Electronic structure studies of the spinel CoFe₂O₄ by X-ray photoelectron spectroscopy

Abstract

The spinel CoFe₂O₄ has been synthesized by combustion reaction technique. X-ray photoelectron spectroscopy shows that samples are near-stoichiometric, and that the specimen surface both in the powder and bulk sample is most typically represented by the formula (Co_0.4Fe_0.6)[Co_0.6Fe_1.4]O₄, where cations in parentheses occupy tetrahedral sites and those within square brackets in octahedral sites. The results demonstrate that most of the iron ions are trivalent, but some Fe²⁺ may be present in the powder sample. The Co 2p_3/2 peak in powder sample composed three peaks with relative intensity of 45%, 40% and 15%, attributes to Co²⁺ in octahedral sites, tetrahedral sites and Co³⁺ in octahedral sites. The O 1s spectrum of the bulk sample is composed of two peaks: the main lattice peak at 529.90 eV, and a component at 531.53 eV, which is believed to be intrinsic to the sample surface. However, the vanishing of the O 1s shoulder peak of the powder specimen shows significant signs of decomposition.

Introduction

The cobalt ferrite CoFe₂O₄ is a very important magnetic materials, which has covered a wide range of applications including electronic devices, ferrofluids, magnetic delivery microwave devices and high density information storage due to its wealth of magnetic and electronic properties, such as cubic magnetocrystalline anisotropy, high coercivity, moderate saturation magnetization, high Curie temperature T_C, photomagnetism, magnetostriction, high chemical stability, wear resistance and electrical insulation, etc. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12].

In structure, the spinel cobalt ferrite CoFe₂O₄ crystallizes in a face-centered cubic structure with a large unit cell containing eight formula units. There are two kinds of lattices for cation occupancy, A and B sites have tetrahedral and octahedral coordination, respectively. In the normal spinel structure Co is a divalent atom, occupying tetrahedral A sites, while Fe is a trivalent atom, sitting on the octahedral B sites. When A sites being Fe³⁺ ions, while B sites equally populated by Co²⁺ and Fe³⁺ ions, the spinel structure is referred to as the inverse kind [13], [14]. Commonly, the CoFe₂O₄ material is considered to be mostly an inverse spinel compound with most divalent Co ions occupying octahedral sites [7], [15], [16]. It means that the Co and Fe cations distribute at both sites. Since the Fe_A³⁺–Fe_B³⁺superexchange interaction is normally different from the Co_A²⁺–Fe_B³⁺ interaction, variation of the cation distribution over the A and B sites in the spinel leads to different magnetic properties of these oxides even though the chemical composition of the compound does not change [14].

The spinel cobalt ferrite CoFe₂O₄ materials have been synthesized by different methods [17], [18], [19]. In most cases, variation of the magnetic properties was obtained due to the different distribution of the Co and Fe cations over the A and B sites. Thus, investigations on the distribution of the Co and Fe cations over the A and B sites in the spinel cobalt ferrite CoFe₂O₄ are important [4], [7], [13], [14].

Quantitative X-ray photoelectron spectroscopy (XPS) gives not only the chemical composition, but also information on the chemical bonding and chemical state. This will help us to understand the distribution of the Co and Fe cations in the spinel cobalt ferrite CoFe₂O₄. Thus, in this paper, homogeneous CoFe₂O₄ powder and bulk were synthesized by combustion technique, and the XPS was taken to study on the valence of the elements and electronic configuration.