Evolution of High Coercivity in CoPt Nanoparticles Through Nitrogen Assisted Annealing (original) (raw)
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Synthesis and magnetic properties of CoPt nanoparticles
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High magnetocrystalline anisotropy CoPt particles with an average size of 8 nm were synthesized by the superhydride reduction of CoCl 2 and Pt͑acac) 2 at a high temperature. As-made particles showed a disordered face-centered cubic lattice and were superparamagnetic. Upon heat treatment at temperatures above 600°C, the particles transformed to the L1 0 phase, as indicated by the appearance of the superlattice peaks in the x-ray diffraction and high magnetocrystalline anisotropy. The temperature dependence of the coercivity of nanoparticles annealed at 650°C was measured from 10 to 300 K and analyzed using a Sharrock formula. After annealing at 650°C, the anisotropy of the nanoparticles was Kϳ1.7ϫ10 7 erg/cm 3 .
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Co 30 Cu 10 Pt 60 nanoparticles were prepared by a simplified polyol process devoid of any surfactants or capping agents. Thermo-magnetic analysis showed that the addition of 10 at. % Cu reduces the Curie temperature of Co 40 Pt 60 from 492 o C to 435 o C. The incorporation of Cu is effective in reducing the threshold ordering temperature for the A1-fcc to L1 0-fct phase transformation resulting in coercivity enhancement. Coercivity close to 10 kOe at a lower annealing temperature of 600 o C in Co 30 Cu 10 Pt 60 demonstrates that high coercivity in CoPt can be achieved in non-equiatomic compositions also. Annealing temperature dependent two phase behavior has been understood by δM studies.
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Co 50 Pt 50 nanoparticles were co-deposited on thermally oxidized Si substrates by electron beam evaporation at 750 1C. The mean particle sizes are between 5and5 and 5and20 nm and depend on the nominal thickness of the layer. Different processing conditions resulted in different structural and morphological properties of the samples which led to superparamagnetic and ferromagnetic behaviors. The post-annealing treatment of the CoPt nanograins resulted in the crystallization of the L1 0 ordered phase and in the magnetic hardening of nanoparticles with a maximum coercivity of $7.4 kOe.
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Various manifestations of the exchange interaction effects in magnetization curves of the CoPt nanostructured particles are demonstrated and discussed. The inter-grain exchange constant A in the sponge-like agglomerates of crystallites is estimated as А = (7 ± 1) pJ/m from the approach magnetization to saturation curves that is in good agreement with А = (6.6 ± 0.5) pJ/m obtained from Bloch T 3/2 law. The fractal dimensionality of the exchange coupled crystallite system in the porous media of the disordered CoPt alloy d = (2.60 ± 0.18) was estimated from the approach magnetization to saturation curve. Coercive force decreases with temperature as Hc ~ T 3/2 which is assumed to be a consequence of the magnetic anisotropy energy reduction due to the thermal spin wave excitations in the investigated CoPt particles.
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Size-Induced Effects in Wet-Chemically Synthesized CoPt3 Nanoparticles
Journal of Nanoscience and Nanotechnology, 2010
CoPt 3 alloy spherical nanoparticles with sizes tuned between 3-7 nm were produced by the simultaneous thermal treatment of proper platinum and cobalt precursors in the presence of surfactants. The final size and composition of the particles are determined by the precursors' ratio, the chemical behavior of Co precursors and the stabilizing efficiency of the surfactants. By employing higher reaction temperatures (∼350 C) better alloying is achieved leading to enhancement of macroscopic magnetic features and decrease of the superparamagnetic limit down to 7 nm.
The Journal of Physical Chemistry C, 2010
CoPt organized cluster assemblies are developed to produce ultra-high-density magnetic media that need the smallest sizes for recording particles. At the nanometer scale (3-4 nm in diameter in our case), surface atoms represent around 50% of the total number of atoms and the segregation effect could dominate the alloying effect. Our purpose is to describe the competition between segregation and alloying effects in asdeposited or embedded bimetallic CoPt clusters preformed in the gas phase. The segregation is studied by X-ray photoemission spectroscopy. First of all, it was found that satellite peaks at high energy disappear when particles are fully embedded in a matrix. Moreover, CoPt clusters exhibit a partial core-shell structure when the clusters are not chemically bonded to the matrix. Conversely, an alloying effect is reported when CoPt particles are embedded in a silicon matrix. These effects are analyzed on the basis of both the phase diagrams and the enthalpy formation in the bulk phase. The bulk phase diagram remains valid for the nanometric scale, taking into account a scaling factor. Furthermore, the asymmetry of the photoemission peaks is carefully analyzed in terms of intrinsic and extrinsic energy losses.
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IEEE Transactions on Magnetics, 2000
Various contributions to the magnetic anisotropy energy (MAE) dispersion for an assembly of CoPt nanoparticles are examined, using the empirical Néel anisotropy model. It is shown that, while small shape and composition variations have negligible effects, the statistical distribution of chemical arrangements can be the major source of MAE dispersion.