Monodispersed and highly ordered L10 FePt nanoparticles prepared in the gas phase

Abstract

We report the direct preparation of monodispersed L10 phase FePt nanoparticles by controlled nucleation and growth using a gas phase aggregation source. These FePt nanoparticles became ordered during their growth in an argon gas flow. They are octahedron faceted with an average size of 5.8nm and a standard size distribution of 11%, as illustrated by transmission electron microscope. Magnetic measurements show that these FePt nanoparticles have coercivities of 8.25kOe at room temperature and 26.5kOe at 50K. This technique provides a novel approach for fabricating nanomaterials with controllable phase and shape in general.

Prospects for nanoparticle-based permanent magnets

“…Compared with chemical methods, the desired crystalline ordering in the case of rare-earth alloy [18,27] and FePt nanoparticles [30] can be obtained directly without subsequent heat treatment using high sputtering powers (≤ 120 W) or modifying the plasma conditions as revealed in the high-resolution TEM (HRTEM) image of the directly ordered YCo 5 nanoparticles (bottom inset of Figure 2). Assembly of cluster-deposited nanoparticles, for example FePt, has been achieved by depositing them onto Si substrates, which are pre-coated with multilayers of amphiphilic phospholipid molecules [33,34].…”

“…For example, the sputtering of atoms from the solid surface of a desired composition (such as YCo 5 or FePt) in a cooled inert-gas atmosphere (mixture of Ar and He)>10 −2 torr leads to successive collisions of sputtered atoms with the inert gas ions, and thus results in the formation of nanoparticles in the gas phase prior to deposition on substrates kept at room temperature [18,27,30,32]. This method produces monodisperse nanoparticles of size d ≤ 10 nm with an rms standard deviation σ/d ≈ 0.15, as shown in the TEM image of YCo 5 nanoparticles ( Figure 2) and corresponding particle size histogram (top inset of Figure 2) [18].…”

“…The cluster-deposition method using physical vapor deposition processes such as sputtering, thermal evaporation or laser ablation is based on the inert-gas-condensation principle and has been successful in producing both FePt and rare-earth alloy nanoparticles of smaller sizes (2-15 nm) with uniform size distribution and a high degree of atomic ordering [18,[26][27][28][29][30][31][32]. For example, the sputtering of atoms from the solid surface of a desired composition (such as YCo 5 or FePt) in a cooled inert-gas atmosphere (mixture of Ar and He)>10 −2 torr leads to successive collisions of sputtered atoms with the inert gas ions, and thus results in the formation of nanoparticles in the gas phase prior to deposition on substrates kept at room temperature [18,27,30,32].…”

“…Similarly, SmCo 5 and Nd 2 Fe 14 B nanoparticles of diameter ~10 nm produced by surfactantassisted ball milling exhibit high H c at room temperature in the range ~2.0-18.6 kOe and 1.2-4.0 kOe, respectively [37,38]. L1 0 -FePt nanoparticles have superior chemical stability and show high H c up to 20 kOe at room temperature for nanoparticles with sizes varying from 4 to 10 nm produced by both the cluster-deposition and wet-chemical techniques [9,22,29,30].…”

“…RCo 5 and L1 0 -FePt nanoparticles prepared by both wet-chemical and clusterdeposition methods are typically found to have M r /M s ≈ 0.5 [9,18,22,29,30], and this is one of the limiting factors affecting (BH) max for nanoparticle-based permanent magnets. M r /M s can be increased to some extent in the case of exchange-coupled nanocomposites [10].…”