There is much interest in crystallographic defects in thin film magnetic recording media and their role in influencing recording performance such as media noise and thermal loss. In this article we report a correlation between the magnetic viscosity in CoPtCr thin film media, which is the origin of thermal loss effects, and the concentration of local fcc-like regions. The concentration of these defects ͑the type and density of stacking faults͒ was varied by growth on different underlayers ͑Cr and CrTa/Cr͒ and was measured with grazing incidence x-ray scattering using synchrotron radiation. We show that a substantial percentage of local fcc regions in an otherwise hcp cobalt alloy film leads to significant magnetic viscosity effects at quite modest magnetic fields. We find that the activation volume is reduced for a sample with a higher percentage of fcc-like regions and suggest that this can be understood in terms of the effect of weak links acting to stabilize local micromagnetic configurations.
In this overview paper the concept of thermal activation of magnetisation reversal is reviewed in terms of the Wohlfarth-Gaunt formalism. This formalism gave rise to the concept of an activation volume of reversal. Other techniques have been developed for the determination of activation or critical volumes of reversal and these are reviewed. It is found that these methods give the same approximate value for the activation volume but the only method based on micromagnetism is the waiting time experiment which is consistent with Gaunt. Factors which affect the activation volume such as incoherent reversal and intergranular coupling are discussed together with measures that may be required in order to generate ultrahigh density recording media that are relatively free from thermal loss of signal.
Intergranular interactions and time dependence effects have been studied in CoCrPt/Cr thin films with two different Cr concentrations and different physical grain sizes. The samples have a bicrystal structure in which Co alloy grains, grown onto a Cr underlayer have subgrains with orthogonal c-axes. It was found via the ⌬I method that exchange coupling was predominant in all the films along the circumferential direction. The samples with higher Cr concentration exhibited lower exchange coupling as indicated by the value of d(⌬I)max/dH which was found to correlate with the integrated medium noise. The study of the time dependence effects revealed that higher Cr concentration caused a reduction in activation volumes indicating a higher degree of segregation in the grain boundaries of the films. A correlation is reported between the activation volumes and the domain images of the samples in the ac demagnetized state.
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