Significant modification of
59
Co hyperfine fields assigned to specific structural changes
in sputtered Co/Au and Co/Cu multilayers
C. Christides
Department of Engineering Sciences, School of Engineering, University of Patras, 26110 Patras, Greece
and Institute of Materials Science, NCSR ‘‘Demokritos,’’ 153 10 Aghia Paraskevi, Attiki, Greece
S. Stavroyiannis and D. Niarchos
Institute of Materials Science, NCSR ‘‘Demokritos,’’ 153 10 Aghia Paraskevi, Attiki, Greece
M. Wojcik, S. Nadolski, and E. Jedryka
Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland
Received 28 April 1998; revised manuscript received 30 November 1998
Magnetron sputtered Co/Cu and Co/Au multilayers MLs with low-field giant magnetoresistance were
grown on SiN
x
underlayers. Spin-echo
59
Co nuclear magnetic resonance NMR spectra reveal an fcc Co layer
stacking for the Co/Cu multilayers whereas the Co/Au spectra exhibit a unique profile that cannot be assigned
to any of the known bulk crystalline or glassy Co structures. Analysis of the Co/Au superlattice reflections,
observed with x-ray diffraction XRD, indicates that Co layers ( d
Co
) expand significantly relative to close-
packed lattice values and provide evidence for a reduced atomic density. The complementary study with XRD
and NMR measurements has revealed a metastable Co nanostructure in the as-prepared Co/Au MLs.
S0163-18299900913-3
I. INTRODUCTION
Since 1985 the evolution of the theory of quantum inter-
ference phenomena
1
has led to experiments on ultrathin Au
films and compositionally modulated films with alternating
Co/Au layers, that have shown
2,3
quantum size effects in the
resistivity and magnetoresistance. To observe such quantum
interference phenomena high-purity ultrathin single-crystal
metal films with atomically flat surfaces or interfaces are
required, and gold has been proved to be an ideal material
for these studies.
2,4
Thus, experimental evidence of quantum
size effects has been established after the observation of os-
cillations in: i the dependence of resistivity in ultrathin,
epitaxial gold films on the film thickness
2
and ii the indirect
exchange coupling in Co/Au111/Co epitaxial trilayers.
4
Be-
sides the scientific interest to investigate the involved quan-
tum well states on the spin-polarized quantum size effects at
the ferromagnetic/no-magnetic interface
5
a great deal of at-
tention has been focused on the technological applications
6
of the resultant giant magnetoresistance GMR effect in
GMR reading heads and sensors.
Today, 111 textured multilayers with an fcc modulated
structure exhibit the largest GMR effect at room temperature
for sputtered
7
Co/Cu multilayers MLs while such effect is
not reported in sputter-grown Co/Au MLs.
8
In epitaxial Co/
Au111/Co trilayers the GMR curves and the magneto-
optical Kerr effect hysteresis loops
4
exhibit a coercive field
( H
c
) of about 0.5 kOe that precludes GMR applications with
epitaxial Co/Au MLs, due to the large magnetocrystalline
anisotropy of hcp Co. Comparatively, sputtered fcc Co/Cu
MLs with appropriate layer thicknesses exhibit GMR ratios
9
up to 60% at room temperature in contrast to epitaxial Co/Cu
MLs which register a much smaller GMR effect, and their
magnetic switching field ( H
s
) is several times larger.
10
Thus,
from the current experimental evidence it is difficult to dis-
tinguish between several contributions to the GMR which
are directly or indirectly linked to the structural properties of
the superlattice.
11
Lately, it was reported
12
that sputter-grown
Co1 nm/Au2.5 nm
30
MLs with 111 texture exhibit at
ambient conditions a GMR ratio of about 3%, an H
c
=0.01
kOe and an H
s
=0.03 kOe, which are an order of magnitude
less than the corresponding H
c
and H
s
values reported pre-
viously. Conventional
12
and, unpublished yet, high-
resolution HR cross-section transmission electron micros-
copy TEM indicate that this result is related
12
to unusual
atomic packing inside the Co layers. Thus, a modified cubic
Co structure give rise to low-field GMR in the as-prepared
Co/Au MLs. The developed Co layering deserves further in-
vestigation because it is relating a kinetically grown Co layer
structure with possible GMR applications of Co/Au MLs.
Spin-echo
59
Co nuclear magnetic resonance NMR has
proved
6
to be a powerful method that distinguishes the com-
ponents from the interfaces and the bulk Co layer
13
and pro-
vides information about the local Co atomic packing in mul-
tilayered structures. Since the NMR spectra of sputtered
GMR Co/Cu MLs have been studied extensively,
6,13
result-
ing in a unique assignment between the spectral lines and the
corresponding Co stacking, we decided to investigate the
59
Co NMR spectra of these, low-field GMR, Co/Au MLs in
comparison with spectra obtained from Co/Cu MLs as well.
In this study, our intent is to probe indirectly the local struc-
ture differences in the Co layers of low-field GMR Co/Cu
and Co/Au MLs as a function of Co layer thickness ( t
Co
) by
using complementary information from
59
Co NMR and XRD
measurements.
II. EXPERIMENTAL DETAILS
Metallic disks of 99.99% pure elements with diameter 5
cm were used as target materials in a high-vacuum Edwards
E360A sputtering system with a cluster of ATOM-TECH
PHYSICAL REVIEW B 1 APRIL 1999-I VOLUME 59, NUMBER 13
PRB 59 0163-1829/99/5913/88129/$15.00 8812 ©1999 The American Physical Society