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Summary form only given. The thermal stability of a Co-based 5-element medium for over 20Gbit/in2 recording is investigated by magnetometry and spinstand methods. Recent quaternary and 5-element alloys for longitudinal magnetic recording media reveal significantly reduced grain sizes compared to Co-Cr-Pl-Ta alloys for the same underlayer structure. Cr segragation was also enhanced and the Cr-rich regions hetwecn grains were wider than that of CoCrRTa media. These resulted in significant gains in SNR necessary for high density and high data rate recording and also in the degradation of magnetic thermal stability. Figures show the accumulated gain area for a CoCrRTa and CoCrPt-XY media on similar underlayers and CoCrPt-X alloy on NiAl. It is shown that the dynamic coercivity Hcr of CuCrPtTa, CuCrPt-X and a 5-element medium which has approximately +2 dB advantage at 400 kfci over the 20Gbit/in2 demonstation medium. The stability factor KuV/ksT is decreased for the new media. The significant reduction in Hcr from 100ns to longer times is thermal in origin and is also reflected in the thermal decay of magnetization.