Cross-relaxation and phonon bottleneck effects on magnetization dynamics in LiYF4:Ho3+

Frequency and dc magnetic field dependences of dynamic susceptibility in diluted paramagnets LiYF$_4$:Ho$^{3+}$ have been measured at liquid helium temperatures in the ac and dc magnetic fields parallel to the symmetry axis of a tetragonal crystal lattice. Experimental data are analyzed in the framework of microscopic theory of relaxation rates in the manifold of 24 electron-nuclear sublevels of the lowest non-Kramers doublet and the first excited singlet in the Ho$^{3+}$ ground multiplet $^5I_8$ split by the crystal field of S$_4$ symmetry. The one-phonon transition probabilities were computed using electron-phonon coupling constants calculated in the framework of exchange charge model and were checked by optical piezospectroscopic measurements. The specific features observed in field dependences of the in- and out-of-phase susceptibilities (humps and dips, respectively) at the crossings (anti-crossings) of the electron-nuclear sublevels are well reproduced by simulations when the phonon bottleneck effect and the cross-spin relaxation are taken into account

Numerical adiabatic potentials of orthorhombic Jahn-Teller effects retrieved from ultrasound attenuation experiments. Application to the SrF2:Cr crystal

A methodology is worked out to retrieve the numerical values of all the main parameters of the six-dimensional adiabatic potential energy surface (APES) of a polyatomic system with a quadratic T-term Jahn-Teller effect (JTE) from ultrasound experiments. The method is based on a verified assumption that ultrasound attenuation and speed encounter anomalies when the direction of propa- gation and polarization of its wave of strain coincides with the characteristic directions of symmetry breaking in the JTE. For the SrF2:Cr crystal, employed as a basic example, we observed anomaly peaks in the temperature dependence of attenuation of ultrasound at frequencies of 50-160 MHz in the temperature interval of 40-60 K for the wave propagating along the [110] direction, for both the longitudinal and shear modes, the latter with two polarizations along the [001] and [110] axes, respectively. We show that these anomalies are due to the ultrasound relaxation by the system of non-interacting Cr2+ JT centers with orthorhombic local distortions. The interpretation of the ex- perimental findings is based on the T2g (eg +t2g) JTE problem including the linear and quadratic terms of vibronic interactions in the Hamiltonian and the same-symmetry modes reduced to one interaction mode. Combining the experimental results with a theoretical analysis we show that on the complicated six-dimensional APES of this system with three tetragonal, four trigonal, and six orthorhombic extrema points, the latter are global minima, while the former are saddle points, and we estimate numerically all the main parameters of this surface, including the linear and quadratic vibronic coupling constants, the primary force constants, the coordinates of all the extrema points and their energies, the energy barrier between the orthorhombic minima, and the tunneling splitting of the ground vibrational states.Comment: 8 pages, 3 figure

High-frequency EPR of Tb3+-doped KPb2Cl5 crystal

High-frequency electron paramagnetic resonance (EPR) spectra of the KPb2Cl5:Tb3+ crystal have been investigated. Three types of spectra were observed in the frequency range of 74-200 GHz. The most intensive spectrum with the resolved hyperfine structure corresponded to transitions between sublevels of the 159Tb3+ ground quasi-doublet with the zero-field splitting (ZFS) close to 48 GHz. Experimental results were analyzed by the exchange charge model of the crystal field affecting terbium ions in low-symmetry Pb2+ positions with the chlorine sevenfold coordination and the charge compensating vacancy in the nearest potassium site. The calculated values of g-factors and ZFS were in agreement with the experimental data. The nature of a broad EPR line with ZFS of about 180 GHz and of additional weak EPR lines observed as satellites of the main Tb3+ lines was discussed

Submillimeter EPR spectroscopy of lanthanide compounds: Pair centers of Ho3+ in CsCdBr3

High-frequency EPR spectra corresponding to magnetic dipole transitions between the doublet-singlet-doublet group of the lowest crystal field levels of the trigonal single ion and the symmetric dimer centers in CsCdBr3 have been studied. For the dimer center, the absorption spectra in the manifold of the lowest 1024 electron-nuclear states are simulated by successive diagonalization of the electron-nuclear Hamiltonian which includes the crystal field, the electronic Zeeman energies, the magnetic and quadrupole hyperfine interactions, and the dipole-dipole interactions between the paramagnetic ions. The crystal field parameters of Ho3+ ions in the symmetric dimer are determined. © 1998 Elsevier Science S.A

Submillimeter electron-nuclear excitation spectra in CsCdBr3:Ln3+ (Ln=Tm, Ho) crystals

The ESR spectra of single and pair impurity centers of thulium and holmium ions in CsCdBr3:Tm3+ and CsCdBr3:Ho3+ crystals are measured in the frequency range 160-400 GHz. Analysis of the characteristic features of the hyperfine structure of the ESR lines and analysis of the variations in the spectra as a function of the temperature and external magnetic field shows that the Ln3+ ions substitute for Cd2+ ions and predominantly form symmetric pair centers of the type Ln3+-(vacancy at a neighboring Cd2+ site)-Ln3+. The ESR spectra of CsCdBr3:Ln3+ crystals are used to make a positive identification of the optical spectra of selective laser excitation. © 1997 American Institute of Physics

Multifrequency EPR study of Cr3+ ions in LiScGeO4

An electron paramagnetic resonance study of a synthetic single crystal of Cr-doped LiScGeO4 was carried out at the X-and Q-bands at 300 K and at the broad band (70-370 GHz) at 4.2 K. It was established that the EPR spectra with the magnetic multiplicity KM = 2 observed in all the frequency bands are due to the Cr3+ substituted for Sc3+ at the mirror symmetry octahedral site. The angular dependences of the two symmetry-related spectra of Cr3+ in the three crystallographic planes were fitted with the spin Hamiltonian (S = 3/2) of monoclinic symmetry. The zero-field splitting of the ground state energy levels was determined as 1.309(5) cm-1 which compares well with that for Cr3+ in forsterite and alexandrite crystals with similar olivine-like structure. Additional weak lines due to Mn2+ and Fe3+ at the mirror symmetry sites were also identified in the X- and Q-band spectra

STABILIZATION ENERGIES OF THE JAHN-TELLER COMPLEXES IN CaF2:Cr2+ CRYSTAL

In CaF2 crystal doped with Cr2+ ions, attenuation of all the normal ultrasonic modes with the wave vector k were investigated at 26 -158 MHz in the temperature region of 4 - 170 K. The observed peaks of relaxation origin were interpreted as manifestation of the Jahn-Teller effect

Submillimeter ESR spectra of Fe²⁺ ions in synthetic and natural beryl crystals

© 2017, Pleiades Publishing, Ltd. Electron spin resonance spectra of non-Kramers bivalent iron (Fe 2+ ) ions have been detected in synthetic and natural beryl crystals with an iron impurity. The observed ESR spectra have been attributed to resonance transitions of Fe 2+ ions from the ground (singlet) state to excited (doublet) levels with the splitting Δ = 12.7 cm –1 between the levels. The experimental angular and frequency dependences of the resonance field of the ESR signal have been described by the spin Hamiltonian with the effective spin S = 1. The analysis of the ESR data and optical absorption spectra indicates that the Fe 2+ ions are situated in tetrahedral positions and substitute Be 2+ cations in the beryl structure