Abstract
To transfer face-centered-cubic (fcc) FePt nanoparticles to the face-centered-tetragonal (fct) 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 (matrix) 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% H2 +7%Ar) 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 400:1) 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 kOe) of the monodispersed fct nanoparticles by the salt-matrix annealing.
Original language | English (US) |
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Article number | 08E911 |
Journal | Journal of Applied Physics |
Volume | 99 |
Issue number | 8 |
DOIs | |
State | Published - 2006 |
Externally published | Yes |
Funding
This work is supported by the U.S. DoD/DARPA through ARO under Grant No. DAAD19-03-1-0038 and by the DoD/MURI program under Grant No. N00014-05-1-0497. FIG. 1. (Color online) XRD patterns of 4 nm FePt nanoparticles (a) as synthesized, and (b) annealed at 600 °C for 2 h with NaCl:FePt ratio of 40:1, (c) at 700 °C for 2 h at a ratio of 40:1, (d) at 700 °C for 4 h with the ratio of 100:1, and (e) at 700 °C for 8 h with the ratio of 400:1. FIG. 2. TEM images of 15 nm FePt nanoparticles (a) as synthesized and annealed in NaCl matrix at 700 °C for 2 h with NaCl-to-FePt ratios of (b) 4:1, (c) 40:1, and (d) 100:1. FIG. 3. TEM images of 4 nm FePt nanoparticles annealed in NaCl matrix at (a) 700 °C for 2 h with a NaCl: FePt ratio of 40:1, (b) 700 °C for 4 h with the ratio of 100:1, (c) 700 °C for 8 h with the ratio of 100:1, and (d) 700 °C for 8 h with the ratio of 400:1. FIG. 4. (Color online) Hysteresis loops of (a) 4 nm FePt nanoparticles annealed at 700 °C for 4 h at a ratio of 100:1, (b) 8 nm FePt nanoparticles annealed at 700 °C for 8 h at a ratio of 400:1, and (c) 15 nm FePt nanoparticles annealed at 700 °C for 2 h at a ratio of 100:1.
ASJC Scopus subject areas
- General Physics and Astronomy