Hard magnetic FePt nanoparticles by salt-matrix annealing

Abstract

To transfer face-centered-cubic ͑fcc͒ FePt nanoparticles to the face-centered-tetragonal ͑fct͒ phase with high magnetic anisotropy, heat treatments are necessary. The heat treatments lead to agglomeration and sintering of the nanoparticles. To prevent the particles from sintering, salts as the separating media ͑matrix͒ have been used for annealing the nanoparticles in our experiments. The fcc nanoparticles produced by chemical synthesis were mixed with NaCl powders. The mixture was then annealed in forming gas ͑93% H 2 +7%Ar͒ in different conditions to complete the fcc to fct phase transition. After the annealing, the salt was washed out by water and monodisperse fct FePt nanoparticles were obtained. Detailed studies on the effect of the NaCl-to-FePt weight ratios ͑from 1:1 to 400:1͒ have been performed. It was found that a suitable NaCl-to-FePt ratio is the key to obtain monodisperse fct FePt nanoparticles. A higher NaCl-to-FePt ratio is needed for larger particles when the annealing conditions are the same. Increased annealing temperature and time should be accompanied by a higher NaCl-to-FePt ratio. Magnetic measurements show very high coercivity ͑up to 30 kOe͒ of the monodispersed fct nanoparticles by the salt-matrix annealing. 1 The chemically synthesized FePt nanoparticles, however, are of face-centered-cubic ͑fcc͒ phase without magnetic anisotropy. To transfer FePt nanoparticles from fcc phase to face-centered-tetragonal ͑fct͒ phase, heat treatments above 600°C are necessary, which undesirably lead to sintering of these nanoparticles.Since 2000, great efforts have been made to produce monodisperse fct FePt nanoparticles 2-8 driven by potential applications of the magnetically anisotropic nanoparticles in high-density recording media and high-performance nanocomposite magnets. Recently, we obtained monodisperse fct FePt nanoparticles with retained size and shape by using salts as the annealing separating media. 9 The salts can be completely removed after the annealing just by washing the samples in water. High coercivity up to 30 kOe of the fct particles has been obtained. In this paper we report detailed results in controlling the particle morphology and properties by adjusting the salt-to-FePt particle ratio. EXPERIMENTThe fcc FePt nanoparticles with size of 4, 8, and 15 nm were synthesized by chemical solution methods.1,10-13 Sodium chloride ͑NaCl͒ was selected as a separating media in this investigation due to its chemical stability and high solubility in water. NaCl was first ball milled for 24 h to reduce the particle size. The ball-milled NaCl powder was then dispersed in hexane and mixed with hexane dispersion of assynthesized fcc FePt nanoparticles. The mixture was stirred until all the solvent evaporates. Then the mixture was annealed in forming gas ͑93% H 2 +7%Ar͒ in different conditions to complete the fcc to fct transition. The annealed powders were washed in de-ionized water and centrifuged for several times to remove all the NaCl.9 Different NaCl-toFePt weight ratios from 1:1 to 400:1 were t...

Phase Transformation of FePt Nanoparticles

“…A new technique has been adopted in our group recently, based on salt-matrix annealing at temperatures up to 750 C, with which monodisperse fct FePt nanoparticles have been successfully produced [21], [22]. In this invited paper we present our recent research results in producing monodisperse ferromagnetic FePt nanoparticles.…”

Magnetic Properties of FePt Nanoparticles Annealed With NaCl

“…EXPERIMENTAL PROCEDURES Two samples of fct FePt nanoparticle powders with varying particle diameter, 4 and 15 nm, have been studied. Each sample was synthesized using the method of Sun et al [1] and annealed with fine ground ( 20-m particles) NaCl at 700 C for 4 h in forming gas 95%Ar 5%H [3], [4]. The FePt powder was set in an epoxy resin for containment and the measurements were performed at 273 K on an Oxford Instruments Vibrating Sample Magnetometer with a 90 kOe field range.…”

Observation of L1-like chemical ordering in a decahedral FePt nanoparticle by Cs-corrected high resolution transmission electron microscopy

“…This salt-matrix annealing technique can produce ordered L1 0 -FePt nanoparticles without aggregation. 8,9 The annealed nanoparticles were washed, dispersed in isopropanol, and fished out with a carbon coated transmission electron microscopy ͑TEM͒ grid. TEM experiments were carried out on a JEOL-2200FS TEM with a field emission gun operating at 200 kV.…”

“…This has prevented a meaningful experimental study on the phase transformation and magnetic properties of the nanoparticles as a function of the particle sizes. One of the exceptions is the coannealing method in a ground salt matrix 8,9 and strong size dependent magnetic properties have been observed as a result. 10 Size dependence in chemical ordering of FePt nanoparticles has been observed.…”