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: Mn50Al50/Al bilayer films were fabricated on a glass substrate by rf magnetron sputtering. The films were subsequently heat-treated in order to transform nonmagnetic MnAl ε-phase to magnetic τ-phase. The addition of Al buffer layer could enhance the adhesion of the MnAl films, and for Mn50Al50/Al bilayer films with Al layer thickness between 5 and 15 nm, we can obtain high saturation magnetization (〉390 emu/cm3), and high coercivity (〉2200 Oe) Mn50Al50/Al films for practical usage. Bending beam method analysis shows that the larger the residual stress of the Mn50Al50/Al bilayer film is, the higher the coercivity is. The relations between the saturation magnetization, the coercivity and the phase transition, the microstructures of the films are discussed.© 1997 American Institute of Physics.

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: 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: 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. © 2000 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.

Notes: The chain-of-spheres model is modified by taking the effect of particle interactions into account to interpret the packing density dependence of the coercivity of elongated particles. The relationship of coercivity versus packing density is derived from Lorentz mean field approximation. Coercivity of the particle assembly is treated in detail with regard to the particle axis orientation. Calculations on the coercivities of randomly oriented Fe3O4 particles at various packing densities are found in good agreement with experimental results of Davis.

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: (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.