We use a technique called MOKE (Magneto Optic Kerr Effect), to analyze and characterize
magnetic thin films. The basic principle behind MOKE is that as polarized light interacts with a
magnetic material the polarization of the light can change. Think of the light as an
electromagnetic wave. It affects the electrons in the material. If something changes with the
electrons, their spin direction perhaps, then the light's polarization can change as well. In fact the
strength of the magnetization of the material affects the change in the polarization of the light
that is reflected off the surface of a magnetic thin film.
In our lab we have a slightly atypical MOKE setup. We have a laser, two polarizers, a magnet,
and a light detector like most setups but we also have what is called a Photoelastic Modulator or
PEM for short. The light goes through the first polarizer and strikes the sample, which is in a
region where the magnetic field can be changed. The light then reflects off the sample through
the PEM to the other polarizer and into the detector. Now the key thing here is that if the
polarization of the light changed when it was reflected off the sample, because it is magnetized,
then the intensity of the light getting through the polarizer must change. By adding the PEM,
which oscillates the polarization of the light at a constant frequency, we can measure how much
the polarization changes on reflection very precisely. This is done using a lock-in technique and
allows us to measure changes in polarization down to about 10-5 radians. By measuring this
change in polarization, as the magnetic field applied to the sample changes, we can measure how
the magnetization of the sample changes with applied field. For a ferromagnet we would see a
hysteresis curve. We can use this information to characterize a sample and find out how the
magnetization along different axes change with applied fields.
Our MOKE setup is very adaptable. Currently it is also setup to do what is called ROTMOKE.
The only difference is that the magnetic field is rotated around the sample. This can tell us a lot
about which directions the magnetization prefers to point in a given material, which is called
magnetic anisotropy and is a very important property of magnetic materials and of key interest in
both fundamental research and applied technologies.