An electroless-plated ferromagnetic Ni-P layer, which is suitable for mass production, was employed as a soft magnetic underlayer (SUL) for a double-layered perpendicular recording media. A Ni-P SUL with a thickness range from 1.5 to 3.0 m was plated on the Al substrate, then the surface of the Ni-P SUL was polished about 1 m to obtain a smooth surface with a roughness Ra of less than 0.3 nm. The saturation flux density and the in-plane coercivity for the Ni-P SUL were about 0.5 T and 15 20 Oe, respectively. Media with a Ni-P SUL at a thickness of more than 0.5 m showed almost the same magnetic properties and recording performances as a medium with a 200-nm thick sputtered CoZrNb SUL. Furthermore, the spike noise commonly observed from an SUL was not found for the media with the Ni-P SUL in this study, indicating great potential for the electroless-plated ferromagnetic Ni-P layer as the SUL of the perpendicular recording media.Index Terms-Electroless-plated Ni-P, perpendicular magnetic recording media, soft magnetic underlayer (SUL), spike noise.
Switching field distributions (SFDs), are presented for longitudinal media of ∼40 Gbit/in.2 and the influence of thermal agitation of magnetization is discussed. Two remanence curves were measured at different sweep rates of applied field of ∼10 and ∼108 Oe/s, respectively, and SFD curves were obtained by differentiating the remanence curves. The SFD becomes significantly narrower at the higher field sweep rate. A SFD curve without the effect of thermal agitation was calculated using Sharrock’s equation. The full width at half maximum of the SFD curve at 10 Oe/s, ΔHr/Hr, is 0.45 for an isotropic medium, while that estimated without the thermal agitation effect ΔH0/H0 is 0.24, indicating that the SFD at the recording frequency is nearly half that measured at vibrating sample magnetometer (VSM) time scales. The ΔH0/H0 of the ∼40 Gbit/in.2 medium is much smaller than that of a 15 Gbit/in.2 medium used for comparison, although the ΔHr/Hr is ∼1.4 times larger than that of the 15 Gbit/in.2 medium due to thermal agitation. The value of ΔH0/H0 is not influenced by the uniaxial anisotropy induced by substrate texture KuTX. This result indicates that the KuTX is not effective for reducing SFD at recording frequencies, although the KuTX strongly affects the SFD at VSM times scales. The improvement of recording performance with increasing KuTX may be related to suppression of thermal relaxation of the remanent magnetization at bit transitions, immediately after the writing process.
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