Magnetism and local order: ab initio tight-binding theory

The effects of the local environment on the electronic structure and magnetic moments of Fe, Co, and Ni have been studied by confining these atoms to assume various structural forms such as chains, surfaces, layers, and crystals. The coordination number of the atoms can thus be changed over a wide range. The local environment of the magnetic atom has also been altered by introducing defects such as impurities, vacancies, and vacancy complexes. A simple method based upon the real space was devised that enables us to calculate the electronic structure of perfect as well as imperfect systems with speed and accuracy. The method is based upon a cross between the molecular-cluster and the tight-binding theories and contains no adjustable parameters. The effect on the magnetic moments due to vacancies, vacancy clusters, and surface relaxations in Fe are studied to illustrate the versatility of the method. The results in chains, slabs, and bulk are compared with earlier theoretical results, as well as available experimental data. The excellent agreement achieved in these comparisons provides room for optimism that our theory can be useful in studying complex systems otherwise inaccessible to modern-day theories.