Magnetisation of inks for magnetic particle inspection

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Abstract
The authors report measurements of the magnetisation of magnetic inks used in the magnetic particle inspection method of non-destructive testing. The shape and size of particle aggregates are found to influence strongly the initial susceptibility of these systems. The volumetric packing fraction has a negligible effect upon the initial susceptibility over the range of concentration studied. At both room temperature and 77 K, magnetic inks exhibit a ln t magnetic viscosity. At 77 K the magnetisation process is of a Neel type, producing a maximum in the magnetic-field-dependent coefficient of magnetic viscosity close to the coercivity. At room temperature the relaxation process is more complex, involving particle rotations and readjustments which dominate any simple Neel-type process. The details of these latter processes are unclear.

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