Structure, magnetism and transport of the Zn-Co-O system

It has been a grand challenge for materials sciences for long, to find a semiconducting material with ferromagnetic order up to room temperature. The availability of such a material would be the technological breakthrough for spintronic devices with novel functionality and/or reduced power consumption. The aim of this project is to address a new perspective for magnetic order in dilute magnetic semiconductor (DMS) materials, specifically Co:ZnO. For that, extremely high dopant concentrations shall be realized, taking advantage of the good solubility of Co in ZnO. In addition, the transition to the ZnCo 2 O4 spinel, a recently established p-type transparent conducting oxide (TCO), shall be investigated, i.e. to study the ternary Zn-Co-O system in the full composition range. The nature of the transition from the n-type wurtzite DMS to the p-type spinel TCO will be investigated. All specimens will be prepared by reactive magnetron sputtering under ultrahigh vacuum conditions which is capable of yielding well- defined epitaxial films without the formation of secondary phases. The samples will be characterized with regard to their structural, electronic, magnetic, and transport properties with emphasis on element-selective, synchrotron- based spectroscopic techniques in combination with simulations of the respective spectra. The magnetic and transport properties will be optimized by means of co-doping the samples with Cu and Ni. In summary, the objectives of the proposed project have wider implications for both fields, realizing magnetic order through coalescence as well as tailoring the transition from n-type to p-type conductivity via the composition in a ternary oxide which will ultimately allow to fabricate functional all-oxide p-n-junctions, which will be an important step forward to realize all-oxide (transparent) electronics.

The FWF project Structure Magnetism and Transport of the Zn-Co-O System has investigated a functional oxide material system which to the end of Co-doped ZnO has been intensively researched as a novel semiconductor material where ferromagnetic order may be induced by magnetic doping. Such materials would be of interest for novel, energy efficient concepts of computing, utilizing the magnetic degree of freedom to improve the performance. However these early predictions have already turned out to be unfeasible. One central aim of the project was to explore the remaining possibilities to finally achieve magnetic order in Co:ZnO, namely by ever increasing the Co content in the material. An unprecedented concentration of 60% was achieved and indeed the material turned out to order magnetically, but only at temperatures far below room temperature. To our surprise the material in addition exhibited another interesting feature. One can imprint a robust magnetization upon cooling it in a magnetic field, a phenomenon known as exchange bias. Usually exchange bias is only found in heterostructures of ferro- and antiferromagnets and it is commonly used e.g. in magnetic recording heads in hard disks. Here we had a system where this effect was occurring in an uncompensated antiferromagnet alone which opened the opportunity to study and model the behaviour of uncompensated magnetic moments which is of relevance for a more in-depth understanding of exchange bias systems in general. Besides this unexpected new finding, the high Co concentrations also enables to transform 60% Co:ZnO into the ZnCo2O4 spinel, where virtually all physical properties are changed. The Co ions are in a different oxidation state (3+ rather than 2+) as well as surrounded by a different number of neighbouring atoms (6 rather than 4). Additionally the color changes from green to brown and the charge carriers responsible for the conductivity are holes rather than electrons. This transition could be studied in great detail and it turns out that all three properties are intimately coupled. Unfortunately, the magnetic properties were found to be antiferromagnetic at low temperatures independent of all other properties. Also the electrical resistance was found to be too high for useful application. However, that opened the possibility to explore these materials with regard to their suitability for another useful application solar cells. Upon doping ZnO with Al it is already known that the material keeps it full transparency while it becomes electrically conducting. So in combination with the n- type, green Co:ZnO and the p- type ZnCo2O4 it is possible to construct an all-oxide solar cell and first working cells are currently under investigation and possibilities are explored to make them more efficient. In summary, during the project many different ways to magnetism in the Zn-Co-O system were explored but instead of ferromagnetism a nice new model system for uncompensated antiferromagnetism was found. Also first real working devices based on oxide heterostructures were fabricated and tested which rely on the in-depth understanding of the material on a microscopic level attained in the course of this project.