Chapter 4 - Magnetic Anisotropy in Transition Metal Systems

Abstract

Publisher Summary Magnetic anisotropy is because of the electronic exchange forces, which can be quite significant in magnetic materials. Even in the absence of external magnetic fields, these forces are strong enough to align magnetic moments present in a given material. Usually an external field is necessary to get these moments to align along, even in materials that are likely to be strongly ferromagnetic. The reason for such behavior is the presence of magnetic domains or subvolumes. Each one of these domains could possess a saturated moment, but different domains may not be aligned with each other. This non-alignment is the cause of unsaturation in typically ferromagnetic materials. An applied (magnetic) field of sufficient strength is necessary to bring such domains to align themselves. The Magnetic Anisotropy Energy (MAE) is the energy associated with the orientation of the magnetic moments in a condensed matter system. For cubic systems, the leading term in anisotropy breaks the lattice (local) symmetry. Magneticanisotropy can be linked with switching the orientation of magnetization of a given magnetic material from its easy axis towards its hard axis. The energy required to change this orientation is identified as the magnetic anisotropy energy. The chapter discusses magnetocrystalline anisotropy (MCA), early work in this field, dipole–dipole interaction and related anisotropy, magnetoelastic anisotropy, perpendicular magnetic anisotropy, x-ray magnetic circular dichroism (XMCD), X-ray magnetic linear dichroism (XMLD) for anisotropy measurements, Mermin–Wagner theorem, spin Hamiltonian, band theoretical treatments, and spin reorientation transitions in multilayers. Examining Hartree–Fock type exchange integrals and nonspherical terms therein shows how orbital polarizations arise. Evaluation of the spin–orbit energy term along various directions of spin quantization in a lattice yields useful information about magnetic anisotropy. There is discussion on the basics of exchange anisotropy and its complexity and its relevance to magnetic data storage.