Magneto Optic Kerr Effect (MOKE) Measurements
The above Figure shows MOKE hysteresis loops of Terfenol-D thin film on Silicon substrate at different strain levels. A four-point bending fixture is employed to strain the thin film. The change in coercivity with strain can be used to calculate the film’s magnetostriction.
Length > 26 mm and < 40 mm
Width > 4 mm and < 8 mm
Length/ Width Ratio > 4
The Magneto-Optic Kerr Effect (MOKE) describes changes to light reflected from a magnetic surface. These changes result from the off-diagonal components of the dielectric tensor. These off-diagonal components give the magneto-optic material an anisotropic permittivity, meaning that its permittivity is different in different directions.
There are two main types of MOKE measurements. Transverse/Longitudinal MOKE to detect in-plane magnetization and Polar MOKE to detect out-of-plane magnetization.
The experimental setup consists of:
An electromagnet with bipolar power supply to generate the magnetic field
A polarized light source for probing the sample's magnetization
A photoelastic modulator to periodically modulate the light between left and right circularly polarized light
A photo detector to detect the reflected light from the sample
A lock-in-amplifier to improve the signal to noise ratio
A GPIB card to control the magnetic power supply and the lock-in amplifier through the computer
Each sample is loaded into the custom MOKE fixture. This fixture enables the sample to be tested per ASTM standard D6272 [Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials by Four-Point Bending].
This test results in Kerr rotation and Kerr ellipticity measured at different fields. These measurements are used to produce a hysteresis loop. By measuring the change in M vs H loops with the applied strain one is able to calculate the coupling between magnetic and mechanical fields and estimate the magnetostriction values. Therefore resulting in a measurement of the magnetostriction in thin magnetic film structures. This technique can be used for the following studies