Source : Webster's Revised Unabridged Dictionary (1913)

Demagnetize \De*mag"net*ize\, v. t.
   1. To deprive of magnetic properties. See {Magnetize}.

            If the bar be rapidly magnetized and demagnetized.
                                                  --Am. Cyc.

   2. To free from mesmeric influence; to demesmerize. --
      {De*mag`net*i*za"tion}, n. -- {De*mag"net*i`zer}, n.

Source : WordNet®

demagnetize
     v 1: erase (a magnetic storage device) [syn: {demagnetise}]
     2: make nonmagnetic; take away the magnetic properties (of);
        "demagnetize the iron shavings"; "they degaussed the ship"
        [syn: {demagnetise}, {degauss}] [ant: {magnetize}, {magnetize}]