EXAFS and magnetic characterization of inverted ball milled zinc ferrite powders (abstract)

Abstract

Divalent zinc is commonly used as a replacement cation in ferrites, as it increases the material magnetization when substituted in moderate amounts. However, pure zinc ferrite (ZnFe2O4) is a normal spinel, as zinc has a strong preference for the tetrahedral (A) site in the spinel structure, with Fe occupying the octahedral (B) sites. Then antiferromagnetic B–B coupling dominates the magnetic behavior yielding a material with a Neel temperature near 10 K. It is typically difficult to produce inverted zinc ferrite due to this strong site preference, but recently we produced fine zinc ferrite powders having an inversion parameter of 0.20 using an aerogel process. Ball milling these powders increased the inversion parameter to 0.55, and the median particle size from 14 to 40 nm. To confirm the x-ray diffraction inversion parameters, extended x-ray absorption fine structure (EXAFS) measurements were taken on these powders and ZnFe2O4 standards for both the Fe Kα and Zn Kα absorption edges. Fourier transformed results were fingerprinted against theroretical radial distrubtion functions generated from multiple-scattering FEFF codes for scattering atoms located on the tetrahedral and octahedral sites. Qualitatively, the zinc ferrite standard appears as a normal spinel, while both the iron and zinc radial distributions for the ball milled sample showed large site occupation of both A and B sites. This sample shows superparamagnetic behavior at room temperature, with some ferrimagnetic particles exceeding the critical volume, as indicated by a 15 Oe coercive field. The saturation magnetization of the metastable ball milled powder exceeds 2100 G.