[LoveMortuus]

Ferromagnetic matter is magnetic by nature. If you look at the periodic table you'll see that most of the elements are actually metals. Below Carbon there's a line which parts metals from semi metals and non metals.

But with proper equipment you can detect non magnetic conductive materials. Not sure what's it called, but if you pass a magnet near eg. copper the magnetic force from the magnet will induce electricity on copper which in turn with also produce a slight magnetic field. There are many good YouTube videos on this topic and they're quite cool! (https://youtu.be/u7Rg0TcHQ4Y)

[eloff]

That's not what they're doing though. There literally going to drag a magnet across the sea floor and examine anything that sticks to it. Which will be largely iron, nickel, and cobalt. Were it to be alien tech, it probably won't have large amounts of those and won't stick to the magnet. At least it seems that way to me. Could you make a magnet that would create a strong enough attraction to pick up a piece of gold? I suspect not, or there would be a reality TV show with rednecks trawling the ocean in Alaska with magnets, searching for gold.

[LoveMortuus]

Even though I would love to see any non-human technology, I would probably bet that they're just going to find a bunch of dust, rocks and of course, human trash.

[wnkrshm]

Paramagnetism. There is also Diamagnetism and Anti-Ferromagnetism.

[a9h74j]

Definitely. But of course a metal detector coil can also look at simple change of inductance, hence responding to conductivity even with magnetic mu = 0.