Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 151

Another type of magnetism, called ferrimagnetism, more common in ionic‐bonded ceramics, refers to a type of ferromagnetism in which the magnetic moment of ions at one type of atomic sites is partly cancelled by antiparallel interactions with ions of another site. However, there remains a net magnetic moment of the material in the absence of a magnetic field. Ferrimagnetic ceramics have a lower saturation magnetization than ferromagnetic metals but their electrically insulating properties provide an advantage in some engineering applications where a low electrical conductivity is required.

The ferrimagnetic iron oxides Fe₃O₄, commonly referred to as magnetite, and ferric oxide, γ‐Fe₂O₃, referred to as maghemite, are the most widely studied nanoparticles for hyperthermia treatment of tumors because of their favorable biocompatibility. Magnetite has a crystal structure that belongs to a class of compounds called spinels. The formula of normal spinels has the general formula AB₂O₄ (or AO∙B₂O₃), where A²⁺ and B³⁺ are divalent and trivalent ions, respectively. These normal spinels have a characteristic crystal structure based on a close packed face‐centered cubic (FCC) lattice of O²⁻ ions, with the A²⁺ ions and B³⁺ ions occupying tetrahedral and octahedral interstitial sites within the lattice. Magnetite has an inverse spinel structure B(AB)O₄ in which the occupancy of the interstitial sites is slightly different. One‐half of the B³⁺ ions in magnetite occupy tetrahedral sites (called a sites) whereas the A²⁺ ions and remaining half of B³⁺ ions occupy octahedral sites (called b sites) (Figure 4.20a). In magnetite, the O²⁻ ions are nonmagnetic due to all their electrons occupying filled shells. Consequently, we need to consider only the interactions between one‐half of the Fe³⁺ ions (with five unpaired electrons in their 3d shell) on the a sites and the Fe²⁺ ions (four unpaired electrons in their 3d shell) and the other half of the Fe³⁺ ions on the b sites (Figure 4.20b). The antiparallel a–b interaction between the ions in the a and b sites dominates and, thus, the saturation magnetization per formula unit (Fe₃O₄) is 4 μ_B.

Figure 4.20 (a) Part of Fe₃O₄ crystal structure showing the tetrahedral a sites and octahedral b sites. (b) Illustration showing the arrangement of the electron magnetic moments of the Fe ions at the a and b sites in the Fe₃O₄ crystal structure.

Maghemite (γ‐Fe₂O₃) has the same crystal structure as magnetite but it is often considered a ferrous ion (Fe²⁺) deficient magnetite because there are no Fe²⁺ ions in the crystal structure when compared to magnetite. Although there is no clear agreement about the distribution of the cations in the maghemite crystal structure, it is often stated that the Fe³⁺ ions occupy both the tetrahedral a sites and octahedral b sites, with five Fe³⁺ ions and one vacancy in the b sites for every three Fe³⁺ ions in the a sites. This can be expressed by the formula Fe[Fe_5/3◻_1/3]O₄ where the symbol ◻ represents a vacant site.