Band-Ferromagnetism Ground-State and Finite-Temperature Phenomena
Written by leading experts in the field of band-ferromagnetism, this book is intended to give a status report on our understanding of this complicated and fascinating problem of solid state physics. Modern developments are presented and explained in a tut
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imension-Reduced Band Ferromagnets
Ferromagnetism is known as a state of collective order in three-dimensional solids. In most elements, the magnetic moments per atom as a consequence of non-completely filled electronic shells lead to a paramagnetic answer of the system to an applied external magnetic field. In only a few cases, most-known for iron, cobalt, and nickel, the magnetic moments are found to be spontaneously ordered below a critical temperature, the Curie temperature TC . This state of ferromagnetic order is caused by the so-called exchange interaction, which is understood as the result of the Coulomb interaction in combination with the Pauli exclusion principle. Ferromagnetism is, therefore, a many-body effect of quantum-mechanical nature. It depends critically upon the overlap of electron wave-functions of adjacent atoms. As a consequence, magnetic properties respond sensistively to a changed number of neighbouring atoms, an altered crystal structure and an enhanced or reduced lattice parameter. The development of smaller and smaller magnetic devices for data storage and retrieval demands for studying the influence of the reduced dimensionality on the magnetic properties. Ultrathin films as quasi two-dimensional systems are grown and characterized by state-of-the-art surface-science techniques under ultra-high vacuum conditions. Rows of atoms as realization of quantum wires and clusters as quantum dots are deposited on carefully selected substrates. The growth conditions and the morphology of these nanometer-scale structures are controlled with high precision. Novel magnetic phenomena have been discovered to be used in a wide spectrum of possible and already commercially available products for application in daily life. A famous example is the giant magnetoresistance (GMR) discovered in exchange-coupled layered structures, but now realized in a variety of spin valve structures. The GMR effect has already revolutionized the data storage and sensor technology [1]. K. Baberschke, M. Donath, and W. Nolting (Eds.): LNP 580, pp. 267–282, 2001. c Springer-Verlag Berlin Heidelberg 2001
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Markus Donath
From a fundamental physics point of view, finite-size effects, i.e. the influence of the reduced dimensionality, altered crystal structure, and modified lattice parameter on the magnetic properties, have to be understood. Primary magnetic properties like the Curie temperature, the spin and orbital magnetic moments per atom, and the magnetic anisotropy have to be explained as well as the electronic states with their spin dependence that underlie the primary magnetic properties. Electron wave functions no longer extend to infinity as in a three-dimensional solid, they are confined by the presence of surfaces and/or interfaces. In this contribution, surface magnetic effects are discussed in relation to the surface electronic structure. It is predicted from theory and confirmed by experiments that surfaces of ferromagnets exhibit an enhanced spin magnetic moment compared with the bulk. In the studied cases, this is explaine
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