Brillouin Light Scattering Studies of Magnetic Multilayers

1
Brillouin Light Scattering Studies of Magnetic
Multilayers
Cyrus Reed, Milton From Department of Physics and
Astronomy, Western Washington University
Magnetic Multilayers
An example case study Fe/Pd multilayers
Brillouin Light Scattering (BLS)
Questions
Samples used in the study

• The volume of material which BLS probes is a few
  cubic microns in size. Thus it is ideally suited
  to magnetic multilayer geometries.
• BLS provides a way of measuring the magnetic
  surface anisotropy of magnetic films.
  Understanding this anisotropy is of key
  importance in the development of spin-injection
  contacts in semiconductor devices.
• BLS is a non-destructive technique which uses
  laser light to investigate magnetic excitations
  in a material.

What are they?
Prepared by magnetron sputtering

• Are the excitations present in a Fe/Pd
  nanostructure compatible with elementary
  spin-wave theory?
• What is the magnetic anisotropy imposed on the Fe
  by the Pd?
• Is the bulk moment of the Fe atoms affected by
  the presence of Pd?

• Ultrathin magnetic layers of ferromagnetic
  material separated by non-magnetic spacer layers.
• Individual layers are typically 1 to 20 atoms
  thick.
• Multilayers are grown by magnetron sputtering or
  by Molecular Beam Epitaxy

6 samples x6, 12, 18, 29, 32, and 43
Dependence of the BLS spectra on applied magnetic
field
Using BLS to characterize a magnetic material a
two-step process
Present uses of Magnetic Multilayers

• Step 2
• Fit a model calculation to the data. Model
  parameters are
• Perpendicular anisotropy (K)
• Interlayer exchange coupling (J)
• Uniform magnetization in each magnetic film (M)

Step 1 When light is scattered from a magnetic
sample the lights frequency will be shifted by
magnetic excitations in the material. Measure
this frequency as a function of applied magnetic
field with a scanning Fabry-Perot Interferometer.

• Magnetic Field Sensors
• Read Heads for computer hard disks
• Galvanic Isolators

Dependence of BLS spectra on Pd thickness
Sample Fe15Å/Pdx x 25, Magnetic field 2kOe
Magnetic multilayers technology of the future?

• MRAM (Magnetoresistive random access memory)
  IBM, Honeywell, Motorola, Hewlet Packard all have
  substantial development activities
• SPINTRONICS One possible road to Quantum
  computing is to use magnetic multilayers to
  inject spin polarized electrons into
  semiconductor devices.

Conclusions
Schematic diagram of the BLS apparatus

• A single parameter fit to our data yields the
  correct BLS peak height ratios and field
  dependence for individual samples. Thus the data
  is in agreement with simple spin-wave theory.
• The observed anisotropy parameter depends on Pd
  thickness. It falls in the range 0.12ltKlt0.4
  ergs/cm2
• The varying K value may be a sign that the moment
  of the Fe atoms is affected by Pd. Mossbauer
  spectroscopy and magnetometry experiments are
  currently being done to test this hypothesis.

Photo of the scanning interferometer being built
at WWU.
Acknowledgements

• The BLS data presented here was collected by us
  on apparatus at Simon Fraser University
  (Vancouver, Canada).
• Samples were made at the Center for Materials
  Research at McGill University (Montreal,
  Canada).Thanks to Li Cheng and Zaven Altounian
  for sample preparation and structural
  characterization.