Activation volumes in CoPtCr-SiO2 perpendicular recording media

CoPtCr-SiO₂ perpendicular recording media with varying levels of SiO₂ were examined by two different methods to determine the activation volume. The first is based on the sweep-rate dependence of the remanence coercivity using Sharrock´s equation. The second is based on the measurement of the fluctuation field from time-dependence data, determined using a magneto-optical Kerr effect (MOKE) magnetometer. The values of V_act measured at the coercivity for both methods are almost the same, with the fluctuation field and activation volumes increasing with the SiO₂ content. The difference between V_act and the grain volume measured directly from bright-field TEM images decreases as the SiO₂ content increases due to the reduction of intergranular exchange coupling. The experimental results indicate that values of V_act obtained from single- and double-layered media are consistent. It was also found that the coercivity and normalized hysteresis loop slope at coercivity varied with SiO₂ content, with the coercivity peaking at 8 at%SiO₂ (nearly 26 vol%SiO₂).