A novel, to the best of our knowledge, ultralow-temperature luminescence thermometry strategy is proposed, based on a measurement of relative intensities of hyperfine components in the spectra of Ho3+ ions doped into a crystal. A (LiYF4)-Li-7:Ho3+ crystal is chosen as an example. First, we show that temperatures in the range 10 - 35 K can be measured using the Boltzmann behavior of the populations of crystal-field levels separated by an energy interval of 23 cm(-1). Then we select the 6089 cm(-1) line of the holmium I-5(5) -> I-5(7) transition, which has a well-resolved hyperfine structure and falls within the transparency window of optical fibers (telecommunication S band), to demonstrate the possibility of measuring temperatures below 3 K. The temperature T is determined by a least-squares fit to the measured intensities of all eight hyperfine components using the dependence I(nu) = I-1 exp (-b nu), where I-1 and b = a +1 /kT are fitting parameters and a accounts for intensity variations due to mixing of wave functions of different crystal-field levels by the hyperfine interaction. In this method, the absolute and relative thermal sensitivities grow at T approaching zero as 1/T(2)and 1/T, respectively. We theoretically considered the intensity distributions within hyperfine manifolds and compared the results with experimental data. Application of the method to experimentally measured relative intensities of hyperfine components of the 6089 cm(-1) PL line yielded T = 3.7 +/- 0.2 K. For a temperature of 1 K, an order of magnitude better accuracy is expected.

We report high-resolution measurements and analysis of the optical transmission and emission spectra of a concentration series of La(1-x)PrxAlO3 (x=10-3, 9 10-3 and 2 10-2) single crystals in a wide frequency range ((2-27) 103 cm-1) at temperatures 5-300 K, below the structural phase transition from the cubic to the rhombohedral phase. Pr3+ ions substitute for La3+ ions at sites with point symmetry D3. All recorded spectral lines are assigned to specific initial and final crystal-field (CF) energy levels, the scheme of CF levels is constructed and described by an appropriate set of CF parameters. The structural phase transition is accompanied by shear strains and the formation of ferroelastic twin domains of four types compressed along one of the four C3 axes in the parent cubic crystal lattice. Specific features of the measured spectra, namely, anomalous broadening of spectral lines and doublet structure of some lines corresponding to transitions between CF singlets and doublets, are considered as a result of interaction of 4f electrons with the field of random shear deformations competing at the boundaries of twin domains. Modeling of the observed line shapes was performed taking into account both hyperfine interactions and random lattice strains, as well as the effect of temperature. The width (6.6±0.7)⋅10− 4 of the introduced two-dimensional distribution function of random strains was found from a comparison of simulated and measured profiles of the split spectral lines. The results of this work can be applied for quantitative assessment of crystal quality.

Experimental and theoretical studies of the dynamic magnetization in swept magnetic fields of the orthorhombic SrY2O4 single crystals doped with the Dy3+ Kramers ions (0.01 at. % and 0.5 at. %) with natural abundances of even and odd Dy isotopes are presented. Impurity ions substitute for Y3+ ions at two nonequivalent crystallographic sites with the same local Cs symmetry but strongly different crystal fields. Well -pronounced double -loop hysteresis is observed at temperatures 2, 4, 5, and 6 K for sweeping rates of 5 and 1 mT/s. The microscopic model of spectral, magnetic, and kinetic properties of Dy3+ ions is developed based on the results of electron paramagnetic resonance, site -selective optical spectroscopy, and magnetic relaxation measurements. The derived approach to the dynamic magnetization in the sweeping field based on the numerical solution of generalized master equations with time -dependent transition probabilities induced by the electron -phonon interaction, quantum tunneling, and cross relaxation allowed us to reproduce successfully the evolution of the hysteresis loop shape with temperature, sweeping rate, and concentration of paramagnetic ions.

Experimental and theoretical studies of the dynamic magnetization in swept magnetic fields of the orthorhombic ${\mathrm{SrY}}_{2}{\mathrm{O}}_{4}$ single crystals doped with the ${\mathrm{Dy}}^{3+}$ Kramers ions (0.01 at. % and 0.5 at. %) with natural abundances of even and odd Dy isotopes are presented. Impurity ions substitute for ${\mathrm{Y}}^{3+}$ ions at two nonequivalent crystallographic sites with the same local ${C}_{s}$ symmetry but strongly different crystal fields. Well-pronounced double-loop hysteresis is observed at temperatures 2, 4, 5, and 6 K for sweeping rates of 5 and 1 mT/s. The microscopic model of spectral, magnetic, and kinetic properties of ${\mathrm{Dy}}^{3+}$ ions is developed based on the results of electron paramagnetic resonance, site-selective optical spectroscopy, and magnetic relaxation measurements. The derived approach to the dynamic magnetization in the sweeping field based on the numerical solution of generalized master equations with time-dependent transition probabilities induced by the electron-phonon interaction, quantum tunneling, and cross relaxation allowed us to reproduce successfully the evolution of the hysteresis loop shape with temperature, sweeping rate, and concentration of paramagnetic ions.

We present the optical and magneto-optical spectroscopy and electron paramagnetic resonance (EPR) investigations of CaMoO4 single crystals doped with the erbium ions. Telecom-wavelength resonance transition inhomogeneous line width of Er3+ is relatively narrow for oxide crystals which makes this material promising for quantum technologies applications. The hyperfine structure in optical spectra of 167Er3+ isotope is well resolved. Energies and symmetries of wavefunctions of 39 energy levels of Er3+ ions in the crystal-field (CF) of S4 symmetry and g-factors of some CF Kramers doublets were measured and successfully simulated on the basis of CF calculations. The obtained set of CF parameters was used for modeling the hyperfine structure profiles observed in the optical absorption spectra.

Experimental and theoretical studies of the dynamic magnetization in swept magnetic fields of the orthorhombic SrY_2O_4 single-crystals doped with the Dy^3+ Kramers ions (0.01 and 0.5 at. odd Dy isotopes are presented. Impurity ions substitute for Y^3+ ions at two nonequivalent crystallographic sites with the same local C_s symmetry but strongly different crystal fields. Well pronounced double-loop hysteresis is observed at temperatures 2, 4, 5 and 6 K for sweeping rates of 5 and 1 mT/s. The microscopic model of spectral, magnetic and kinetic properties of Dy^3+ ions is developed based on the results of EPR, site selective optical spectra and magnetic relaxation measurements. The derived approach to the dynamic magnetization in the sweeping field based on the numerical solution of generalized master equations with time-dependent transition probabilities induced by the electron-phonon interaction, quantum tunneling and cross-relaxation allowed us to reproduce successfully the evolution of the hysteresis loop shape with temperature, sweeping rate and concentration of paramagnetic ions.

EPR spectra of impurity Ho3+ ions in oriented SrY2O4 single-crystals are registered at the temperature 4.2 K in the frequency range from 70 to 180 GHz. The results of measurements evidence for the substitution of Ho3+ ions for the Y3+ ions at the structurally nonequivalent sites R1 and R2 with the local Cs point symmetry. The values of g-factors, hyperfine structure constants and the energy gaps between the ground and the first excited non-degenherate crystal-field sublevels of the ground 5I8 multiplet are determined. The observed specific features of the ground states of Ho3+ ions (non-Kramers doublets with the zero-field splittings of 4.30 and 1.67 cm–1) open a possibility to identify transitions in optical spectra of SrY2O4:Ho and inelastic neutron scattering spectra of SrHo2O4 crystals.

Detailed studies of emission and excitation spectra of impurity Ho3+ centers in the SrY2O4 single crystal performed by means of the site-selective laser spectroscopy are reported. The energy levels patterns for the 5I8, 5I7, 5I6, 5S2 and 5F4 multiplets of Ho3+ ions which substitute for the Y3+ ions at the two structurally nonequivalent sites Y1 and Y2 with the local Cs symmetry are constructed by making use of the measured spectra. These energy schemes are well described in the framework of the crystal field theory, the sets of 15 crystal field parameters are determined for Ho3+ ions at both Y1 and Y2 sites. The calculated parameters of the EPR spectra agree with the earlier published experimental data.

EPR spectra of impurity Ho3+ ions in oriented SrY2O4 single-crystals are registered at the temperature 4.2 K in the frequency range from 70 to 180 GHz. The results of measurements evidence for the substitution of Ho3+ ions for the Y3+ ions at the structurally nonequivalent sites R1 and R2 with the local Cs point symmetry. The values of g-factors, hyperfine structure constants and the energy gaps berween the ground and the first excited non-degenherate crystal-field sublevels of the ground 5I8 multiplet are determined. The observed specific features of the ground states of Но3+ ions (non-Kramers doublets with the zero-field splittings of 4.30 and 1.67 cm-1) open a possibility to identify transitions in optical spectra of SrY2O4:Ho and inelastic neutron scattering spectra of SrHo2O4 crystals.

EPR spectra of impurity Ho3+ ions in oriented SrY2O4 single-crystals are registered at the temperature 4.2 K in the frequency range from 70 to 180 GHz. The results of measurements evidence for the substitution of Ho3+ ions for the Y3+ ions at the structurally nonequivalent sites R1 and R2 with the local Cs point symmetry. The values of g-factors, hyperfine structure constants and the energy gaps berween the ground and the first excited non-degenherate crystal-field sublevels of the ground 5I8 multiplet are determined. The observed specific features of the ground states of Но3+ ions (non-Kramers doublets with the zero-field splittings of 4.30 and 1.67 cm-1) open a possibility to identify transitions in optical spectra of SrY2O4:Ho and inelastic neutron scattering spectra of SrHo2O4 crystals.

We analyze the field-dependent intensities of the coupled electron-phonon modes observed in the low-temperature far-infrared (terahertz) reflection spectra of PrFe3(BO3)4 and develop a theory based on the Green’s function approach. An excellent agreement between the experimental and theoretical data is achieved. The developed theory of the intensity transfer from phonons to quasi-electronic excitations can be applied to the electron-phonon modes in other compounds, in particular, in magnetodielectric materials, where it can be used to analyze the magnetodielectric response.

Resolved hyperfine structure and narrow inhomogeneously broadened lines in the optical spectra of a rare-earth-doped crystal are favorable for the implementation of various sensors. Here, a well-resolved hyperfine structure in the photoluminescence spectra of LiYF4:Ho single crystals and the anticrossings of hyperfine levels in a magnetic field are demonstrated using a self-made setup based on a Bruker 125HR high-resolution Fourier spectrometer. This is the first observation of the resolved hyperfine structure and anticrossing hyperfine levels in the luminescence spectra of a crystal. The narrowest spectral linewidth is only 0.0022 cm(-1). This fact together with a large value of the magnetic g factor of several crystal-field states creates prerequisites for developing magnetic field sensors, which can be in demand in modern quantum information technology devices operating at low temperatures. Very small random lattice strains characterizing the quality of a crystal can be detected using anticrossing points.

We present the low-temperature magnetic structures of SrGd 2 O 4 combining neutron diffraction methods on polycrystalline and single-crystal samples containing the 160 Gd isotope. In contrast to other members of the Sr Ln 2 O 4 family ( Ln = lanthanide) this system reveals two long-range ordered magnetic phases, which our diffraction data unambiguously identify. Below T N1 = 2.73 K, a q 1 = (0 0 0) magnetic structure is stabilized where ferromagnetic chains along the c axis (space group Pnam ) are coupled antiferromagnetically with neighboring chains. On cooling below T N2 = 0.48 K, an additional incommensurate component modulated by q 2 = (0 0 0 . 42) evolves and is aligned along either of the perpendicular axes for the two different Gd sites, resulting in a fanlike magnetic structure. The identification of the particular Gd sites with the magnetic order observed with neutron diffraction is facilitated by a detailed analysis of the crystal fields acting on the sites. The observed ordering phenomena underline the complex multiaxial anisotropy in this system. DOI

We report high-resolution infrared Zeeman spectra in the region of transitions between electronic singlets of the ground 5I8 and the first excited 5I7 crystal-field manifolds of Ho3+ in 7LiYF4 in a magnetic field directed along the tetragonal c axis of the crystal. Well-resolved hyperfine structure exhibiting non-linear Zeeman hyperfine splittings and crossings of components is observed. The experimental data are successfully explained on the basis of crystal-field calculations.

Comprehensive spectroscopic, magnetization, and theoretical studies of a LaAlO3:Tm3+ single crystal in the ferroelastic R3‾c phase are reported. The Tm3+ ions substitute for the La3+ ions at sites with the D3 symmetry. High-resolution absorption, photoluminescence, and site-selective emission and excitation spectra were measured in the broad spectral range from 4000 to 28000 cm−1 at temperatures 4.2–5 K. The two-fold degeneracy of the ground state of Tm3+ was uncovered by magnetization measurements. Energies and symmetry properties of wave functions of crystal-field levels of the Tm3+ ions were determined and successfully reproduced by crystal-field calculations. Specific profiles with a dip at the center of spectral lines corresponding to transitions involving non-Kramers doublets give evidence for random strains in the studied multidomain sample. The value of 0.1–0.8 cm−1 of deformational splitting of non-Kramers doublets exceeds hyperfine splittings by more than an order of magnitude. The observed line shapes were successfully modeled assuming the interaction of Tm3+ ions with random deformations described by the generalized two-dimensional Lorentzian distribution with the width of (7 ± 0.5)⋅ 10−4. The simulation was performed using the electron-deformation coupling constants calculated in the framework of the exchange-charge model.

EPR spectra of impurity Ho3+ ions in oriented SrY2O4 single-crystals are registered at the temperature 4.2 K in the frequency range from 70 to 180 GHz. The results of measurements evidence for the substitution of Ho3+ ions for the Y3+ ions at the structurally nonequivalent sites R1 and R2 with the local Cs point symmetry. The values of g-factors, hyperfine structure constants and the energy gaps berween the ground and the first excited non-degenherate crystal-field sublevels of the ground 5I8 multiplet are determined. The observed specific features of the ground states of Но3+ ions (non-Kramers doublets with the zero-field splittings of 4.30 and 1.67 cm-1) open a possibility to identify transitions in optical spectra of SrY2O4:Ho and inelastic neutron scattering spectra of SrHo2O4 crystals.

We measured the heat capacity, magnetic susceptibility, magnetization, magnetoelectric effect, and optical spectra of samarium chromium borate crystals. It was found that, in SmCr3(BO3)(4) single crystals, structures with the space groups R32 and C2/c may coexist. Magnetic phase transitions were discovered at temperatures T-N1 = 7.8 +/- 0.5 K, T-N2 = 6.7 +/- 0.5 K, and T-3 = 4.3 +/- 0.2 K, and assumptions about their nature were made. Based on the high-resolution optical spectra and magnetometry data, the crystal-field parameters in the Sm3+ and Cr3+ positions and the parameters of Cr3+-Cr3+ and Cr3+-Sm3+ exchange interactions were determined. The magnetic properties of the quasi-one-dimensional chromium subsystem were analyzed in the frame of the previously developed self-consistent four-particle cluster model. Despite a strong suppression of Cr3+ magnetic moments by the interchain antiferromagnetic exchange interactions, the contributions of the chromium subsystem to the bulk magnetization are dominant.

The magnetic structure is usually determined by the neutron diffraction measurements. However, in the case of complex multisublattice magnetics, this method fails to give an unambiguous result. Here, on the example of multiferroic ${\mathrm{HoFe}}_{3}{({\mathrm{BO}}_{3})}_{4}$, we show that in the case of rare-earth (RE) compounds the right magnetic structure can be determined by additionally using optical spectroscopy and a theoretical analysis based on spectroscopic data. ${\mathrm{HoFe}}_{3}{({\mathrm{BO}}_{3})}_{4}$ demonstrates a series of phase transitions and interesting magnetic and magnetoelectric properties. The available information on the magnetic structure of the compound, necessary for understanding and utilizing these properties, is contradictory. To resolve the existing ambiguities, we apply a combined approach. The high-resolution spectroscopy data deliver a set of the ${\mathrm{Ho}}^{3+}$ crystal-field (CF) levels in the paramagnetic and both easy-plane and easy-axis magnetic phases. These data are used to determine CF and ${\mathrm{Ho}}^{3+}\text{\ensuremath{-}}{\mathrm{Fe}}^{3+}$ exchange parameters and, then, to calculate the temperature dependencies of the magnetic susceptibility tensor of ${\mathrm{HoFe}}_{3}{({\mathrm{BO}}_{3})}_{4}$. Based on these calculations, we suggest an easy-plane antiferromagnetic structure with a collinear arrangement of the Fe spins along the $a$ axis and induced noncolinear moments of magnetically nonequivalent Ho ions. The suggested structure is further confirmed by single-crystal elastic neutron scattering experiments. We argue that specific features of the magnetic properties of RE iron borates isostructural to ${\mathrm{HoFe}}_{3}{({\mathrm{BO}}_{3})}_{4}$ are governed by the energy patterns and the symmetry properties of the wave functions of the lower CF levels of the RE ground multiplet in the crystal field of the ${C}_{2}$ symmetry.

High-resolution spectroscopic studies of $\mathrm{LaAl}{\mathrm{O}}_{3}$ single crystal doped with holmium ions are reported. Polarized and unpolarized absorption and luminescence spectra were measured in the broad spectral range from 2000 to $23\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--1}$ at temperatures of 4.5--5 K. Additional measurements were fulfilled using site-selective laser spectroscopy. Energies and symmetry properties of the corresponding wave functions of crystal-field levels of ${\mathrm{Ho}}^{3+}$ ions which substitute for ${\mathrm{La}}^{3+}$ ions in $\mathrm{LaAl}{\mathrm{O}}_{3}$ at sites with the ${D}_{3}$ symmetry were determined with high accuracy, and, on this basis, crystal-field calculations were performed. A thorough analysis of spectral line profiles with the fine doublet structure corresponding to singlet-doublet transitions in the trigonal crystal field was made, which showed the existence of the random deformation splitting phenomenon. The value of deformation splitting lies in the region $0.3--0.8\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--1}$ that substantially exceeds the widths of the doublet hyperfine splitting. The observed line shapes were successfully modeled, assuming the interaction of the ${\mathrm{Ho}}^{3+}$ ions with random deformations of the crystal lattice induced by point defects and ferroelastic domain boundaries.