Abstract
We present the results of a study related to phase transformation of chemically synthesized FePt nanoparticles under high pressure from face-centered cubic into face-centered tetragonal structure. As-synthesized nanoparticles are around 4.5 nm and show superparamagnetic behavior at 300 K. After annealing under 60 bar pressure of hydrogen at 400 ∘ role=presentation> ∘ ∘ ^{\circ} C for 2 h, nanoparticles exhibit strong ferromagnetic behavior with 5391 Oe coercivity. Results show that high-pressure annealing lowers the decomposition temperature of the surfactants surrounding nanoparticles and partially hinders agglomeration arising from heat treatment. The promising ferromagnetic properties of the FePt nanoparticles after annealing under high pressure make them suitable for ultrahigh-density memory devices.
Highlights
Equiatomic FePt nanostructures with face-centered tetragonal structure are very promising candidates for high-density magnetic storage devices beyond 1 Tbit/in 2 ( Ku = 7 × 106 J/m3) . 1,2 Owing to their high magnetocrystalline anisotropy, the critical size at which FePt nanoparticles (NPs) become superparamagnetic at room temperature is as low as 3 nm. 3 In the fct structure, the Fe and Pt atoms form alternating layers along the c-axis and the alloy is ferromagnetic as a result of the hybridization between the 3d orbital of the Fe atoms and the 5d orbital of their Pt neighbors
Heat treatment under high hydrogen pressure is expected to decrease the phase transition temperature of FePt NPs from disordered fcc into ordered fct, as previously observed for bulk FePt films by Lai et al 15 They reported that hydrogen atoms in the FePt structure promote the diffusion of Fe and Pt atoms by occupying the interstitial sites of FePt and inducing a local strain or an agitation that accelerates the ordering of FePt and decreases the phase transition temperature
Here we investigate the effect of heat treatment of chemically synthesized FePt NPs under high hydrogen pressure on the phase transition and sintering behavior of the NPs
Summary
1,2 Owing to their high magnetocrystalline anisotropy, the critical size at which FePt nanoparticles (NPs) become superparamagnetic at room temperature is as low as 3 nm. Here we investigate the effect of heat treatment of chemically synthesized FePt NPs under high hydrogen pressure on the phase transition and sintering behavior of the NPs. For this purpose, chemically synthesized FePt NPs were drop-casted on Si(100) substrates and annealed under 60 bar of H 2 atmosphere, at 400 ◦ C for 2 h.
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