Local Electronic Properties of Doped Correlated Electron Systems

In this project the local properties of doped correlated electron systems will be investigated by nuclear magnetic resonance in selected materials. In order to conduct the measurements a coherent, broad band NMR pulse spectrometer tailored for measurements in magnetically ordered systems will be set up on basis of a spectrometer provided by courtesy of the Forschungszentrum Jülich (Germany) to support this project. Three compounds were chosen for the considerable interest in them from a fundamental as well as from an applicational point of view. The studies of the doping mechanisms in PrBa2 CU 3 O6+x are expected to shed light on the origin of the outstanding role of Pr among the rare earths in this structure and, thereby, on the microscopic carrier properties in the isostructural high temperature superconductors. Investigations of crystals treated thermally to induce changes in the microstructure are planned in order to clarify the origin of the correlation between microstructure and transport properties. A better understanding of the carriers and their relation to details of the structure in this family of materials is of eminent interest in improving the transport properties of insulator/superconductor heterostructures of the cuprates, which are not well understood at the present time. The investigation of magnetic properties in the perovskitic manganates R1-x Ex MnO3 (R = Rare Earth, E = earth- alkaline) on a local scale will contribute to the understanding of the microscopic mechanisms underlying the remarkable correlation between magnetism and transport observed in this class of materials. For applications the occurrence of a field induced metal-insulator transition may become of great interest, since it leads to the so-called colossal magnetoresistance phenomenon. Up to now the effect is limited to high fields and low temperatures, which makes a microscopic study of the processes involved highly desirable. From measurements in thin films a detailed characterization of such technically important samples is expected, and information on the applicability of the method to investigations of artificially structured samples will be obtained. Finally, the work in the Kondo lattice YbCu 5-x AIx addresses the problem of the spin dynamics in dense Kondo systems with disorder. The systems recently received large interest from fundamental solid state physics in view of the non-Fermi liquid behaviour observed above a quantum transition at T = 0. In this regime spin fluctuations dominate the macroscopic properties in an extended temperature range, and the low energy part of the fluctuation spectrum can be studied by NMR.

The aim of the projekt was the design of a nuclear magnetic resonance (NMR) spectrometer fulfilling the special requirements connected with investigations of cooperative phenomena in doped materials, and in the presence of strong electronic correlations. The strong current interest in correlated electron systems from both, the fundamental physics and the technical point of view, stems from the close relation of conducting and insulating, magnetic and superconducting phases in their comlex phase diagrams. The nuclear spin used in NMR is a very sensitive probe for microscopic information on the solid state which was not available in Austria up to now. During the first part of the project a home-made digital pulse generator was designed for a commercial receeiver and transmitter unit, and the software for automatic computer ontrole of the experiment, for data acquisition, and analysis was developed. The spectrometer was tested successfully with mesurements in the temperature range 2.0 - 130 K, at frequencies between 8.0 - 130 MHz, and in external magnetic fields up to 8 T. In addition, an extension to frequencies up to 500 MHz was designed. The first investigations were concerned with the structure and the magnetic phase transition of CeNiGa2. We then studied the metamagnetic transistion of Co in ErCo3, and have been able for the first time to detect the NMR-signal across this transition. The transition is understood theoretically as a localization of band electrons into states at the Co which enhance the Co-moment at low temperature. We found that the local probe of Co-NMR provides a very sensitive check of this model. The most successful part of the project was the investigation of doped hexaferrites. SrFe12O19 hexaferrites have been the most important permanent magnet material for several decades, because of their low price compared to rare-earth based systems. The finding of a significant anisotropy enhancement upon simultaneous doping with La and Co in 1999 attracted, therefore, a renewed interest in the poorly understood microscopic mechanism of the magnetic anisotropy in the ferrites. With the new spectrometer we were not only able to examine the well known Fe-NMR spectra, for the first time we investigated the defect sites themselves using Co- and La-NMR. In this way we could prove that Co2+ replaces Fe3+ on f2-sites in the complex structure, compensating the excess charge of La3+ on Sr2+-sites. Furthermore we could show that Co enters a low- spin configuration. It is expected that the information on the electronic configuration will provide an important guideline for the search of a better microscopic understanding of the magnetic anisotropy in these technicalyy important structures.