= Solution
Excessive feedback gain can make the closed loop underdamped or unstable. Edge transitions then produce overshoot, ringing or oscillatory streaks in the recorded height. The apparent structure is a controller response rather than a real series of surface ridges. The exact <amplitude> and frequency depend on the actuator, cantilever, loop delay and controller settings; without those data one can sketch the qualitative ringing but cannot predict a unique quantitative trace.
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