Monodisperse face-centred tetragonal FePt nanoparticles with giant coercivity

Abstract

Monodisperse face-centred tetragonal (fct) FePt nanoparticles with high magnetic anisotropy and, therefore, high coercivity have been prepared via a new heat treatment route. The as-synthesized face-centred cubic FePt nanoparticles were mixed with salt powders and annealed at 700˚C. The salts were then removed from the particles by washing the samples in water. Monodisperse fct FePt particles were recovered with the particle size and shape being retained. Coercivity of the isolated particles up to 30 kOe at room temperature has been obtained.

Prospects for nanoparticle-based permanent magnets

“…Wet-chemical techniques are suitable for producing monodisperse and self-assembled nanoparticles of average sizes tunable from 3 to 15 nm, which are generally produced by the reduction or decomposition of metal salts in the presence of stabilizing or capping agents [9,10,[20][21][22][23]. For example, the reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of oleic acid and oleylamine stabilizers results in monodisperse FePt nanoparticles [9], which can be deposited as self-assembled nanoparticle arrays on suitable substrates using spin or dip-coating.…”

“…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].…”

“…It is worth noting that these nanoparticles are often exchange-coupled to each other, which leads to a reduction in H c through the magnetic reversal by the propagation of interaction-domain walls. However, H c can be further improved by producing exchange-decoupled nanoparticles, which are expected to show Stoner-Wohlfarth-type behavior with H c scaling directly with their magnetic anisotropies according to 2K 1 /M s [5,6,16,22,29]. For example, cluster-deposited FePt nanoparticles dispersed in a non-magnetic carbon matrix exhibited a high H c = 29 kOe at room temperature, as shown in Figure 3b, and H c increases further to 40 kOe at 4.2 K [29,32].…”

“…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].…”

Rapid thermal annealing of FePt nanoparticles

“…For example, the salt-matrix annealing is an effective method to produce monodisperse L1 0 structured FePt nanoparticles by using salts as the separating media. [9][10][11][12][13] Coating of high-melting-point materials on FePt particles can also be a good technique to prevent agglomeration and sintering. 14,15 Controlling annealing time is another option to reduce the sintering by limiting interdiffusion between nanoparticles.…”