FePt nanoparticles

The recent liquid phase synthetic techniques provide us the metal nanoparticles with the standard deviation smaller than 10%. So a lot of scientists have been attracted by an investigation on the transition from molecular to bulk properties from both the fundamental and technological points of view. Here we present our recent liquid phase techniques to control the size and composition of Au and FePt nanoparticles. 

This alcohol reduction method is applied to the control of size and composition of not only the noble metal/noble metal [7] but the 3d-transition metal/noble metal nanoparticles [8] like magnetic FePt nanoparticles. 

In 2000, Sun and co-workers succeeded in synthesis of monodispersed Fe/Pt nanoparticles by the reduction of platinum acetylacetonate and decomposition of Fe(CO)5 in the presence of oleic acid and oleylamine stabilizers [18]. The Fe/Pt nanoparticle composition is readily controlled, and the size is tunable from 3 to 10 nm in diameter with a standard deviation of less than 5%. For practical use, we developed the novel symthetic method of FePt nanoparticles by the polyol reduction of platinum acetylacetonate (Pt(acac)2) and iron acetylacetonate (Fe(acac)3) in the presence of oleic acid and oleylamine stabilizers in di- -octylether [19,20]. The Fe contents in FePt nanoparticles can be tuned from 23 to 67atomic%, and the particle sizes are not significantly affected by the compositions, retaining to be 3.1 nm with a very narrow size distribution, as shown in Figure 6. 

Figure 6. Influence of the molar ratio of precursors on the Fe content of FePt nanoparticles. The dashed line indicates the Fe content in feeing precursors. (Reprinted from Ref. [19], 2004, The Chemical Society of Japan.)...

Very recently, we have developed one-pot synthesis of FePt nanoparticles larger than 5 nm with controlled composition by the polyol reduction of Pt(acac)2 and Fe(acac)3 in excess ligands without using the conventional solvents [23]. Figure 8 presents the TEM images... 

Sun, S.H. (2006) Recent advances in chemical synthesis, self-assembly, and applications of FePt nanoparticles. Advanced Materials, 18 (4), 393-403. 

Tanase, M. and Laughlin, D. (2004) Polyol process synthesis ofmonodispersed FePt nanoparticles. Journal of Physical Chemistry B, 108 (20), 6121-6123. 

Sun, S. Murray, C. B. Weller, D. Folks, L. Moser, A. 2000. Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices. Science 287 1989-1992. 

A ferritin-FePt nanoparticle bionanocomposite was fabricated in later studies, which shows the integrated magnetic behavior of the synthetic and biological components (Fig. 6.5 Srivastava, Samanta, Jordan, et al. 2007). A transmission electron... 

FePt Nanoparticle Higher dipole-dipole interaction... 

Figure 6.5 (a) The formation of ferritin-mediated self-assembly of FePt nanoparticles via... 

Srivastava S, Samanta B, Arumugam P, Han G, Rotello VM. DNA-mediated assembly of iron platinum (FePt) nanoparticles. J Mater Chem 2007 17 52-55. 

Sun SFl, Anders S, Thomson T, Baglin JEE, Toney MF, Hamann HF, Murray CB, Terris BD. Controlled synthesis and assembly of FePt nanoparticles. J Phys Chem B 2003 107 5419-5425. 

Figure 6.5 (a) The formation of ferritin-mediated self-assembly of FePt nanoparticles via electrostatic interactions, (b) magnetic dipole-dipole interaction of ferritins assembled with FePt nanoparticles, and (c) zero field cooling and field cooling results for the ferritin-FePt nanoparticle composite film and individual components. Reprinted with permission from Srivastava, Samanta, Jordan, et al. (2007). Copyright 2007 American Chemical Society. 

The multilayers were deposited at temperatures ranging from 150-350°C. The t phase appears at 200 °C and ceases to form when deposited at 350°C [8]. The x phase is the only ferromagnetic phase of MnAl and hence for the two deposition temperatures for which the phase is formed there is a development of coercivity. Reports in the literature indicate a coercivity approaching 2 kOe for thin films [9]. As seen in Fig. 15 the coercivity increases up to 3.7 kOe. The formation of MnAl nanoparticles would provide an answer to the problem of agglomeration of FePt nanoparticles due to the lower annealing temperatures required to transform the particles to the FCT. 

In the case of the FePt nanoparticle system the nanoparticles must be subjected to elevated temperature in order to undergo the Zl0 phase transformation. This is achieved by passing the particles through a furnace region as seen in Fig. 17, which consists of an assembly of halogen lamps surrounding a quartz tube situated above the particle formation region. 

Figure 18. SAD pattern, Bright field image and particle size distribution of FePt nanoparticles with a) lights off and b) lights on.

FORMATION AND SUPERSTRUCTURAL DEVELOPMENT OF EPITAXIALLY GROWN FePt NANOPARTICLES... 

Silver has been reported to reduce the transformation temperature of FePt nanoparticles made by both sputtering and chemical synthesis [15, 16]. Furthermore, Ag, MgO and FePt have very similar lattice parameters (aAg = 0.40 nm) and therefore allows for a study of the transformation temperature without destroying the perpendicular texture. Here we describe the study of FePt/Ag multilayers deposited on MgO single crystal substrates at temperatures up to 400 °C. The X-ray diffraction patterns of the (FePt 2 nm/Ag 16 nm) pseudo-multilayers deposited on MgO [001] substrates at different temperatures are shown in Fig. 23. The superlattice (001) peak is present in all samples at temperatures above 280 °C, indicating the formation of Z,lo phase. 

FePt C layer with FePt nanoparticles embedded in a C matrix and soft FeCoNi underlayer. 

One assembly example is polyethylenamine (PEI)-mediated self-assembly of FePt nanoparticles [56]. PEI is an all -NH-based polymer that can replace oleate/oleylamine molecules around FePt nanoparticles and attach to hydrophilic glass or silicon oxide surface through ionic interactions [52], A PEI/FePt assembly is readily fabricated by dipping the substrate alternately into PEI solution and FePt nanoparticle dispersion. Figure 10 shows the assembly process and TEM images of the 4 nm Fes8Pt42 nanoparticle self-assemblies on silicon oxide surfaces. Characterizations of the layered structures with X-ray reflectivity and AFM indicate that PEI-mediated FePt assemblies have controlled thickness and the surfaces of the assemblies are smooth with root mean square roughness less than 2 nm. 

Figure 10. (A) Schematic illustration of PEI-mediated self-assembly of FePt nanoparticles by alternately adsorbing a layer of PEI and a layer of nanoparticles on a solid surface and TEM images of PEI-mediated assembly of 4 nm Fe58Pt42 nanoparticles on silicon oxide surface (B) one layer of assembly and (C) three layers of assembly [56].

---