Oxidation controlled phase composition of FeCo(Zr) nanoparticles in CaF2 matrix

• A selective oxidation of Fe in Fe–Co nanoparticles, NPs, is revealed.
• Oxidation controlled composition of Fe–Co NPs in CaF2 matrix is determined.
• “Metallic core–amorphous oxide shell”-like NPs are obtained at low oxygen pressure.
• “Crystalline core–amorphous shell” oxide structures appear at high oxygen pressure.
• A scheme of phase occurrence and structure of nanoparticles is proposed.

Results of the study of oxidised (FeCoZr)х(CaF2)100 − х films in a wide composition range (30 ≤ х ≤ 74 at.%) sintered at oxygen pressures PO = 4.3 and 9.8 mPa with X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), 57Fe Mössbauer spectroscopy (MS), and vibrating sample magnetometry (VSM) are reported.A “crystalline metallic core–amorphous oxide shell” structure of nanoparticles (NPs) in films oxidised at lower PO = 4.3 mPa is observed in TEM images. A combined use of XRD, MS and XAS techniques provides identification of NP phase composition evidencing α-FeCo(Zr) core of the variable content depending on x and a complex oxide shell including amorphous α-Fe3 +2O3 together with Co2 +1 − yFe2 +yO at low x (30–36 at.%). Full oxidation of NPs at higher PO = 9.8 mPa is indicated by TEM, XRD, MS and XAS patterns evidencing the formation of “crystalline Co2 +1 − yFe2 +yO core–amorphous α-Fe3 +2O3 shell” structure. Progressive oxidation of NPs with decreasing x at PO = 4.3 mPa as well as with PO increase is followed by variation of average centre shifts < δ > obtained from deconvoluted 57Fe Mössbauer spectra. Magnetometry measurements of films sintered at PO = 4.3 mPa reveal an effect of exchange bias confirming “ferromagnetic metallic core–antiferromagnetic oxide shell” structure of NPs.The results show that phase composition of oxide shells is governed both by concentration of oxygen available during synthesis and on the x. Schemes of progressive NP oxidation with identified phase composition as a function of PO and x values are proposed.

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