Correlation effects in half-metallic ferromagnets

Spintronics is a new branch of electronics in which the electron spin in addition to charge is manipulated at the nano- and atomic level. Spintronic devices are promising elements to be possibly used in quantum computers and quantum communication. Spintronics is, thus, believed to strongly influence the perspectives of information in the future. A class of materials which has been considered for spintronic applications is the one of half-metallic ferromagnets (HMF). In principle, due to the fact that the Fermi energy of one spin direction lies in a gap, the current in a HMF is completely spin polarized, promising great advantages for a variety of device applications. The present research project helps the construction and application of new theoretical techniques to investigate spintronic materials in order to understand their finite-temperature behavior and to achieve some technically important characteristics like multifunctionality and tunable properties. Realistic predictions for these materials can be made on the basis of their electronic structure using Density Functional Theory within the Local Density Approximation, DFT-LDA, in combination with different many-body approaches, such as Dynamical Mean Field Theory, DMFT, which takes into account local correlations. However, in order to characterize the finite- temperature behavior, which is mainly determined by magnetic excitations, a theory which captures non-locality is required. Among the already known techniques making DMFT less local, a new approach, variational cluster perturbation theory V-CPT, was introduced recently. On the methodological side, the current project aims at implementing V-CPT, as a faster and reliable solver of the many body problem in combination with the DFT-LDA approach. In the applicative part of the project, we mainly address the issues of finite-temperature polarization, magneto- optical properties and quantum transport in spintronic materials based on HMF. Apart from increasing the fundamental knowledge in the field of material science, the computational schemes developed in this project will give quantitative explanations and predictions necessary to design future electronic devices based on HMF.

The central problem in the theory of ferromagnetic materials containing transition metals is the proper treatment of the strong interaction between electrons in the d-band at finite temperature. The present project addresses the finite temperature properties of Half-metallic ferromagnets, a peculiar class of materials with unusual magnetic and electronic properties: they are metallic for one spin channel, and insulating or semi-conducting for the opposite one. These properties make half-metals possible candidates to what we call today "Spintronics": electronics based on the manipulation of spins. For this class of technologically important materials we have demonstrated for the first time that polarization follows a completely different temperature behavior than magnetization. This result is in contrast to what was known for most ferromagnetic materials. We explained it within a modern approach that treats on equal footing the realistic material description and the many-body electron-electron interactions. Our results demonstrate the importance of electronic interactions in establishing materials properties: by including these interactions, physical properties are changed drastically. In particular, we showed for several half-metals that in the presence of electron-electron interaction the half-metallic gap closes and the spin polarization is completely lost. On the other hand, technological applications require as large polarizations as possible. In this respect, understanding the influence of many-body effects in these compounds allowed us to discuss possible ways to optimize half-metallic properties at finite temperatures. The strategy adopted was to investigate the effects of impurities that minimize the detrimental effect of electronic interactions enforcing the half-metallic gap. The results of the project gave quantitative and qualitative predictions in these directions. Our theoretical efforts were supported by collaborations with experimental groups that provided evidence on the existence of many-body states through magnetic tunnel junction spectroscopy. In addition, experimental investigations are on the way to apply and further develop the idea of optimizing half-metallicity. In order to enlarge the project`s area of applicability, the present research was extended naturally to interfaces and heterostructures. In this sens the activity is continued in the framework of several follow-up projects.