Photothermally modulated stray field imaging of magnetic materials

Magnetic stray fields of ferromagnetic surfaces were modulated by focused laser beam heating using a chopped Ar-laser beam. The changes of the magnetic flux close to the surface due to the temperature dependence of the magnetization are measured by a pickup coil. Modulation frequencies were between 100 Hz and 30 kHz. The heating beam and pickup coil were rastered over the sample surfaces to produce microscopic images. A lateral resolution of about 10 μm has been achieved, which is determined mainly by the size of the laser beam. The technique was applied to samples of iron containing cementite and austenite phases. The phase signals are revealing the different signs of the local magnetization. Measurements as a function of the external magnetic field show the reversal of the magnetization in the ferromagnetic phases.