Oscillations of the Curie temperature and interlayer exchange coupling in magnetic trilayers
A. Ney, F. Wilhelm, M. Farle, P. Poulopoulos,* P. Srivastava, and K. Baberschke
Institut fu ¨r Experimentalphysik, Freie Universita ¨t Berlin, Arnimallee 14, D-14195 Berlin-Dahlem, Germany
Received 19 October 1998
The onset of long-range magnetic order in exchange-coupled epitaxial Co/Cu/Ni trilayers, 2–4 monolayers
ML each, on Cu001 is studied by element-specific x-ray magnetic circular dichroism between 30 and 300
K in ultrahigh vacuum. Oscillations of the enhancement of the ordering temperature of Ni ( T
Ni
) by more than
40 K are measured as a function of interlayer exchange interaction by varying the Cu001 spacer thickness.
Below a Cu thickness of 2.3 ML antiferromagnetic coupling is measured. The period, phase, and amplitude of
the T
Ni
oscillations are in excellent agreement with the theoretical prediction for the short- and long-period
oscillations of the interlayer exchange coupling. S0163-18299950306-8
The discovery of oscillatory exchange coupling between
two ferromagnetic layers separated by a nonmagnetic spacer
material has attracted a lot of interest over the last several
years.
1
Theoretical approaches based on the Ruderman-
Kittel-Kasuya-Yosida RKKY model or quantum well states
have been able to describe the experimentally observed
short- and long-period oscillations of the interlayer exchange
coupling J
inter
( d ) as a function of the spacer thickness d.
2,3
Most of the attention has focused on the understanding of
giant magnetoresistance and on the dependence of the phase,
amplitude, and periodicity of J
inter
on the thickness and ma-
terial of the spacer and ferromagnetic layers and the orienta-
tion of the substrate. Very little work has addressed the effect
which the oscillatory exchange might have on the Curie tem-
perature of the ferromagnetic layer. As already pointed out
by Bayreuther’s group,
4
simple mean-field theory would pre-
dict an oscillation of the Curie temperature which correlates
with the exchange coupling. Evidence for this was found
experimentally in polycrystalline Ni/Au multilayers.
4
In that
experiment a set of different samples with nominally the
same thickness of the magnetic layer and a variable spacer
thickness was used. However, the reported uncertainty in the
magnetic layer thickness could lead to T
C
variations due to
finite-size effects
5
similar in magnitude to the observed T
C
oscillation amplitude.
On the contrary, epitaxial trilayers with two different fer-
romagnetic layers e.g., Co and Ni having separate ordering
temperatures are ideal prototype systems for illustrating the
relation between J
inter
and T
C
: We recently could demon-
strate by means of the element specificity of the x-ray mag-
netic circular dichroism XMCD technique that the ordering
temperature T
Ni
* of Ni in Co/Cu/Ni/Cu001 trilayers is in-
creased by T
Ni
compared to T
C
of Ni in Cu/Ni/Cu001 due
to the presence of Co by up to 40 K. Despite the strong
interlayer coupling that one expects for very thin spacers,
the Co and Ni magnetizations still vanish at different
temperatures.
6
Such a behavior could be interpreted as two
different ordering temperatures of the exchange coupled lay-
ers, T
C
of Co, the Curie temperature of the whole system,
and T
Ni
* , the ordering temperature of the Ni layer.
6,7
Such an
indirectly coupled system must be distinguished from di-
rectly coupled ferromagnets which have been discussed
theoretically.
7
This has triggered our interest to investigate
the following questions: i Is there an oscillation of T
Ni
with the spacer thickness d
Cu
and can it be correlated with
theoretical models of the oscillatory interlayer coupling? ii
Is it possible to observe antiferromagnetic coupling for mag-
netic trilayers with Cu001 spacer thickness in the range
2–4 monolayers ML and how does antiferromagnetic
AMF versus ferromagnetic FM interlayer coupling shift
the Curie temperatures of the ferromagnetic layer?
The latter question goes beyond the specific interest for
the relation between J
inter
and T
C
. Theoretical works for
noble-metal spacers based on the RKKY model have pre-
dicted two oscillations of the interlayer coupling with the
spacer thickness reflecting the topological properties of the
spacer Fermi surface:
2
J
inter
d =
1
d
2
A
1
sin 2 d /
1
+
1
+A
2
sin 2 d /
2
+
2
. 1
The earlier multilayer literature has reported only the longer-
period oscillations of J
inter
.
8
The short-period oscillations
were recorded later on in high-quality epitaxial trilayers with
Cr Ref. 9 or Cu001Ref. 10 spacers. Moreover, it is only
for few trilayers that short-period oscillations and AFM cou-
pling have been observed for spacers thinner than 3–4 ML,
e.g., for Fe/Au/Fe.
11
For the case of Cu001 spacers a sys-
tematic spin-polarized scanning electron microscopy spin-
SEM study on M/Cu/Co trilayers M=Fe,Co,NiRef. 12
has revealed both short- and long-period oscillations in good
agreement with theoretical values of
1
=2.56 ML 1 ML
=0.18 nm and
2
=5.88 ML, respectively.
2
The phases and
the amplitude ratio A
1
/ A
2
have been found to depend criti-
cally on sample quality and ferromagnetic layer thickness.
For the Ni/Cu/Co/Cu001 trilayer an amplitude ratio of
A
1
/ A
2
=1.30.5 was observed.
12
However, no oscillation or
AFM coupling was ever found for Cu001 spacers thinner
than 5 ML. This had been attributed to the existence of
pinholes
13
since the growth of Co on Cu001Refs. 14 and
15 is known to be problematic due to the segregation of Cu
to the top for the above system.
In this work we provide experimental evidence for an
AFM interlayer coupling in trilayers with Cu001 spacers
thinner than 5 ML. We measure the enhancement of the
element-specific ordering temperature T
Ni
which is found
RAPID COMMUNICATIONS
PHYSICAL REVIEW B 1 FEBRUARY 1999-II VOLUME 59, NUMBER 6
PRB 59 0163-1829/99/596/39383/$15.00 R3938 ©1999 The American Physical Society