Self-Assembled FePt Nanocrystals with Large Coercivity:  Reduction of the fcc-to-L1₀ Ordering Temperature

Abstract

Herein we report the synthesis and properties of Fe(55)Pt(45) nanoparticles, both monodisperse and self-assembled into hexagonal close-packed and cubic arrays of 4.0 +/- 0.2 nm size in an L1(0) structure, obtained by a modified polyol process. The new synthetic route improved the control over the particle composition, thereby reducing the temperature required to convert from face-centered cubic (fcc) to face-centered tetragonal (fct) phase by some 30-50 degrees C without additives. Annealing at 550 degrees C for 30 min converts the self-assembled nanoparticles into ferromagnetic nanocrystals with large coercivity, H(C) = 11.1 kOe. Reducing the fcc-to-fct (L1(0)) ordering temperature avoided particle coalescence and decreased the loss in particle positional order without compromising the magnetic properties, as is generally observed when additives are used.

Temperature dependence and magnetocrystalline anisotropy studies of self-assembled L10-Fe55Pt45 ferromagnetic nanocrystals

“…As shown in a previous article, 8 the high coercivity value of about 12 kOe was observed in a self-assembled Fe 55 Pt 45 nanoparticle in a cubic array. In this work, we report the temperature ͑blocking temperature͒ and the uniaxial magnetocrystalline anisotropy constant dependence of coercive field as a function of the annealing temperature for a chemically synthesized L1 0 -Fe 55 Pt 45 nanoparticle array.…”

“…8 Monodisperse Fe 55 Pt 45 nanoparticles were synthesized as follows: in a three-neck round-bottom flask, a mixture of Pt͑acac͒ 2 ͑0.4 mmol͒ and 1,2-hexadecanediol ͑1.2 mmol͒ in octylether ͑20 mL͒ was heated at 100°C. Then, oleic acid ͑0.4 mmol͒, oleylamine ͑0.4 mmol͒, and isopropanolic hotsolution preheated at 70°C of Fe͑acac͒ 3 ͑0.49 mmol in Fe͒ were added to the mixture via syringe.…”

“…1, 3 Recently, there has been much attention placed on the chemical synthesis of self-assembled, monodispersed, and chemically ordered L1 0 ͑tetragonal phase͒ FePt nanoparticles for future ultrahigh-density magnetic storage. [4][5][6][7][8] In fact, recent advances in magnetic recording technology have indicated that, if self-assembled in a tightly packed, exchangedecoupled array with controlled magnetic easy axis direction, these FePt nanoparticles could support high-density magnetization reversal transitions and would be a candidate for future ultrahigh-density data storage media [8][9][10] with potentially one bit per particle corresponding to storage densities of 20 Tbit/ in. 2 The tetragonal phase of an FePt system is of particular interest because of the high magnetocrystalline anisotropy ͓K U ϳ͑6.6− 10͒ ϫ 10 7 erg/ cm 3 ͔ that should allow the use of smaller particles, but yet avoid thermal instabilities that give rise to superparamagnetic ͑SPM͒ behavior.…”

“…2,3,12 Although the structural and magnetic properties of FePt have been intensively studied as a function of composition, 13 there are a few number of works that have studied these nanoparticles when the FePt composition is set in Fe 55 Pt 45 , which is reported as expected to present the highest magnetic properties in the ordered L1 0 phase. 8,13 For bulk alloys, these structural and magnetic properties are known. However, due to the difficulty of stabilizing Fe 55 Pt 45 phase as nanoparticle material, the small size and the larger surface area of the particles affect these properties and, consequently, the ordering process during the L1 0 phase formation could be also affected.…”

Direct chemical synthesis of L10-FePtAu nanoparticles with high coercivity

“…However, high temperature annealing leads to complete decomposition of the surfactant on the surface of each NP and consequently produces an undesirable aggregation and sintering. [25][26][27][28] As a result, the NPs lose their solubility and most importantly their size and shape homogeneity. 29 Therefore, realizing the fcc-to-fct phase transformation without sintering of FePt NPs is one of the major challenges in the production of hard magnetic FePt NPs.…”