Hajime Aoi

Optimization of the SiO/sub 2/ content in CoPtCr-SiO/sub 2/ perpendicular recording media for high-density recording

Optimization of the SiO/sub 2/ content for high-density recording was discussed for {(Co/sub 90/Cr/sub 10/)/sub 80/Pt/sub 20/}/sub 100-Z/-{SiO/sub 2/}/sub Z/ perpendicular recording media. The grain size D/sub grain/ decreases significantly from 8.8 to 5.4 nm as the SiO/sub 2/ content Z increases from 0 to 14.4; however, no significant difference in c-axis orientation was observed. Magnetocrystalline anisotropy of grains maintains a large value of nearly 8/spl times/10/sup 6/ erg/cm/sup 3/, even at 10 at % SiO/sub 2/ addition, indicating a high potential of perpendicular anisotropy K/sub u/ to resist thermal agitation of magnetization. The coercivity H/sub c/ increases significantly as the SiO/sub 2/ increases, which is coincident with the reduction of the slope of magnetization curve /spl alpha/=4/spl pi/(dM/dH) due to the enhancement of grain isolation. The H/sub c/ shows a maximum at around 11 at % SiO/sub 2/ (nearly 30 vol % SiO/sub 2/). A further increase of SiO/sub 2/ reduces the value of H/sub c/ significantly, although the /spl alpha/ maintains a constant value of nearly 2, due to thermal agitation of magnetization. The media noise reduces significantly as the SiO/sub 2/ increases; however, the addition of SiO/sub 2/ beyond about 13 at % leads to a slight increases in media noise probably due to a significant increase of thermal agitation. The recording resolution D/sub 50/ shows a broad maximum of /spl sim/420 kFCI at around 11 at % SiO/sub 2/, suggesting that the D/sub 50/ is related to the media noise performance. Experimental results indicate that there is a tradeoff to be made between the thermal stability and media noise performance as a function of SiO/sub 2/ composition.

Fabrication of L11-type (Co–Ni)–Pt ordered alloy films by sputter deposition

L11 type Co-Pt ordered alloy films with a large uniaxial magnetic anisotropy, Ku, of the order of 107erg∕cm3 were successfully fabricated at relatively low substrate temperatures of 270–390°C using ultrahigh vacuum sputter film deposition. L11 type ordered Co-Pt films, with the ⟨111⟩ direction (easy axis of magnetization) perpendicular to the film plane, were fabricated on MgO(111) single crystal substrates and glass disks. The ordered structure was formed in a wide Pt content region of 40–75at.%, and Ku showed a maximum at around 50at.% Pt content. The values of the order parameter S and Ku for L11 type Co50Pt50 films increased as the substrate temperature Ts, increased. Ku reached about 3.7×107erg∕cm3 (S=0.54) at Ts=360°C for the single crystal films deposited on MgO(111) substrates, indicating a potential increase in Ku by enhancing the ordering. The values of Ku for polycrystalline films deposited on glass disks were smaller than those for the single crystal films on MgO(111) substrates, however, Ku r...

High perpendicular magnetic anisotropy of CoPtCr/Ru films for granular-type perpendicular media

The perpendicular magnetic anisotropy K/sub u/ of CoPtCr films deposited on Ru seed layers is discussed as a function of film composition. Moreover, the change in K/sub u/ by the addition of SiO/sub 2/ is examined in light of thermal stability of CoPtCr-SiO/sub 2/ media. The K/sub u/ of Co-10 at % Cr shows a high value of /spl sim/5/spl times/10/sup 6/ erg/cm/sup 3/, which is much higher than that reported by Bolzoni et al. The addition of Pt to the Co-10 at % Cr film results in a further enhancement of K/sub u/. A maximum K/sub u/ of nearly 10/spl times/10/sup 6/ erg/cm/sup 3/ is observed at 25/spl sim/30 at % Pt. All series of films with various Cr contents show maximum values of K/sub u/ at 25/spl sim/30 at % Pt. The maximum value for CoPt (Cr=0) films reaches /spl sim/15/spl times/10/sup 6/ erg/cm/sup 3/. The value of magnetocrystalline anisotropy of grains K/sub u//sup g/, calculated by taking account of the volume fraction of CoPtCr grains, decreases as the SiO/sub 2/ content increases. However, the K/sub u//sup g/ maintains a large value of more than 7/spl times/10/sup 6/ erg/cm/sup 3/, even at 10 at % SiO/sub 2/ in addition to (Co/sub 90/Cr/sub 10/)/sub 80/Pt/sub 20/, which indicates a high potential of CoPtCr-SiO/sub 2/ media to resist thermal agitation of magnetization.

High-potential magnetic anisotropy of CoPtCr-SiO/sub 2/ perpendicular recording media

The magnetic anisotropy of CoPtCr-SiO/sub 2/ perpendicular recording media, including higher energy terms, was studied as a function of film composition and seed layer materials. All series of CoPtCr films with various Cr content, deposited on Ru seed layers, show maximum values of total anisotropy K/sub u/ at 25-30 at%Pt. The maximum value for CoPt(Cr=0) films reaches /spl sim/15/spl times/10/sup 6/ erg/cm/sup 3/. The addition of SiO/sub 2/ to the CoPtCr films reduces the grain K/sub u/, however the grain K/sub u/ maintains a large value of 8/spl times/10/sup 6/ erg/cm/sup 3/ even when 10at%SiO/sub 2/ is added to (Co/sub 90/Cr/sub 10/)/sub 80/Pt/sub 20/, for instance, which indicates the high-potential thermal stability. Theoretical calculations for media designs of 400 Gbits/in/sup 2/ revealed that the ratio of the high-energy anisotropy term K/sub u2/ to K/sub u1/(K/sub u/=K/sub u1/+K/sub u2/) is required to be 0.2-0.35 to enhance the energy barrier for the remanent state, without a notable change in switching field. The films deposited on Ru seed layers were found to show negligibly small K/sub u2/ values, however, the values of K/sub u1/ and K/sub u2/ vary significantly with the seed layer material used. K/sub u1/ decreases almost linearly as the K/sub u2/ value increases. It is concluded that CoPtCr films have a sufficient potential in the values of K/sub u1/ and K/sub u2/ for high-density perpendicular media.

Estimation of Maximum Track Density in Shingled Writing

In this paper, the feasibility of two-dimensional magnetic recording is discussed from the viewpoint of high track density. 747 curves of shingled track were measured using existing heads and media and were interpreted with a squeezing model dominated by the SN ratio of the read head output. As long as a reader can be placed on the median line of the squeezed data area, wider OTC than that of conventional writing will result. The modeled 747 curves were in good agreement with the experimental data for both conventional writing and shingled writing. The feasibility of narrow track pitch shingled writing was shown, however, write-to-write misregistration should be minimized to maximize the track density.

Microwave assisted magnetization switching in Co/Pt multilayer

In this study, we have experimentally investigated the microwave assisted magnetization by switching (MAS) on the microstructured Co/Pt multilayer. The sample exhibits the typical magnetization curve peculiar to perpendicular anisotropy films, that is, a steep reversal initiated by nucleation of a reversed domain followed by its subsequent gradual expansion by the domain wall displacement. By applying microwaves with the frequency of GHz order, the nucleation field Hn is significantly reduced at three frequencies. Taking into account the effective anisotropy field of our sample, the first dip of Hn at the lowest frequency probably corresponds to the Kittel mode excitation, and the other two dips at higher frequencies correspond to unidentified excitation modes other than the Kittel mode. Among them, the last dip of Hn at the highest frequency reaches about 1/3 of that without microwave application. These results suggest the existence of more effective excitation modes for MAS than the Kittel mode.

Large uniaxial magnetic anisotropy by lattice deformation in CoPt/Ru perpendicular films

A large uniaxial magnetic anisotropy Ku of about 2×107erg∕cm3 was obtained in Co–(20–30) at. % Pt perpendicular films deposited on Ru seed layers. The films showed good c-axis orientation perpendicular to the film plane, and no evidence of a Co3Pt-type-ordered structure was found in the X-ray-diffraction patterns. The value of Ku (the sum of the first- and second-order terms of uniaxial anisotropy, Ku1 and Ku2) increased as the thickness of the CoPt layer decreased, and reached about 2×107erg∕cm3 at 2 nm (Co–20 at. % Pt films) or 5 nm (Co–(25–30) at. % Pt films). The ratio of Ku2 to Ku1 was 0.16 in 40 nm films, but reduced to less than 0.03 for thicknesses below 5 nm. The experimental results suggest that, besides the surface anisotropy, the increase in Ku on reducing the thickness is mostly related to the reduction of the hcp-CoPt lattice c∕a ratio, probably due to epitaxial growth of CoPt on the Ru seed layer.

A single-pole-type head design for 400 Gb/in/sup 2/ recording

A single-pole-type head design for 400 Gb/in/sup 2/ based on finite-element method (FEM) simulations is presented. The conditions required to generate a sufficient field for this areal density are shown. To reach this density various media were considered and written track widths, erase bands, signal-to-noise ratios, and written bit patterns were derived from Laudau-Lifshitz-Gilbert (LLG) simulations.

Thickness reduction in CoPtCr-SiO/sub 2/ perpendicular recording media to improve media performance

Magnetic properties and recording performance of thin CoPtCr-SiO/sub 2/ media were examined in relation to thermal stability. Magnetic anisotropy K/sub u/ of {(Co/sub 90/Cr/sub 10/)/sub 80/Pt/sub 20/}/sub 89/-(SiO/sub 2/)/sub 11/ media maintains a constant value of around 5.5/spl times/10/sup 6/ erg/cm/sup 3/, even at film thickness of 4 nm. This K/sub u/ value corresponds to the anisotropy energy of the grains K/sub u//sup g/ calculated by taking account of the volume fraction of about 8/spl times/10/sup 6/ erg/cm/sup 3/, indicating a high potential to resist thermal agitation. The high K/sub u/ derives a high remanence coercivity H/sub r/ of /spl sim/4 kOe, even at film thickness of 8 nm, resulting in a loop squareness of nearly 1. The ratio of magnetic anisotropy energy to thermal energy K/sub u/V/sub act//kT maintained a value of more than 70, even at this thickness. The media noise at low recording density increases with decreasing film thickness below 8 nm, probably due to the thermal agitation. The recording resolution D/sub 50/ showed a broad maximum of /spl sim/420 kFCI at around this thickness. The reduction in film thickness improves recording resolution; however, it is likely that the large thermal agitation in the thin-film region degrades the recording resolution, resulting in an optimum thickness to show the highest recording resolution.

Modeling of Writing Process for Two-Dimensional Magnetic Recording and Performance Evaluation of Two-Dimensional Neural Network Equalizer

Modeling of a simple writing process considering intergranular exchange fields and magnetostatic interaction fields between grains is studied for two-dimensional magnetic recording (TDMR). A new designing method of a two-dimensional neural network equalizer with a mis-equalization suppression function (2D-NNEMS) for TDMR is also proposed. The bit-error rate (BER) performance of a low-density parity-check coding and iterative decoding system with the designed 2D-NNEMS is obtained via computer simulation using a read/write channel model employing the proposed writing process under TDMR specifications of 4 Tb/in2, and it is compared with those for one- and two-dimensional finite impulse response equalizers (FIREs). It is clarified that the BER performance for the designed 2D-NNEMS is far superior to those for the FIREs.