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Magnetism is not my specialty,  but since someone asked - not all stainless steels are non-magnetic.

Stainless steel is a generic term for steel alloys with a minimum of 10.5% chromium. The chromium gives the steel its 'stainless' properties - i.e. corrosion resistance. A very thin chromium-rich oxide layer forms on the surface of the metal; this is what provides the corrosion resistance.  The layer is 'self-healing' if it is scratched.  In general, the higher the proportion of chromium, the stronger the corrosion resistance of the steel. In addition to chromium, other metals are added to give the steel particular properties such as strength and malleability. Nickel is added to strengthen the oxide layer.
A basic stainless steel is formed from the addition of chromium.  This is what is typically used in stainless steel silverware, kitchen sinks, etc. It  has a 'ferritic' structure and is magnetic.  It can also be hardened through the addition of carbon (making it 'martensitic'.)
However, the stainless steels commonly used in construction are 'austenitic' - these have a higher chromium content and also contain nickel. The nickel modifies the physical structure of the steel and makes it non-magnetic. I think it has something to do with the spin of the electrons, but that is kind of a complete guess.
The austenitic (300 series) of steels contain nickel and are non-magnetic. Grade 304 may become magnetic after cold working such as wire drawing or rolling but  can be returned to a non-magnetic condition by stress relieving with heat.  Grades 310  and Grade 316 have higher nickel contents and have very little tendency to become magnetic.
Gail Kelley