Magnetism in Perovskite Manganites and Cobaltites at the Nano Scale
Cornelia E. Hintze – Hector Fellow Hilbert von Löhneysen
The exact position of atoms in the crystal structure of lanthanum manganites and cobaltites (both anorganic ionic compounds) significantly influences the electronic structure and thus the magnetic transition temperature and the type of magnetic ordering. Even small changes in the lattice e.g. caused by pressure or doping with various elements can instigate a transition from antiferromagnetic to ferromagnetic. Since nanoparticles have a large surface compared to their volume, they can be treated as “bits” of surface, having a very different crystal structure compared to traditional bulk materials. The variation in size can thus be used to directly control the variation in the magnetic transition temperature.
In lanthanum cobaltites the energetical closeness of different spin states adds an extra degree of freedom, which can change at small crystallite size, whereas in manganites it is only the atomic distances, crystal symmetry and oxygen stoichiometry that controls magnetism. Under the supervision of Hector Fellow Hilbert von Löhneysen, this PhD project will focus on the preparation of nanoparticles using microemulsions (see Figure). In contrast to previously published studies, this method allows exact control of the nanoparticle size via a simple mixing ratio. Hence, crystal quality, oxygen stoichiometry, and defect density are constant throughout the sample and changes in the magnetic properties can be directly related to size.