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We have studied the lattice parameter changes of L10 FePt nanoparticles annealed to near equilibrium as a function of composition by x-ray diffraction. We have found that the (111) diffraction peak shifts linearly with composition, however, the c parameter mostly changes in the Pt rich compositions and the a parameter mostly changes in the Fe rich compositions with respect to the equiatomic composition. This causes the tetragonality of the L10 structure to be maximized near the Fe 50%/Pt 50% composition. The magnetic properties were measured at room temperature and at 5 K and are correlated to the structural changes occurring as a function of composition.

The influence of Cu, Ag, and Au additives on the L1 0 ordering, texture, and grain size of FePt thin films has been examined. Lattice parameter data indicated that Au and Ag additives tended to segregate from FePt, but Cu alloyed with FePt. FePt films with Au or Ag additive showed 1-2 kOe higher coercivity values compared to a pure FePt film after annealing at 450°C and above for 10 min. The addition of at least 20 vol. % Cu to FePt boosted average coercivity values and increased ͑001͒/͑002͒ x-ray peak intensity ratios, suggesting an accelerated L1 0 ordering process for annealing temperatures exceeding 350°C. Decreasing the film thickness promoted ͑001͒ film texture in FePtϩ20% Cu films, but higher annealing temperatures were required to achieve large coercivity. Au and Ag limited the average grain size compared to a pure FePt film. Cu additive increased the average grain size and film roughness.

Fe-Pt thin-film alloys have been grown by electrodeposition at potentials positive to that required to deposit elemental Fe. X-ray diffraction studies indicate the formation of fine grained face centered cubic alloys, while Rutherford backscattering spectrscopy and energy-dispersive X-ray spectroscopy reveal substantial incorporation of oxygen in the FePt deposits. The Fe-Pt codeposition process is driven by the negative enthalpy associated with alloy formation. The experimentally determined relationship between alloy composition and the iron group underpotential was found to be in reasonable agreement with free energy calculations for the binary alloy system, based on thermochemical data.There is currently considerable interest in FePt as a high-density perpendicular recording medium, due to the high magnetocrystalline anisotropy of the L1 0 phase. The significant challenges of achieving an appropriately oriented L1 0 phase, while maintaining the required grain ͑or particle͒ size of less than 5 nm, remain unsolved, despite considerable effort. 1-3 FePt has attracted additional interest due to its shape-memory properties, and Invar effects, both of potential utility in microelectromechanical systems ͑MEMS͒. 4 In addition to these useful physical properties Fe-Pt and related alloys have potential application as CO-tolerant electrocatalyst in polymer electrolyte fuel cells. 5,6 In all the above applications, process control during synthesis is of central importance.A variety of means have been used to produce Fe-Pt and similar alloys ranging from vacuum methods like MBE and sputtering 2,3,7,8 to electrodepositon 9-13 of thin films or fine particle production by solution phase chemical reduction. 1,14-16 One particular advantage of electrochemical methods is the ability to easily specify and control the supersaturation while monitoring its effect on growth kinetics.Herein we examine the factors affecting alloy composition during electrodeposition from an aqueous electrolyte containing chlorocomplexes of platinum and iron. Traditional alloy deposition studies largely focus on growth in the overpotential domain. 17 In this case, the composition is controlled by the relative rate of reduction of the constituents occurring in a potential regime where both species can be deposited in their elemental form. The desired differential activity, required for a particular alloy composition, is achieved by judicious choice of component concentrations and complex forming ligands. In contrast, in this study the use of the free energy of alloy formation to control alloy composition is demonstrated.The thermodynamic basis for alloy formation is well established. In fact, high temperature electrochemical potential ͑emf͒ measurements have contributed significantly toward the understanding of phase equilibria and the construction of phase diagrams. A necessary condition for binary alloy A 1Ϫx B x formation is equality of the electrochemical potential of the respective constituentswhere E i is the Nernst potential given by ͓2͔The free e...