Solid State Amorphization Reaction by Rod-Milling Al_xTa_<1-x> Powders and the Effect of Annealing

High thermal stable amorphous Al_xTa_ alloy powders with wide amorphization range (10≤x≤90) have been synthesized by rod-milling technique using a mechanical alloying (MA) method. During the first few kiloseconds (11-173 ks) of the MA time, the layered-composite particles of Al and Ta are intermixed and form an amorphous phase upon heating at about 680 K in a differential thermal analyzer by thermally assisted solid state amorphization (TASSA). The heat formation of an amorphous Al_xTa_ alloy via the TASSA process, ΔH_ has been measured as a function of the MA time. The crystallization characteristics indexed by the crystallization temperature, T_ and the enthalpy of crystallization, ΔH_ of the amorphous phase formed via the TASSA process are also investigated as a function of the MA time. Comparable with the TASSA process, a homogeneous amorphous Al_xTa_ alloy is formed after longer MA time (1080 ks). The amorphization process in this case is attributed to a mechanical solid state amorphization (MDSSA). At the end of the MA time (1080-1440 ks), the maximum heat of formation of an amorphous Al_xTa_ alloy via the MDSSA process, ΔH_, has been calculated. Moreover, the thermal stability characterized by the crystallization temperature, T_ and the enthalpy of crystallization, ΔH_, are also estimated. The role of amorphization via each process has been discussed

Structure and magnetic properties of Co/CoO and Co/Si core-shell cluster assemblies prepared via gas-phase

Plasma-gas condensation cluster deposition systems have been introduced and applied for preparation of Co/CoO and Co/Si clusters assemblies. In Co/CoO cluster assemblies prepared by the single source PGC system with introduction of O-2 gas into the deposition chamber, fee Co cores are covered with NaCl type CoO shells, showing marked enhancement of unidirectional and uniaxial magnetic anisotropy and a clear cross-over phenomenon in the magnetic relaxation from the high temperature thermal regime to the low temperature quantum tunneling regime. In Co/Si cluster assemblies prepared by the double source PGC system, fee Co cores are also covered with amorphous Si rich shells, showing rather small magnetic coercivity. Since Co/CoO and Co/Si core-shell clusters are stable in ambient atmosphere, they will be used as building blocks for novel nano-structure-controlled materials. (c) 2004 Elsevier Ltd. All rights reserved

Magnetic-Field- and Temperature-Dependent Characteristics of Fe/Cu Granular Films Produced by Sputter- and Cluster-Beam-Deposition(Research in High Magnetic Fields)

The magnetoresistance (MR) and magnetization (M) have been measured as functions of temperature, T, and magnetic field, H, in sputter(SP)- and cluster-beam(CB)-deposited Fe_XCu_ alloys. The MR for the SP-deposited Fe_Cu_ film exhibits a maximum at around the Curie temperature (T_C=150 K) and increases rapidly below 100 K owing to a spin-glass transition. For the CB-deposited granular Fe-Cu films with low Fe content, the MR is also enhanced at low temperatures being attributable to a cluster-glass behavior of small fcc Fe clusters. At low temperatures, moreover, a T^ dependence is found for both MR and M versus T curves. For the SP-deposited Fe_Cu_ sample, the T^ coefficient of the MR roughly corresponds with that of the M. For the CB-deposited Fe_Cu_ sample, however, they are inconsistent each other. These results originate from different status of the Fe atoms : the Fe atoms are rather randomly dispersed in the Cu matrices in the SP-deposited sample, while they form clusters in the CB-deposited one

Effect of heat treatment on structure and magnetic properties of the Fe-N and Fe-Ti-N alloy films

Fe-N and Fe-Ti-N alloy films have been prepared by reactive sputtering. The structure and magnetic properties of the Fe-Ti-N and Fe-N films have been studied as a function of the N-2 flow rate R(N-2) and annealing temperature T-A by X-ray diffraction (XRD) and a vibrating sample magnetometer. The as-prepared and annealed Fe-N films consist of the alpha-Fe and Fe4N phases but the Fe-Ti-N films are composed of the alpha-Fe and Ti2N phases. The coercivity, H-c, of the Fe-N films changes drastically with R(N-2) and T-A, while that of the Fe-Ti-N films does not change with T-A up to 500 degrees C. These results indicate that the addition of Ti suppresses the formation of iron nitride phases and improves the thermal stability of Fe-N films. (C) 1997 Elsevier Science S.A

Thermomagnetic behaviors of Fe-Cr-N films with perpendicular magnetic anisotropy

The temperature dependence of magnetization has been determined for sputter-deposited Fe-Cr-N films with perpendicular magnetic anisotropy. Decomposition and phase transformation with heating have been determined by X-ray diffraction, differential scanning calorimetry, and thermomagnetometry. There are three magnetic transformation stages in the temperature-rising thermomagnetic curves. The first stage which occurs below 350 degrees C corresponds to the paramagnetic transition of the ferromagnetic alpha-Fe-Cr phase. The second stage (350-550 degrees C) is the decomposition of the alpha-Fe-Cr and nonmagnetic gamma'-(Fe,Cr)(4)N-x phases into the pure alpha-Fe and sigma-FeCr phases, leading to an increase of the magnetization and the disappearance of the perpendicular magnetic anisotropy. The final magnetic transformation stage is the paramagnetic transition (T-c=735 degrees C) of the pure alpha-Fe phase. Since there is no rapid magnetization change between liquid helium temperature and room temperature, the gamma'-(Fe,Cr)(4)N-x phase is nonmagnetic at low temperatures. (C) 1998 Elsevier Science S.A

Formation and magnetic properties of Fe-Pt alloy clusters by plasma-gas condensation

Size-monodispersed FexPt1-x alloy clusters were synthesized using a plasma-gas-condensation technique which employs two separate elemental sputtering sources and a growth chamber. The composition of the alloy clusters was controlled by adjusting the ratio of the applied sputtering power. We found that high-temperature disordered fcc-FexPt1-x clusters whose mean diameters of 6-9 nm depend on the Ar gas flow ratio were formed for a wide average composition range (xapproximate to0.3-0.7), and the lattice constant of as-doposited clusters increases almost linearly with decreasing x, being extrapolated to the value of pure Pt metal. For Fe49Pt51 cluster-assembled films, high coercivity (8.8 kOe) was obtained by annealing at 600 degreesC within 10 min due to improved chemical ordering, although as-deposited cluster-assembled films have lower blocking temperatures than room temperature, and show a small coercivity value (similar to25 Oe) at room temperature due to intercluster magnetic interaction. (C) 2003 American Institute of Physics

Effects of O-2 gas on the size and structure of Cr clusters formed by plasma-gas-condensation

Cr clusters have been produced by a plasma-gas-condensation type cluster deposition apparatus, and studied using a time-of-flight mass spectrometer and a transmission electron microscope. The Cr clusters formed in high pressure inert (Ar and/or lie) gas atmosphere are of an A15-type structure. When an O-2. Ps is mixed with the inert gases in the source (sputtering) chamber, a bcc phase is formed together with Cr2O3. The O-2 gas introduction leads to an increase in the gas temperature of the source chamber probably due to release of the formation enthalpy of the oxide. The A15 phase is annealed by such excess heat and becomes the equilibrium bee phase. The sizes of bee clusters are smaller than those of the A15-clusters, probably due to the heterogeneous nucleation promoted by the oxide formation

Exchange anisotropy of monodispersed Co/CoO cluster assemblies

Monodispersed Co/CoO cluster assemblies with the mean cluster size of 13 nm have been prepared using a plasma-gas-condensation-type cluster beam deposition apparatus. The structural analysis and magnetic measurement indicate that the Co cluster is covered by an oxide shell composed of CoO. The effect of the oxygen gas flow rate during deposition and that of temperature on the coercivity and hysteresis loop shift induced by field cooling were measured. The effect of the CoO shell on the loop shift and the temperature dependence of the exchange anisotropy are discussed. The unidirectional anisotropy is negligible above 200 K for the present assemblies. This is ascribed to the rapid decrease of the anisotropy of the antiferromagnetic interfacial layers near the inter-face of the Co cores and CoO shells

Compositional partition in Ag-Nb alloy clusters produced by a plasma-gas-condensation cluster source

We have produced Ag-Nb clusters by a facing-target type plasma-gas-condensation cluster source as our first step toward alloy cluster formation. The Ag-Nb clusters have been deposited on substrates and examined by a transmission electron microscope with a nano-beam energy dispersive X-ray analysis. We have obtained Ag-Nb alloy clusters with the sizes range between 5 and 10 nm in diameter. Their chemical compositions are broadly dispersed and partitioned into Ag-rich and Nb-rich ones, being consistent with the immiscible type equilibrium phase diagram. This result suggests that alloy cluster formation is driven by the alloy phase stability. (C) 2000 Acta Metallurgica Inc

Electrical properties of oxide-coated metal (Co, Cr, Ti) cluster assemblies

Oxide-coated metal (Co, Cr and Ti) cluster assemblies whose mean cluster sizes are 8-13 nm have been fabricated by a plasma-gas-condensation type cluster beam deposition technique. With increasing oxygen gas flow rate R-O2, the oxide-coated metal cluster-assembled films exhibit a metal-nonmetal transition. In the metallic regime, the resistivity reveals In T dependence at low temperature due to weak localization of conduction electrons and/or electron-electron interactions in the disordered oxide-coated cluster-assembled films. The In T dependence still remains for the very thick oxide-coated metal-cluster-assembled films (the actual thickness t(c) = 2400 nm) which is clearly a three-dimensional system. This behavior can be interpreted by a low dimensionality of the three-dimensional oxide-coated cluster assemblies because of a porous cluster stacking and imperfect or non-uniform oxide shell