To understand the occurrence of either preferred direction, one
has to assume that the AF layer exhibits a small magnetic moment
at the interface that can be aligned by the cooling field. After
cooling, this moment will not change its direction, even upon reversal
of the external field, since it is now strongly anchored (pinned)
in the magnetic system of the AF. This very small moment at the
interface then aligns the FM via an internal magnetic field called
magnetic exchange coupling. This exchange-bias phenomenon is useful
in modern magnetic devices.
Two years ago, the Stanford group was able to provide the first
direct experimental evidence for the existence of this small
interfacial moment in the AF by using Beamline 4.0.2. To explain the anomalous
behavior with FeF2, it has been postulated that the coupling between
the pinned moment in FeF2 and a FM layer is not parallel but antiparallel
and thus reverses the favored direction of the FM. In the new experiment,
the researchers verified this proposition in a high-quality 2.5-nm
FeF2–Co bilayer grown at West Virginia University.
They used x-ray magnetic circular dichroism (XMCD) spectroscopy,
a technique that can selectively detect the magnetic properties
of sites close to the interface and distinguish between the magnetic properties of cobalt
and iron atoms.
Hysteresis loops (magnetization vs. applied magnetic field) of
the interfacial iron spins and the FM cobalt layer taken at 300
K (well above the ordering temperature, TN =78 K) for FeF2 show
that both loops are symmetric and the entire moment at the FeF2
interface follows the Co moment. Since one would expect the orientations
of all iron moments within the FeF2 lattice to be in constant motion
above TN, it is clear that the magnetic moment of the cobalt layer
can be used to induce magnetic order in the topmost layer of the
FeF2.
Ideal hysteresis loops (magnetization vs. applied
magnetic field) for the FM component of “typical” field-cooled
AF-FM bilayers with “positive” remanent magnetization
and of FeF2–FM bilayers with “negative” remanent
magnetization. The magnetizations in each of the component
layers and the interface are represented by the arrows in
the diagrams.
At 15 K, the investigators observed a partial shift in the cobalt
hysteresis loop indicating a positive remanent magnetization. Most
of the spins at the interface simply seem to follow the FM Co layer
as they do above TN, and the overall shape of the interfacial hysteresis
loop closely resembles that of the cobalt layer. However, it appears
that the interfacial iron loop shifts down towards negative magnetization.
This shows that a small fraction of the spins at the interface
generate a magnetic moment that always point opposite to the favored
magnetization of the FM.
Left: Element-specific cobalt (black) and iron
(red) hysteresis loops (as measured by XMCD) hysteresis loops
acquired at 300 K and 15 K after cooling in a weak field that
only partially aligns the surface moment. Right: Schematic of
moments in the cobalt and at the interface.
In sum, the group found that a small fraction of interfacial
magnetic moments tend to align the FM layer opposite to their moment,
which can lead to a reversal of the preferred magnetization direction
of the FM with respect to the cooling field. These moments appear
only below the antiferromagnetic ordering temperature. On the other
hand, the FM is able to align some of the moments at the AF surface
so that they point in the same direction as the FM even above TN.
All in all, the magnetic properties at AF–FM interfaces are
the result of complicated and competing processes. The present
results provide clear evidence to distinguish between these two
types of coupling mechanisms.
Research conducted by H. Ohldag and J. Stöhr (Stanford Synchrotron
Radiation Laboratory); H. Shi and D. Lederman (West Virginia University);
and E. Arenholz (ALS).
Research funding: National Science Foundation and U.S. Department
of Energy, Office of Basic Energy Sciences (BES). Operation of
the ALS is supported by BES.
Publication about this research: H. Ohldag, H. Shi, E. Arenholz,
J. Stöhr, and D. Lederman, “Parallel versus antiparallel
interfacial coupling in exchange-biased Co/FeF2 bilayers,” Phys.
Rev. Lett. 96, 027203 (2006).
ALSNews
Vol. 268, August 30, 2006 |