ALBERT

Notes: Formation of acicular α-FeOOH particles from the reaction system of FeCl2 -NaOH is studied. It is found that acicular α-FeOOH particles with a particle size of about 0.5 μm and axial ratio of about 10 can be obtained by oxidizing the mixed solution of FeCl2 and NaOH with a molar ratio of NaOH/FeCl2 larger than 2. New acicular Co-γ-Fe2 O3 particles are synthesized by absorbing Co2+ ions on the surface of acicular α-FeOOH particles followed by dehydrating, reduction, and oxidation. We find that using N2 as dehydrating atmosphere is better than air or H2. After reduction, variations of the coercivity of Co-Fe3 O4 particles with various cooling rates are investigated. The transformation temperature of Co-Fe3 O4 →Co-γ-Fe2 O3 is about 300 °C for the particles with cobalt content (Co/Co+Fe)〈12 mol %. Distributions of Co2+ and Fe2+ ions in the particles are measured by the dissolution method. Coercivity of the acicular Co-γ-Fe2 O3 particles between 400 and 1200 Oe can be controlled by the cobalt content and additional anneal in N2.

Notes: The chemical coprecipitation annealing method is shown to be suitable for preparing BaFe12−2xCoxSnxO19 powders with a hexagonal fine platelet structure and a narrow particle size distribution. The proper annealing temperatures are between 800 and 900 °C. The saturation magnetization decreases slowly with increasing x; however, the coercivity decreases rapidly with increasing x. When x=0.8, the coercivity is reduced to below 1000 Oe, while the value of the saturation magnetization is near 52 emu/g. The temperature coefficient of coercivity decreases roughly from 2.6 to −1.0 Oe/°C, when x increases from 0.0 to 1.2.

Notes: The effects of Zn ion on magnetic properties Fe3O4 magnetic colloids were investigated in this study. Fe3O4 magnetic colloids were produced by the chemical coprecipitation method, i.e., mixing an acidic solution containing FeCl2⋅4H2O, FeCl3⋅6H2O, ZnCl2⋅4H2O, with a NaOH alkali solution at 70 °C, and then centrifuging them from the mixed solution. Various reaction times, solution pH values, and Zn ion contents were also used. Fe3O4 magnetic fluid was obtained by adding ammonium oleate into the mixed solution, precipitating the colloids from the solution, neutralizing the colloids by hydrochloric acid, and dispersing the colloids in n-hexane. XRD, EDX, TEM, and VSM were used to determine the structure, chemical compositions, particle sizes, and magnetic properties of the colloids and the magnetic fluid. The spinel colloids was easily form at a higher pH value in solutions where the pH value ranged from 7 to 12. Fe3O4 colloids were completely formed within the first minute of mixing and the particle size of Fe3O4 colloids did not increase with time after the first minute.The lattice parameter of Fe3O4 colloids increased linearly with the Zn ion content because the diameter of Zn ion is larger than that of Fe ions. The particle size of Fe3O4 colloids was found to be 10 nm by TEM. For an initially fixed Zn content of 8 wt % in solutions, the Zn content in the Fe3O4 colloids ranged from 3.32 wt % at pH=5 to a maximum value of 7.85 wt % at pH=10. Later, it reduced to 7.51 wt % at pH=12 because Zn ion has the lowest solubility at pH=10. At 8 wt % of zinc ion in the solution, the σs of the Fe3O4 colloid increase sharply from 0 at pH=3 to 92 emu/g at pH=8 and then reach a maximum value of 94 at pH=10. The σs value and Hc value of the Fe3O4 colloid were found significantly improved by adding a suitable amount of Zn ions, e.g., ranging from 70 emu/g and 48 Oe at Zn=0 wt % to a maximum 94 emu/g and 50 Oe at Zn=7.14 wt %. Later they reduced to 70 emu/g and 44 Oe at Zn=12.52 wt % when prepared at pH=10. The σs value of the magnetic fluid was found linearly proportional to the colloid content in the magnetic fluid. For a colloid containing 7.51 wt % of Zn ion, the σs value of the magnetic fluid is 9.8 emu/g at 25 wt % of colloid. © 1996 American Institute of Physics.

Notes: Mn–Al thin films with high coercivity and high saturation magnetization were successfully fabricated by rf magnetron sputtering with properly controlled chemical composition, substrate temperature, and annealing temperature. A high coercivity of about 3000 Oe and a saturation magnetization of about 420 emu/cc have been achieved. We have observed that during annealing at 410 °C, the nonmagnetic ε phase with a grain size of roughly 100 nm transforms into a metastable ferromagnetic τ phase with a platelike grain size of roughly 300 nm. From the continuous measurement of the stress of the films in vacuum as a function of temperature, we observed a compression stress during heating below 220 °C, and a tension stress above 220 °C during cooling. The structure phase transformation from ε to τ phases was related to the stress variation from compression to tension. The high coercivity can be explained by the high magnetocrystalline anisotropy constant of the τ phase and the magnetoelastic energy arises from the residual stress of Mn–Al films after the shear transformation. © 1997 American Institute of Physics.

Notes: Samples of (Cu/Zn)-bonded and unbonded Sm2Fe17MxNy with M=B or C (x=0, 0.25, and 0.5; y〈3) were fabricated. All the samples, besides those with B, show single Curie temperature TC and with Sm2Fe17-type crystal structure; however, multiphase structure and double TC were observed in all the samples with B. For all the heating runs the electrical resistivity roughly above 600 K increases abruptly for all Zn-bonded samples; and decreases abruptly for all Cu-bonded samples. After these high temperature runs, the residual electrical resistivity increases for all Zn-bonded samples, and decreases for all Cu-bonded samples. The effects of Cu segregation and Zn reaction with samples are identified by the EPMA analyses.

Notes: Acicular α-(FeO)OH particles were synthesized using the chemical coprecipitation method with the reaction system of Fe2Cl2-NaOH. The Mo-modified γ-Fe2O3 particles are obtained by adding the proper amount of sodium molybdate to the solution of geothite, followed by the processes of dehydration, reduction, and oxidation. The thermal stability was investigated by thermal differential analysis. It was found that the transition temperature of γ-Fe2O3→α-Fe2O3 increases from 544 °C for the unmodified particles to 720 °C for particles containing 4% by weight of molybdenum. The improved thermal stability has the advantage that the temperature for the oxidation of Fe3O4 particles can be considerably increased without the risk of forming nonmagnetic α-Fe2O3 particles. The magnetization (σs) and coercivity (Hc) of the magnetic acicular γ-Fe2O3 have been determined from room temperature to 200 °C with a vibrating sample magnetometer. The results indicate that both σs and Hc at room temperature decrease with increasing Mo content. The dependence of Hc can be approximately described by the linear equation Hc=Hc,0(1−AT) over that temperature range. The temperature coefficient A decreases with Mo content until the Mo content reaches 4 wt. %. The origin of improved thermal stability is discussed.

Notes: Acicular α-FeOOH particles with particle length of about 0.2 μm and axial ratio of about 8 were used as the starting material to prepare acicular Fe-Co alloy particles. They were made by adsorbing Co(OH)2 on the surface of α-FeOOH particles, followed by dehydration, annealing, and reduction. In order to prevent sintering at high temperatures, the Co-α-FeOOH particles were first dehydrated at 300 °C and then coated with a thin layer of silica before high-temperature treatments. Furthermore, after reduction, the metallic powder was immersed in toluene to avoid oxidation. The particle structure of Fe-Co was determined from x-ray diffraction. It has been found that, up to 50 at. % Co, all the Fe-Co particles have the same bcc structure and similar lattice constant. The magnetic properties of Fe-Co alloy particles with different Co concentrations were investigated using a vibrating sample magnetometer. The specific saturation magnetizations σs are about 30% below those of bulk materials. The maximum value of σs at room temperature is obtained with 42 at. % Co. The coercivity that mainly originates from shape anisotropy is as high as 1250 Oe for the particles with 33 at. % Co. Furthermore, σs drops sharply with Si content while Hc peaks at 0.6% Si. The coercivities of the particles also depend on the crystallite size. Hc shows a maximum when the crystallite size is about 300 A(ring).

Notes: (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 thickness is small. In contrast, the magnetic hardening mechanism and coercivity of the FePt–Si3N4 composite film are dependent on the Si3N4 volume fraction. © 2000 American Institute of Physics.

Notes: 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. © 1999 American Institute of Physics.

Notes: MnxAl100−x−yCy thin films with x=35–65 at. % and y=0–2.4 at. % were prepared by rf magnetron sputtering. Effects of the chemical composition and annealing temperature on the magnetic properties and microstructure of Mn–Al–C films were investigated. X-ray analysis shows that the as-deposited Mn–Al–C thin films are amorphous, and their saturation magnetization is very low. After annealing at temperatures between 400 and 550 °C in vacuum for 30 min, the magnetic phase with higher carbon concentration shows better thermal stability. The best annealing condition was found to be at 410 °C for 30 min. A ferromagnetic τ phase with a grain size of roughly 200–250 nm appeared at a composition range between 40 and 60 at. % Mn for MnxAl99−xC1 thin films; and the sample with Mn50Al49C1 has high coercivity and moderate saturation magnetization. The carbon addition can increase the thermal stability of the coercivity of the Mn–Al thin films. © 1999 American Institute of Physics.