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 (∼ 6.6 MJ m) of Fe–Pt alloys makes their nanostructured forms attractive for potential use as nanoscale magnetic bits in ultrahigh-density information storage devices or as fillers to create new types of magnetocomposites. The high magnetic anisotropy and coercivity arise from the chemically ordered face-centered-tetragonal (fct) L10 phase, which is sensitive to both chemical composition and the size of the nanoparticles. Control over particle size and dispersity 13] is essential to form ordered arrays of nanoparticles, and preserve these features against agglomeration during high-temperature annealing (e.g., 550 °C) treatments used to obtain the L10 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 or chemical compositional control 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. But, the chemical composition of the nanoparticle is different from the initial molar ratio of the metal precursors due to different precursor decomposition rates. Size control is further complicated by the strong influence of parameters such as Ar gas flow rate, heating rate, and temperature. For example, a Fe(CO)5/Pt(acac)2 ratio of two yields Fe-deficient (Fe44Pt56) 9 nm nanoparticles or Fe-rich (Fe53Pt47) 6 nm nanoparticles, depending on the heating rate. In contrast, coreduction of FeCl2 and Pt(acac)2 results in nanoparticles with chemical compositions within 2 % 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 L10 phase. Although a Fe3O4 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, 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 molecularly capped core/shell particles exhibit high coercivity Hc ∼ 850 mT due to L10 ordering in the particles upon annealing to 650 °C, without any observable changes to the overall nanoparticle size and shape. Synthesis and integration of high-coercivity nanomagnets of controlled shape, size, composition, surface chemistry, and stability, with molecularly engineered surfaces and matrices (e.g., polymers and biomaterials) open up new possibilities for realizing novel flexible memory devices and magnetocomposites. K2PtCl4 and FeCl3 were reduced simultaneously with N2H4, in reverse micellar microemulsions composed of nanoscopic water droplets stabilized by nonionic surfactant molecules, brij 56 or brij 52, in iso-octane. Scheme 1 illustrates the partiC O M M U N IC A IO N S