Electroless-plated CoNiReP/NiFeP double-layered (DL) media with various underlayer coercivity (Hc(u)) were examined to study perpendicular recording performance using ring-type heads. For the case of DL medium with a combination of ring-type heads, it is found that the enhancement of the reproduced voltage by the “phase synchronization (PS) effect[1, 2]” is significantly influenced by varying the perpendicular layer thickness (δp) and head parameters. In the recording performance using a Mn-Zn ferrite head, the reproduced voltage is drastically enhanced by the PS effect as far as the head field penetrates sufficiently into the bottom of the DL media and magnetizes the underlayer, and the reproduced voltage for the DL medium with He(U) =160 Oe is higher than that for the DL medium with Hc(U)=50 Oe. However, such an effect became obscured with an increase in δp, and in turn, the DL medium with lower Hc(u) value demonstrated higher reproduced voltage than that of the DL medium with He(u)=160 Oe, due to the “double layer effect[3, 4]” which eminently subjects to the DL media with soft magnetic and quasi-soft magnetic underlayers whose coercivity is lower than several tens of Oe. In the case using a metal-in-gap (MIG) head with Bs=15 kG, on the other hand, the penetration of the head field was further improved, sufficiently magnetizing the underlayer, and the DL medium with even thicker δp (0.3 μm) and higher Hc(u) (160 Oe) showed higher reproduced voltage over 240 nVp-p due to the PS effect.
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