Erasure of signals on magnetic recording media

This paper explores some aspects of the erasure of signals from magnetic recording media. The first part gives the erasure characteristics for several different types of materials in current use for magnetic recording. A novel method of displaying the results shows that γ-Fe2O3and Cobaloy metal alloy particles behave substantially the same, while CrO2and Co-doped gamma ferric oxide are different from them and from each other. The second part explores anomalous signal recurrence after complete erasure. It is shown that under certain conditions of storage a non-magnetic memory of the signal is left on the tape. This signal can be converted back to the magnetic form and measured, although its level is quite low. It is subsequently posible on most tapes to remove the non-magnetic signal. The plausibility of the "non-magnetic signal" hypothesis is supported by calculations of the expected level which are of the same order of magnitude as the levels actually measured.