Monel is an alloy that is mostly nickel but can have a significant amount of copper (not magnetic). Also Nickel is not as magnetic as iron (steel). Martin is not likely going say what they are using but a set of electric guitar strings would be a cheap experiment. I would try nickel plated steel strings, the most common type of electric guitar string and likely the most magnetic. The nickel plate is to keep the steel from rusting as stainless steel is more expensive and can be less to not magnetic.
https://physics.stackexchange.com/qu...th-to-a-magnet