Electron Paramagnetic Resonance of Tm3+ Ions in Lanthanide Nicotinate Dihydrates

E.p.r. at microwave frequencies in single crystals of lanthanum nicotinate dihydrate (LaND) doped with ca. 0.01 mole fraction of thulium shows no spectrum of isolated Tm³⁺ ions, but does exhibit spectra from coupled pairs of nearest-neighbour Tm³⁺ ions. The spin hamilton parameters are approximately the same as those measured for such pairs in pure TmND; in the dilute material the ¹⁶⁹Tm hyperfine structure is resolved and a small zero-field splitting of the ground doublet of each Tm³⁺ ion is measured, described by a term δS_x in the spin hamiltonian. A careful measurement of the conditions for excitation of e.p.r., and of the relative intensities of the lines, indicates that the transitions are driven by matrix elements of the Zeeman operator, involving components of the microwave magnetic field parallel to the principal axis of the g matrix, which are non-zero only if there is a term in δ. It is suggested that there is a similar term in δ in pure TmND, whose influence upon line positions is unresolved, but which is necessary to produce the observed transition intensity. However, such a term would render Δ M_s = ± 1, Δ m_I = 0 transitions unobservable at r.f. frequencies (15-30 MHz) as it would produce too large a separation between the energy levels; so it would leave us unable to account for the magnetic resonance at r.f. frequencies in pure TmND. Further experiments are required to produce an understanding of the transition mechanisms in pure TmND.