Generally speaking, iron, nickel, cobalt and nickel are all magnetic – or formally speaking, ferromagnetic. Stainless steels are all magnetic except those based upon a purely austenitic microstructure, so ferritic, martensitic and duplex/super duplex grades are magnetic.
An austenitic microstructure refers to the crystallographic structure of the alloy, which is the lattice structure that the atoms align themselves too in the solid state. The significant additions of nickel to this family of stainless steels forces the microstructure to exist as an austenitic structure rather than a ferritic for instance.
The ferromagnetic property of metals i.e. their ability to be attracted to a permanent magnet, are associated with the control of electrons that orbit the nucleus of an individual (iron) atom. In other microstructures, the ‘spin’ of these orbiting electrodes can be aligned by an external magnetic, and the metal appears to be magnetic or ferromagnetic. In an austenitic structure, the microstructure appears to be more random at the atomic layer, meaning that the electrons are unable to align their rotation in a consistent direction. Therefore, they appear as non-magnetic or non-ferromagnetic.