Magneto-optic Kerr effect magnetometry and microscopy (MOKE)
The magneto-optical Kerr effect (MOKE) is a well established technique to study magnetization properties. The effect is based on the fact, that the plane of polarization of light is rotated when the light is reflected from a magnetic material. The physical origin of MOKE is the magnetic circular dichroism effect: exchange and spin-orbit coupling in a magnetic material lead to different absorption spectra for left- and right-circularly polarized light. Measuring the change of the polarization of the reßected beam (often referred to as Kerr angle ΘKerr) gives access to the magnetization state of the sample.
With MOKE it is possible to study quasi-static magnetization reversal properties and magnetic anisotropies. When using a pulsed laser system it is also possible to study the time dependence of the magnetization under the influence of, e.g., a pulsed magnetic field or a microwave field. Since it is an optical technique it is non invasive, and the spatial resolution is only limited by the optical resolution. Thus, we are able to study the static and dynamic properties of magnetic thin films and magnetic structures with lateral dimension down to 1 μm.
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| Fig. 1: Schematic setup of a longitudinal Kerr magnetometer. |
Our group uses four different MOKE setups, all of them using the same principle. The light of a laser source is s-polarized through a thin-film polarizer. The beam is focused onto the sample. The polarization of the reflected light is analyzed by a detector unit that was developed and is built in our laboratory. A Wollaston prism divides the beam into two orthogonally polarized beams, which are monitored by a pair of photodiodes. The detector works as an opto-electrical bridge circuit to increase the signal-to-noise ratio. The obtained normalized differential signal (I1−I2)/(I1+I2) is directly proportional to the Kerr angle ΘKerr.
Four experimental setups are available to investigate different scientific aspects:
Longitudinal Kerr magnetometer: Longitudinal MOKE geometry to probe quasi-static properties of magnetic thin films. Optical resolution ∼ 100 μm, magnetic field up to 0.2 T, automated sample positioning and rotation.
Microfocused Kerr microscope with rotation unit: Longitudinal MOKE geometry to probe quasi-static properties of micro-structured magnetic elements. Optical resolution < 1 μm, magnetic field up to 60 mT, automated sample positioning, rotation and stabilization.
Dual MOKE magnetometer: Two combined MOKE magnetometers working in parallel, one in longitudinal and one in polar geometry to study the quadratic MOKE effects on magnetic thin films. Optical resolution ∼ 100 μm, two orthogonal pairs of magnet coils to provide any in-plane field direction up to 25 mT, automated sample positioning and rotation.
Time resolved scanning Kerr microscope: Longitudinal or polar MOKE geometry to study dynamic magnetization reversal properties of micro-structured elements. Optical resolution < 500 nm, time resolution ∼ 60 ps, magnetic field up to 15 mT, automated sample positioning and stabilization.