Higher - order field - dependent terms in spin Hamiltonians for transition ions : implications for high - magnetic field and high - frequency EMR measurements

An overview of theoretical background of the higher-order field-dependent (HOFD) terms in the generalized spin Hamiltonians for transition ions and current status of experimental studies of the HOFD terms is provided in the nutshell. The terms nonlinear in the magnetic field (B) of the type: B2S2, B3S, B5S or B2I2, B3I, B5I, where S and I is the electronic and nuclear spin, respectively, are of particular importance as compared with the usual linear Zeeman term B.g.S. This is due to the high B values, up to 100 T (pulsed fields), achievable at present in the high-magnetic field and/ or high-frequency electron magnetic resonance (HMF-EMR) measurements of magnetic and spectroscopic properties. A blueprint for future theoretical and experimental studies of the HOFD terms is proposed taking into account their implications for HMF-EMR measurements