The result is what you want -- an alternating magnetic field. Since you cannot purchase an "AC battery" I don't know what you mean. All batteries are DC, but conversion circuits can produce AC from DC, of course (these boxes are called "power inverters" if a relatively low DC voltage is used to produce a mains-level AC output).

I don't mean to be insulting, but the fact that these basic sorts of things are unclear to you is a strong indicator that this project is simply not for you. There are too many ways in which you can fail, or in the worst case, even injure yourself.

I urge you simply to purchase some 720K PC floppies from an online vendor. A 10-minute Google search should turn up a suitable -- and reasonably-priced -- source. Or go to Radio Shack and see if they still sell AC-powered bulk erasers for cassette tapes. Those would do nicely.