Oscillatory Coupling
The magnetic exchange coupling [EPG Pub# 610] between magnetic layers separated by nonmagnetic spacer layers is currently both of fundamental scientific interest and of potential technological significance in giant magnetoresistance-based magnetic sensor applications. We have studied the coupling in Fe/xx/Fe(100) sandwich structures where the interlayer material, xx=Cr [EPG Pub# 580], Ag [EPG Pub# 613], Au [EPG Pub# 675], Cu, Al, or V. The sandwiches were prepared by molecular beam deposition of the interlayer material on top of a nearly perfect, Fe single crystal whisker substrate. In order to investigate the thickness dependence of the coupling, the spacer layer was grown as a wedge of linearly increasing thickness. The thickness of the wedge was measured by scanning the specimen with the microscope in the RHEED mode.
The wedge was then coated with an Fe film and the coupling was determined from
SEMPA images of the magnetic domain structure of the film.
The high spatial resolution of SEMPA allowed us to use the small Fe
whisker substrates and precisely measure the magnetic coupling along the wedge.
All of the sample preparation was done in situ so that the
magnetization, composition, and atomic-scale roughness and thickness of the
same specimen could be monitored by using SEMPA, Auger spectroscopy, and RHEED,
respectively. In addition, we have made extensive
STM measurements of the Cr
growth on the Fe whiskers. For all of the interlayers studied,
we have found that the atomic scale structure of these films strongly affects the magnetic coupling of the
multilayer. Cr, Ag and Au grow as smooth, epitaxial layers and produce the
magnetic exchange coupling summarized in the figure.
For these smooth interlayers, the coupling oscillates between ferromagnetic and
antiferromagnetic with a period that is determined by the size and shape of
the interlayer material's Fermi surface. The following table compares the
coupling periods derived from the SEMPA measurements with periods predicted by
calculations using Fermi
surface nesting features:
| Exchange Coupling Periods (layers) | |||
| Measured | |||
| Stiles24 | Bruno & Chappert25 | ||
| Fe/Cr/Fe (0.144 nm/layer |
2.105 ± 0.005 12 ± 1 |
2.10 12.1 (and others) |
|
Fe/Ag/Fe (0.204 nm/layer) |
2.37 ± 0.17 5.73 ± 0.05 |
2.45 6.08 |
2.38 5.58 |
Fe/Au/Fe (0.204 nm/layer) |
2.48 ± 0.05 8.6 ± 0.3 |
2.5 9.36 |
2.51 8.60 |
Cu, Al, and V do not grow in a smooth layer by layer manner and the coupling
reflects this disorder. Cu does not grow as a continuous film at all, but
instead forms large crystallites at substrate defects. Al ;grows as a very
rough film which results in 90°, biquadratic coupling between the Fe
layers. V ;also grows as a rough film producing only long period coupling
oscillations.
The nearly perfect Fe/Ag, Au, Cr/Fe multilayer structures also can be used to
test theoretical predictions of the magnitude of the exchange coupling
strength. In order to measure the exchange coupling strength, the applied
magnetic field required to switch the Fe film magnetization from antiparallel
to parallel alignment with the whisker is measured. This measurement is done in
a magneto-optic Kerr effect, MOKE, microscope, since
the large applied magnetic fields would distort the tractories and
polarizations of the secondary electrons used in SEMPA. Samples are therefore
prepared in the SEMPA apparatus, coated with a thin Au passivation film, and
then imaged in air in the MOKE microscope. The following figure compares SEMPA
and MOKE images from a Au wedge grown on an Fe whisker and then coated with
12 ;layers of Fe followed by 6 ;layers of Au.
The figure below shows a composite of MOKE images taken at many different applied magnetic fields. The dark regions correspond to antiferromagnetically coupled Fe. This is, effectively, an image of the Fe/Au/Fe exchange coupling strength.
From this series of images a coupling strength as a function of the Au thickness can be derived, but this strength is the average of the true coupling strengths from the various thicknesses that are present at any particular average thickness. Using roughness data derived from RHEED measurements of the wedge, the average coupling strength was modelled and fit [EPG Pub# 675] to the data in order to extract the true coupling strengths.
The derived coupling strengths for Fe/Au/Fe agree well with the theoretical predictions by Stiles [EPG Pub# 657]. Thus, the large, order of magnitude, differences observed previously between measured and theoretical coupling strengths can be mostly attributed to the roughness of the films.
Investigation of Exchange
Coupled Magnetic Layers by Scanning Electron Microscopy with Polarization Analysis (SEMPA)
Observation of Two Different
Oscillation Periods in the Exchange Coupling of Fe/Cr/Fe(100)
Oscillatory Magnetic Coupling
in Fe/Ag/Fe(100) Sandwich Structures
Determination of the Exchange
Coupling Strengths for Fe/Au/Fe
Spin-Dependent Interface
Transmission and Reflection in Magnetic Multilayers
John Unguris
Daniel T. Pierce
Robert J. Celotta
Mark Hart - IBM, Almaden
David Tulchinsky - Naval Research Laboratory
Gary Prinz - Naval Research Laboratory
Conrad Busman - Naval Research Laboratory
Supported in part by the Office of Naval Research
Online: May 1996
Last Updated: February 2008
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