Evolution of Structural and Magnetic Properties and the Electronic Structure of Spinel FexCo3-xO4 Thin Films.

Evolution of structural, optical, and magnetic properties has been investigated for sol-gel-grown spinel Fe_x Co_3-x O₄ thin films as the Fe composition (x) increases from 0 to 2. The crystal structure of Fe_x Co_3-x O₄ is found to remain cubic with the lattice constant increasing with increasing x. Coexistence of two phases is observed by X-ray diffraction for 0.76 ≤ x ≤ 0.93, interpreted as due to normal spinel, dominant for x ≤ 0.55, and inverse spinel, dominant for x ≥ 1.22. Analysis on the measured optical absorption spectra by spectroscopic ellipsometry for the samples indicates the dominance of the normal spinel phase for low x in which Fe³⁺ ions mostly substitute octahedral Co³⁺ sites. X-ray photoelectron spectroscopy measurements revealed that both Fe²⁺ and Fe³⁺ ions exist with similar strength in the x = 0.93 sample. Vibrating sample magnetometry measurements revealed that the Fe_x Co_3-x O₄ films start exhibiting magnetic hysteresis behaviors for x ≥ 0.76 with the saturation magnetization increasing with increasing x. Conversion electron Mössbauer spectra measured on the x = 0.93 sample showed that Fe²⁺ ions prefer the octahedral sites, indicating the formation of the inverse spinel phase. The remarkable change of the lattice constant for x ≥ 0.76 is derivable from the site preference of the Fe²⁺ and Fe³⁺ ions. [ABSTRACT FROM AUTHOR]