To deprive of magnetic properties; to render nonmagnetic. See Magnetize.

To demagnetize would be to remove from a magnet or magnetic stripe any preponderance of magnetization in any particular orientation. Generally, the magnet is subjected to a strong alternating magnetic field (such as a bulk tape eraser) which is gradually withdrawn. The magnet is then said to be "AC erased". Otherwise, subject the magnet to a magnetic field that is constant, not alternating; the magnet is then said to be "DC erased". AC or DC erased magnetic stripes do not produce an output signal in a magnetic read head.

erase (a magnetic storage device)

To free from mesmeric influence; to demesmerize.