Manipulation of magnetic and chemical properties of cobalt nanoparticles studied by means of x-ray photo-emission electron microscopy

Investigation of structure and magnetic properties of nanoparticles is important for application in catalysis, data storage, chemical sensing, energy conversion and drug delivery. Laser manipulation of magnetization of the nanoparticles can be promising for next generation data recording technology.
Large scattering of magnetic properties of cobalt nanoparticles makes it difficult to apply simple scaling laws. Most probably this is due to the measurement techniques averaging over a large number of the nanoparticles with different crystal structures, internal defects and morphology. In our approach we use an outstanding combination of characterization techniques that allow us to directly correlate magnetic properties in individual cobalt nanoparticles with their crystal structure and morphology. We use x-ray photo-emission electron microscopy (XPEEM) for magnetic characterization of the nanoparticles and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) for atomic resolution structural characterization. Our results show that magnetic blocking in cobalt nanoparticles occurs independently of the particles size and the orientation of the magnetization does not correlate with crystallographic axes. Our structural investigations suggest also that many of the particles have defects which modify the magnetic anisotropy. We have developed atomistic models for STEM simulations and compare them to the STEM data to prove the nature of the defects and their positions in the nanoparticle. Still ongoing is the development of a theoretical approach for calculating the magnetic properties of nanoparticles with defects.
We combine XPEEM and HAADF-STEM approach to correlate magnetic properties and chemical composition of cobalt nanoparticles with the actual morphology upon in situ oxidation. Understanding the role of the surface is important for revealing the origin of magnetically blocked states of cobalt nanoparticles smaller than 15 nm, oxidation kinetics and the products of the reaction is important for catalysis. Most of the studies rely on x-ray absorption spectroscopy and modelling of the spectra. However, early oxidation kinetics of cobalt nanoparticle remained unclear. We show that reduction of magnetic volume upon oxidation lowers the magnetic energy barrier. Our STEM data show a surprisingly complicated oxidation kinetics, which is not properly reflected in simulated x-ray absorption spectra. The early stage of oxidation leads to formation of inhomogeneous shell on the nanoparticles, dosing more oxygen improves the shell morphology, even further oxidation leads to thickening of the shell.
Purely optical magnetization orientation reversal with femtosecond laser pulses (all-optical switching) was shown in thin films and granular media of various materials. So far no such results were presented for single nanoparticles with diameter smaller than 15 nm. We combine XPEEM with femtosecond laser pulse exposure to investigate the effect of ultrashort laser pulses on cobalt nanoparticles. No deterministic switching is found independently on laser fluence and polarization. Also, no thermal switching of nanoparticles magnetization is observed. Instead, we find that laser triggers a chemical reaction with the substrate which alters magnetic energy barrier in the nanoparticles. Our results suggest that for a successful laser-induced switching of the magnetic nanoparticles, nanoparticles with lower Curie temperature TC either defined by size effects or by choosing different materials are required.
Summarizing, our investigations show that structural defects are important for magnetic properties of cobalt nanoparticles, especially for stability of their magnetization and orientation of the magnetic moment. We found complex oxidation kinetics, which is important for better understanding of catalysis and magnetic behavior. Femtosecond laser excitation of magnetic nanoparticles seems promising but for materials with lower TC. Higher resolution x-ray imaging is needed to reveal spin configuration of the individual nanoparticles time and better investigate the chemical composition and magnetic properties of oxidized cobalt nanoparticles.