Nanostructured composite thin films showing magnetic exchange coupling at the material interface have attracted great interest for the development of electronic components such as spin-valves. Besides the commonly performed fabrication of multilayer systems, the utilization of nanoparticle building blocks holds great potential for thin films with tailored magnetic properties and allows the facile but controlled combination of materials with complementary magnetic characteristics. In this work, we present the use of prefabricated highly crystalline iron platinum (fcc-FePt) and iron oxide (Fe x O y ) nanoparticles for the preparation of nanocomposite thin films with varying compositions by wet processing from mixed dispersions. The resulting multiphase coatings showed high homogeneity, low surface roughness, and superparamagnetic behavior. By the variation of the amount of incorporated iron oxide, a precise adjustment of the magnetization at high field strength could be achieved. Furthermore, calcination under inert gas atmosphere resulted in a controlled phase transition of the magnetic phases and thus, in purely metallic coatings composed of ferromagnetic fct-FePt and antiferromagnetic fcc-FePt 3 , a decrease in surface roughness as well as high magnetic coercivity at room temperature. Field-cooling below the Neél temperature of fcc-FePt 3 led to an exchange bias effect with a strong increase in coercivity and the characteristic hysteresis shift. In comparison to the literature, our nanocomposite thin films showed fully ordered phases without the occurrence of phase impurities, a distinctly higher coercivity, and an exchange bias shift of 38.7 mT.
The synthesis of FePt nanocrystals is typically performed in an organic solvent at rather high temperatures, demanding the addition of the in situ stabilizers oleic acid and oleylamine to produce monomodal particles with well-defined morphologies. Replacing frequently-used solvents with organic media bearing functional moieties, the use of the stabilizers can be completely circumvented. In addition, various morphologies and sizes of the nanocrystals can be achieved by the choice of organic solvent. The kinetics of particle growth and the change in the magnetic behavior of the superparamagnetic FePt nanocrystals during the synthesis with a set of different solvents, as well as the resulting morphologies and stoichiometries of the nanoparticles were determined by powder X-ray diffraction (PXRD), small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), inductively coupled plasma optical emission spectroscopy (ICP-OES)/mass spectrometry (ICP-MS), and superconducting quantum interference device (SQUID) measurements. Furthermore, annealing of the as-prepared FePt nanoparticles led to the ordered L10 phase and, thus, to hard magnetic materials with varying saturation magnetizations and magnetic coercivities.
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Die Kombination von Materialien mit unterschiedlichen magnetischen Eigenschaften auf der Nanoskala führt durch Kopplungseffekte zu Phänomenen wie der sogenannten unidirektionalen Austauschanisotropie, die z. B. für Speichermedien oder magnetische Sensoren großes Potenzial besitzt. Während solche Strukturen bislang nur über komplexe Prozesse der alternierenden Abscheidung aus der Gasphase zugänglich sind und daher noch keine breite Anwendung gefunden haben, würde die Herstellung aus Dispersionen von Nanopartikeln nicht nur eine wesentliche Vereinfachung darstellen, sondern auch eine breite Vielfalt von Materialkombinationen und Strukturen ermöglichen. Hierbei stellt die Prozessierung hartmagnetischer Nanopartikel eine Herausforderung dar, die jedoch durch die Verwendung von Vorläufermaterialien mit anschließender Phasenumwandlung gelöst werden kann.
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