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Title: In situ investigation of ordering phase transformations in FePt magnetic nanoparticles

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USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
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Journal Article: Publisher's Accepted Manuscript
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Journal Volume: 176; Journal Issue: C; Related Information: CHORUS Timestamp: 2018-01-11 18:19:37; Journal ID: ISSN 0304-3991
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Wittig, James E., Bentley, James, and Allard, Lawrence F. In situ investigation of ordering phase transformations in FePt magnetic nanoparticles. Netherlands: N. p., 2017. Web. doi:10.1016/j.ultramic.2016.11.025.
Wittig, James E., Bentley, James, & Allard, Lawrence F. In situ investigation of ordering phase transformations in FePt magnetic nanoparticles. Netherlands. doi:10.1016/j.ultramic.2016.11.025.
Wittig, James E., Bentley, James, and Allard, Lawrence F. 2017. "In situ investigation of ordering phase transformations in FePt magnetic nanoparticles". Netherlands. doi:10.1016/j.ultramic.2016.11.025.
title = {In situ investigation of ordering phase transformations in FePt magnetic nanoparticles},
author = {Wittig, James E. and Bentley, James and Allard, Lawrence F.},
abstractNote = {},
doi = {10.1016/j.ultramic.2016.11.025},
journal = {Ultramicroscopy},
number = C,
volume = 176,
place = {Netherlands},
year = 2017,
month = 5

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Publisher's Accepted Manuscript

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  • No abstract prepared.
  • The Fe{sub x}Pt{sub 100−x} nanoparticles (NPs) with different nominal atomic rations (30≤x≤80) were synthesized at 700 °C by the sol–gel method. The structure, morphology and magnetic properties of the samples were investigated. When the Fe content in the Fe–Pt alloy NPs was 30 at%, FePt{sub 3} NPs were successfully synthesized. With the increase in Fe content up to 50 at%, it was found that the superlattice reflections (0 0 1) and (1 1 0) appeared, which indicated the formation of the L1{sub 0}-FePt phase. Meanwhile, the FePt{sub 3} fraction was reduced. When the Fe content increased to 60 at%, single-phasemore » L1{sub 0}-FePt NPs were synthesized. The coercivity (Hc), saturation magnetization (Ms) and chemical order parameter S for Fe{sub 60}Pt{sub 40} NPs were as high as 10,200 Oe, 17.567 emu/g and 0.928, respectively. With the further increase of the Fe content to 80 at%, only Fe{sub 3}Pt phase existed and the Hc of the Fe{sub 3}Pt NPs decreased drastically to 360 Oe. - Graphical abstract: Fe{sub 3}Pt, FePt and FePt{sub 3} nanoparticles was obtained by sol–gel method. The effect of iron and platinum content on structural and magnetic properties of the FePt nanoparticles was investigated. Display Omitted - Highlights: • L1{sub 2}-FePt{sub 3}, L1{sub 0}-FePt and L1{sub 2}-Fe{sub 3}Pt NPs were synthesized by sol–gel method. • The chemical order parameter S affects the magnetic properties of the Fe–Pt alloy. • Structural and magnetic properties of the Fe–Pt alloy NPs were studied. • The synthetic route in this study will open up the possibilities of practical use.« less
  • First, second, and third nearest-neighbor mixing potentials for FePt alloys were calculated from first principles using the Connolly-Williams approach. Using the mixing potentials obtained in this manner, the dependency of equilibrium L1{sub 0} ordering on temperature was studied for bulk and for a spherical nanoparticle with a 3.5-nm diameter at equiatomic composition by use of Monte Carlo simulation and the analytical ring approximation. The calculated order-disorder temperature for bulk (1495-1514 K) was in relatively good agreement (4% error) with the experimental value (1572 K). For nanoparticles of finite size, the (long-range) order parameter changed continuously from unity to zero withmore » increasing temperature. Rather than a discontinuity indicative of a phase-transition we obtained an inflection point in the order as a function of temperature. This inflection point occurred at a temperature below the bulk phase-transition temperature and which decreased as the particle size decreased. Our calculations predict that 3.5-nm diameter particles in configurational equilibrium at 600 deg. C (a typical annealing temperature for promoting L1{sub 0} ordering) have an L1{sub 0} order parameter of 0.83 (compared to a maximum possible value equal to unity). According to our investigations, the experimental absence of a (relatively) high L1{sub 0} order in 3.5-nm diameter nanoparticles annealed at 600 deg. C or below is primarily a problem of kinetics rather than equilibrium.« less
  • Partial chemically ordered FePt nanoparticles were produced by annealing the nanoparticles in transit through the furnace from the source to the substrate. The equiaxed or polyhedral morpohology of FePt nanoparticles can be produced by tuning the temperature of nanoparticle-forming chamber. The equiaxed FePt particles were amorphous whereas the polyhedral particles showed good crystallinity. Both FePt particle assemblies were superparamagnetic without on-line heating of particles. With on-line annealing, the assembled films showed ferromagnetic behavior. The coercivity of the polyhedral FePt particle assembly was about 600 Oe, whereas that of equiaxed FePt particles was 450 Oe. The larger coercivity was attributed tomore » the increased particle size.« less
  • Arrays of FePt particles (diameter 7 nm) with mean interparticle distances of 60 nm are prepared by a micellar technique on Si substrates. The phase transition of these magnetic particles towards the chemically ordered L1{sub 0} phase is tracked for 350 kV He{sup +} ion irradiated samples and compared to a nonirradiated reference. Due to the large separation of the magnetically decoupled particles the array can be safely annealed without any agglomeration as usually observed for more densely packed colloidal FePt nanoparticles. The He{sup +} ion exposure yields a significant reduction of the ordering temperature by more than 100 K.