Synthesis and study of L10 magnetic nanostructures
Date
2018
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Chemically ordered FePt and CoPt nanoparticles, with the L10 structure have attracted great scientific and technological interest during the last few decades because of their potential applications in a variety of fields such as permanent magnets, magnetic storage media, catalysis, and biomedicine. The high magnetocrystalline anisotropy (Ku ~5-7 Merg/cc) of the L10 phase, allows for ferromagnetism below 5 nm making them ideal candidates for ultra-high density magnetic storage applications. Furthermore, FePt and CoPt in the ordered L10 crystal structure serve as highly active and durable electrocatalysts for various reactions such as oxygen reduction reaction (ORR) in the polymer electrolyte membrane fuel cells. The L10 structure has been shown to be more catalytically active and chemically durable compared to the disordered (fcc) phase, making it a promising candidate for Pt-based catalysts in fuel cells. ☐ In this thesis we studied the effect of bismuth additive on the magnetic and structural properties of FePt and CoPt nanoparticles and nanostructures produced by chemical synthesis. The results showed for the first time that highly ordered L10 FePt and partially ordered CoPt can be produced at liquid synthesis temperatures, below 400 degrees C, without using a noble metal additive. The mechanism of bismuth enhancing the fcc to L10 phase transition was extensively studied by electron microscopy; the results suggest that surface segregation of bismuth leads to enhanced mobility of the Fe, Co, and Pt atoms allowing for the lower temperature phase transition (fcc to L10), and increased ordering. ☐ The intrinsic properties (saturation magnetization, magnetocrystalline anisotropy and Curie temperature) of the nanoalloys were studied using high field magnetometry, and thermomagnetic measurements. The anomalous Curie temperature behavior observed in bulk and thin film alloys of FePt and CoPt was explored in nanoparticles. The results showed that for FePt nanoparticles the Curie temperature increased going from the fcc to L10 phase, and the opposite behavior was observed in the CoPt nanoparticles. The experimental data was modeled using an atomistic exchange based simulation to determine whether rearrangement of Fe/Co and Pt atoms during the phase transformation is responsible for the changes in Curie temperature. ☐ The highest coercivities obtained in these samples are 16.9 kOe for FePt and 1.7 kOe for CoPt, 15.8 kOe after annealing. The values are shown to contradict coherent rotation based on Stoner and Wohlfarth. To study the origin in coercivity and hysteresis behavior, magnetic viscosity and remanence curves were used to give insight into the magnetic interactions within the FePt and CoPt nanostructures. Single particle atomistic simulations are used to model the hysteresis behavior of the nanostructures based on experimentally measured parameters, and compared to the experimentally determined values of coercivity. The experimental and simulated results suggest a non-coherent rotation behavior, with mixtures of L10 and fcc phases that result in reduced coercivity. These results are compared to models for exchange interactions in nanocrystalline magnets and nanocomposite magnets to explain the experimentally measured coercivities.