Introduction

Abstract

The phenomenon of spontaneous collective order of the magnetic moments in some solid materials (ferro-, ferri-, antiferromagnetism), still attracts the interest of researchers working in experiment and theory alike. Experimentalists carefully characterize the magnetic properties of these interesting materials as a function of the structure, morphology, composition, magnetic field, pressure, and temperature. The ultimate goal is to tailor the magnetic properties and optimize them for certain applications. The theoretical description is not a trivial task because collective magnetism is a many-body phenomenon of quantum-mechanical nature. So far, no complete theory is available which could describe all kinds of ferromagnetic materials. Two major classes of ferromagnets are distinguished according to the kind of electrons carrying the magnetic moments: itinerant or band ferromagnets on the one side and local-moment ferromagnets on the other side. In the latter case, the exhange interaction is not a direct interaction due to the localization of the electrons with no significant overlap of their wave functions from one atomic site to the next. An interaction between the localized magnetic moments and the itinerant charge carriers or interspaced anions is needed for a so-called indirect exchange interaction. The contributions of this book concentrate on three different subjects within the topic of local-moment ferromagnetism. The first part deals with concentrated local-moment systems comprising classical local-moment ferromagnets as well as manganites, which show the colossal magnetoresistance (CMR) effect. The second part covers a relatively new class of materials, the diluted magnetic semiconductors. The origin of ferromagnetic order in these materials is subject of an intense debate today. The third part focuses on half-metallic ferromagnets, an interesting class of materials, well-known for decades, but with new perspectives for applications in magnetoelectronic and spintronic devices.