Clyde A. Hutchison

Electron paramagnetic resonance, optical and magnetic studies of Tb3+ in lanthanide nicotinate dihydrates

Optical absorption and fluorescence, magnetic susceptibility and electron magnetic resonance experiments have been made of Tb3+, 4f8, ions in lanthanide nicotinate dihydrate single crystals. Optical absorption spectroscopy in undiluted terbium nicotinate dihydrate (TbND) indicates that the ground state of Tb3+, 7F6, is a singlet exhibiting a quadratic Zeeman effect, and this is confirmed by the observation of a temperature-independent Van Vleck paramagnetism below 4.2 K. This is what one would expect for a non-Kramers ion in a crystal field of no symmetry. In spite of this, electron paramagnetic resonance (epr) is observed from Tb3+ ions both in dilute LaND containing 0.01 mole fraction of Tb and in TbND. In the dilute material, in spite of the absence of site symmetry, the results show a nearly degenerate non-Kramers doublet with a g value of 17.2(8), close to the maximum possible value of 12 gj; the ground state must therefore be predominantly Mj = ±6. The epr in TbND arises from some type of defect site involving two interacting dissimilar Tb3+ ions. The spectrum of Tb3+ ions observed by optical fluorescence is also attributed to defect sites.

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 δSx 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 ∆MS = ±1, ∆mI = 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.

The Crystal Field in the Lanthanide Nicotinates

The majority of the experimental results of previous measurements of optical absorption and fluorescence, including the Zeeman effect, of magnetic susceptibility and electron paramagnetic resonance, on lanthanide nicotinate dihydrates of the heavier half of the lanthanide group, have been accounted for by a model of the crystal field. This crystal field is constructed within the framework of the exchange charge model in an approximation with only four fitted parameters.

Magnetic Resonance, Optical and Magnetic Studies of the Nicotinate Dihydrates of Dysprosium and Erbium

Measurements have been made in lanthanide nicotinate dihydrate (LnND) single crystals, where Ln ═ Dy and Er, of optical absorption, including the Zeeman effect, of magnetic susceptibility between 1.4 and 4.2 K, of electron paramagnetic resonance between about 9 and 35 GHz, and of magnetic resonance at 22 MHz. As in the case of TmND (Baker et al. 1986a), the measurements are consistent with a ground state with a large amplitude of Mj ═ ±J, an almost Ising-like g-matrix, and a purely magnetic dipole-dipole interaction between nearest-neighbour Ln3+ ions. In this respect, the Kramers ions Dy3+ and Er3+ exhibit features similar to the non-Kramers ions Tb3+ and Tm3+ in the nicotinate dihydrates.

Optical and Magnetic Studies of Thulium Nicotinate Dihydrate

The series of compounds with stoichiometric formula [Ln(C5H4NCO2)3(H2O)2]2, in which Ln is a trivalent lanthanide ion, have a dimeric molecular crystal structure with space group P21/c.Optically clear single crystals of the thulium compound, with typical dimensions 3 mm x 2 mm x 1 mm, have been grown from aqueous solution. The structure is such that Tm3+ ions occur in dimeric pairs with separation 0.4346 nm. The two ions in the pair are related by inversion symmetry, but there is no point symmetry at each thulium ion site. The crystals have been studied by optical absorption (partly with the use of a tuneable dye laser) at 490 nm wavelength; magnetic resonance spectroscopy at 35, 17, 10 and 0.02-0.06 GHz ; and magnetic susceptibility measurements. In spite of the absence of site symmetry, the results show that the ground state of each Tm3+ ion, 4f12, 3H6 is a non-Kramers doublet with a single g value of 13.59(5), Close to the maximum possible value 12 gJ = 13.97. The ground state must therefore be predominantly MJ = | ± 6 >. The axis of the unique g value lies about 10° from the inter-ion vector. The magnetic properties are dominated by interaction between the two Tm ions within each dimer ; in zero applied field this splits the fourfold ground state of the pair by 0.927(15) cm-1, i.e. 27.79(45) GHz, almost identical with the value of 0.926(72) cm-1 corresponding to pure dipolar interaction. Although the lower level is that with parallel spins, there is no evidence of longer range cooperative ordering down to 0.55 K.

Magnetic resonance studies of Dy3+ and Er3+ in lanthanum nicotinate dihydrate

Electron paramagnetic resonance (EPR) measurements have been made on Dy3+ and Er3+ in lanthanum nicotinate dihydrate (LaND) single crystals, for comparison with the information described in the previous paper about single crystals of DyND and ErND. For both DyND and ErND, as for TmND, the measurements were found to be consistent with a ground state with a large amplitude of Mj ═ ±J, an almost Ising-like g-matrix, and a purely magnetic dipole-dipole interaction between nearest neighbour (NN) Ln3+ ions, where Ln is either Dy or Er. In contrast, for Ln3+ dilutely substituted into LaND, although the g-matrix has one principal value much larger than the other two, the principal direction is different from that for LnND. EPR in crystals of LaND containing between 0.01 and 0.1 mole fraction of Ln shows that isolated ions have a g-matrix very similar to that in more dilute crystals, and NN pairs have a g-matrix similar to that for LnND and a purely magnetic dipole-dipole interaction. Hence, the Kramers ions Dy3+ and Er3+ are seen to exhibit the same features as were found for the non-Kramers ions Tb3+ and Tm3+ in the lanthanide nicotinate dihydrates.