Shinji Kondoh

Magnetic and structural properties of CoNiPt(SiO2) alloy films for high‐density longitudinal recording

Magnetic and structural properties of sputtered CoNiPt(SiO2) alloy films have been studied for high‐density longitudinal recording media. In‐plane coercivity remarkably increases from 1400 to 2500 Oe with an increase of SiO2 content up to 4 at. % and coercive squareness S* slightly decreases from 0.90 to 0.87, while the perpendicular magnetic anisotropy is not changed significantly. Media noise is markedly reduced by the addition of SiO2. Si in the film is shown to be in the form of SiO2 by x‐ray photoelectron spectroscopy. In x‐ray diffraction, hcp‐CoNiPt lattice is observed and the lattice constants of this hcp structure are expanded by the SiO2 addition. Relative integrated intensities of the diffraction peaks from (100) and (002) planes to (101) plane are not dependent on the SiO2 content, which indicates that the average orientation of the c axis of the hcp lattice is not influenced by the SiO2 addition. This agrees with the result that the perpendicular anisotropy is not significantly changed. From ...

Preparation of Y-Ba-Cu-O Superconductors with High Critical Current Density by Unidirectional Melt Solidification

Y-Ba-Cu-O superconductors with high Jc have been prepared by a unidirectional melt solidification method. Uniformly oriented plate-shaped YBa2Cu3O7-y crystals containing small particles of Y2BaCuO5 have been grown successfully using the material with the nominal composition of Y1+xBa2-xCu3-2xOz (x0.4) as a starting material. The Jc values of the unidirectionally solidified Y-Ba-Cu-O samples, measured at 77 K using a four-probe method with continuous direct current, were 4600 and 12000 A/cm2 in 1 and 0.25 T, respectively. These values nearly coincided with those estimated from magnetization hysteresis loops using Beans critical state model.

Addition of New Pinning Center to Unidirectionally Melt Solidified Y-Ba-Cu-O Superconductor

Y-Ba-Cu-O bulk superconductor including small particles of BaSnO3 was synthesized by the unidirectional melt solidification method. Finely dispersed Y2BaCuO5 precipitates and BaSnO3 particles were observed in YBa2Cu3Oy matrix. The critical current density at 77 K in 8 kOe was 1.8×104 A/cm2, which was about four times higher than that of the BaSnO3-free specimen. Fine particles of BaSnO3 (0.1–1 µm) can act as effective flux pinning centers in the specimen and result in an increase of the critical current density.

Effects of oxide addition on magnetic and structural properties of CoNiPt alloy films

The effects of oxide addition on magnetic and structural properties of sputtered CoNiPt alloy films for high density longitudinal recording with a random c‐axis orientation of hcp Co have been studied. It is found that the addition of SiO2 up to 4.2 at. % results in a significant increase in the in‐plane coercivity from 1700 to 2400 Oe and a signal to media noise ratio with a slight decrease in coercive squareness from 0.90 to 0.87, while the perpendicular magnetic anisotropy is not significantly changed. X‐ray diffraction shows that the c‐axis orientation normal to the film plane does not change and the hcp lattice is strained due to the addition of SiO2. A marked decrease in the grain size is observed with the addition of 4.2‐at. % SiO2, which causes the development of grain separation and therefore enhances the coercivity. The origin of the increase in coercivity with 2.1‐at. % SiO2 is also discussed.

MAGNETIC INTERACTION AND REMANENCE PROPERTIES IN CONIPT(SIO2) GRANULAR FILMS FOR HIGH-DENSITY LONGITUDINAL RECORDING

Magnetic interaction and remanence properties in (Co81Ni7Pt12)100−x(SiO2)x films (x=0–8) have been studied for high‐density longitudinal recording. Films with SiO2 contents of 6 and 8 mol %, in which sufficiently isolated CoNiPt grains with a diameter of about 10 nm and a random orientation of the c axis of hcp structure are observed, show a significant low‐noise property as well as high coercivity. Both frequency dependence of the media noise and magnetic domain structure in the remanent‐magnetized state show that the exchange interaction between grains is eliminated by the grain boundary of the SiO2 phase with the width of 2 nm. In such films, high coercive squareness of 0.9 on the remanence curve is obtained, while the squareness on the hysteresis curve decreases as the SiO2 content increases. Henkel plot indicates that ‘‘ferromagnetic’’ interaction among the grains sustains such high squareness on the remanence curve. The strength of the interaction depends on the film thickness and is proportional to...

CHEMICAL SEGREGATION IN CONIPT(SIO2) ALLOY FILM

Chemical segregation in SiO2‐added CoNiPt alloy films for high‐density longitudinal recording media have been studied by high‐resolution transmission‐electron microscopy‐energy dispersive spectroscopy with a spatial resolution of 20 A. Sputtered CoNiPt(SiO2) films consist of well‐separated grains with diameters less than 200 A. Si is clearly detected only in a grain boundary region with the width from 12 to 30 A. Oxygen concentration in the grain boundary is also much higher than that inside the grain. X‐ray photoelectron spectroscopy shows that Si atoms in the film are in the form of SiO2. Therefore, it is concluded that SiO2 is segregated to the grain boundary in the CoInPt films. The development of the grain separation due to this chemical segregation explains the excellent magnetic and recording properties of the CoInPt(SiO2) films, such as high coercivity beyond 2400 Oe and low media noise. Inhomogeneity of CoNiPt alloy elements are also discussed.

A Simple Method of Fabricating Preferentially Oriented YBa2Cu3O7-x Film on Silver Substrates

Porous YBa2Cu3O7-x film stuck to a flexible silver substrate was easily fabricated by sintering a doctor-blade-cast green sheet of YBa2Cu3O7-x placed on the silver substrate. Furthermore, preferentially oriented dense YBa2Cu3O7-x film was also fabricated by adopting a copper plated silver substrate and then sintered below the melting temperature of silver. A possible mechanism for the appearance of the preferentially oriented dense YBa2Cu3O7-x is discussed based on the liquid-phase sintering in the YBa2Cu3O7-x-CuO-Ag system.

GRAIN STRUCTURE AND MAGNETIC CLUSTERING IN SIO2 ADDED CONIPT GRANULAR FILMS

Grain structure and its relation to the magnetic properties in SiO2 added CoNiPt granular films have been studied, with the SiO2 content varied from 0 to 8 mol %. With SiO2 more than 4 mol %, significant grain boundaries with a width of a few nm are formed by the segregated SiO2. The average grain size in the film plane decreases from 16 to 9 nm with the SiO2 increased up to 8 mol %. The media noise significantly decreases with increasing SiO2 and remanent magnetized domains observed by magnetic force microscopy (MFM) also become finer. With 8 mol % SiO2, MFM shows that weak magnetic clustering spreads over the grains, with a diameter of about 100 nm. This clustering indicates the collaborative behavior of the magnetization between grains is responsible both for the steep magnetization reversal as well as the slight increase in the media noise as the recording density is increased.

Anisotropic magnetic-field dependence of the critical current density and microstructure for YBaCuO thin films

Abstract Anisotropic magnetic-field dependence of the critical current density (J c ) of polycrystalline YBa 2 Cu 3 O 7−δ thin films deposited on (100)MgO. (100)SrTiO 3 and a-axis oriented BaSnO 3 was studied. For the films on MgO 2 . J c changes from J c (B//substrate surface)≈1×10 5 A/cm 2 to J c (B⊥ substrate surface)≈4×10 4 A/cm 2 at 77.3K and 0.6T. On the other hand, for the films on SrTiO 3 , J c is almost constant at about J c (B//)≅J c (B⊥)≈3×10 5 A/cm 2 under the same conditions. Transmission electron microscopy shows that the films on MgO consist of c-axis oriented small YBa 2 Cu 3 O 7−δ grains (∼ 1 μm in size), while the films on SrTiO 3 consist of a mixture of a-axis and c-axis oriented small YBa 2 Cu 3 O 7−δ grains (∼0.5μm in size). These facts suggest that the dependence of J c on the direction of the applied magnetic field strongly reflect the microstructure of the thin films.