Document authentication by means of exchange-anisotropic magnetic material

An apparatus for autheticating a recording medium which is to be safeguarded against falsification and which possesses discrete magnetizable regions of an exchange-anisotropic material whose Neel temperature T.sub.N is below the Curie temperature T.sub.C of the ferromagnetic or ferrimagnetic component, said magnetizable regions of the recording medium being separated from each other by non-magnetic regions, said apparatus comprising an arrangement for successively passing said recording medium at a constant speed through two groups of stations each including at least four individual stations in tandem, the stations of said first group including

• a first station having magnetic head means for exposing said magnetizable regions of said recording medium to a DC field in one direction,
• a second station having heating means for heating said regions to a temperature T.sub.2 which is below T.sub.C and below the Neel temperature T.sub.N,
• a third station having magnetic head means for exposing said regions, subsequent to cooling to a temperature T.sub.1 below T.sub.2, to a strong magnetic AC field which decreases to zero, and
• a fourth station having magnetic head means for determining the polarity of the residual magnetization, and
• said second group including at least four stations corresponding to the stations of the first group and having magnetic head means and heating means, respectively, for cooperating with said regions in a respective manner, except for the magnetization in the first station of said second group being in the direction opposite to that in the first station of the first group and the polarity of the residual magnetization determined in the fourth station of the second group being opposite to that determined in the fourth station of the first group.