Magnetic oxide thin films can be prepared by physical or chemical methods. In physical deposition methods, a source of material is separated by a distance d from the substrate, which is often heated in the range 400–1000 ◦C to facilitate growth. Some variants are indicated in Figure 1.5. At low pressure, the atomic species from the source arrive at the substrate without collision, but at higher pressure, they are thermalized by collision with the gas atoms in the chamber. At room temperature, λ=6/P, where λ is the mean free path of the atom in millimeters
and P is the pressure in pascals. Oxide thin films are used as tunnel barriers, and as functional elements in thin film stacks (ferromagnetic, antiferromagnetic, ferroelectric oxides). The methods are now described in more detail. In situ measurement of the thicknesses of the thin films may be achieved using optical reflectometry or a quartz crystal monitor.