Abstract
We report on new metastable γ-FeNi nanoparticles produced by mechanical alloying of melt-spun ribbon using a high energy ball mill followed by a solution annealing treatment in the γ-phase region and water quenching in of the face-centered cubic γ-phase. In the Fe–Ni phase diagram there is a strong compositional dependence of the Curie temperature, Tc, on composition in the γ-phase. This work studies the stabilization of γ-phase nanostructures and the compositional tuning of Tc in Fe–Ni alloys which can have important ramifications on the self-regulated heating of magnetic nanoparticles in temperature ranges of interest for applications in polymer curing and cancer thermotherapies. To date we have achieved Curie temperatures as low as 120 °C by this method.
Highlights
Suppression of phase transformations in metastable nanostructures can be used to produce materials with properties that are not obtainable in equilibrium structures
Important recent examples of this can be found in the suppression of the nucleation of the stable ␥-phase in Co–Fe-based nanocomposite systems produced from the primary nanocrystallization of amorphous precursors at compositions where the binary Fe–Co phase diagram would predict that the ␣-phases and ␥-phases should coexist
We discuss the merits of the synthesis route on the compositional tuning of Tc which can have important ramifications on the self-regulated heating of magnetic nanoparticles6 in temperature ranges of interest for applications in polymer curingϳ100 ° C,7 cancer thermotherapiesϳ42 ° C, and the design of efficient magnetocaloric refrigerants.8 phases
Summary
Suppression of phase transformations in metastable nanostructures can be used to produce materials with properties that are not obtainable in equilibrium structures. Important recent examples of this can be found in the suppression of the nucleation of the stable ␥-phase in Co–Fe-based nanocomposite systems produced from the primary nanocrystallization of amorphous precursors at compositions where the binary Fe–Co phase diagram would predict that the ␣-phases and ␥-phases should coexist.. In Fe–Ni-based nanocomposite systems, a similar phenomenon is observed in Fe-rich alloys where the nucleation of the equilibrium ␣-phase is suppressed in favor of the metastable ␥-phase. This can have profound effects on technical magnetic properties because on the Fe-rich side of the Fe–Ni phase diagram there is a strong compositional dependence of the Curie temperature, Tc, on composition in the ␥-phase.. The heat generated from the constant grinding of the steel ballsabout 300 ° Cagainst the powder causes these powders crystallize and form a twophase mixture of 36.51% fcc FeNi3 and 63.49% bcc ␣-Fe, as predicted from the Fe–Ni binary phase diagram
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