Marakanetz, yes of course there are magnets present in the cartridges. But they do not cause any permanent magnetization of the coils. They cause the coils to be energized by the movement in the field, and induce a current. But, when movement stops, the signal generation stops, and no magnetizing force is left residing in the coils, other than that which is created by the magnet's fields.
However, there is some possibility that there is some content of nickel or other magnetizable metal in some of the solder joints. That could become magnetized. However, I really think that is a low possibility, and would only comprise a negligable amount of material that could be magnetized extremely weakly, and is not in the vicinity of the main field anyway. I think that is not a real factor in this, but I mentioned it anyway.
However, there is some possibility that there is some content of nickel or other magnetizable metal in some of the solder joints. That could become magnetized. However, I really think that is a low possibility, and would only comprise a negligable amount of material that could be magnetized extremely weakly, and is not in the vicinity of the main field anyway. I think that is not a real factor in this, but I mentioned it anyway.