Satoshi Ohta

New preparation process for sputtered γ-Fe 2 O 3 thin film disks

A new preparation process for sputtered γ-Fe 2 O 3 thin film disks has been developed. The new process is composed of Fe 3 O 4 film formation by reactive sputtering and oxidation to γ-Fe 2 O 3 film. The characteristic of the process is that the Fe 3 O 4 single phase film is formed directly by reactive sputtering of an Fe-alloy target in Ar-O 2 atmosphere. The magnetic properties and recording results of the newly developed γ-Fe 2 O 3 thin films were almost equal to those of the reported sputtered films. Recording density D 50 reached 1,100 bits/mm.

Direct preparation of γ-Fe2O3 thin film recording media by reactive r.f. sputtering technique

A method for the direct preparation of γ-Fe2O3 thin films from iron metal was studied. The technique employs an intermittent iron sputter deposition process using a partially masked revolving substrate and a subsequent oxidation process of each deposited layer. Electron diffraction analysis of the films obtained shows superlatice reflections due to iron vacancy ordering, indexed as (110), (210) and (211), together with the reflections of the spinel structure. No reflections from an α-Fe2O3 structure can be detected in the films. From their electrical resistivity and lattice parameter values the films are identified as being in an intermediate state between stoichiometric Fe3O4 and γ-Fe2O3. They exhibit a typical coercive force of 600 Oe, a squareness ratio of 0.75, a saturation magnetization of 3600 G and a coercive squareness ratio of 0.8 for an Fe0.945Co0.025Cu0.03 target. These characteristics confirm that the films are applicable to magnetic recording media.

Signal-to-noise ratio studies on γ-Fe 2 O 3 thin film recording disks

The signal-to-noise ratio SNR w of γ-Fe 2 O 3 thin film disks has been investigated in a linear bit density range of 77-1500 flux reversals per millimeter (FRPM), for several media whose average crystallite sizes were changed by Ti doping. The noise voltage was found to increase as the bit-cell length became shorter, and as the average crystallite size became larger. A SNR w relationship to the average crystallite size, the resolution, and the bit-cell length was empirically determined as a simple formula. For a 10-μm core width and for a 1.0-μm bit-cell length, an SNR w of more than 35 dB was estimated to be attainable from the medium which had a 0.90 resolution and had crystallites averaging less than 0.06 μm in size.

Wear-resistance and signal-to-noise ratio in γ-Fe 2 O 3 thin film disks

Wear-resistance in an Al 2 O 3 pin-disk wear-test and SNR for γ-Fe 2 O 3 thin film disk composites (γ-Fe 2 O 3 thin film/substrate) were investigated. It was found that wear-resistance enhancement for the films is essentially accompanied by SNR deterioration due to crystallite growth. On the other hand, an increased Knoop hardness, H K , for the substrate surface greatly enhanced wear-resistance. Use of heat-resistant, defect-free CrO 3 anodization substrates, which have 420 kg/mm2H K , successfully minimized the wear-depth. Finally, it was found that the γ-Fe 2 O 3 thin film disks with a high-SNR can have higher wear-resistance than metallic thin film disks, because the oxide films themselves have high hardness.

Temperature hysteresis during reactive r.f. sputtering

Abstract Temperature hysteresis phenomena in reactive r.f. sputtering were studied. The temperature, measured using a thermocouple set in the plasma, changed markedly at critical r.f. power densities (Pf 1 and Pf 2 ) during the reactive sputtering of an iron target in an ArO 2 mixed atmosphere. The hysteresis phenomena are qualitatively explained by a variation in the sputtering rate caused by oxidation of the target surface, assuming that the temperature change depends mainly on the number of secondary electrons bombarding the thermocouple.

Anisotropy field distribution in Os-doped γ-Fe 2 O 3 thin films

Unusually-large, widely-distributed H k S are found on Os-doped γ-Fe 2 O 3 thin films. The average H k s are 10, 18 and more than 20 kOe for 0.9, 2.0 and 7.0 at.% Os-doped films, respectively. Large W r (3-12×105erg/cc ) remain even at a high magnetic field of 14 kOe, because of the H k distributions. Uniaxial anisotropy energy of 2×105erg/cc, which is induced through magnetic-annealing, is because of the orientation of a small portion of the intrinsic anisotropy axes. Film characteristics indicate that lattice spacing of the γ-Fe 2 O 3 structure and Mossbauer parameters of Fe3+ions stay constant, irrespective of Os content. Os4f photo-electron binding energies agree with those for Os metal. An Os-deficient layer 200-400 A in thickness at the top surface is detected.

Aging effect on coercivity of iron oxide thin films

Changes in magnetic properties for Fe/sub 3-x//spl square//sub x/O/sub 4/ (x: oxidation degree) thin films made by reactive sputtering and subsequent heat treatments have been examined under room temperature aging and constant temperature annealing. Aging causes variations in coercivity of insufficiently oxidized films which have a specific resistance of less than 1/spl times/10/sup 1/ /spl Omega//spl middot/cm, while the coercivity of /spl gamma/-Fe/sub 2/O/sub 3/ (x=1/3) did not change. This phenomenon did not depend on additive elements or preparation method. Other magnetic properties such as saturation magnetization, residual magnetization, squareness ratio and coercive squareness, were not affected by aging for any Fe/sub 3-x//spl square//sub x/O/sub 4/ composition. The activation energy for a coercivity change is 0.72-0.95 eV near room temperature for films with a specific resistance below 1/spl times/10/sup 1/ /spl Omega//spl middot/cm. It was confirmed that only the coercivity varied at 20/spl deg/C, while both coercivity and the degree of oxidation changed with annealing at 100/spl deg/C. >

High Coercive Thin Films of Co-Doped γ-Fe2O3–Fe3O4 Solid Solution

Fe3+[Fe3+1+2δFe2+1-3δΔδ]O4 thin films have been prepared by oxidizing sputter-deposited magnetite, where Δ and δ indicate the vacancy and its content, respectively. The coercive force, Hc, exhibited the maximum value when δ was approximately 0.20. Films with a δ of more than 0.18 had only an infinitesimal Hc increase even after 6 years of aging at room temperature. It was surmised that enhanced Fe2+-Δ ordering stabilizes the Hc change with time. (Co0.023Fe0.977)2.80Δ0.20O4 films had a 1300 Oe Hc and 0.85 squareness ratio. These films are thought to be useful for high bit density magnetic disk application.