STRUCTURE AND ANISOTROPY OF [001] Co/Pd
ARTIFICIAL SUPERLATTICES
F. Den Broeder, D. Kuiper, H. Donkersloot
To cite this version:
F. Den Broeder, D. Kuiper, H. Donkersloot. STRUCTURE AND ANISOTROPY OF [001]
Co/Pd ARTIFICIAL SUPERLATTICES. Journal de Physique Colloques, 1988, 49 (C8),
pp.C8-1663-C8-1664. <10.1051/jphyscol:19888759>. <jpa-00229002>
HAL Id: jpa-00229002
https://hal.archives-ouvertes.fr/jpa-00229002
Submitted on 1 Jan 1988
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JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, decembre 1988
STRUCTURE AND ANISOTROPY OF [OOl]Co/Pd ARTIFICLAL SUPERLATTICES
F. J. A. den Broeder, D. Kuiper and H. C. Donkersloot
Philips Research Laboratories, 5600 J A Eindhoven, The Netherlands
Abstract. - Single crystalline [001] fcc Co/Pd artificial superlattices were prepared by vapour deposition in UHV. X-ray
diffraction revealed periodic structures even for films containing only one atomic layer Co and three atomic layers Pd per
period. Perpendicular anisotropy, which is found only for superlattices containing Co monolayers, increases sharply with
substrate temperature, indicating a strong dependence on atomic layer roughness.
It has recently been found that polycrystalline
[Ill] textured fcc Co/Pd multilayers, acquire, by the
existence of interface anisotropy, a perpendicular easy
axis below a Co layer thickness tco of 8 A [I, 2, 31.
To study the effect of another crystallographic orientation, this paper deals with the structure and anisotropy
of single-crystalline [OOl] fcc Co/Pd artificial superlattices.
They were prepared by e-beam evaporation in UHV
onto cleaved [OOl] NaCl at substrate temperature T,
after deposition of a 1000 [001] epitaxial Pd base
layer at 300 OC. Shutters were used t o alternate the
constituents, while deposition rates ( w 1 A/s) were
monitored by quartz oscillators.
The samples are designated by ComPdn,in which m
and n are numbers of (001)-monolayers per modulation
period, assuming for Co and Pd an fcc structure with
lattice constants ac, = 3.55 A and spa = 3.89 A. Total
numbers of bilayers were chosen to obtain a total Co
thickness of about 700 A.
I
t
a
15
20
25
-L
d (A)
30
Fig. 1. - X-ray diffractogram (CuKa) of a [001] ColPdlo
superlattice deposited at T, = 520 OC on NaCl (001); d =
sin
'.
The following ComPd, series were prepared:
a) n = 10 with m = 1,2, ..., 6; T. = 50 OC;
b ) m = l w i t h n = 1 , 2 , ...,5; T,=50°C;
c) m = 1with n = 1,2; T,= 100 OC, 150 OC, 200 OC.
For the whole series a and for series b with n > 2, Xray diffraction (XRD) showed a periodic structure. As
an example, figure 1 is an XRD pattern for ColPdlo.
Although the Co layers are nominally one monolayer
thick, the pattern displays six satellites around the
main 200 reflection, indicating a well-defined superlattice. When the substrate had been rotated during layer
deposition, the XRD rocking curve of the strongest superlattice reflection was symmetrical, showing a mosaic spread of only 0.32 deg around [OOl]. However,
when no rotation had been applied,
the rocking
curve
-+
.
was asymmetrical, indicating that the mean [001] axis
was somewhat inclined towards the original position
of the Pd source. This points to shadowing effects,
caused by limited surface diffusion at T, = 50 OC.
For all films plan-view transmission electron diffraction (TED), showed a single crystalline fcc structure,
as shown in figure 2 for Co3Pd10. Despite the 9 % difference in interatomic distance for Co and Pd, only
-
Fig. 2. - Plan-view electron diffraction pattern of Co3Pdlo
superlattice.
single, circular spots from planes which are perpendicular t o the film plane were observed. This means that
the layers are structurally coherent and consequently
tetragonally deformed.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888759
C8 - 1664
JOURNAL DE PHYSIQUE
For the series a with n = 10, the saturation magnetization Is per unit volume Co, measured with a vibrating sample magnetometer, was found to be higher
= 1.76 T) and to increase with
than for bulk Co
lower m up to about 2,65 T for m = 1. Such an enhancement has already been explained by polarization
of Pd-interfacial atoms [3].
Of all the multilayers in series a, only those containing Co monolayers (m = 1) had a perpendicular anisotropy. As an example, figure 3 shows the
(2
- Uniaxial anisotropy energy Ku of a) ColPdn
multilayers prepared at Ts= 50 OC. b) ColPdl and C01Pd2
multilayers prepared at increasing substrate temperature
Fig. 4.
rn
Fig. 3. - Magnetic hysteresis curves measured in fields
to the film plane of a)
parallel ([I) and perpendicular (I)
Co2Pd10 and b) ColPdlo superlattices.
highly different hysteresis curves for Co2Pd10 (in-plane
anisotropy) and ColPdlo (perpendicular anisotropy).
This result differs from [ I l l ] multilayers, for which
the anisotropy was perpendicular up to tco = 8 [2,
31. In the latter films magnetocrystalline anisotropy,
probably originating from a [00.1] hcp-Co stacking,
contributed significantly to the anisotropy. In the
present [OOl] fcc multilayers the thicker Co layers
have a cubic stacking, for which magnetocrystalline
anisotropy is probably absent. Also anisotropy induced by coherency stresses may play a role. The origin of the perpendicular anisotropy of the Co monolayers is thought to be interface anisotropy or magnetocrystalline anisotropy associated with the specific
atomic arrangement.
From the area between the magnetization curves,
measured in fields perpendicular and parallel to the
film, the uniaxial anisotropy constant K,, per unit volume Co was determined. Figure 4a shows for the case
of Co monolayers (series b) that Ku increases with n.
This may indicate a magnetic infiuence of the Pd thickness layers on Ku. However, since for n = 1 and n = 2
XRD did p o t show clear superlattice reflections, the
increase of K, may also originate from a better definition of the superlattice structure with larger n. This
prompted us to investigate ColPdl and ColPd2 made
at higher T, (series c).
Figure 4b shows that there is no systematic change
of Ku for ColPdl, but for ColPdz Ku rises sharply
with T,. For the latter films XRD gave weak superlattice reflections whose intensity increased with T,.Evidently at higher T,, increased surface diffusion during
growth makes the layers smoother while their perpendicular anisotropy increases. It is also noted that for
ColPdl, prepared at 100 OC and 200 OC, for which Ku
is also positive, TED gave (110) superlattice spots.
If the perpendicular anisotropy is due to interface
anisotropy, the latter thus appears to be very sensitive
to layer roughness. For the smoothest layers prepared
~ ,
at Ts = 200 "C, we obtain from Ku=3.28 M J / ~ after
correcting for the demagnetization energy of pure Co,
an interface anisotropy constant Ks = 0.39 m ~ / m ~ .
This higher value compared to [ I l l ] Co/Pd (K, =
0.26 m3;/m2, Ref. [2]) may reflect an orientation dependence of K., but it is more likely caused by a
greater smoothness. This agrees with a recent geometric model based on Co-Co and Co-Pd pair interactions which predicts a strong dependence of K,
on
interface diffuseness [4].
In conclusion, we succesfully prepared [001] fcc
Co/Pd artificial syperlattices. Only with Co monolayers they show perpendicular anisotropy, which increases when the Co layers become smoother.
[I] Carcia, P. I?., Meinholdt, A. D. and Suna, A.,
Appl. Phys. Lett. 47 (1985) 178.
[2] Draaisma, H. J. G., den Broeder, F. J. A. and de
Jonge, W. J. M., J. Magn. Magn. Muter. 66
(1987) 351.
[3] Den Broeder, F. J. A., Donkersloot, H. C.,
Draaisma, H. J. G. and de Jonge, W. J. M., J.
Appl. Phys. 61 (1987) 4317.
[4] Draaisma, H. J. G., den Broeder, F. J. A. and de
Jonge, W. J. M., J. Appl. Phys. 63 (1988) 3479.