Thermodynamics of post-erasure signal effect in γ-Fe2O3magnetic recording tapes

Some γFe2O3-based magnetic recording tapes, particularly those using larger particle size γFe2O3, tend to retain a low-level or latent residual recorded signal after erasure with a large alternating magnetic field. The residual signal effect increases with time and temperature of storage after recording. The effect varies with the FeO content of the γFe2O3particles, and is a maximum at an FeO content of about 10 percent by weight. An explanation is presented of the phenomenon in terms of migration of cation vacancies from the interior of a magnetic domain to a domain wall in a magnetized γFe2O3particle thereby decreasing the free energy of the system. This sets up concentration gradients of Fe²⁺ions in regions of domain walls which remain after demagnetization of the tape and results in some residual magnetization. Reasonable agreement is obtained between calculated and observed residual magnetic effects as a function of FeO content.