Field dependent exchange coupling in NiOÕCo bilayers
J. Camarero,
1,
* Y. Pennec,
1
J. Vogel,
1
S. Pizzini,
1
M. Cartier,
2
F. Fettar,
2
F. Ernult,
2
A. Tagliaferri,
3
N. B. Brookes,
3
and B. Dieny
2
1
Laboratoire Louis Ne ´el, CNRS, BP166, 38042 Grenoble Cedex, France
2
CEA/Grenoble, DRFMC/SPINTEC, 38054 Grenoble Cedex, France
3
European Synchrotron Radiation Facility (ESRF), 38043 Grenoble Cedex, France
Received 12 December 2002; published 31 January 2003
Dynamic magnetization reversal measurements have revealed a strong dependence of the exchange-coupling
strength on the maximum applied field for NiO/Co bilayers. Time-resolved Kerr measurements, performed at
room temperature, show that the coercive field increases linearly with the maximum applied field between two
reversals. If the maximum applied field is different in the positive and negative directions, an exchange bias is
observed as well as an asymmetry in the magnetization reversal behavior between the two sides of the
hysteresis loop. X-ray magnetic circular dichroism measurements at the Ni L
2,3
edges indicate that these effects
are related to an increase of the net uncompensated Ni moment with increasing field. These effects should also
be present in other low anisotropy antiferromagnetic/ferromagnetic systems.
DOI: 10.1103/PhysRevB.67.020413 PACS numbers: 75.30.Et, 75.50.Ee, 75.70.Cn, 78.70.Dm
The nature of the magnetic interaction at the interface
between an antiferromagnet AF and a ferromagnet F is a
long-debated issue.
1
Experimentally, when a AF/F bilayer is
grown in a magnetic field or field cooled from above the
Ne
´
el temperature of the AF layer, the hysteresis loop is offset
from zero applied magnetic field by an exchange bias H
E
.
An enhancement of the coercivity ( H
C
) and an asymmetry in
the magnetization reversal process are also generally ob-
served in AF/F systems. The physics of exchange coupling is
not fully understood, but it is well established that the
atomic-level morphological, chemical, and spin structures at
the interface, as well as the spin structure inside the AF layer,
play an essential role.
In the original model,
2
a perfect, flat interface with un-
compensated spins at the AF surface and a rigid spin struc-
ture in the AF layer were used to explain the existence of an
exchange bias. The exchange bias expected from this model
is an order-of-magnitude larger than experimentally ob-
served. In order to remove this discrepancy, several modifi-
cations to this model have been proposed. Mauri et al.
3
sug-
gested the formation of AF domain walls parallel to the
interface. For rough interfaces, frustration of the AF/F inter-
actions can also lead to vertical domain walls in the AF
layer.
4,5
In polycrystalline AF layers, the granular structure
also leads to a multitude of AF domains, with different an-
isotropy directions.
6
A repopulation of the different domains
upon field cooling,
7,8
imposed by the interaction with the F
layer
9
and mediated by uncompensated moments at the AF
interface,
10
is likely to be a crucial ingredient for the ex-
change bias mechanism. This repopulation is thermally acti-
vated, leading to a strong influence of the temperature but
also of the waiting time on the exchange coupling.
11–14
In this Communication we show that also the external
magnetic field can influence the reorganization of the AF
magnetization and thus the exchange anisotropy. This influ-
ence originates from the net uncompensated AF moment,
which was observed to increase with increasing magnetic
field. A simple picture based on spin-glass-like behavior of
the uncompensated AF spins is used to account for the ex-
perimental results.
The study has been carried out at room temperature on
polycrystalline NiO/Co bilayers which have already been the
subject of magnetic force microscopy
15
MFM and magne-
tization dynamics
16
studies. These bilayers exhibit a well-
defined uniaxial anisotropy resulting from the deposition at
oblique incidence of the NiO layer, and, at room tempera-
ture, an enhancement of the coercive field with respect to a
single Co thin film without a significant shift of the hyster-
esis loops. The results of the magnetization dynamics study
could be explained with a model of exchange anisotropy
invoking the randomness of the coupling between AF and F
layers which originates from the high frustration of exchange
interactions at the AF/F interface.
16
The dependence of the coercive field H
C
on the external
magnetic field, for different field sweep rates, has been ob-
tained using the longitudinal Kerr effect. Figure 1a shows
typical hysteresis loops of a NiO/Co bilayer for an applied
field with triangular shape and fixed sweep rate dH /dt . H
C
increases for increasing values of the maximum applied field
H
max
whereas the shape of the transition stays identical. The
latter indicates that there is no change in the magnetization
reversal process. H
C
follows a linear relationship with H
max
,
with the largest variation for the highest dH /dt , as shown in
Fig. 1b. This could explain why this effect has not been
observed in quasistatic measurements. Figure 1c shows that
this linear relationship can be written as
H
C
=H
0
1 + H
max
, 1
where is a constant and only H
0
depends on dH /dt . H
0
can hence be addressed as the intrinsic dynamic coercive
field of the Co layer for a given dH /dt . The same behavior
has been observed for several NiO and Co thicknesses.
We discuss three possible explanations for the dependence
of H
C
on H
max
. The first is that the observed effect is due to
an incomplete saturation of the F layer for the maximum
applied field H
max
.
17
MFM images taken at zero field after
applying a field higher than the Co coercive field
15
and x-ray
RAPID COMMUNICATIONS
PHYSICAL REVIEW B 67, 020413R2003
0163-1829/2003/672/0204134/$20.00 ©2003 The American Physical Society 67 020413-1