PHYSICAL REVIEW B, VOLUME 64, 144431

Influence of strain on the magnetocrystalline anisotropy in epitaxial CrÕCoÕPd 111... films

S. Boukari, E. Beaurepaire, H. Bulou, B. Carrie re, J. P. Deville, and F. Scheurer
Institut de Physique et Chimie des Mate´riaux de Strasbourg, UMR 7504, CNRSUniversite´ Louis Pasteur, 23 rue du Loess,
67037 Strasbourg, France

M. De Santis and R. Baudoing-Savois
Laboratoire de Cristallographie, CNRS (UPR 5031) associe´ a l'Universite´ J. Fourier et a l'Institut National Polytechnique de Grenoble,
25 av. des Martyrs, 38042 Grenoble, France
Received 6 February 2001; published 24 September 2001
We studied the correlations between the structure in particular strain and magnetic anisotropy in thin
Co/Pd 111 films uncovered and covered with Cr. Measurements were done by grazing incidence x-ray dif-
fraction and magneto-optical Kerr effect. To properly describe these correlations, one has to consider a surface
magnetoelastic coefficient. We demonstrate that Cr capping leads to an enhanced anisotropy strength due to
strain effects, and an extended perpendicular anisotropy thickness range due to an interface contribution.

DOI: 10.1103/PhysRevB.64.144431 PACS number s : 75.70.Ak, 75.80. q, 75.30.Gw

Magnetic multilayers and ultrathin films often present en- Co/Pd 111 films. In particular, the influence of the capping
hanced perpendicular anisotropy that make them suitable for on the strain state in the Co layer is analyzed.
application in the field of magnetic recording or storage The magnetic characterization was performed by in situ
devices.1 Their anisotropy properties are usually described and ex situ magneto-optical Kerr effect MOKE . The in-
phenomenologically by the Ne´el pair interaction model.2 In plane and out-of-plane strains as well as the stacking of the
this picture, the tendency for the magnetization to be out-of- Co layers were determined by in situ grazing x-ray diffrac-
plane results from the competition between a surface or in- tion GIXD , performed at the SUV station of the French
terface anisotropy (Ks), due to the broken symmetry, and CRG-IF beamline BM32 at the European Synchrotron Ra-
the volume (Kv) plus the dipolar anisotropy (Kdip). The ef- diation Facility in Grenoble.9 The chamber allows simulta-
fective anisotropy is given by neous in situ film growth and diffraction measurements. The
energy of the impinging beam was 11 keV and the incident
angle was kept above the critical angle for total reflection.
K Kv Kdip Ks /d, 1 The reciprocal lattice was described by A*, B*, and C* with
A*, B* in the surface plane making an angle of 60° and
where d is the thickness of the film.3 C* normal to the surface: A* B* 4 2/aPd 3
However, epitaxial films usually present a strain depen- 2.642 Å 1 and C* 2 /aPd 3 0.9342 Å 1. For
dence with thickness so that Kv is no longer constant but more details on sample preparation and diffraction experi-
depends on the thickness d because of magnetostriction.46 If ments, see Ref. 10.
this is not taken into account, as in Eq. 1 , it results in an The Co was deposited by molecular beam epitaxy at 370
apparent contribution to Ks from the strain dependence of K on a Pd 111 single crystal to obtain smoother films than
Kv , although the energy is located throughout the film and those deposited at 300 K.11 The magnetic properties of un-
not only at the interfaces.7 The surface anisotropy Ks ob- covered Co/Pd 111 were measured in situ by MOKE in the
tained from such an analysis must therefore be viewed as an polar geometry during the growth at 370 K. The loops show
effective anisotropy. full remanence between 2.0 and 4.3 monolayers ML , indi-
Strain induced modification of the anisotropy could also cating a perpendicular easy axis Fig. 1 . Upon covering the
appear upon capping a film with a protective layer to per- Co layer with Cr, the perpendicular region with full rema-
form ex situ magnetic measurements for instance , as it is nence is extended to 7 ML. This behavior is due to a change
known that capping can have a strong effect on the under- in the effective interface anisotropy, favoring a perpendicular
laying film structure.8 easy axis. However, to know whether the change is located at
We see therefore that the connection between the macro- the Co-Cr interface or in the volume of the film, one has to
scopic anisotropy and the structural parameters can be com- check if Cr deposition induces structural changes in the un-
plex. Often one could be tempted to infer, from the depen- derlying Co film.
dence of anisotropy on thickness, the structural properties at The in-plane lattice parameter of uncovered Co films
the origin of this anisotropy. However such a process can grown on Pd 111 was measured in situ as a function of Co
easily lead to erroneous conclusions about the structure, thickness by GIXD during the growth by locating the maxi-
which in the end hinder the efforts made to engineer the mum of the so-called Co truncation rod see Fig. 2 . A rod
anisotropy. denotes the intensity diffracted between Bragg points be-
To disentangle the origin of the different contributions to cause of finite size effects. The position of a rod perpendicu-
the anisotropy, we made a study of the interplay between the lar to the surface is related to the in-plane lattice
magnetic anisotropy and the structure in Co/Pd 111 and Cr/ parameter.10,12 Since the penetration depth of the beam is

S. BOUKARI et al. PHYSICAL REVIEW B 64 144431

FIG. 3. X-ray intensity along the Co truncation rod for a 14 ML
FIG. 1. Remanence divided by saturation for uncovered Co/ Cr/ 11 ML Co/Pd 111 film. The L scale is in reciprocal lattice units.
Pd 111 films crosses , and covered wih Cr squares . Open squares represent the data points. The best fit represented by
full circles was obtained with a 50% hcp and a 50% disordered
much larger than the film thickness, an average lattice pa- stacking. Indicated is the intensity which would arise from a pure
rameter of the Co film is obtained. We tried several models disordered, twin-fcc, hcp, or fcc film.
to fit the relaxation with thickness d of the in-plane lattice
parameter: a 1/d decrease,7 a (1/d)2/3 decrease,13 and a expansion at low Co thicknesses and strain release at higher
model proposed by Basson and Ball.14 In our case, these thicknesses. To account for the Cr capping on the in-plane
models do not give a satisfactory description of our data. The Co strain, we kept the same functional description for the
in-plane strain is rather described by the following phenom- strain as for uncovered Co/Pd 111 and added two terms. The
enological law when d di new strain is given by
d 1 dc / d di , 2 d d a/d b, 3
where 9.8% is the natural misfit between Co and Pd, and with a 4.6 %* ML and b 0.5 %. We emphasize that the
accounts for a residual strain. Similar descriptions of strain above formula is used as a purely phenomenological descrip-
relaxation were made in the case of Ni/Cu 100 ,15 tion of strain modifications. Our point is not to analyze the
Co/W 110 ,16 and Co/Cu 100 .17 The best fit is obtained with relaxation mechanism but to obtain numerical values for the
0.14, dc 0.42 ML, and di 1.72 ML. Comparable val- strain as a function of Co thickness.
ues of residual strain about 1.5% were also observed on Scanning along the Co truncation rod perpendicular to the
Co/Pd 111 multilayers.18 film plane makes it possible to distinguish between the dif-
After Cr deposition on the Co/Pd 111 films, the in-plane ferent stacking sequences of the Co planes and to determine
Co lattice parameter is modified see Fig. 2 . There is an the spacing of the planes. Using Guinier's model,19 the data
are fitted with a combination of hcp, fcc and twinned fcc, and
disordered contributions Fig. 3 . No significant evolution in
the stacking or in the interlayer parameter deduced from the
position of the maximum of the peaks is noticed upon Cr
deposition. On average, the covered films are slightly ex-
panded out-of-plane by about 1.2 % with respect to the bulk
value for hcp Co 2.04 Å 1 ML . According to the macro-
scopic elasticity laws, an expansion both in-plane and out-
of-plane is unexpected but has already been observed. It has
been attributed to the defects in the film.20,21 The films are
composed of about 50% hcp Co and 50% disordered Co see
Fig. 3 . If the stacking sequence of an fcc structure is char-
acterized by the letters ABC . . . and the one for an hcp struc-
ture by ABAB . . . , then by disordered Co we mean that on
top of a plane A, there is the same probability to have a plane
B or C. Regarding the magnetic anisotropy, a disordered film
FIG. 2. In plane strain for an uncovered open squares Co/ is equivalent to a film made half of fcc Co and half of hcp
Pd 111 film and covered full squares with 14 Cr ML. The lines
are fits to the data see text . The inset shows the diffracted intensity Co, so that the magnetic properties of our film will be de-
around the Pd K 1 and Co K 1.6 truncation rod at H 0 and scribed by a film with 25% fcc Co and 75% hcp Co.
L 0.4 for a 5.7 ML Co film. Note the change in position of the Co The modification of magnetic properties upon Cr capping
truncation rod right before open squares and after full squares is therefore a complex effect. Kv is no longer constant but
Cr coverage. The K scale is in reciprocal lattice units. depends on the Co thickness through the thickness depen-

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INFLUENCE OF STRAIN ON THE . . . PHYSICAL REVIEW B 64 144431

TABLE I. Tabulated material dependent parameters used to de-
scribe the anisotropy.

K hcp hcp hcp hcp fcc
dip Kmc B1 2B3 B2 B2
106 erg/cm3 106 erg/cm3 106 erg/cm3 106 erg/cm3 106 erg/cm3

12.6 4.1a 570b 220c 770d

aReference 23.
bReference 24.
cReference 24.
dReference 25.

dence of strain. To estimate the strain contribution to the
effective anisotropy K, we modeled the anisotropy starting FIG. 5. Anisotropy times Co thickness versus Co thickness for a
from a structural description and including parameters found Co wedge covered with Cr. The full line is a fit to the data full
in the literature about magnetic properties of Co see Table squares using Eq. 4 and material dependent parameters given in
I . The model result was then compared to the measured Table I. The dashed curve results from a fit with no interface mag-
anisotropy, which was obtained by fitting polar hysteresis netoelastic coupling constant (Bs 0). The crossed line is obtained
loops with a Stoner-Wohlfarth coherent rotation model as- with a b 0 in Eq. 3 to estimate the strain induced anisotropy
suming a magnetization of 1420 emu/cm3 for the Co. The change upon Cr capping see text .
loops were recorded ex situ on a wedge-shaped Co/Pd 111
film covered with Cr. To ensure coherent rotation of the mag- late the high thickness data gives an effective interface an-
netization they were acquired by polar MOKE in two differ- isotropy Ks of 1.38 erg/cm2 and a volume anisotropy Kv of
ent geometries: perpendicular magnetic field when the easy 4.4 106 erg/cm3.
axis of magnetization was in-plane, and with a magnetic field Note that the effective volume anisotropy is close to the
at 79° from the normal for a perpendicular easy axis.22 Typi- one for an unstrained hcp Co thin film see Table I , so that
cal magnetization loops for both geometries are represented from the slope alone one would conclude that there is no
in Fig. 4 with the best fit in full line taking into account a strain in the film.
fourth order term . However, the effective anisotropy could From the detailed structural description given above, we
not be determined in the thickness range from 7 to 11 ML, modeled the magnetic uniaxial anisotropy using the follow-
for which there is evidence of noncoherent magnetization ing expression:
rotation.
On the plot K*d versus thickness, one observes a nonlin- K K hcp hcp hcp hcp
dip 0.75 Kmc B1 2B3 B *
2
ear behavior at low thicknesses which is not compatible with fcc
Eq. 1 Fig. 5 . This curvature shows the tendency for the 0.25*B2 /d, 4
2 Ks Bs*
magnetization to go back in-plane. Using Eq. 1 to extrapo- where everything is known except two parameters which are
designed in bold in the equation: Kdip is the dipolar anisot-
ropy, Khcp
mc the hcp Co magnetocrystalline anisotropy, Ks the
Ne´el interface anisotropy, and the Bi's respectively Bs) are
first order bulk respectively surface magnetoelastic cou-
pling constants. The Co fcc magnetocrystalline anisotropy
and the effect of roughness on the anisotropy were neglected.
The convention is that a positive K denotes a perpendicular
easy axis. Values used for the material dependent parameters
are given in Table I. The two unknown parameters-the in-
terface anisotropy Ks and the surface magnetoelastic con-
stant Bs-are used to fit the data.
A first attempt to fit the data was done with Bs fixed to
zero. The best result dashed curve of Fig. 5 was obtained
with Ks 0.08 erg/cm3. Clearly the fitting is not satisfactory,
especially at low thicknesses where the curvature is opposite
to the measured one. At higher thicknesses however the
slope is close to the one given by the data.
A second attempt was therefore needed where the two
FIG. 4. Normalized magnetization curve recorded in a canted parameters Ks and Bs were free to vary during the fit. Now
and polar geometry on a Co wedge covered with Cr. The uniaxial the data are well described full line of Fig. 5 with Ks
anisotropy was deduced from fits to the data full lines using a 0.34 erg/cm3 and Bs 11.5 erg/cm2. Also the tendency
coherent rotation model including first and second order anisotropy. for the magnetization to go back in-plane is accounted

144431-3

S. BOUKARI et al. PHYSICAL REVIEW B 64 144431

for through the negative value of Bs . The influence of the of region with full perpendicular remanence upon Cr depo-
surface magnetoelastic constant is therefore opposite to the sition see Fig. 1 is therefore mainly due to an increase of
interface anisotropy. Note that the as fitted interface anisot- the interface anisotropy.
ropy Ks 0.34 erg/cm3 is much smaller than the one de- In summary, structural and magnetic investigation of un-
duced from the simple model Eq. 1 , leading to Ks covered Co/Pd 111 films and then covered with Cr has been
1.38 erg/cm3 , which demonstrates how large the mag- performed. Through a detailed analysis of the structure of the
netoelastic contribution to the effective interface anisotropy film, we were able to show up the correlations between the
can be. structure and the anisotropy. Capping the Co film with Cr has
To try to understand how Cr deposition changes the mag- two effects: first an increase of the interface anisotropy
netic anisotropy, the function K*d is plotted with the previ- which leads to an extension of the region with full perpen-
ously evaluated Ks and Bs , taking a b 0 in Eq. 3 dicular remanence, and second a deformation of the under-
crossed curve of Fig. 5 to virtually hinder the deformations laying Co film which increases the strength of the anisotropy.
in the magnetic film due to capping. The slope of the as To properly describe the magnetic anisotropy, in particular
obtained curve is smaller than the one corresponding to the towards the low thicknesses, one has to consider a surface
data. The deformation induced by Cr increases thus the magnetoelastic coefficient.
strength of the anisotropy over the whole thickness range.
However, the thickness where the easy axis switches from Beam time at ESRF French CRG-Interface beamline is
perpendicular to parallel to the surface is nearly the same as acknowledged for the present study via the French Scientific
when Cr induced deformations are included. The extension Committee.

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