Abstract
Antiferromagnetic (AF) nanostructures from Co3O4 were prepared by the nanocasting method from two-dimensional hexagonal (SBA-15) and three-dimensional cubic (KIT-6) silica templates and investigated using magnetometry experiments. The properties of this AF system are governed by core-shell behavior. The core behaves regularly AF, whereas the shell behaves as a two-dimensional diluted antiferromagnet in a field (DAFF) (Figure 9.1).
Magnetometry results on Co3O4 nanostructures of different sizes showed that below a critical structure size of 8 nm, core and shell decouple magnetically and yield independent magnetization signatures. The origin of the decoupling could be due to a crossover of the ratio of coupling energies, i.e. intra-shell and core and inter-shell and core.
Focusing on the core-shell behavior, the studies were extended to CoO and Cr2O3 AF nanostructures. The properties of these two AF systems are in agreement with the core-shell behavior proposed for Co3O4 nanostructures.
Furthermore, this study reports about a "magnetic fingerprint" method to characterize the magnetic behavior, i.e. it was demonstrated how thermo-remanent (TRM) and isothermo-remanent (IRM) magnetization curves can serve to identify the irreversible magnetization contributions usually encountered in nanosized magnetic structures. Figure 9.2 shows the TRM/IRM fingerprint of superparamagnetic systems, superspin glasses, 3d DAFFs and Co3O4 nanowires (NWs). Using TRM/IRM plots vs. field one can confirm a core-shell behavior, i.e. a regular AF core and a 2d DAFF shell for all three systems.
Monodispersed iron oxide nanoparticles with an average size of 20 nm were synthesized by thermal decomposition of metal-oleate precursors in high boiling solvent and subsequently self-assembled on Si substrate. Structural characterization of the self-assembled iron oxide nanoparticles dried at 80 ◦C showed that the nanoparticles consist of a mixture of iron oxide phases, wüstite and spinel. By annealing the nanoparticles at 170 ◦C in air it is possible to favor the spinel phase in the nanoparticles without the unwanted particle coalescence. Furthermore, using electron beam lithography, patterned trenches of 40-1000nm width for templated self-assembly were fabricated to study the influence of the confinement of the nanoparticles. Circles up to 60 nm diameter of iron oxide nanoparticles were also fabricated (Figure 9.3).
Magnetometry results on iron oxide films showed a strong dependence of the magnetic properties on the thermal treatment. Depending on the annealing temperature nanoparticles exist in either FeO, γ-Fe2O3, Fe3O4 or mixed phases. Furthermore, magnetometry studies were performed on trenches with widths of 130 nm. Samples containing mainly nanoparticles in the FeO-phase show some influence on the structuring. At low temperatures, the magnetic properties are influenced by the direction of the applied magnetic field during the measurement. However, systems annealed at higher temperatures containing either γ-Fe2O3 or Fe3O4 do not show modified behavior.
The results discussed in this thesis have important consequences for the understanding of the surface behavior of magnetic nanosystems in general. Moreover since many exchange-bias models consider the understanding of the AF surface to be crucial for the interpretation of the exchange-bias effect, the present results might lead to a refined consideration of the AF interfacial magnetization contribution.
Increasing density requirements in the microelectronics and magnetic-storage data continue to motivate the production of devices at ever smaller dimensions. The results presented in this thesis show that combining conventional lithography, chemical synthesis, and self-assembly it is possible to fabricate sub-100-nm arranges of iron oxide NP. These results can be extended to other NP, such as CoPt or FePt. It has been reported by Black et al. that self-assembled devices composed of periodic arrays of 10-nanometer-diameter cobalt nanocrystals display spin-dependent electron transport [249] with magnetoresistance ratios on the order of 10% below 20 K. Electron transport properties of magnetite nanocrystal arrays should be very interesting as well. Magnetite is a half-metal, where the electronic density of states is spin polarized at the Fermi level. The conductivity is dominated by spin-polarized charge carriers. At the time of this writing, magnetoresistance experiments on these iron oxide NPs are carried out in our group.
