Susceptibility of magnetic information storage to power frequency magnetic fields

The potential for loss of magnetically stored information in the presence of power frequency magnetic fields is described in terms of fundamental physical mechanisms. Three important mechanisms are identified: (1) short term erasure when external fields are high enough to be a significant fraction of the demagnetizing field, (2) long term thermal degradation when external fields exceed a magnetic field associated with the acceleration of the natural thermal degradation process to an unacceptable level, and (3) concentration of magnetic flux by recording equipment, leading to erasure, augmented thermal degradation or corruption of the recording process. Quantitative expressions are developed for evaluation of the first two mechanisms in terms of fundamental magnetic properties of the recording medium, and a 3-D finite element simulation is used to quantitatively assess the third mechanism for a modern recording head. The first two mechanisms typically require field levels much larger than ordinary power frequency fields, but the third mechanism can lead to local fields large enough to bring the prior mechanisms into play. Results are put in the context of prior published standards and descriptions.