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

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

Structure and magnetic properties of FePt alloy cluster-assembled films

We studied structure and magnetic properties of FexPt1-x alloy clusters fabricated by a plasma-gas-condensation technique which employs two separate elemental sputtering sources and a growth chamber. Fe and Pt metal vapors generated were cooled rapidly in an Ar atmosphere, and grown into alloy clusters. Most of the as-deposited FexPt1-x alloy clusters are multiply twinned and have predominantly an icosahedral structure. The experimental results also show that there is a narrow distribution of the chemical composition among individual clusters but the Fe and Pt atoms are distributed homogeneously in as-deposited alloy clusters, which is discussed on the basis of a formation process of the alloy clusters in the inert gas-condensation process. The optimal magnetic hardening or the chemically ordered FCT FePt clusters can be achieved at proper annealing temperature for very short annealing time. (C) 2003 Elsevier B.V. All rights reserved

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

Structural and magnetic characteristics of monodispersed Fe and oxide-coated Fe cluster assemblies

We systematically studied structural and magnetic characteristics of size- monodispersed Fe and oxide-coated Fe cluster assemblies with the mean cluster sizes of 7-16 nm. Transmission electron microscopy and scanning electron microscopy (SEM) observations show that the Fe clusters in the assemblies maintain their original size at room temperature. In the SEM images, a random stacking of the Fe clusters and a porous structure with a low cluster packing fraction of about 25% are observed. For the Fe cluster assemblies, magnetic coercivity (H-c) at room temperature increases from 4x10(1) to 4x10(2) Oe by increasing the mean cluster size from 7.3 to 16.3 nm. Using the experimental values of the coercivity at Tgreater than or equal to100 K and the fitting values of blocking temperature T-B from H-c=H-c0[1-(T/T-B)(1/2)], we estimated the values of magnetic anisotropy constant K of the order of 10(6) erg/cm(3) from T-B=KV/25k(B), which is larger by an order of magnitude than the bulk Fe value (5x10(5) erg/cm(3)). Such a large effective anisotropy at Tgreater than or equal to100 K is ascribed to the large surface anisotropy effects of the small clusters and the low cluster-packing fraction of the Fe cluster assemblies. For the oxide-coated Fe cluster samples, the coercivity strongly depends on the oxygen gas flow rate during deposition, cluster size, and temperature. In the case of a high oxygen gas flow rate (namely high surface-oxidized clusters), the ferrimagnetic oxide shell crystallites also affect the coercivity at T>50 K: The hysteresis loop shift disappears, leading to a complex change in the coercivity and an enhancement of the effective anisotropy constant. (C) 2002 American Institute of Physics

Composite deposition of Co and Si clusters by rf/dc plasma-gas-codensation

A double-glow discharge cluster source system has been made by modification of a conventional co-sputter-deposition apparatus. Using this equipment, we tried to produce Co clusters generated by a dc glow discharge mode, Si clusters by an rf glow discharge mode, and deposit them simultaneously on a substrate. Putting a separate plate between these two glow discharge rooms, we have obtained a mixture of Co and Si clusters. Here, the Co clusters are distributed rather at random, while the Si clusters are aggregated to form a larger group. Taking off the separate plate, we have obtained core-shell clusters, in which small Si clusters surround Co core clusters. These features are quite different from the instantaneous alloying and/or very rapid atom diffusion that has been expected at contact interfaces between nanometer-sized small Co and Si clusters. They suggest that this double-cluster source system is useful to fabricate various sorts of cluster composites that cannot be prepared by thermodynamical methods, such as co-evaporation and precipitation. (C) 2003 American Institute of Physics