Stability of amorphous transition metal-rare earth films for magneto-optic recording

The purpose of this work is to determine the process parameters which control the lifetime of magneto-optic recording materials and to evaluate their useful life in terms of the fundamental physical parameters which control the domain size. The amorphous films were prepared by r.f. bias sputtering, about 100 nm thick, to provide an easy axis perpendicular to the plane of the film. Their composition was approximately Co80Gd17Tb3. They were overcoated with ∼100 mm of SiO2to provide environmental protection as well as signal enhancement. Direct measurements of the diameters of an 8 by 8 array of 1 μm diameter domains were made at 100°C for up to 6 months. No change in diameter was detected to within ±10%. Higher temperatures of exposure could not be used without the domains disappearing. The stability of domains exposed to higher temperatures was inferred from measurements of changes in the perpendicular anisotropy, K and coercivity, Hc, by using do=4(AK)^1/2/Ms(Hc-H) for the minimum stable domain size. The stability has arbitrarily been defined as the time or temperature to reach 0.8 of the original value of K or Hc. The stability of both K and Hchas been shown to be strongly dependent on the bias used during the film preparation. These changes are directly correlated to the stability of the argon content in the film. The bias and argon content are inferred to be the factors which control the structure of the film which in turn controls the stability. We conclude: (1) The rate limiting factor in the life of these films is the change in K which is found to have a lifetimet = t_{o} \exp(\DeltaE/RT), where ΔE = 1.26 eV and to= 5.0 × 10^-11sec for samples prepared near the optimum bias. (2) The projected shelf life at 20°C is then 3000 years. (3) The projected number of write cycles of 2 × 10¹¹for the 60 ns laser pulse used for heating the material is quite adequate.