Kazuhiro Ouchi

Co-Cr recording films with perpendicular magnetic anisotropy

For a new perpendicular magnetic recording system, a Co-Cr recording film with perpendicular anisotropy has been developed by an RF sputtering. The Co-Cr films are found to show some suitable properties for high density recording such as perpendicular anisotropy, a rectangular M-H loop, and fine grain structure. An extremely high recording density of 100,000 bits/inch was realized by using the Co-Cr film. The crystal and microscopic structure of the films are also discussed, and the perpendicular anisotropy of the Co-Cr films is mainly originated from the uniaxial magneto-crystalline anisotropy.

Perpendicular magnetic recording with a composite anisotropy film

For a recently proposed perpendicular recording system, a composite anisotropy medium has been developed to improve the recording sensitivity of the perpendicular recording head. The medium is composed of a Fe-Ni soft magnetic film and a Co-Cr perpendicular anisotropy film, which are successively deposited on a base by an r. f. sputtering. By using the new double layer medium, an extremely high recording sensitivity could be obtained, compared with the single layer Co-Cr medium. The recording current needed to saturate the double layer film decreased to one-tenth of that for the single layer Co-Cr film. Although the Fe-Ni layer was soft magnetic material, neither deterioration of the frequency response nor peak shift was observed for the double layer film. The reproduction with a perpendicular head was also investigated, and a high output voltage and a high signal-to-noise ratio were obtained.

Studies of the perpendicular magnetization mode in Co-Cr sputtered films

The extremely high resolution properties in a perpendicular magnetic recording system were studied with a recently developed Co-Cr film medium. Bitter technique was used to observe magnetization distribution in the medium, and the transition of magnetization was directly observed on the top and the bottom surfaces of the film. It was proved that an ideal perpendicular magnetization mode, having a very narrow transition width, takes place even in a high recording density over 40 kBPI. From the angular variation of H c and the micro-structure of the Co-Cr film, it was concluded that the high recording resolution of the film results from the fine columnar particles which show the rotational mechanism of magnetization reversal accompanied by a perpendicular magnetocrystalline anisotropy. The Co-Cr film was also found to be superior in perpendicular anisotropy to any other Co-M films studied here in.

Magnetization reversal process in polycrystalline ordered Fe–Pt(001) thin films

Magnetization reversal processes in Fe–Pt(001) thin films prepared by a high-pressure sputter deposition method were studied. Samples were classified in four types of domain patterns. Type I, with maze-like domain patterns, has a mixing mode of nucleation and wall motion for magnetization reversal. While, type II, with large island domain patterns, shows wall motion in its magnetization reversal. Type III has small island domain patterns, and type IV has fine discrete domain patterns showing rotational modes with inclined M–H loops. Type IV is expected to be one of the candidates for future ultrahigh-density magnetic recording media with high resolution and low noise.

Overview of latest work on perpendicular recording media

Recent works on perpendicular recording media were reviewed, Co-Cr alloy films are still under improvment and show a recording ability of 20 Gbit/in/sup 2/ and beyond. New perpendicular recording media that have high perpendicular anisotropy have been developed. These would be candidate media for ultrahigh density recording beyond 100 Gbit/in/sup 2/.

Design consideration of ultrahigh-density perpendicular magnetic recording media

Design consideration of double-layered perpendicular magnetic recording media for ultrahigh-density recording was studied based on a micromagnetic model. It was found that the introduction of an appropriate exchange coupling between grains remarkably improves the recording resolution of the media. The essential role of the exchange coupling could be understood that it increases the perpendicular M-H loop slope, which has the same contribution to the resolution as the head field gradient. Large values for the product of the M-H loop slope parameter and the head field gradient are expected to result in small transition widths less than 10 nm. Consideration of thermal stability of the media including effects of the demagnetizing field and the M-H loop slope suggested that Co-Cr based media would meet the magnetic properties for recording of 200 Gb/in/sup 2/ but may not for higher densities. Media with higher anisotropy fields like Fe-Pt would be the candidate.

Perpendicular magnetization structure of Co-Cr films

In a perpendicular recording system, a Co-Cr film as a medium is capable of storing very high density signals. Lorentz microscopy of 1000 kV TEM was used to observe the structure of recorded magnetizations in Co-Cr films having perpendicular anisotropy. A composite medium of a Co-Cr film with a soft magnetic back layer was shown by Lorentz microscopy to have a horseshoe magnetization structure. The stable antiparallel magnetization of transition in the Co-Cr layer determined the head-on magnetization structure of the soft magnetic back layer, which consists of a new straw-rope domain structure. The perpendicular magnetization structure of the Co-Cr film was found to consist of small domains magnetized through the film thickness which correspond to the columnar microstructure of the film. Since the intrinsic hysteresis loop of a Co-Cr film was shown to essentially have an ideal rectangular shape, it can be concluded that the Co-Cr layer of a composite film can be recorded by an ideal magnetizing process with negligible demagnetizing field at the transition.

Sputter-deposited (Fe-Pt)-MgO composite films for perpendicular recording media

Aiming for improvement of a pure Fe–Pt perpendicular double-layered medium, a composite film of (Fe–Pt)–MgO with ordered FePt-fct(001) phase was prepared by sputtering on heated substrates without any post-annealing. It was found that the (Fe–Pt)–MgO films exhibited a large perpendicular magnetic anisotropy in the preparation condition of lower substrate temperature than that were required for the films with other additives such as Al2O3 and SiO2. The (Fe–Pt)–MgO composite films showed hysteresis loop with relatively steep slope at and smaller magnetic domains than that of the pure Fe–Pt film. A higher recording resolution was achieved for the perpendicular double-layered medium with the (Fe–Pt)–MgO composite film.

Recent subjects and progresses in research on Co-Cr perpendicular magnetic recording media

Recent advances in the Co-Cr media are discussed regarding their potential in recording and physical properties. The head reproducing resolution is most important for high density recording in perpendicular recording. The Co-Cr medium, if it has a fine columnar structure, shows very high recording resolution, since the columns are predicted to show a rotational mode of magnetization reversal, and also offer an ideal perpendicular magnetization mode. Stress is placed on the remaining issues : improvement of the reliability through improvements in durability ; and supply of high quality base materials.

Properties of high rate sputtered perpendicular recording media

Double layered media for perpendicular magnetic recording were made by dc magnetron sputtering. The coercivity of the Co‐Cr layer was independent of deposition rate over the range from 0.006 to 1.2 μm/min, when the substrate temperature was kept constant during sputtering. Micro columnar structure of the Co‐Cr film was studied by chemical etching. Only the columns were dissolved leaving the column boundaries showing a microhoneycomb structure. From the composition analysis of the etchant, Cr segregated column boundaries were confirmed to exist even in the high rate sputtered films. The Permalloy back layer with coercivity below 1 Oe shows large noise pulses in the reproducing process. Increasing the coercivity to several Oersteds reduced the noise to the amplifier noise level without changing the signal level. Finally, a high recording performance was obtained for the media made by high rate sputtering.