Wataru Yamagishi

Magnetic properties of Fe-Co alloys produced by powder metallurgy

Powder metallurgical techniques were applied to the production of Fe-50wt%Co alloy which, because of its high magnetization, shows the greatest promise for applications such as magnetic cores and pole pieces. To obtain a sintered Fe-50wt%Co alloy with good magnetic properties, we tried to improve the sintering process by using a mixture of pre-alloyed Fe-rich Fe-Co powder and fine Co powders. The effect of the Fe content of the pre-alloyed Fe-Co powders was investigated over a range of 50 to 90 wt%. The effect of the Co powder particle size at -325 and -400 mesh was also examined. As a result, we obtained a sintered Fe-50wt%Co alloy with a relative density of 95%, using a mixture of pre-alloyed 80wt%Fe-Co powder and -400 mesh Co powder. This sintered Fe-50wt%Co alloy has the following magnetic properties; a magnetization of 2.15 T in a magnetic field of 4 kA/m, and a coercive force of 160 A/m.

Synthetic ferrimagnetic media: effects of thermally assisted writing

Thermally assisted writing on high-coercivity synthetic ferrimagnetic media (SFM) was demonstrated using a conventional spin stand equipped with an optical head for commercial magnetooptical drives. The laser light (/spl lambda/ = 685 nm) was focused through a glass substrate onto a recording layer. The optical spot size was 1.1 /spl mu/m and a commercial magnetic head had a writer width of /spl sim/0.25 /spl mu/m. The recording properties were measured as a function of the writing current (I/sub w/) and the laser power (P/sub w/). For the thermally stable medium with H/sub c/ = 6 kOe, a laser irradiation with an optimum power significantly improved the overwrite performance and the signal-to-noise ratio (SNR) values. The SNR values were improved by optimizing P/sub w/ over a wide writing current range. The improvements with the assist were found in both the signal and the noise. The media with a large dynamic coercivity value or with thick magnetic layers clearly showed the advantages with thermal assist.

Exchange-assisted nonlinear bit shift reduction at high linear densities in synthetic ferrimagnetic media

Synthetic ferrimagnetic media comprised of two magnetic layers tend to exhibit larger nonlinear transition shift (NLTS) compared to single magnetic layer conventional media. NLTS is reduced by increasing the antiferromagnetic exchange field (Hex) and especially by employing a lower layer with high magnetization and thickness product (Mst). Due to the lower switching field of the stabilization layer, the position of the transition during writing is farther downstream than that of the upper layer. We suggest that, at high densities, the pinning of the lower layer transition below the nearest neighboring upper layer transition leads to a reduction of demagnetizing fields and therefore NLTS.

Percolation studies in synthetic ferrimagnetic recording media

We have studied the percolation limit of synthetic ferrimagnetic media (SFM) by the measurement of noise power and third harmonic ratio and by imaging with a high-resolution magnetic force microscope (MFM). The noise power spectrum, which helps to find the onset of percolation, is significantly different in SFM structures compared to the conventional recording media. Properties of the stabilization layers, such as switching field distribution (SFD), are highly influential in deciding the integrated noise power spectrum. With suitable selection of high moment stabilization layers, the noise at higher densities, within the percolation density limit, can be substantially reduced. Third harmonic ratio studies indicate that the partial erasure occurs above /spl sim/900 kfci. Using a high-resolution high-Q MFM, it is possible to obtain stable and nonpercolating high density patterns up to 1000 kfci on media with tBr=0.22 memu/cm/sup 2/ and Hc/spl sim/4500 Oe. This results in practical designs of media for recording densities >150 Gbits/in/sup 2/.

Tilted magnetization in Cr/CoCrPt thin film recording media

The epitaxial growth of Cr/CoCrPt thin film recording media using sputter deposition was studied in this paper. Its structural properties were investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) to analyze the crystallographic texture and granularity of the films, respectively. SQUID and Kerr magnetometers were used to measure the magnetisation of the film. Also, read/write properties of the film were also studied.

Optically assisted magnetic recording targeting over 1 Tbit/in2

Optically-assisted magnetic recording (OAMR) can solve fundamental problems concerning thermal fluctuation and write capability in magnetic recording, and it is regarded as the key technology in achieving density exceeding 1 Tbit/in2. OAMR is classified into magnetic dominant recording and optical dominant recording, and differences with respect to the recording method and effect between them are described. Magnetic dominant recording was conducted on longitudinal synthetic ferrimagnetic media to prove the fundamental effectiveness. Both SNR and overwritability without thermal erasure were assured. A theoretical estimation in optical dominant recording suggests that OAMR enables 10 times density compared with conventional magnetic recording. Butted grating should provide good optical characteristics as the heating element for optical dominant recording, and it also has compatibility with the conventional magnetic head for integration.