Wen Ming, Zhu Yuanzheng, Meng Xiangguo
(Department of Chemistry, Tongji University, Shanghai 200092,China)

Abstract Three platinum alloy nanoparticles in the range of 2–4 nm were synthesized by phase- transfer solvothermal process. The characteristics of as-obtained nanoalloys were correlated by HRTEM, EDS, XRD, DSC and VSM. The additions of Ni lower L1₀ kinetic ordering temperature in Fe-Ni-Pt system. Magnetic specificity is discovered that as-synthesized nanoalloys changed from paramagnetic to ferromagnetic with Hc change from 60 to 1900 Oe as Ni content increse, but the variation trend of annealed ones are inverse case with Hc varied from 4218 to 228Oe.
Keyword phase-transfer solvothermal, platinum alloy nanoparticles, magnetic

1. INTRODUCTION  
Synthesis of alloy magnetic nanoparticles has attracted great interest for their potential applications in ultrahigh-density magnetic storage^[1]. Especially, FePt nanoparticles, as an important candidate storage media, have been under intensive study in recent years for their magnetic properties^[2] due to their L1₀ phase with large uniaxial magnetocrystalline anisotropy [Ku≈7×10⁶ J/m³] and good chemical stability^[3–6]. Catching the advantages of platinium alloys and striking characteristic of nanosized materials, platinium alloys nanoparticles can get prominent properties in ferromagnetism^[3,7], catalytic activity^[8,9] and corrosion resistance^[10]. Some work report on the synthesis of binary alloy nanoparticles such as FePt ^[11], CoPt ^[12], NiPt^[13] and so on. However, to the best of our knowledge, few reports were concerned with synthesis and magnetic properties of FeNiPt ternary alloy nanoparticles. Because Ni can lower the L1₀ curie temperature in FeNiPt films^[14], meanwhile low phase transformation temperature is desired for the L1₀ platinium alloys as recording media. Thus FeNiPt ternary alloy became better candidate than FePt for heat-assisted magnetic recording to overcome the high coercive fields. Here for understand the relationship of the composition, phase transition and magnetic property, three alloys nanoparticles of FePt, FeNiPt and NiPt are synthesized through reduction of Fe₂(C₂O₄)₃·5H₂O, NiCl₂·6H₂O and H₂PtCl₆·6H₂O by propylene glycol involving sodium oleate and OA as surfactant and stabilizer in the solvothermal system^[15].The structure, morphology, kinetic phase transition and magnetic properties are discussed detail in this paper.

3. RESULTS AND DISSCUSSION
3.1 Structure                     
Fig.1 shows the TEM images and EDS spectra of the as-synthesized nanoparticles of FePt (Fig.1a), FeNiPt (Fig.1b) and NiPt (Fig.1c). The size of plantinium alloy monodispersed nanoparticles generated in this reaction was in the range of 2–4 nm with narrow deviation. The measurement results of composition analysis give Fe₅₂Pt₄₈ for S1, Fe₄₁Ni₁₂Pt₄₇ for S2 and Ni₅₄Pt₄₆ for S3, respectively. Strong peaks for Fe, Ni, and Pt undoubtedly confirmed that the as-synthesized nanoproducts are FePt for S1, FeNiPt for S2 and NiPt for S3. The atom ratio are about 1/1 for Fe₅₂Pt₄₈ in Fig. 1(a), 3/1/4 for Fe₄₁Ni₁₂Pt₄₇ in Fig.1(b) and 1/1 for Ni₅₄Pt₄₆ in Fig.1(c).

We summarized the mechanism as follows. Three phases formed in system: sodium oleate (solid), the liquid phase of ethanol and oleic acid (liquid), and the water-ethanol solution containing transition metal ions (solution). The phase transfer of Mⁿ⁺ occurred spontaneously across the interface between solid sodium oleate and ethanol solution based on Mⁿ⁺ and Na⁺ exchange. At the designated temperature, the propylene glycol in the liquid phases reduced the metal ions at the liquid-solid interfaces. Thus, the platinium alloy nanoparticles could be obtained at the bottom of the container. All as-synthesized nanoalloys have disorder structures. Annealing induced the phase transformation to chemically ordered fct phase.
3.2 Thermal stability
In the interesting of the kinetic ordering temperature, the nanoproducts were characterized by DSC (Fig.3). There are two exothermic peaks at 451.6K and 682.3K for Fe₅₂Pt₄₈, 475.0K and 678.1K for Fe₄₁Ni₁₂Pt₄₇, 485.2K and 668.7 K for Ni₅₄Pt₄₆. For a given Pt content, with the Ni content increasing, the kinetic ordering temperature of A₁ phase shift to high temperature field in the order of Fe₅₂Pt₄₈(Fig.3a)＜Fe₄₁Ni₁₂Pt₄₇ (Fig.3b)＜Ni₅₄Pt₄₆ (Fig.3c), and of L1₀ phase shift to low temperature field in the reverse order of Fe₅₂Pt₄₈(Fig.3a)＞Fe₄₁Ni₁₂Pt₄₇ (Fig.3b)＞Ni₅₄Pt₄₆ (Fig.3c). So the additions of Ni in FePt higher kinetic ordering temperature of A₁ phase and lower that of L1₀ phase. In the ternary FeNiPt alloys^[14], because Ni atoms are approximately the same size as Fe atoms, a size argument is not sufficient to explain the observed behavior. Therefore, a chemical content must also contribute to the activation energy. In FeNiPt, it exhibits substantial decreases in the activation energy for even small additions of Ni. So the addition of Ni acts to accelerate the L1₀ phase transformation, which does not agree with the previous reported^[14].
3.3 Magnetic properties
The Hc of as-obtainzed Fe₅₂Pt₄₈, Fe₄₁Ni₁₂Pt₄₇ and Ni₅₄Pt₄₆ are 60Oe, 830Oe and 1900Oe, respectively. After annealed, the Hc are changed to 4218Oe for Fe₅₂Pt₄₈, 2450Oe for Fe₄₁Ni₁₂Pt₄₇, 228Oe for Ni₅₄Pt₄₆( Table.1). Interestingly, the magnetism of as-synthesized nanoalloys changed from paramagnetic with Hc of 60Oe to ferromagnetic with Hc of 1900Oe as Ni content varied from 0at.% to 54at.%, but the variation trend of Hc of annealed ones are inverse case, Hc decrease from 4218Oe (ferromagnetic) to 228Oe (paramagnetic).

4. CONCLUSIONS              
Three monodispersed platinium alloys nanoparticles with the size of 2-4 nm were synthesised through the reduction of Fe₂(C₂O₄)₃·5H₂O, NiCl₂·6H₂O and H₂PtCl₆·6H₂O by a propylene glycol in the solvothermal system. The additions of Ni lower L1₀ kinetic ordering temperature in Fe-Ni-Pt. Magnetic specificity indicated that as-synthesized nanoalloys changed from paramagnetic to ferromagnetic as Ni content increase, but the annealed ones are inverse case.

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