1. Introduction

2. Structure and Properties of Magnetite

Magnetite, Fe₃O₄, an iron oxide, has an inverse spinel structure (Figure 1). The details of its structure were first established in 1915 by W.H. Bragg ^[33], who provided one of the first mineral structures determined using the X-ray diffraction method. Magnetite has a cubic unit cell with a lattice constant of a = 0.839 nm and is an iron oxide of mixed valences Fe³⁺ and Fe²⁺ within the stoichiometry Fe²⁺/Fe³⁺ = 0.5. The main feature of the spinel crystal lattice is the presence of two crystallographic positions of iron ions: Fe²⁺ and half of the Fe³⁺ ions occupying octahedral sites (surrounded by six oxygen O²⁻), and the other half of the Fe³⁺ ions occupying tetrahedral sites (surrounded by four oxygen O²⁻). Magnetite is frequently non-stoichiometric, resulting in a cation-deficient Fe³⁺ layer. Fe²⁺ could also be fully or partly replaced by other divalent ions (Mg²⁺, Co²⁺, Ni²⁺, etc.), which leads to changes in the lattice constant.

Figure 1. Visualization of the magnetite unit cell identified using octahedral Fe^2,5+ (dark grey), tetrahedral Fe²⁺ (light grey), and oxygen (red). The local site symmetries are shown by the octahedral and tetrahedral shapes around fully coordinated Fe sites within the unit cell. The different bond angles between the Fe sites lead to dominant antiferromagnetic coupling between the tetrahedral and octahedral sites, giving a bulk ferrimagnetic order (adapted from ^[34] with permission from Springer Nature).

The magnetic properties of magnetite are determined by its crystal structure. Namely, the ordering of magnetic spin moments of iron cations in the crystal lattice occurs through a special interaction between electrons of the 3d shell of neighboring cations. The ordering of magnetic spin moments leads to the overall magnetic ordering in magnetite. Magnetite possesses magnetocrystalline anisotropy, a special case of magnetic anisotropy that is related to spin-orbit coupling. Magnetocrystalline anisotropy depends on the magnetite crystal structure, shape, and chemical composition and plays a critical role in the magnetic ordering of magnetite. The unique magnetic and structural properties of magnetite, in comparison, for example, with ferrites, are determined also by the presence of so-called hopping electrons. Briefly, 3d electron transfer occurs in the Fe²⁺–Fe⁺³ octahedral cation pair, which could be described in short as Fe²⁺(3d⁶) ↔ Fe³⁺(3d⁵). The concentration of hopping electrons in magnetite is high in comparison with spinel ferrites, which have similar lattice structures, and thus makes a significant contribution to the formation of its magnetic properties. To conclude, magnetite at room temperature is ferrimagnetic with a Curie temperature of 580 °C (Table 1), which has a rather high saturation magnetization value of 92 A·m²/kg.

Table 1. Properties of magnetite.

This entry is adapted from the peer-reviewed paper 10.3390/ma15072601