Bit patterned media have attracted considerable interest as future high density magnetic recording media, since they provide a promising technology to postpone the problem of superparamagnetic limit, i.e., thermal instability of the recorded bits in the media. Major problems for the practical use of the bit patterned media are topography of discrete magnetic bits defined by lithographical fabrication which disturbs stable flying of the HDD head and switching field distribution which is considered to be due to the re-deposition of the etched materials. Ion beam irradiation has been proposed as a new approach to pattern magnetic materials locally without etching magnetic materials, and ion irradiation into Co/Pt and Co/Pd multilayers (MLs) has been reported for the local modification of their perpendicular anisotropies. However, in the Co/Pt and Co/Pd MLs patterned by ion irradiation, the adjacent magnetic bits are not magnetically isolated due to the exchange coupling through in-plane magnetized spacing, which will limit the ultimate density of the media.
    We have succeeded in the fabrication of the ion-beam patterned medium using a CrPt₃ alloy film. The CrPt₃ shows ferrimagnetism when it has an ordered L1₂ phase, while paramagnetism when disordered fcc phase, and we found that the very low Kr⁺ ion irradiation of 2 x 10¹⁴ ions/cm² induces such a phase change. Right figure shows MFM image of patterned CrPt₃ with square bit size of 50 x 50 nm separated by spacing of 30 nm fabricated by local irradiation of 30 keV Kr⁺ with 2 x 10¹⁴ ions/cm². Even in the sub-100 nm bit size, clear magnetic contrast is seen in the image. Bright and dark contrasts come from the non-irradiated bit where CrPt₃ remains L1₂ phase and exhibits ferrimagnetism. On the other hand, no magnetic contrast is seen in the irradiated area (spacing), which indicates the magnetization of CrPt₃ is suppressed locally by ion irradiation.

MFM image of CrPt₃ bit patterned medium fabricated by ion irradiation method (bit size: 50 nm x 50 nm).