Magnetic and structural properties of chemically synthesized Fe(Co)-Pt(Ni, Bi) nanoparticles
Date
2021
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Publisher
University of Delaware
Abstract
This work is focused on the study of FePt and CoPt nanoparticles synthesized by a chemical route, and their ternary alloys made with small substitutions with Ni and Bi. Chemically ordered L10 FePt and CoPt nanoparticles, with face-centered tetragonal (fct) phase structure have attracted extensive attention for potential applications in ultrahigh-density magnetic recording media, magnetic energy storage, and electrocatalysis because of their large anisotropy constant (𝐾𝑢~5 − 7 Merg/cc). In the first part of the work, Fe-Pt-Ni nanoparticle were synthesized by the co-reduction of iron(iii) acetylacetonate (Fe(acac)3), platinum(ii) acetylacetonate (Pt(acac)2) and nickel(ii) acetylacetonate (Ni(acac)2) in a standard solution synthesis. Structural analysis showed that Ni replaces Pt in the lattice to form FePt1-xNix. For low Ni content, high chemical ordering was obtained after annealing at 700°C for at least 15 minutes, with a coercivity of 10.7 kOe, compared to 14.9 kOe for L1o FePt. At higher Ni content the L10 chemical ordering decreased, and this correlated with a decrease in coercivity. However it was found that when Bi was used in the synthesis, it improved the chemical ordering the L10 ordering to the point that the coercivity in the 𝑥 = 0.4 nanoparticles increased from 1.1 kOe to 2.8 kOe in one of the heat treated sample. Ni substitution led to an increased saturation magnetization and a decrease in magnetocrystalline anisotropy consistent with the decrease of coercivity. This study demonstrated the possibility of creating ternary L10 structure derived from FePt through Ni substitution of Pt and still retain the desired hard magnetic properties. The Ni and Bi substitutions were performed with CoPt, where in one part, Ni was used to form CoPtxNi1-x and in another part, Bi was used in the synthesis to attempt to obtain the L10 structure. However, it was found that Ni substitution does not work in the same way with CoPt, resulting in a significant loss of the hard magnetic properties of CoPt. ☐ In the second part of the thesis, FePt3 nanoparticles were synthesized using iodine as an intermediary in our reaction We found that iodine helped the formation of the L12 FePt3 phase in the as-synthesized particles, and that the L12 ordering was further improved by heat treatment at 700°C. L12 FePt3 is antiferromagnetic below 160K and, whereas the disordered state is ferromagnetic at room temperature. The degree of ordering of the L12 phase in the as-synthesized nanoparticles could be controlled by modifying the reaction parameters. The structural and magnetic properties, including antiferromagnetic and ferromagnetic phase transitions, of the various nanoparticles were studied. ☐ In the final part of this work, different stages of the fcc to L10 phase transformation of FePt nanoparticles were studied, along with their effect on the magnetic properties. Their properties were compared to those of L10 CoPt nanoaparticles. A ternary alloy of PtFeCo was synthesized and annealed, and its transformation to the L1o stage and the resulting changes in magnetic properties compared to those of CoPt and FePt. The dependence of coercivity, anisotropy and saturation magnetizations with temperature of samples of FePt, CoPt and PtFeCo at various stages of the L10 transformation were also investigated and compared with theoretical models.
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Keywords
CoPt, fct FePt, Fe-Pt-Ni alloys, L12 FePT3