The special magnetic field is a game mechanic introduced in generation 4 where particular Pokemon evolves only in presence of strong magnetic field (Nosepass and Magneton in Platinum).
The special magnetic field in Sinnoh is located inside Mt Coronet (the whole of it). You can evolve your Magneton into Magnezone there by levelling it up.
Source 1 - PokemonDB
Source 2 - Bulbapedia
Hope this helps!!