Calculation and analysis of the forces on ferro-fluid particles on the surface of a Permalloy overlay bar

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Abstract
The formation of the ferro-fluid particles on the domain walls of a Permalloy overlay bar is due to the forces exerted on each particle by the stray field of the bar. Domain wall observation by Interference Contrast Technique reveals that the ferro-fluid particles do not form at some parts of the 180° domain walls. A further investigation by normal illumination technique has shown in fact that the particles are repelled from these areas as they look brighter than the grayish background. By calculation of the magnetic force from magnetic stress tensor and employing the domain wall model for an overlay bar, it is shown that a magnetization reversal inside the 180° domain wall is responsible for repelling the ferro-fluid particles. Inside the volume of an overlay bar, the forces exerted on the magnetization inside a 90° domain wall is also calculated and it is shown that these walls rotate under the torques applied by these forces until the resultant torque becomes zero. The force lies along the wall at this stage.