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We investigated the mechanism of nanometer-depth scratches triggering adjacent track interference (ATI) by applying DC erasure magnetic fields to scratch areas in recording media and measuring demagnetization by imaging magnetic bits using magnetic force microscopy. We found that the magnetic coercivity and anisotropy (Ku) of the scratch area is decreased to about half compared to that of a normal area. Section analysis of the recording layer under and along the scratch by transmission electron microscope revealed that the nanometer-depth scratch causes both c-axis tilt and slip of the (0001) plane of the close-packed hexagonal lattice structure of the grains. Micro-magnetic simulation indicated that the c-axis tilt only had a secondary effect on ATI but that the Ku decrease had a significant effect. On the basis of these transmission electron microscope analyses and the micro-magnetic simulation, we then concluded that the slip of crystal plane (0001) reduced Ku by introducing a stacking fault and essentially reduced coercivity, resulting in ATI.