Antiferromagnetic properties of 3d transition metal oxide nanoparticles

Antiferromagnetic (AFM) materials have attracted significant attention in the past years due to emerging fields such as antiferromagnetic spintronics, the development of ultrafast switching magnetic random access memory devices, spin valves, and new ultrahard magnetic materials. In order to scale down such new devices for its technological implementation a fundamental understanding of antiferromagnetism at the nanoscale is of crucial interest. While antiferromagnetic materials have been widely investigated in their respective bulk and thin film forms, only very few attempts were made to understand nanoscaled antiferromagnets. This is mostly due to the fact that probing magnetism of thin films and at the nanoscale usually utilized magnetic stray fields which do not appear in nano-sized antiferromagnetic materials. Although there have been some studies of the antiferromagnetic properties of nanoparticles by means of magnetometry, Mössbauer spectroscopy, neutron scattering etc. Those methods usually integrate over large ensembles of nanoparticles making it very challenging to disentangle effects of sizes and shapes distributions as well as the chemical composition. Further, in typical powder measurements it is extremely challenging to directly separate the influence of intrinsic magnetic properties and eventual magnetic inter-particle interactions.
In this thesis, this issue is overcome using x-ray photoemission electron microscopy utilizing the x-ray magnetic linear dichroism effect. This effect allows one to directly probe the magnetic ordering parameter of a antiferromagnetic material exploiting the fact that linearly polarized x-ray within a antiferromagnetic media show an orientation dependent absorption. Further, scanning electron microscopy is used to investigate the morphology of individual nanoparticles and small agglomerates. In order to be able to correlate the morphology and the chemical and magnetic properties of the nanoparticles, samples with a very low surface concentration on silicon substrates are produced. Since the silicon substrates contain unique gold marker structures it is possible to identify the very same nanoparticles and agglomerates in complementary microscopy measurements. This thesis addresses the magnetic properties of acicular goethite ($\mathrm{\alpha -FeOOH}$) nanoparticles and $\mathrm{CoO/Co_3O_4}$ core-shell nanooctahedra. In case of the $\mathrm{CoO/Co_3O_4}$ nanoparticles the emergence of possible uncompensated magnetic surface/interface moments are investigated using the x-ray magnetic circular dichroic effect which directly probes the direction of the magnetic moments.
It is found that the antiferromagnetic spin axis of individual goethite nanoparticle have a tendency to be oriented out of the sample plane. This behavior suggests that an interface effect with the non-magnetic substrate influences the magnetic properties of the goethite nanoparticles.
In case of the $\mathrm{Co_3O_4/CoO}$ core-shell nanooctahedra it is demonstrated that antiferromagnetic spin axis can be determined without any prior knowledge about the crystal directions. The axis are found to align closely to the crystal direction, as it would be expected for CoO in its bulk form. Additionally, indications for uncompensated magnetic moments are detected. The magnetic moments are found to behave superparamagnetically down to 100 K. Such superparamagnetic behavior is not found for the antiferromagnetic CoO core. Although, it is possible that the antiferromagnetic order in the core can switch its sublattice magnetization by 180° which can intrisically not be observed by studying the x-ray magnetic linear dichroism.