Herstellung, mikrostrukturelle Eigenschaften und Magnetismus von nanokristallinen Cobalt- und Zinkchromit-Spinellen in Abhängigkeit von Partikelgröße und Zusammensetzung

In the course of this work the properties of zinc chromite ZnCr2O4 and cobalt chromite Co1+xCr2-xO4 (x = 0-2) were analysed depending on composition and particle size. Synthesis was carried out by a bottom-up process, the sol-gel route from Pechini. Additionally, CoCr2O4 nanoparticles were synthesised by a tribomechanical top-down process. The primary focus of this research were the magnetic properties of the chromite spinels. For the first time, the magnetic characteristics like the ferrimagnetic Curie temperature TC, the spiralmagnetic ordering TS, the incommensurable to commensurable transition TL, the Curie-Weiss temperature ΘCW and the effective magnetic moment μeff and exchange integrals were determined of several phases over the whole composition range from Co1+xCr2-xO4 (x = 0-2). Remarkable changes in the course of the magnetic properties of the bulk phases are at x = 0.5 and 1.375 of Co1+xCr2-xO4. This correlates with the percolation limit pc 0.4 and 1.36 for the magnetic BB- and AB-interaction of cubic spinels. The nanocrystalline spinels Co1+xCr2-xO4 (x = 0‑2), synthesised by a bottom-up process, show pure phase in the x-ray diffractogram. Because of the inhibition of crystallisation of chromium-rich spinels, a spinel with chromium deficit was built at smaller particle size, whose lattice parameters were less than those of the predetermined composition, in agreement to Vegards behaviour of cobalt chromite. Nanoparticles have an amorphous byproduct, presumably cobalt chromate/dichromate and chromium acid anhydride. Magnetic properties of the nanocrystalline phases of Co1+xCr2-xO4 change significantly in comparison to bulk. With particle size reduction the manifestation of the occurring ordering rapidly diminishes. Furthermore an increase of ΘCW and a decrease of μeff with smaller particle size sit detected. The exchange integrals are not significantly influenced up to a particle size reduction to 15 nm. Below 15 nm all three magnetic interactions decrease, which can be traced back to rising disorder and respectively deviation of the composition. Also superparamagnetic behaviour is found. Beside the particle size, the size dependent molar ratio of nCo/nCr, the byproducts further increase the influence on the magnetic properties, for example by the creation of Cr+6 compounds. There are several property changes for nc-Co1+xCr2‑xO4 according to composition and particle size. A distinct separation between size and composition dependent effects on nanocrystalline cobalt chromite is not easy. Changes of lattice parameter, density, TC, μeff and ΘCW from nc-CoCr2O4 correlate with the specific surface. Thereby these changes follow the fundamental principle of nanoscience. Tribomechanically synthesised nanoparticles of CoCr2O4 show divergent properties to particles which were synthesised by sol-gel route. The strain for tribomechanically synthesised particles are three times larger than for particles with same size from the sol-gel route. A high strain is, due to the local high energy impact, not unusual for tribomechanical synthesis and normally leads to a broad particle size distribution. Likewise the via sol-gel route synthesised cobalt chromite, decreases the density, μeff and the maximal molar magnetisation with particle size. ΘCW increases. However these changes do not correlate with the specific surface. It could be caused by the influence of strain and disorder. These could also cause TS and TL to not be detected at a particle size of 30 nm and lead to a decrease of the exchange integrals of the magnetic interactions. There is no influence on the ferrimagnetic AB-interaction through particle size, TC remains at 100 K. Via sol-gel route synthesised nanocrystalline zinc chromite shows, contrary to previous reports, a clear size-dependent influence onto the lattice parameter. With decreasing size the lattice parameter increases. The change of lattice parameter and density with particle size correlates with the specific surface. Nanoparticles of zinc chromite have a byproduct, presumably chromium acid anhydride. The magnetisation from zinc chromite increases with decreasing particle size. This can be attributed to increasing distortion of the long-range magnetic interaction and increasing defects with decreasing size. Accordingly the expression of the Néel temperature is weaker with particle size reduction, which is 12.5 K for the bulk material. It shift to lower temperature with decreasing size and is not detectable below ~8 nm. The antiferromagnetic signatures diminish with decreasing particle size, indicated by the change of ΘCW and μeff.