Here, I have a couple of 'magnetizers / de-magnetizers'. They're about 2" square by 1" thick with an irregular-shaped hole that goes right through. They are red in colour. In one part of that hole, you insert the tool you want magnetized; in the other part, the reverse - for de-magnetization. I believe they came from Farnell or RS many years ago: may still be available; very useful gadget.
As for screwdrivers that seem to have strangely become magnetized, yes, I've had that - and, occasionally, still do. Why it happens, I don't know - and I've never attempted to find out why. I suspect that one screwdriver (or any other tool) gets in the zone of a magnetic field somehow and then with typical juggling around of tools on the bench, its proximity to other ferrous tools makes them magnetic, too. I suppose that the sudden collapse of a substantial a.c. current could have same magnetizing effect. In the old days of 3½" FDDs, that 'magnetic screwdriver' syndrome was a real hazard to us PC technicians!
When in the field, I always carried more that
one diagnostic floppy disk!
Al.