Subject: What core material for an electromagnet would not have a residual effect?

Years ago I worked on a device that used an electromagnet with an unusual core
material. When energized, the EM produced quite a powerful field. But when the
current stopped flowing, the core had NO residual magnetic effect. I'm now
trying to show this to a bunch of 6th graders but the only material I've found
even similar is brass bolts. We are trying to make a EM that has a powerful
field when energized but nothing when power is removed. Help!

Re: What core material for an electromagnet would not have a residual effect?