Synthesis of monodisperse iron-platinum (FePt) nanoparticles by reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of oleic acid and oleyl amine stabilizers is reported. The FePt particle composition is readily controlled, and the size is tunable from 3- to 10-nanometer diameter with a standard deviation of less than 5%. These nanoparticles self-assemble into three-dimensional superlattices. Thermal annealing converts the internal particle structure from a chemically disordered face-centered cubic phase to the chemically ordered face-centered tetragonal phase and transforms the nanoparticle superlattices into ferromagnetic nanocrystal assemblies. These assemblies are chemically and mechanically robust and can support high-density magnetization reversal transitions.

/pdf/monodisperse-fept-nanoparticles-and-ferromagnetic-fept-12j5xql32h.pdf

Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices

Hard-Magnet L10-CoPt Nanoparticles Advance Fuel Cell Catalysis

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling 'real' materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily on average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. This work combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure-property relationships at the fundamental level.

/pdf/deciphering-chemical-order-disorder-and-material-properties-1suwzujzlv.pdf

Deciphering chemical order/disorder and material properties at the single-atom level

Rapid point-of-care (POC) diagnostics that enable specific cellular and molecular detection are currently being developed while some have already become clinical reality. These diagnostics are often based on portable, handheld instruments and reagent-containing test kits. Overall, the development has largely been driven by technological advances, medical needs and cost-saving initiatives. For example, POC systems allow care providers to obtain test results quicker1, which in turn enables immediate clinical management decisions, elimination of costly delays to result in better care. The introduction of POC systems into primary and home care will ultimately preempt unnecessary hospitalization, improve inefficiencies associated with expensive hospital-based medical care and reduce dependence on large, centralized clinics for routine diagnosis.2, 3 POC technologies are also expected to have major impacts in resource-limited settings and low/middle income countries where access to healthcare is often limited.4

/pdf/recent-developments-in-magnetic-diagnostic-systems-1juq8z5o3u.pdf

Recent Developments in Magnetic Diagnostic Systems.

High temperature solution phase decomposition of Fe(CO)/sub 5/ and reduction of Pt(acac)/sub 2/ in the presence of stabilizers, oleic acid and oleyl amine, are employed to produce 4 nm diameter FePt nanoparticles. The Fe and Pt composition of the nanoparticle materials can be tuned by adjusting the molar ratio of Fe(CO)/sub 5/ to Pt(acac)/sub 2/, and the compositions ranging from Fe/sub 30/Pt/sub 70/ to Fe/sub 80/Pt/sub 20/ are obtained. The nanoparticle materials are easily dispersed into alkane solvent, facilitating their self-organization into nanoparticle superlattices. As synthesized FePt nanoparticles possess a disordered fcc structure and show superparamagnetic behavior. Thermal annealing induces a change of internal particle structure and thus of the magnetic properties of the particles. Composition dependent structure analysis shows that an annealed FePt nanoparticle assembly with a composition around Fe/sub 55/Pt/sub 45/ will lead to the highly ordered fct phase. This Fe/sub 55/Pt/sub 45/ nanoparticle assembly yields high coercivity, and will be a candidate for future ultra-high density magnetic recording media applications.

Compositionally controlled FePt nanoparticle materials

Grain growth and ordering kinetics have been studied in a 10‐nm‐thick CoPt alloy film of equiatomic composition annealed in the temperature range 550–700 °C by quantifying the grain size, grain size distribution, ordered fraction and ordered domain size. The mean grain size of the as‐deposited films is 5 nm and the film is fully face‐centered cubic. Upon annealing in the temperature range 550–600 °C, the mean grain size reaches a stagnation limit of 27 nm and the grain size distribution is lognormal. Grain growth resumes beyond 600 °C and the mean grain size reaches as high as 55 nm at 700 °C. Ordering occurs by nucleation and growth of L10 ordered domains, with a mean size of 3 nm at 550 °C and 19 nm at 700 °C. The ordered fraction shows a dramatic increase from 1% to ∼28% between the two extremes of annealing temperature. The increase in the coercivity of the annealed films follows the increase in the ordered fraction more closely than the increase in grain size. The shape of the M–H loop shows evidence...

Grain growth and ordering kinetics in CoPt thin films

The microstructure of sputtered 10-nm thin films of equiatomic binary alloys of CoPt and FePt was characterized using transmission electron microscopy (TEM). Grain growth kinetics was examined using manual and digital analysis of bright-field TEM images and was seen to take two stages during annealing in these films. A rapid growth stage concurrent with the formation of a [111] fiber texture was observed to occur within the first 5–10 min of annealing, followed by a much slower growth stage after the fiber texturing was well advanced. Differences in grain growth rate and ultimate grain size were also observed to depend on heating rate.

Grain growth in ultrathin films of CoPt and FePt

An EFTEM and conical dark field investigation of co-sputtered CoPt+Yttria stabilized zirconia thin films

This study relates the microstructure of equiatomic binary alloys of CoPt and FePt with their room-temperature hysteretic magnetic properties, particularly their high coercivity. A transformation from an atomically disordered, face-centered-cubic structure to the Li0 ordered structure occurred during post-deposition annealing and was characterized using digital analysis of dark-field transmission electron microscopy (TEM) images. The transformation was observed to follow first-order nucleation and growth kinetics, and the ordered volume fraction transformed was quantified at numerous points during the transformation. The ordered volume fraction was then compared to the magnetic coercivity data obtained from a superconducting quantum interference device (SQUID) magnetometer. Although the relationship most commonly described in the literature is that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in this study was found to correspond to the fully ordered state. Furthermore, in samples that were less than fully ordered, a direct relationship between ordered volume fraction and coercivity was observed. The proposed mechanism for the high coercivity in these films is an increasing density of magnetic domain wall pinning sites concurrent with an increasing fraction of ordered phase.

A Study On High Coercivity And Lio Ordered Phase In Copt And Fept Thin Films

The high magnetic anisotropy and high coercivity of equiatomic CoPt thin films make them attractive as potential materials for magnetic recording applications. Magnetic coercivity (Hc) over 10 kOe has been measured in films in which the as-deposited fee phase has been partially transformed to the atomically ordered Ll0 phase. Very high Hc has been related to high volume fraction and small size of the Ll0 precipitates. A better understanding of the Ll0 phase formation and quantification of volume fraction is critical to optimizing the magnetic properties of this material. As we have previously reported, an increase in Hc was observed with an increase in Ll0 volume fraction in 10 nm thick, equiatomic CoPt films. In our current investigation we have observed that, at anneal temperatures far from the order/disorder transition temperature, e.g. at T = 0.6 Tc, numerous, very fine Ll0 precipitates are seen, some of which cluster at the parent phase grain boundaries. At T = 0.85 Tc, very few, larger Ll0 regions were seen. As precipitates of the ordered phase grow to impingement, antiphase boundaries (APB) are formed. This is consistent with a heterogeneous nucleation and growth mechanism for the formation of the Ll0 phase throughout the temperature range studies.

Llo Phase Formation in CoPt Thin Films

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