Dear Tapash Chandra Paul,
Porosity of a material is a topological concept.
As porosity increases, the saturation magnetisation must change. But the type of porosity is one of the important factors that leads to saturation magnetism.
We know that magnetisation is due to orientation of dipoles by the torque produced by the applied magnetic field.
The torque due to the surrounding dipoles opposes or accelerates the orientation. This opposing or accelerating torque depends on the microscopic structure of the material. This means porosity is one of the factors on which saturation magnetisation depends.
Thanks
N Das
Hence,
Spontaneous magnetisation (i.e. the material magnetisation extrapolated at zero magnetic field) is the magnetic moment per unit volume. If porosity can be accessed through a measurement of density then studying spontaneous magnetisation as a function of density and comparing it to the magnetic moment per unit mass could be a way to evaluate the role of porosity i.e. just a dilution effect or a more elaborate effect due to an increase of surface to bulk contributions. Surface atoms could have enhanced or reduced moments for example. [Saturation magnetisation is a tricky measurement, it should be the 0K spontaneous M. At higher T, it depends on the maximum field available and magnets have a high field susceptibility which makes saturation a difficult quantity].
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