Nobuyuki Inaba

Investigation of 2 Gb/in/sup 2/ magnetic recording at a track density of 17 kTPI

Developments of magnetic head, recording medium, positioning, and signal processing technologies made it possible to perform magnetic recording at an areal density of 2 Gb/in/sup 2/ on a prototype 3.5-in diameter hard disk drive. A track density of 17 kTPI, which is comparable to or even greater than that used in optical storage systems, is used to minimize the reduction in the physical clearance between the medium and the head. The authors outline the system and its component technologies. The read/write characteristics of the 3.5-in prototype disk system are also reported. >

Microstructural segregation in CoCrTa and CoCrPt longitudinal magnetic recording media

Abstract Microscopic segregation structures of Co-15.2 at% Cr-4.3 at% Ta and Co-13.4 at% Cr-13.0 at% Pt longitudinal magnetic recording media are investigated by using an analytical transmission electron microscope equipped with an energy dispersive X-ray analyzer. Cr in the CoCrTa medium is extremely localized near the magnetic grain boundaries with a maximum Cr concentration of about 25 at%, while the Cr concentration inside the grains is much lower at about 10 at%. The width of the Cr-enriched region is estimated to be about 1 nm. The segregation structure of the CoCrTa medium is represented by a model consisting of a magnetic hexagonal grain made of a Co-rich core with high saturation magnetization M s covered by a Cr-enriched thin shell with low M s . The Cr-enrichment is also observed in the CoCrPt medium near the grain boundaries, but with a broader Cr-rich width of more than 2 nm and a maximum Cr concentration of at most 20 at%. The difference in the Cr segregation structure in these two media is related to the difference in the noise characteristics between the CoCrTa and the CoCrPt media.

Compositional and temperature dependence of basic magnetic properties of CoCr-alloy thin films

Compositional and temperature dependencies of saturation magnetization (M/sub s/) and uniaxial magnetocrystalline anisotropy (K/sub u/) are determined using single crystal CoCr-alloy thin films. Temperature dependencies of M/sub s/ and K/sub u/ are stronger with increasing Cr content. M/sub s/(OK) and Curie temperature (T/sub c/) decrease linearly with increasing Cr content in a Co-Cr-X (X=Ta, Pt) system. M/sub s/(OK) and T/sub c/ become zero at the Cr concentration of 33 at.%, Co-Cr-Pt specimens have higher T/sub c/ and higher K/sub u/ than those of Co-Cr-Ta specimens with similar Cr concentration. The magnetic properties of polycrystalline medium are confirmed to depend strongly on those of Co-rich core crystalline grains in Cr segregated microstructure.

Magnetic and crystallographic properties of CoCrPt thin films formed on Cr–Ti single crystalline underlayers

By using an epitaxial growth technique, Co73Cr15Pt12(1120) bi‐crystalline thin films formed on Cr–Tix(001)[x=0–15 at. %] single crystalline underlayers were prepared to investigate the relationship between the magnetic properties and the lattice distortion in the magnetic layer. X‐ray diffraction measurement shows that the best lattice matching between the c axis of the CoCrPt and the [110] direction of the Cr–Ti underlayer is realized and the CoCrPt layer shows an ideal symmetrical hcp. structure when the Ti concentration is 10 at. %, where the maximum magneto‐crystalline anisotropy energy and coercivity values are observed. Distortion in the hcp. CoCrPt magnetic crystal grains decreases the uniaxial magneto‐crystalline anisotropy energy, thus lowering the attainable medium coercivity.

Fluctuation fields and medium noise in CoCrTa and CoCrPt films

Abstract The correlation between magnetic viscosity and medium noise in CoCrTa and CoCrPt longitudinal thin-film media was investigated by measuring the time dependence of the remanence coercivity H r and the read/write characteristics. The media were prepared by do magnetron sputtering under various conditions. Fluctuation fields H f of the magnetic viscosity at H / H r = 1 were obtained from the slopes of the H r versus In t plots. The medium noise decreases with kT / H f (the product of activation volume and saturation magnetization per unit volume), and is independent of other magnetic properties, such as the coercivity and the remanent magnetization per unit area. The medium noise thus primarily depends on the size of the minimum unit of the magnetic moment in reversal.

Magnetic and recording characteristics of bicrystalline longitudinal recording medium formed on an MgO single crystal disk substrate

An epitaxial thin film growth has been applied to prepare bicrystalline CoCrPt longitudinal recording medium using an MgO(100) single crystal disk substrate. X-ray and TEM measurements have revealed that the magnetic layer has a bicrystalline structure which consists of two variant domains with their c-axes lying in-plane and perpendicular-to each other. The maximum medium coercivity is observed at (2n+1)/spl pi//4, while the minimum coercivity is observed at n/spl pi//2 from the MgO[010] when measured along the circumferential direction. The highest signal to noise ratio, the highest linear resolution, and the best off-track performance are observed at (2n+1)/spl pi//4 where the direction of the recording magnetic head field lies between the easy magnetization axes of the two variant domains. >

Effects of Pt and Ta addition on compositional microstructure of CoCr-alloy thin film media

Compositional segregation structures of Co–Cr–Ta, Co–Cr–Pt, and Co–Cr–Pt–Ta thin film media with Cr concentrations of about 15 at. % were studied using an analytical transmission electron microscope. Cr atoms in the CoCrTa specimen are extremely localized around the grain boundaries with a maximum concentration of about 23 at. % with a width of 1 nm, while that inside the grain is about a half of the average composition. The Co–Cr–Pt specimen shows Cr segregation at grain boundaries with a maximum concentration by 4 at. % greater than the average composition. The Cr concentration inside the grain is kept nearly same with the average concentration. The Co–Cr–Pt–Ta specimen shows a degree of Cr segregation between the Co–Cr–Ta and the Co–Cr–Pt systems. The Cr enrichment at bicrystal grain boundaries is broader and weaker than those at the normal grain boundaries.

Thermal stability of magnetic recording in perpendicular thin film media

Recording and signal decay characteristics of perpendicular CoCr-alloy thin film media are investigated in comparison with those of longitudinal media. A perpendicular medium shows recording resolution 25-32% higher than that of a longitudinal medium with similar low density output. The medium S/N of a 25 nm thick perpendicular medium is comparable to that of a 16 nm thick CoCrPt longitudinal medium at high linear densities. The signal decay rate of the high Mr/Ms perpendicular medium is below 0.2% per decade of time for all linear densities. These results and thermal stability considerations based on the temperature dependence of KuV/kT indicate that CoCr-alloy perpendicular recording media are promising to extend the linear density well beyond 400 kFCI.

Structure and Magnetic Properties of Co Epitaxial Thin Films Grown on MgO Single-Crystal Substrates

Co epitaxial thin films were prepared on single-crystal MgO substrates of (100), (110), and (111) planes heated at 300degC by ultra high vacuum molecular beam epitaxy. The film crystal structure varies depending on the substrate orientation. On MgO(100) substrate, a Co(112macr0) epitaxial bi-crystalline film with an hcp structure is obtained. A Co(110) single-crystal film with an fcc structure is formed on MgO(110) substrate, whereas a Co(0001) single-crystal film with an hcp structure is grown on MgO(111) substrate. Co film growth on MgO substrates follows the Volmer-Weber mode. The in-plane magnetization properties reflect the magnetocrystalline anisotropy of bulk hcp-Co or fcc-Co crystal. The Gilberts damping constants of these 40-nm-thick epitaxial Co films are determined.

Temerature Dependence of Magnetocrystalline Anisotropy Energy Determined Using Co-Cr-Ta Single Crystalline Thin Films

The composition and temperature dependencies of uniaxial magnetocrystalline anisotropy constants K/sub u1/ and K/sub u2/ are investigated by torque magnetometry for (1010)-oriented Co/sub 96-x/Cr/sub x/Ta/sub 4/ single crystal thin films (x=8-19 at.%). The K/sub u1/ and the Ku/sub u2/ values of Co/sub 88/Cr/sub 8/Ta/sub 4/ are determined at 290 K to be 2.2/spl times/10/sup 6/ and 5.9/spl times/10/sup 5/ erg/cm3, 6.9/spl times/10/sup 5/ and 5.3/spl times/10/sup 4/ erg/cm/sup 3/, respectively, with increasing Cr concentration from 8 to 19 at.%. When the temperature is increased from 290 to 450 K, K/sub u1/ of Co/sub 88/Cr/sub 8/Ta/sub 4/ thin films decreases by 27% to 1.6/spl times/10/sup 6/ erg/cm/sup 3/. The temperature dependencies of K/sub u1/ and K/sub u2/ increase with increasing Cr content in Co-Cr-Ta films. The compositional and the temperature dependencies of K/sub u1/ and K/sub u2/ are related to the decrease in Curie temperature of Co-Cr-Ta material with increasing Cr concentration.