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Figure 18. Molecular structure of Fe9O3(OC2H5)21(C2H5OH)

Figure 20. XRD patterns of magnetite (a) and hematite (b) iron oxide films deposited on silicon. Reprinted with permission from [428], S. Mathur, et al. Chem. Vap. Deposition 8, 277 (2002). © 2002, Wiley-VCH.

generally by an intramolecular process, ferf-butyl alcohol, isobutene, and dihydrogen as the by-products [143]. This pathway was first proposed by Bradley and Maxfactor in their pioneering work on the thermolysis of Zr-ferf-butoxide [443]. In other studies [227], it has been shown that isobutene results from fi-H elimination of adsorbed ferf-butoxy groups, whereas ferf-butyl alcohol is formed by reaction of a ferf-butoxide fragment with an adsorbed hydroxy group. All of these compounds are volatile and can easily be removed from the deposition zone to obtain oxides with minimum carbon contamination. The carbon content in the CVD deposits was shown to be low (<0.2 at.%), which exhibits a clean ligand stripping in the thermolysis of [Fe(O(Bu)3]2. SEM analysis of the CVD films reveals characteristic morphologies for different iron oxide phases (Fig. 21). The hematite films display nanocrystalline spindle-like grains, whereas densely packed faceted crystallites were observed for the magnetite layers.

The nanostructured nature of the films is reflected in their magnetic behavior. The hysteresis loop of the Fe3O4 film shown in Figure 22 can be considered as the superimposition of a paramagnetic line a and a ferrimagnetic loop b. The weak magnetization when compared with polycrystalline Fe3O4 probably results from the presence of a superpara-magnetic phase. If a sample consists of small particles, its total magnetization decreases with particle size. As the size of the magnetic particle approaches the critical size required for a single-domain behavior, it becomes difficult to switch the magnetization from one direction to another. In the case of nanostructured materials (<50 nm), the total magnetization decreases with decreasing particle size, owing to the increased dispersion in the exchange integral and finally reaches the superparamagnetic state, where each particle acts as a spin with suppressed exchange interaction between the particles. In the case of magnetite films, the mean particle size (ca. 10-12 nm) is smaller than the critical (50-80 nm) size reported in the literature for single-domain behavior [444]. The higher coercivity of the Fe3O4 film (4000 Oe) when compared with coarse-grained Fe3O4 material (ca. 400 Oe) provides additional evidence for superparamagnetic behavior. The advantages of having molecular control over the growth process was also demonstrated by the deposition of different iron oxide phases on the silicon cantilever of an atomic force micrscope with a conformal coverage and homogeneous morphology (Fig. 23).

[Fe(O'Bu)3]2, when used in a sol-gel process, produced hematite nanocrystals with a regular size and narrow size

Figure 21. SEM images of iron oxide phases. (a) Hematite. (b) Magnetite.

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