Computational design of new magnetic materials
Abstract
Tailoring properties of materials based on computational simulations has become an important part in materials design. In this project the focus will be on magnetic materials aiming to study novel molecular magnetic materials (a) which can overcome size limits of conventional electronics. A smaller part of the project will be dedicated to bulk materials (b) where the goal is to find replacements for hazardous components in todays permanent magnets. Both topics will be targeted using density functional theory (DFT) methods for the optimization of the geometry and the analysis of the magnetic properties. Plane wave codes will be employed to relax the atomic structure of the systems. In case of the molecular systems investigations beyond standard DFT will be performed including van der Waals corrections to describe the molecular bonding and DFT+U (or beyond) corrections to account for the localized electrons. Especially for the bulk materials on top of that a full-potential electronic structure method using Linear Muffin-Tin Orbitals will be used to investigate the magnetic anisotropy and the exchange constants. The latter ones are needed as input for subsequent Monte Carlo simulations to obtain the finite temperature behavior.
(a) New molecular magnetic materials
Here molecular macrocycles with transition metal centers will be studied. They serve as model systems for building blocks for future molecular based magnets. The starting point is the investigation of molecular complexes in gas phase. One example are sandwiches of 2 or more phthalocyanine molecules with (different) metal centers. After the optimization of the geometry the magnetic properties, e.g. the exchange coupling between the molecules will be analyzed. In a second step ligands will be added to allow for a fine-tuning of the magnetic coupling. In view of applications free standing molecules are difficult to handle. To model a realistic environment the molecular complexes will be deposited on substrates. Here, preferably metal substrates will be taken into account which serve in experiment as electric contact layers.
The key question is then how the magnetic and electronic properties of the molecules are influenced by the substrate. Even though phthalocyanine molecules have been intensively studied there are still open questions and a detailed understanding of the molecule-molecule coupling on the one hand and the molecule-substrate interaction on the other hand is necessary to identify suitable systems which might then be synthesized in experiment.
(b) New magnetic phases in bulk materials
Permanent magnets are needed on quite a large scale, e.g. in cars or wind turbines. The aim is to remove critical materials such as rare earth but keeping the high performance. Here transition metal based binary and ternary systems will be in the spotlight. The goal is to identify new uniaxial phases and to increase the magnetic anisotropy in these materials by using impurities or changing the composition. Therefore, super cell calculations will be performed to understand the influence of impurities and local order/disorder of the atomic arrangement on the magnetic moments and the magnetic anisotropy.
