Composition‐Mediated Order‐Disorder Transformation in FePt Nanoparticles

Abstract

While great progress has been made in the synthesis, property optimization, and applications of chemically synthesized functional nanoparticles (NPs), a signifi cant remaining challenge is to uniformly control the composition and phase of multicomponent NPs. Phase equilibria for NPs may be altered from bulk behavior due to the higher proportion of surface atoms and contributions from surface chemistry, such as ligand monolayers, resulting in altered thermodynamic and metastable phases for NPs. For Co [ 1,2 ] and Ag, [ 3 ] new phases have been observed in NPs and nanowires, respectively. There have also been several reports of non-equilibrium phases in semiconductor NPs. [4][5][6][7] For multicomponent metal NPs, metastable phases [8][9][10][11] are well known. In some instances, metastable phases frustrate efforts to obtain equilibrium phases. FePt NPs are such a system, wherein an undesired metastable alloy phase and multiple intermetallic phases have hindered the preparation of intermetallic, phase-pure, chemically synthesized FePt NPs with a monodisperse size distribution. [ 12 ] New insights about these challenges and how to overcome them will also be applicable to other kinds of multicomponent metal NPs.FePt NPs are of interest for bit-patterned media (BPM), a prospective architectural paradigm shift for the magnetic recording industry that overcomes the storage density limits of current recording media by using discrete magnetic bits rather than continuous media. [ 13 ] While reducing the bit size increases the storage density, it also increases bits' susceptibility toward thermal demagnetization, which would cause data loss. Superparamagnetism occurs when thermal energy ( k B T ) is suffi cient to overcome a NP's magnetocrystalline anisotropy energy ( KV ), where K is the magnetocrystalline anisotropy constant and V is the volume, which induces random reorientation of the magnetization direction. Magnetocrystalline anisotropy is the coupling of the moment to a favored crystal direction. Other magnetic anisotropies are known, such as those arising from the NP surface, anisotropic shapes, or strain, but for unstrained NPs of approximately spherical shape, magnetocrystalline anisotropy is usually the predominant magnetic anisotropy. The L1 0 intermetallic phase of FePt has an extremely large K of ≈ 10 7 erg cm −3 , [ 14 ] which makes is attractive for BPM, as well as for permanent magnet applications. [ 12 ] In addition to thermal stability, another requirement for BPM is for the variance in the fi eld required to change the magnetization direction of the bits (commonly referred to as the switching fi eld distribution) to be small, in order to avoid high rates of read and write errors. [ 15 ] A narrow switching fi eld distribution can be achieved with periodically spaced bits that have similar magnetic properties. [16][17][18] Monodisperse FePt NPs synthesized using wet-chemical approaches are stabilized by ligands that facilitate self-assembly into highly ordered structures. [ 8 ] Such methods produce disord...

Surface Segregation of Fe in Pt–Fe Alloy Nanoparticles: Its Precedence and Effect on the Ordered‐Phase Evolution during Thermal Annealing

“…From the results presented in Figure , it is clear that nanoparticles emerged with a surface‐segregated Fe‐rich shell upon thermal annealing (see Supporting Information, Figure S1, for additional statistics). However, this finding is in apparent contradiction to earlier reports that suggested a surface segregation of Pt . In general, the segregation phenomenon of bimetallic systems is based on the surface energies and atomic radii of two metals, with the element having a larger atomic radius and a lower surface energy segregating towards the free surface .…”

“…Pt–Fe system, in particular, has gathered a lot of interest in recent years not only on the account of its enhanced ORR electrocatalysis, but also because of its magnetic properties that are deployable in ultrahigh‐density information storage . The PtFe (L1 0 ) ordered phase exhibits a very high uniaxial magnetocrystalline anisotropy along the c direction of the crystal structure.…”

“…Hence, the thermal annealing procedure (typically at 800 °C, 1 h) is generally performed to transform disordered A1 phase into L1 0 or L1 2 phases. It is widely reported that the thermal annealing also promotes a preferential surface segregation of Pt in addition to the ordering process . The preference of Pt over Fe is ascribed to the larger atomic radius and low surface energies of Pt atoms as compared to Fe.…”

Effects of high magnetic field annealing on FePt nanoparticles with shape-anisotropy and element-distribution-anisotropy

“…The possible reasons are as follows: (1) the composition of each wet-chemical synthesized NPs may be not uniform, some Fe-rich NPs will transform to L 1 2 -Fe 3 Pt phase. 37 (2) If the Pt atoms diffuse from the in-side to the out-side of the NPs, the L 1 0 ordered transition is more favourable. However, the Pt atoms are rich in the out-side (corner) of the concave-cube FePt NPs.…”

Post-treatment Method for the Synthesis of Monodisperse Binary FePt-Fe3O4 Nanoparticles

“…This method has many advantages, including its facile synthesis, economical approach, and potential for mass production [6]. In general, the performance of FePt nanomaterials strongly depends on their composition [7–9] To obtain the optimal 1:1 ratio of Fe:Pt, the Fe precursor must be used in excess (at twice the amount of the Pt precursor) because Fe(CO) 5 exists in the vapor phase at the typical temperatures used for FePt synthesis and cannot be consumed completely [6]. Many researchers have studied the form that the excess iron takes and have tried to make full use of the iron precursor.…”