# Antiferromagnetic properties of 3d transition metal oxide nanoparticles

David, Bracher. Antiferromagnetic properties of 3d transition metal oxide nanoparticles. 2021, Doctoral Thesis, University of Basel, Faculty of Science.

 Preview

9Mb

Official URL: https://edoc.unibas.ch/84505/

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.
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.