for

the initial magnetic layers, which shows the magnetic

properties are improved with the increasing thickness of the

Au buffer layer. The smaller Kerr ellipticities on the initial

magnetisation can be explained by two mechanisms. First, it

is possible that the increased local roughness of the thicker

Au layer hinders magnetism of thin Co films [16]. Secondly,

Fig. 5. STM topography images with Au buffer layers. (a) 3.5MLs Au layers, (b) 4.5MLs Au layers, (c) surface of 10MLs Co/3.0MLs Au/Si

(111), and (d) 20MLs Co/3.0MLs Au/Si (111). The scan size is 100 nm.

H. Xu et al. / Solid State Communications 126 (2003) 659–664 663

a partial increase in perpendicular magnetisation of the thin

Co films grown on the thicker Au layers might result in a

reduced remanence observed in plane [14,15]. Unfortu-

nately, we lack a polar MOKE set-up in our system.

Comparing with the coercivity variation in pure substrate

without buffer layer, in the case of growth with an Au buffer

layer, the smooth interface and reduced in-plane anisotropy

promote domain wall motion so that Hc is small over a wide

range of thickness as shown in Fig. 6(b).

4. Summary

In summary, by using molecular-beam epitaxy, ultrathin

cobalt films have been prepared on clean (7 £ 7) recon-

structed and Au covered Si (111). Using STM, it was shown

that Co film shows a strongly preferred growth along the

step edges of the Si substrate, which leads to a strong in-

plane uniaxial anisotropy along the steps. When the step

density of the Si substrates is increased, the magnetic

properties of the films deteriorate which indicates that the

silicide reaction is triggered at step edges. MOKE

measurements showed that cooling of the substrate reduced

silicide formation. Furthermore, the number of magnetically

dead layers can be controlled in some cases after introducing

an Au buffer layer. The Au buffer layer also removes the

preferred growth along step edges, which results in the

reduction of the in-plane uniaxial anisotropy.

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Fig. 6. Remanent Kerr signal (a) and coercivity (b) of Co film

deposited on Au/Si (111) with different Au buffer layers. Filled

circles and filled squares represent measurements with Au thickness

of 4.5MLs and 3.0MLs, respectively. The inset in (a) shows a

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Filled circles and filled squares show hard axis (½11�2� direction) and

easy axis (½�110� direction) loops, respectively.

H. Xu et al. / Solid State Communications 126 (2003) 659–664664