P. C. Kuo

Effects of SiO/sub 2/ on magnetic-properties of Al/sub 2/O/sub 3/-coated acicular /spl alpha/-Fe particles

(SiO/sub 2/, Al/sub 2/O/sub 3/)-coated acicular /spl alpha/-Fe particles were obtained by first coprecipitating Si and Al ions on the surface of /spl alpha/-FeOOH particles and next the /spl alpha/-FeOOH particles were dehydrated and reduced by hydrogen. The effects of Si ion on Al/sub 2/O/sub 3/-coated /spl alpha/-Fe particles are as follows: (1) it prevented the acicular /spl alpha/-Fe particles from particle growth and grain growth during reduction; (2) a suitable amount of Si ion (0.3 wt%) in the particles only slightly decreased the /spl sigma//sub s/ value but significantly increased the /sub i/H/sub c/ value of the particles; (3) 0.3 wt% of Si ion effectively prevented the /spl sigma//sub s/ value of the Al/sub 2/O/sub 3/-coated /spl alpha/-Fe particles from deteriorating with time, higher Si ion content was required to prevent the /sub i/H/sub c/ from deteriorating: and (4) Si ion could not prevent the magnetics of the Al/sub 2/O/sub 3/-coated /spl alpha/-Fe particle from deteriorating under severe surroundings.

Magnetic properties and microstructure of FePt–Si3N4 nanocomposite thin films

(Fe50Pt50)100−x–(Si3N4)x (x=0–50 vol. %) nanocomposite thin films are prepared by dc and rf magnetron cosputtering of FePt and Si3N4 targets on silicon wafer substrates, then annealed in vacuum at various temperatures. The effects of Si3N4 volume fraction, film thickness, and annealing temperatures on the magnetic properties are investigated. Transmission electron microscopy analysis indicated that structurally the film is an amorphous Si3N4 matrix with spherical FePt particles dispersed in it. The particle size of FePt increases with the annealing temperature but decreases with increasing Si3N4 content. Magnetization measurements indicated that maximum in-plane squareness and coercivity occurs at 30 vol. % of Si3N4 after annealing the film at 750 °C for 30 min. The average particle size of FePt in this film is about 40 nm. Saturation magnetization of the FePt–Si3N4 film is independent of film thickness but inversely proportional to the Si3N4 volume fraction. Variation of the films’ coercivity with film t...

Microstructure and magnetic properties of Fe100−xPtx alloy films

Polycrystalline Fe100−xPtx alloy thin films with Pt composition x=25–67 at. % were prepared by dc magnetron sputtering on natural-oxidized silicon wafer substrates, then postannealed in vacuum at various temperatures and over different periods. The effects of film composition, magnetic layer thickness, annealing temperature, and annealing time on the magnetic properties parallel and normal to the film plane were investigated. Films with high in-plane coercivity could be obtained by annealing the films in the temperature range between 400 and 650 °C. Optimum in-plane squareness and coercivity of the film occurred at a film composition of Fe50Pt50 with thickness of about 200 nm after annealing at 600 °C for 30 min. The analyses of transmission electron microscopy diffraction patterns indicated that this annealed Fe50Pt50 film was nearly entirely in the fct γ1-FePt phase. The average grain size of this Fe50Pt50 film was about 85 nm. The grain size was almost independent of the cooling rate after annealing, b...

Magnetic and optical properties of Fe/sub 3/O/sub 4/ nanoparticle ferrofluids prepared by coprecipitation technique

Nanometer size Fe/sub 3/O/sub 4/ particles were fabricated by chemical coprecipitation technique. The particle shape and size are affected by the PH value of the reactive solutions. The ferrofluids were fabricated with the Fe/sub 3/O/sub 4/ particles as magnetic particles, ammonium oleate as surfactant, and de-ionized water as solvent. Optical transmission of Fe/sub 3/O/sub 4/ nanoparticle ferrofluids was investigated as a function of incident optical wavelengths between 450 and 750 nm and applied magnetic fields up to 150 Oe. In general, for samples with lower PH value, the particles are small and agglomerate together, however, for samples with higher PH value, the particles are larger and distributed uniformly. Samples precipitated with higher PH value show larger variation of the transmittance (/spl Delta/T). This can be understood by our transmission electron microscope and X-ray diffraction pattern studies. The behavior of agglomeration for samples with lower PH value relates to the smaller variation of the optical transmission to the magnetic field.

Magnetic hardening mechanism study in FePt thin films

Fe100−xPtx alloy thin films with x=25–67 at. % were prepared by dc magnetron sputtering on naturally oxidized Si substrates. Effects of film composition, annealing temperature (300–650 °C), annealing time (5–120 min), and cooling rate (furnace cooling or ice water quench cooling) on the magnetic properties were investigated. Optimum conditions for saturation magnetization and coercivity of the Fe100−xPtx alloy films were found with x=50 at.%, annealed at 600 °C for 30 min and cooled by ice water quenching. Our experimental data suggests that the magnetic hardening in Fe100−xPtx alloy thin films is mainly due to the fct γ1-FePt phase and the domain wall pinning effect. The domain nucleation mechanism is dominated in samples with furnace cooling; the domain wall pinning mechanism dominates in samples cooled with ice water quenching.

Microstructure and magnetic properties of the (FePt)100−xCrx thin films

The (FePt)100−xCrx alloy thin films with x=0–16 at. % were fabricated on natural-oxidized Si(111) substrate by dc magnetron sputtering. The as-deposited films were annealed between 300 and 750 °C in order to transform the soft magnetic fcc γ-FePt phase to the hard magnetic fct γ1-FePt phase. The addition of Cr in the FePt thin films will reduce its saturation magnetization and coercivity, however, it could inhibit the grain growth during annealing of the samples. The optimum condition for high-density magnetic recording purpose of the (FePt)100−xCrx alloy films was found with x=5 at. %, annealing at 650 °C for 15 min, and ice water quench cooling. According to the transmission electron microscopy study, the average grain size in the annealed (FePt)100−xCrx alloy thin films decrease from 60 to 5 nm with increasing x from 0 to 16.

Epitaxial growth mechanism of L10 FePt thin films on Pt/Cr bilayer with amorphous glass substrate

Ordered L10 FePt films with magnetic perpendicular anisotropy were fabricated with a Pt∕Cr bilayer. The squareness of the L10 FePt film with a Cr underlayer and a Pt buffer was close to one when a magnetic field was applied perpendicular to the film’s plane, because a semicoherent epitaxial growth was initiated from the Cr (002) underlayer; continued through the Pt buffer layer, and extended into the L10 FePt (001) magnetic layer. Without the Pt buffer layer, the Cr atoms may diffuse directly into the FePt magnetic layer. Consequently, an epitaxial barrier of the Cr-rich FePtCr alloy formed between the Cr underlayer and the FePt magnetic layer, degrading the magnetic performance and epitaxial growth of the latter.

Ge2Sb2Te5 Thin Film Doped with Silver

(Ge2Sb2Te5)100-XAgX films (where x=0~3 at.%) with 100 nm thickness were deposited on natural-oxidized silicon wafer and glass substrates by dc magnetron co-sputtering of Ge2Sb2Te5 and Ag targets. The effects of Ag content on the optical properties of the (Ge2Sb2Te5)100-XAgX film were examined by X-ray diffraction, reflectivity and crystalline temperature analysis. It was found that the reflectivity and thermal properties of the Ge2Sb2Te5 film could be improved by doping a small amount of Ag in the Ge2Sb2Te5 film. The crystallization activity energy of the film decreased as the Ag content was increased. However, the crystalline temperature of the film increased with Ag content. The phase-transition temperature of the fcc to hcp phase increased to at least 450°C as x>0.3. We found that the contrast of (Ge2Sb2Te5)98.2Ag1.8 film is suitable for blue laser wavelength recording.

Microstructure and magnetic properties of FeCoN thin films

Effects of nitrogen contents and substrate temperatures to the microstructure and magnetic properties of the FeCoN films have been investigated. According to the TEM and x-ray Scherrer’s equation analyses, we found that the grain size of films with substrate temperature below 200 °C is roughly about 13 nm, however, it increases very fast for films with substrate temperature above 300 °C. N content in the films is saturated to 30 at. %, as N2 flow ratio N2/(Ar+N2) is higher than 5 vol. %. From the magnetization studies, we have found that the saturation magnetization 4πMs of the optimum samples (with the substrate temperature near 200 °C) is 23.9 kG. The improvement of the magnetic properties is attributed to the combination of α-Fe with N to form the high magnetic moment FeN phases.

Microstructure and magnetic properties of nanocomposite FePtCr–SiN thin films

[(FePt)100−xCrx]100–δ –[SiN]δ nanocomposite thin films with x=0–25 at. %, and δ=0–30 vol. % were fabricated on a natural-oxidized Si(100) substrate by dc and rf magnetron cosputtering of FePt, Cr, and Si3N4 targets. The thickness of the films was kept at 10 nm in order to examine the possibility for applying in high-density magnetic recording media. Transmission electron microscopy (TEM) and electron diffraction analyses indicated that the face-centered-cubic (fcc) γ-phase FePt, body-centered-cubic (bcc) Cr, and amorphous SiN coexisted in as-deposited films. The as-deposited films were annealed in vacuum between 350 and 750 °C for 30 min, and then ice-water quench cooling, in order to transform the soft magnetic fcc γ-FePt phase to the hard magnetic face-centered-tetragonal (fct) γ1 phase. Cr was added to inhibit the FePt grain growth, and was observed by TEM and energy disperse spectrum analysis in the grain surface area of FePt grains. The TEM observation indicated that the structure of the film was an ...