1 Introduction to Magnetic Oxides

Oxides are ubiquitous. The Earth’s crust and mantle are largely made up of compounds of metal cations and oxygen anions. Looking at the composition of the crust in Figure 1.1, we see that oxygen is the most abundant element and the most common metals are aluminum and silicon. Most rocks are therefore aluminosilicates. The next most abundant element, and the only transition metal other than titanium to feature among the top ten, which account for over 99% of the crust, is iron (Table 1.1). Remarkably, the same electronic configuration, 2p6, is shared by five of the top ten ions, which account for 92% of the atoms in the crust. Usually, only iron, with its two common charge configurations, Fe2+ (3d6) and Fe3+ (3d5), forms ions with a partially filled shell containing electrons of unpaired spin that exhibit a net magnetic moment. At 2.1 at. % (5.7wt.%), iron is 40 times as abundant as all the other magnetic elements put together; the runners up – manganese, nickel, and cobalt – trail far behind. For over 20 centuries, up until about 1740 [1], the only useful permanent magnets known to man were lodestones. These prized natural magnetic rocks were largely composed of impuremagnetite, the black spinel-structure oxide Fe3O4 with a ferrimagnetic structure that had been magnetized by a fortuitous lightning strike [2]. The other common rock-forming iron oxide is hematite, the reddish corundum-structure sesquioxide αFe2O3. Hematite is also magnetically ordered, in a canted antiferromagnetic structure, but its magnetization is about 200 times weaker than that of magnetite. The weak remanent magnetism imparted to rocks as they cooled in the Earth’s magnetic field has allowed us to read the record of fluctuations of the magnitude and direction of the field at the Earth’s surface. The remanence is largely due to segregated nano crystallites of titanomagnetite in the rock [3]. The Earth’s field is a precious shield that has protected us from the solar wind and allowed life to develop on our planet over the past 3.5 billion years. We learn from the magnetic record that it has reversed numerous times on a geological timescale. The tectonic movements of the plates were thereby pieced together, leading to the first unified theory of Earth sciences. It was actually a quest to understand the magnetism of rocks and baked clay that motivated Louis Néel to formulate the molecular field theory of ferrimagnetism [4], completing the theory of antiferromagnetism that