Hard Disk Drive as a Magnetomechanical Logic Device
  We consider the conditions how two binary numbers can be superimposed on the same track with the use of different recording magnetic fields. As a result the average magnetization of longitudinal medium along the track can have three states: -M, 0 and +M. Possibility to perform logic operations with these states is considered. We demonstrate OR, AND, XOR and NOT operations and discuss a modification of a recording device. 
Vladimir L. Safonov