A Modulating Aperture by Domain Wall Displacements in Magneto-Optical Materials

Two laser beams are focused onto a thin permalloy film. By changing the direction of the magnetization of the permalloy film the magneto-optic Kerr-effect is exploited either to rotate the plane of polarization or to modulate the amplitude of the reflected light. The domain wall behaviour is determined by the applied field and demagnetizing fields in order to compensate the inner field. Acting as a modulation aperture, the displacements can be controlled accurately by the amplitude or the frequency of the applied field. In a first step, the film is characterized for our application by measuring the velocity of the domain wall displacements by two focused beams with known distances. By measuring the delay between the modulation of the reflected beams it is possible to accurately determine the beam separation under the condition of reproducible wall movements. Therefore, a moving domain wall acts like a high-speed modulator. The method could be used for many applications, for instance refraction measurements by dispersion of the refractive index in the atmosphere or rapid quality control of magnetic materials.