Tuning hard and soft magnetic FePt nanocomposites

• Chemical one-pot preparation of multi-phase magnetic soft and hard FePt nanoparticles.
• Easily tuning the phase composition by varying the concentration of the Fe reagent.
• Stable chemical attachment of the nanoparticles on Si substrate.
• Good agreement between the magnetic and structural data.

Nanocomposites formed by hard and soft magnetic phases are very promising for magnetic energy storage and biomedical applications. Highly challenging is the development of simple synthesis methods able to tune the phase composition and a thorough structural, morphologic and magnetic characterization in order to understand and optimize the interactions between hard and soft magnetic phases. Mainly depending on Fe:Pt atomic ratio, multi-phase or single phase FePt nanocomposites have been prepared by thermal treatment of core–shell FePt(Ag)@Fe3O4 nanoparticles at 750 °C for 1 h under flow of a Ar + 5% H2 gas mixture (H2 is necessary to reduce Fe2+ and Fe3+ ions of Fe3O4 to Fe0 atoms and the thermal treatment to form the crystalline soft and hard magnetic FePt phases). Performing Rietveld refinement of the XRD data as well as HR-TEM and electron diffraction analyses, the different phases have been singled out. Besides single phase hard L10 FePt and soft magnetic L12 Fe3Pt nanoparticles, two phase soft α-FePt and γ-FePt and hard and soft magnetic L10 FePt and L12 FePt3 nanocomposites have been formed and the structure and morphology correlated to their magnetic behavior.Moreover, for possible applications, it is important to form stable nanoparticle layers; as-prepared FePt(Ag)@Fe3O4 nanoparticles have been chemically attached on a Si substrate, thermally annealed and the morphology, structure and magnetic properties of the layered nanoparticle sample investigated.