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We present a novel fabrication process for a soft magnetic underlayer of a CoNiFe-based alloy for a double-layered perpendicular magnetic recording medium using electroless deposition. The electroless-deposited CoNiFeP film is focused on as the soft magnetic underlayer. The CoNiFeP soft magnetic underlayer was fabricated on a 2.5-in glass disk with a Ni/Ti seed layer and the substrate was rotated in an electroless deposition bath under a magnetic field applied. The CoNiFeP underlayer as deposited was flattened using a chemical mechanical polishing, and the film thickness thereby became about 1 μm. The in-plane coercivity and saturation magnetic flux density of the CoNiFeP underlayer were less than 2 Oe and more than 12 kG, respectively. The magnetic domain observation revealed that the CoNiFeP underlayer electroless deposited under the magnetic field applied and the substrate rotated exhibited no marked domain boundary, vanishing a spike noise in read-write experiments. A double-layered perpendicular magnetic recording medium with the electroless-deposited CoNiFeP film showed a sufficient overwrite performance and high signal-to-noise ratio as compared with a typical soft magnetic underlayer fabricated using sputtering.