Spin-Hamiltonian formalisms in electron magnetic resonance (EMR) and related spectroscopies

The historical development leading to, and the current status of, the spin- Hamiltonian (SH) formalisms. characterizing electron magnetic resonance (EM R), also referred to as electron paramagnetic (or spin) resonance (EPR or E SR), is reviewed. The spin-Hamiltonian concept is briefly outlined to set a framework for definitions of relevant terms. Meanings of the terms which a re often confused with each other, e.g. physical versus effective Hamiltoni an, real versus effective versus fictitious spin, microscopic SH (MSH), zer o-field-splitting (ZFS) Hamiltonian, generalized SH (GSH), and phenomenolog ical SH (PSH), are elucidated. Various general approaches to 'derive' MSH a s well as to 'construct' GSH for transition ions are discussed, This enable s clarification of relationship between the ZFS Hamiltonian and (a) the cry stal-field (CF), (b) electronic spin-spin, and (c) nuclear-quadrupole Hamil tonians. The inadmissibility of the odd-order ZFS terms (1 = 3, 5) is also discussed, A brief general classification of the major operator and paramet er notations used in the literature to describe ZFS Hamiltonian is provided . Other important areas relevant to EMR, where the SH concept plays a centr al role, are outlined.