Title

Data recovery from magnetic media using magnetic force microscopy

Semester

Spring

Date of Graduation

2008

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Lane Department of Computer Science and Electrical Engineering

Committee Chair

Matthew C. Valenti.

Abstract

Magnetic tapes are widely used to store voice and other data. Due to the effect of hysteresis, modification of the magnetization pattern written to a tape by overwriting, erasing, or physical damage leaves remanent information about the previous magnetization state that can not be discerned by standard audio playback devices. Recently, a high resolution microscopy technique known as magnetoresistive microscopy was used by Pappas et al. in an attempt to recover data from audio tapes which were physically damaged. However, the device, while having a higher resolution than tape playback heads, still averages over entire audio track widths when collecting data. The purpose of this thesis is to propose and investigate a methodology for the recovery of erased data using a microscopy technique known as magnetic force microscopy (MFM). This is a novel approach because MFM can achieve much higher resolution than previous techniques, on the order of nanometers. The primary analysis technique utilized is the fast Fourier transform, which is applied to MFM images of strongly written, weakly written, and erased sinusoidal tones recorded by a standard audio tape deck. To verify our data collection and pre-processing techniques, a numerical simulation of tape media and MFM response is presented and compared with measurement. Several processing techniques, such as windowing to reduce FFT spectral leakage, thresholding to remove the effects of destructive magnetic interference, and tip response deconvolution to remove instrument dependent features from the data, are applied. It was found that at certain frequencies recovery of tones after erasure is possible, suggesting that erasure is frequency dependent. By investigating and altering audio tape samples in situ, the same physical section of tape was imaged several times, and it was found that the erasure process varies with position, even down to the order of microns.

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