Evolution of structural and magnetic properties and the electronic structure of spinel Fe/sub x/Co/sub 3-x/O/sub 4/ thin films

Evolution of structural, optical, and magnetic properties has been investigated for sol-gel-grown spinel Fe/sub x/Co/sub 3-x/O/sub 4/ thin films as the Fe composition (x) increases from 0 to 2. The crystal structure of Fe/sub x/Co/sub 3-x/O/sub 4/ 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/spl les/x/spl les/0.93, interpreted as due to normal spinel, dominant for x/spl les/0.55, and inverse spinel, dominant for x/spl ges/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 Fe/sup 3+/ ions mostly substitute octahedral Co/sup 3+/ sites. X-ray photoelectron spectroscopy measurements revealed that both Fe/sup 2+/ and Fe/sup 3+/ ions exist with similar strength in the x=0.93 sample. Vibrating sample magnetometry measurements revealed that the Fe/sub x/Co/sub 3-x/O/sub 4/ films start exhibiting magnetic hysteresis behaviors for x/spl ges/0.76 with the saturation magnetization increasing with increasing x. Conversion electron Mo/spl uml/ssbauer spectra measured on the x=0.93 sample showed that Fe/sup 2+/ ions prefer the octahedral sites, indicating the formation of the inverse spinel phase. The remarkable change of the lattice constant for x/spl ges/0.76 is derivable from the site preference of the Fe/sup 2+/ and Fe/sup 3+/ ions.

I. Introduction

Transition-Metal oxides with spinel structure have received a great deal of attention over decades due to their interesting structural, electronic, and magnetic properties, useful for various technological applications. CoO is known as a normal-spinel compound exhibiting antiferromagnetism below 40 K [1]. It has been found that the octahedral Co ions in the compound have a configuration of low-spin state with zero magnetic moment, while the tetrahedral Co ions have a configuration of high-spin state [2].