Thin Solid Films, 189 (1990) 359-367
PREPARATION
STRUCTURE
C. MARLIfiRE,
hstitut
OF THIN IRON LAYERS ON GOLD SUBSTRATES
J. P. CHAUVINEAU
d’optique.
(Received
359
AND CHARACTERIZATION
October
CNRS
27,1989;
AND D. RENARD
WA 14, B&iment 503, B.P. 147,91403 Orsay Ckdex (France]
revised January
16, 1990; accepted
February
21, 1990)
Polycrystalline texturized Au/Fe/Au “sandwiches” are prepared by vacuum
deposition on glass substrates. The surface structure and surface roughness are
monitored during the layer growth by reflection high energy electron diffraction and
soft X-ray reflectometry. The crystalline structure and the relative orientation of
gold and iron crystals are determined by transmission electron microscopy.
Iron films with thicknesses ranging from 1 to 20nm have the bulk b.c.c.
structure with a (110) plane parallel to the (I 11) surface plane of the gold crystals.
The free surface roughness is constant for very thin iron films and increases when the
iron thickness is greater than 1.5 nm.
1. INTRODUCTION
Experimental studies were recently devoted to magnetic ultrathin films which
exhibit new properties induced by a decrease in lattice dimensionality and by
interfaces effects. As well as their substantial fundamental interest, these films have
potential applications for high density storage which needs a magnetization
perpendicular to the plane of the film. This is not the usual case because of the
demagnetizing field. However, in single ultrathin films, interface anisotropy
sometimes overcomes the effect of the existence of a demagnetizing field, leading to a
perpendicular magnetization’-‘.
On the theoretical side, Ntel was the first, in 1954, to propose a phenomenological model introducing the concept of magnetic surface anisotropy6. Other
determinations using band theory were performed later for transition metals. They
give more or less reliable results. All these calculations deal with perfectly flat
surfaces. Recently, Bruno introduced a model which takes account of the roughness
of the surface and the lattice deformations near the interfaces7-‘.
It appears now that a detailed knowledge of the atomic-scale structure of the
samples is required in order to explain their magnetic properties.
In the present paper, we report structure studies of iron thin layers protected
from oxidation by two gold films. Gold was chosen as a substrate for iron layers
because it yields conveniently oriented crystals with low surface roughness, a
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C. MARLICRE.
360
J. I’. (‘HAUVlNE:.Ali.
Il. RENARD
necessary condition for the observation
of bidimensionnal
magnetic effects. These
films were studied in situ by soft X-ray retlectometry (SXRR) during growth and by
reflection high energy electron diffraction (RHEED). The Au;‘FciAu sandwiches
were also examinated
t”r sitn by transmission
electron microscopy
(TEM). The
magnetic properties are studied in other laboratories
and will be reported later.
_.
’
SAMPLE
PREPARATIO’L
A first gold film, about 25 nm thick, is deposited at room temperature
in
ultrahigh vacuum conditions
on a float glass or on a quartz substrate. During the
layer growth, the soft X-ray reflectivity is recorded in order to calibrate the quartz
monitor and to measure the surface roughness variations. The annealing of the gold
layer at 150 C for about 30 min induces a recrystallization
which reduces the surface
roughness and gives a texturized polycrystalline
film which is monocrystalline
in
depth: the grains have an average lateral size of about 50 nm. The (1 I 1) compact
planes of the f.c.c. structure are preferentially
parallel to the surface film. On float
glass substrates, the average roughness of the gold layer is about 0.6 nm as measured
by grazing X-ray reflectometry’“.
Electron microscopy
shows that the surface
consists of terraces with an average lateral size of 30 nm separated by monoatomic
steps’ ’
Iron is then deposited on the gold layer at room temperature
at a rate of
0.01 nm s ’ while the soft X-ray reflectivity variation is measured. RHEED patterns
are recorded before and after the iron evaporation.
For all these experiments
a
30 keV electron energy was used. The second gold layer having a thickness of about
25 nm is then deposited on top of the iron film at room temperature
without further
annealing.
3.
REFLEC‘TION
HIGH ENERGY
ELWTRON
DIFFRACTION
A recrystallized gold RHEED pattern taken just before the evaporation
of iron
is shown in Fig. l(a). As said above, the crystals of the gold layer have a common
[Ill l] axis nearly perpendicular
to the surface, but they are randomly
oriented
around this axis. As the surface illuminated by the electron beam (about 0.01 mm2) is
much larger than the average crystallite area, all the azimuths could be observed
simultaneously
on the RHEED pattern. In fact, the lines observed on the pattern can
be attributed
to the three azimuths [I lo], 121 I] and [312], corresponding
to the
more compact rows of the (111) plane.
RHEED patterns obtained during the deposition
of iron on gold films are
shown in Figs. l(b) and l(c). For the iron film with a thickness of 1.1 nm (Fig. l(b))
there is a slight intensity modulation
along the streaks while for the thickness of
19 nm (Fig. I(c)) the pattern is made of spots and can be indexed in a threedimensional
space (Fig. l(d)). Thus the iron surface roughness is increasing with the
film thickness as will be confirmed by SXRR measurements.
These two patterns correspond
to a polycrystalline
iron film with the b.c.c.
structure (X-Fe); the (110) plane is parallel to the sample surfaces and so parallel to
the (111) plane ofgold. Within the precision of the measurements
(about 2”;,), they do
Ar imuths:
(a)
(c)
I 3
1
Pczimuthr
(4
001
011
113
111
133
i
1ii!Z
112
Fig. 1. (a) RHEED pattern of a polycrystalline
gold thin film with a thickness of about 25 nm after an
annealing at 150 ‘C for 30 min (E = 30 keV). (b) RHEED pattern of 1.1 nm of iron deposited on annealed
gold. (c) RHEED pattern of 19 nm of iron deposited on annealed gold. (d) Indexation of the RHEED
pattern of(c); the-scale factor is 3.
(
362
MARLltiRE.
J. 1’. (‘HAUVINEAU,
Il. REi%ARI)
not display obvious differences for the iron layer structure. Thus we find that.
beyond a mean thickness of about 1 nm. iron has nearly the same in-plane spacing
between the atomic rows in the (110) plane as the bulk, in contrast with cobalt on
Au( 111) which presents an expanded surface lattice at small thicknesses12. Each
pattern is the superposition
of six different crystal orientations
having the common
[l lo] direction as indicated in Fig. l(d). The observed azimuths are [ill],
[OOl],
[Oil], [113], 11331 and [112]. The spots corresponding
to the [OOl] azimuth are
very weak and the second-order
spots do not appear.
4.
SOFT X-RAY
REFLECTOMETRY
The reflectivity of the layers is measured continuously
during the growth by a
soft X-ray reflectometer built in the ultrahigh vacuum chamber. For fixed values of
the wavelength (;. = 4Snm) and of the incidence angle (i = 60 I of the soft X-ray
beam, the film reflectivity depends on the thickness and on the free surface roughness
which can vary during the film deposition.
Thus, the damped oscillating curve
obtained (Fig. 2(a)) can be simulated by taking the thickness as a fitting parameter
for the abscissae of the extrema and the surface roughness 0 for their intensities’“.
The optical constants are taken from ref. 14.
The model used for the calculation of the reflectivity of a rough surface is based
on the so-called Debye-Wailer
attenuation
factor. The mean square deviation a2 of
the surface height 2(x. y) is supposed to obey a gaussian law of distribution:
P(Z) = (21UJ2) ’ 2 exp( __~L,~a’)
The model is valid when the value of CTis small compared with the value of the
apparent
wavelength
i./cos i. a condition
which is satisfied in the present
experiments.
The results of the simulation
of the reflectivity curves recorded during the
deposition
of the iron layer and the second gold layer are shown in Fig. 2(b). The
roughness of the first gold layer after recrystallization
is 0.65 nm, slightly above that
of the bare substrate (0.4 nm).
The surface roughness of the iron layer is nearly constant at the beginning of the
deposition and then it increases when the mean thickness is larger than 1.5 nm. in
accordance with the three-dimensional
appearance of the RHEED diagrams.
The surface roughness is then reduced by the deposition of a thickness of gold
equivalent to a few atomic layers on top of the iron film. This smoothing could result
from the preferential nucleation of gold clusters in the holes of the iron surface.
5.
ELECTRON
MICROSCOPY
Films peeled from their dielectric substrates
were observed by TEM. The
electron beam travels through the sandwich structure made of the thin iron layer
between the two gold layers having a total thickness of about 50 nm. The bright field
photograph
of a large gold single crystal can be seen in Fig. 3(a) and the
corresponding
diffraction pattern is presented in Fig. 3(b). The direct beam position
is denoted by 0 and is the centre of the figure. In this sample the iron thickness is
6.5 nm.
STRUCTURE
OF
Fe ON Au
363
AU
I
J
19
0
30
28
THICKNESS
L
1
-
Fe
i
I
(nm)
RU
.s -
.B
-
.7 -
0
10
30
23
THICKNESS
(nm)
Fig. 2. (a) Soft X-ray reflected intensity (0) measured during iron and gold growth on a gold layer. The
intensity calculated (-)
with the surface roughness variation of(b) is also shown. (b) Surface roughness
variation used to calculate intensity in (a).
The black area on the micrograph
corresponds
to the zone where the gold
crystal is exactly in a symmetrical orientation, with the [ll l] axis strictly parallel to
the electron beam. It gives six equivalent (220) spots such as that denoted by G in
Fig. 3(b); these spots are used as a reference for the indexation of the other spots of
(
(Lil
200
nm
MARLI~RI~,
J. I’. 1‘lIAli\:INt:At:.
I). RWARD
(h)
Au fc
d.d.
AU
I 3(4X)
(I IO) Fe
(220) Au
(3ll)AU
(I 12) J-c
the diagram. The a, a’. b. b’, c and c’ spots determine three directions each at 30 with
respect to the three Au(220) spots; these spots do not correspond
to reciprocal
vectors of the gold lattice but are in fact elongated (1 10) spots of iron. A single crystal
of b.c.c. structure in the (1 10) orientation
would give only two symmetrical
(I II))
STRUCTURE
0~
Fe ON Au
365
spots; thus for iron there are three different [l lo] directions at 120” from each other.
We cannot observe the (200) spots of iron because they are too close to the very
bright (220) spots of gold. They probably lead to the apparent deformation
of the
latter spots. Some Fe(211) iron spots are present but as expected they are weak.
Another way to prove the existence of three directions for iron crystallites is to
make dark field images successively with each of the a, a’, b, b’, c and c’ spots. One of
these images obtained from the b spot is shown in Fig. 3(c) with two points of
reference A and B marked on the dark and the bright field images. The same image
was obtained from the symmetrical b’ spot. On this dark field image, some elongated
areas are bright. Different areas appear bright on the two similar images obtained
from the two other pairs of spots a, a’ and c, c’. The whole crystal would be lit by
superposition
of these three dark field images. We can thus conclude that the surface
of each single gold crystal is covered by small iron crystallites which are in three
orientations
at 120” with respect to each other, around a [llO] common axis: the
elongated shape of the a, a’, . . spots arises from a small angular dispersion of the
120” rotation angle between crystallites.
The ct, p, . . . spots in Fig. 3(b) result from double diffractions between Au(220)
planes and Fe( 110) planes: for example, p is obtained from the relation
As a consequence of the elongated shape of the Fe(ll0) spots, the ~1,p, . . . spots also
appear elongated.
These observations
made on a very large gold single crystal are consistent with
the general diffraction pattern presented in Fig. 3(d). The smallest ring (denoted by
Au-Fe d.d.) is thick because it is made of elongated spots such as ~1,8, . . . . The next
weak ring results from the so-called “forbidden” 8422) reflections of gold which
appear only under certain conditions”.
The two following rings are respectively the
(110) iron and the (220) gold rings,
From these results, we propose in Fig. 4 a scheme of the (111) compact atomic
plane of gold covered by (110) b.c.c. iron. We have drawn on this plane the positions
of iron atoms for one of the three orientations
experimentally
observed. The two
others are obtained by rotations of 120” of the iron lattice in the plane. We must
emphasize that this scheme is only valid for iron thicknesses above about 1 nm and
does not represent the real atomic structure of the iron-gold interface. Because of the
small size of iron crystals and of the large gold thickness, high resolution studies are
very difficult on these samples.
6. CONCLUSION
Iron films with thicknesses ranging from 1 to 20nm deposited between two
polycrystalline
texturized gold layers were studied by means of RHEED, SXRR and
TEM.
The first gold layer deposited on glass or quartz substrates has crystals with a
common (111) axis nearly perpendicular
to the surface. The free surface is very
smooth, as shown by soft X-ray reflectivity measurements
and the RHEED pattern
appearance.
366
(‘. MARLIF:RE,
J. P. C‘HAUVINEAII.
D. RENARD
o+o
P o+o+.9 o+”
At the beginning of the iron deposition, at room temperature.
the free surface
roughness is found to be nearly constant but it increases significantly when the mean
thickness is higher than 1.5 nm. The iron layer is polycrystalline
with the bulk b.c.c.
structure and with the (I 10) plane parallel to the substrate surface: in contrast with
cobalt, there is no expanded surface lattice at small thicknesses in the case of iron.
TEM diagrams proved that on the same gold single crystal there are several iron
crystallites
with three different [I lo] directions
at 120 from each other. The
orientations
of the latter have been determined
with respect to the [l IO] atomic
rows of gold.
The deposition of a few atomic layers of gold on top of the iron film strongly
reduces the free surface roughness: this smoothing could be due to the preferential
nucleation of gold clusters in the holes of the iron surface.
The second gold layer protects iron from oxidation
while the samples are
transferred
to the electron microscope
or to systems suitable for studying their
magnetic properties.
STRUCTUREOF
Fe
ON
Au
367
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