The influence of Sb doping in achieving high magnetic coercivities in CoPt nanoparticles for micromagnet applications
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The use of magnetic elements within microelectronic devices are increasingly required in the fabrication of miniature magnetic structures with high energy densities. A synthesis technique is reported that yields Sb-doped CoPt nanoparticles possessing magnetic coercivities as high as 1671 kA/m and magnetic remanences of 295 kA/m, providing an energy product of 20 kJ/m2. Antimony doping was shown to influence the atomic ordering within the alloy sublattices, which allowed the tetragonalization temperature of the nanoparticle structure to be lowered by 200 C to 400 C, thereby reducing crystallite growth and sintering during annealing. The “as-synthesized” particles had average diameters of 4.5 nm, which rose to 25 nm on annealing at 600 C. Synthesis of the doped CoPt particles with high-energy products together with control of particle size distributions in the range of 25 nm allows fabrication of micromagnetic structures by conventional microfabrication techniques such as spin coating and ink-jet printing.
I. INTRODUCTION
The fabrication of integrated microdevices including various magnetic micromechanical systems (MAGMEMS) have required advances increasing magnetostatic energies within structures of low dimensionality and scale. In the recent past, silicon technologies have been used for embedded permanent magnetic structures.1,2 Micromagnetic structures fabricated using physical deposition methods such as sputtering and vacuum techniques suffer from residual stresses and the other disadvantages such as slow deposition rates. The alternative method is to adapt the macroscale-bonded magnetic powder technology consisting of polymers, resins, and inks loaded with micron-size magnetic particles. At the submicron scale, nanoparticle-based composites can be spin coated or screen printed and cured to assemble microdevice boards. These applications require highquality magnetic nanoparticles with strong ferromagnetic properties with the preferred choice being transitionmetal–platinum (TM–Pt) binary alloys and rareearth–transition metal (RE–TM) alloys, because of their high magnetic coercivity and magnetic remanence values that are required to achieve high energy density with volume efficiency. Such systems also have high Curie temperatures, which allow applications at elevated temperatures. Compared with RE–TM-based magnetic a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0053 J. Mater. Res., Vol. 24, No. 2, Feb 2009
materials, TM–Pt binary alloys have better corrosion resistance and chemical stability.3 Among the TM–Pt alloys, CoPt exhibits improved resistance to corrosion and oxidation as well as higher magnetocrystalline anisotropy compared with FePt.4 Chemically synthesized CoPt nanoparticles have attracted considerable attention in the last decade because of their potential applications as ultrahigh-density data storage devices,5,6 lab-on-a-chip magnetic biobeads,7 electrocatalysts for fuel cells,8 giant magnetoresistance materials (GMR),9 magnetic actua
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