Synthesis and Assembly of Monodisperse High‐Coercivity Silica‐Capped FePt Nanomagnets of Tunable Size, Composition, and Thermal Stability from Microemulsions

Abstract

The large uniaxial magnetic anisotropy energy ) [1,2] of Fe-Pt alloys makes their nanostructured forms attractive for potential use as nanoscale magnetic bits in ultrahigh-density information storage devices [3][4][5][6][7] or as fillers to create new types of magnetocomposites. The high magnetic anisotropy and coercivity arise from the chemically ordered face-centered-tetragonal (fct) L1 0 phase, which is sensitive to both chemical composition and the size of the nanoparticles. [4,8,9] Control over particle size and dispersity [10][11][12][13] is essential to form ordered arrays of nanoparticles, and preserve these features against agglomeration [14,15] during high-temperature annealing (e.g., 550°C) treatments used to obtain the L1 0 phase. Therefore, it is crucial to develop protocols to produce FePt nanoparticles with control over composition, particle size, and dispersity, and thermal stability for exploiting their assemblies for data-storage media applications. The synthesis technique described here is a promising approach to realize all these features simultaneously.Prior works have demonstrated FePt nanoparticles synthesis with either excellent size control [3][4][5]10,16] or chemical compositional control [17][18][19] but not both. Thermal decomposition of Fe(CO) 5 vapor, and reduction of Pt(acac) 2 , produces monodisperse nanoparticles with tunable sizes in the 2-15 nm range. [3][4][5]20] But, the chemical composition of the nanoparticle is different from the initial molar ratio of the metal precursors due to different precursor decomposition rates.[10] Size control is further complicated by the strong influence of parameters such as Ar gas flow rate, heating rate, and temperature.[ [19] of the initial molar ratio of the metal precursors, but the particle size is constrained to a limited range of approximately 3-5 nm. Neither of these two synthesis routes is easily amenable to coating the nanoparticles with a protective shell of a thermally stable material (e.g., silica or titania) that could prevent particle shape and size changes during postsynthesis annealing for obtaining the L1 0 phase. Although a Fe 3 O 4 shell can be formed [15] by the thermal decomposition method, the shell degrades at temperatures less than 600°C and destroys the nanoparticle structure and shape. Techniques to coat a silica shell on clusters of magnetic nanoparticles that have been presynthesized using a separate processing method have been demonstrated, [21][22][23] but these techniques are not well suited for creating organized assemblies of well-separated nanoparticles for recording media applications. Here, we report the room-temperature synthesis of monodisperse FePt nanoparticles with tunable particle size over an extended range of 4-20 nm and excellent compositional control using microemulsions. We further use the same microemulsion to enhance the nanoparticle thermal stability by forming a 2 nm thick silica shell, which is also used for functionalization with organosilanes. Organized assemblies formed from the molecul...

Molecularly Protected Bismuth Telluride Nanoparticles: Microemulsion Synthesis and Thermoelectric Transport Properties

“…40-50 nm; see Supporting Information) suggesting that the nanoparticle size is determined by the water nanodroplet size, which in turn is manipulated by controlling the water/surfactant ratio. [19,22,23] X-ray diffraction patterns of as-prepared nanoparticles confirm the presence of trigonal Bi 2 Te 3 ( Fig. 2a).…”

Recent Progress on Silica Coating of Nanoparticles and Related Nanomaterials

“…[48][49][50] Similar approaches were designed for coating magnetic nanoparticles. Ramanath and co-workers [51] employed a molecular capping with dimethoxyorganosilanes for silica coating FePt nanoalloys of tunable size, composition, and high thermal stability against clustering and coalescence, additionally facilitating the formation of ordered nanoparticle assemblies. Tan and co-workers [52] successfully coated Fe 3 O 4 and maghemite (g-Fe 2 O 3 ) magnetic nanoparticles with silica by W/O microemulsion method, through analysis of the effect of different nonionic surfactants on the particle size.…”

Size effect on order-disorder transition kinetics of FePt nanoparticles

“…[17][18][19] Various methods including solution phase decomposition/reduction and salt co-reduction have been continuously perfected, yielding increasingly better size, composition and dispersity control. [20][21][22][23][24] Yet unfortunately, as-synthesized FePt particles are in the disordered A1 form, which is magnetically soft. High temperature annealing is thus applied to induce order-disorder transition in the nanoparticles so the desired L1 0 structure can be obtained.…”