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

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

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

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

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

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

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

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

Depleted Kondo Lattices

We consider a two dimensional Kondo lattice model with exchange J and hopping t in which three out of four impurity spins are removed in a regular way. At the particle-hole symmetric point the model may be studied with auxiliary field quantum Monte Carlo methods without sign problems. To achieve the relevant energy scales on finite clusters, we introduce a simple method to reduce size effects by up to an order of magnitude in temperature. In this model, a metallic phase survives up to arbitrarily low temperatures before being disrupted by magnetic fluctuations which open a gap in the charge sector. We study the formation of the heavy-electron state with emphasis on a crossover scale T* defined by the maximum in the resistivity versus temperature curve. The behavior of thermodynamic properties such as specific heat as well as spin and charge uniform susceptibilities are studied as the temperature varies in a wide range across T*. Within our accuracy T* compares well to the Kondo scale of the related single impurity problem. Finally our QMC resuls are compared with mean-field approximations.Comment: 12 pages, 13 figures. Submitted to Phys. Rev.