Measurement and interpretation of magnetic time effects in recording media

Magnetic time effects are highly relevant to magnetic information storage because of the large difference (up to 16 or 17 orders of magnitude) between the time scales of the recording process and the required storage stability. Magnetic time effects become more pronounced as the volume of the switching unit becomes smaller, and thus become of more practical importance as the microstructure of recording media is made finer in the pursuit of greater information storage density. Magnetic time effects taking place on time scales longer than about 10(-9) s can be explained by a model of thermally assisted crossing of an energy barrier (Arrhenius-Neel formalism). Many recording phenomena can be explained within this regime. Observation of magnetization changes during exposure to a constant field ("magnetic viscosity") can be interpreted to yield an estimate of the switching volume; in most cases, this volume is larger than the frequently cited "activation volume." The dependence of coercivity on the time scale of the magnetic reversal precess (e.g., on the field sweep rate of a hysteresis loop) can also be used to deduce the volume of the switching unit and to estimate time effects relevant to information storage. A model and procedure for analysis of time-scale dependence of coercivity are described here and applied to typical advanced tape media, of both metal-particulate (MP) and metal-evaporated (ME) composition. The resulting switching volume for the MP tape (where the microstructure is well defined) is in approximate agreement with the particle size seen by electron microscopy, The magnetic time effects are significantly stronger in the ME tape than in the MP tape. Measurement of coercivity on at least two very different time scales (e.g,, by vibrating-sample magnetometer and 60-Hz hysteresis loops) provides a convenient means of estimating the effective switching-unit volume, and hence the magnitude of the time effects to be expected in a recording application. The model also allows the estimation of the minimum particle or grain sizes that could be used with adequate stability of magnetic transitions in high-density information storage. The practical lower limit for metal-particle volume is found to be about one-fifth to one-fourth of the volume of current advanced particles.