Crystal field effects in TmCu2 compound

The splitting of the³H₆ multiplet has been estimated for the Tm³⁺ ion in the crystal electric field of the orthorhombic TmCu₂ compound. Using the energy levels and appropriate eigenfunctions the crystal field only susceptibility has been calculated along the principal orthorhombic axes at temperatures 10 to 300 K. The obtained results are compared with our measurements of specific heat and paramagnetic susceptibility on polycrystalline sample.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.     

Absorption spectra and Zeeman effects of Tm3+ in CaF2

The absorption spectra of Tm³⁺ in CaF₂ were analyzed by the concentration series method at 4·2^°K. Systems of lines, belonging to Tm³⁺ion centers of different structure, are isolated from the general spectra. Some of structures of Tm³⁺ ion centers were made clear by studying optical Zeeman effects and a change of absorption lines during thermal treatment. The tetragonal crystal field parameters and g-value of the upper state are also discussed.     

Skew scattering in CeAl2: Evidence of crystal field effect

Hall effect measurements of polycristalline CeAl₂ were performed between 4.2K and 300 K. The zero-field temperature dependence of the extraordinary Hall coefficient exhibits influence of Ce³⁺ ground state multiplet splitting by crystal field.     

TmTO3 (T = Al, Fe, CrandV). A 169Tm Mössbauer study of magnetic and crystal field properties

¹⁶⁹Tm Mössbauer measurements are reported for the distorted perovskite compounds TmAlO₃, TmFeO₃, TmCrO₃ and TmVO₃ and have been analyzed to provide information concerning the crystal field and magnetic properties of the Tm³⁺ ion. Tm³⁺ has no magnetic moment in TmAlO₃ and low temperature saturated moments of 0.09, 0.8 and 2.1μ_B in TmFeO₃, TmCrO₃ and TmVO₃, respectively. In TmFeO₃ and TmCrO₃ the magnitudes and directions of the Tm³⁺ moments are examined in terms of previously proposed magnetic structural models. In TmVO₃, the crystal field properties are very different to those in the ferrite and chromite and a reorientation of the Tm³⁺ magnetic moments, accompanied by a sharp increase in their size, is observed near 10 to 15 K.     

On the influence of dopant ions on blue emission in kgw crystal excited by infrared laser radiation

The role of doping by Nd³⁺ and Tm³⁺ ions in the generation of blue emission in a KGW crystal excited by infrared radiation has been analyzed. Experimental measurements of the emission characteristics and theoretical results of modeling emission spectra of the doping ions are compared. A qualitative difference is shown between the calculated Tm³⁺ luminescence spectrum and that observed in the experiment. Data on branching ratios of luminescence from the ²P_1/2 level of the Nd³⁺ ions suggest a low emission efficiency from this multiplet in the blue spectral region.     

First observation of a negative elastic constant in intermediate valent TmSe

The sound velocities v_L, v_T1 and v_T2 have been measured at 15 MHz on Tm³⁺_0.87Se and Tm^2.8+_0.99Se between 300 K and 4.2 K and the elastic constants c_ij have been derived. In intermediate valent Tm^2.8+_0.99Se c₁₂ turned out to be negative. This sign is interpreted as being typical for intermediate valent compounds. Also for the first time experimental evidence is given for crystal field effects in Tm³⁺_0.87Se. Strong elastic nonlinearities are observed in intermediate valent Tm^2.8+_0.99Se with uniaxial pressure.     

EPR and optical spectroscopy of the Tm2+ ion in the KMgF3 single crystal

Paramagnetic Tm²⁺ ion centers in the KMgF₃ single crystal have been studied by electron paramagnetic resonance and optical spectroscopy. The Stark level energies of the cubic Tm²⁺ multiplets have been established from the luminescence spectra and the crystal field parameters have been calculated. Information about the phonon spectra of KMgF₃ crystals has been obtained from the electron-vibrational structure of the optical luminescence spectra.     

Van Vleck paramagnetism of the thulium garnet Tm3Al5O12

The magnetic susceptibility of the garnet-type single crystal Tm₃Al₅O₁₂ exhibits the typical Van Vleck temperature independent paramagnetism below ≈8 K. The temperature dependence of the susceptibility over the range 2.0-300 K has been analyzed on the assumption that the cubic crystal-electric-field dominates the energy level on ³H₆ (J=6) ground multiplet for Tm³⁺ ion having 12-electrons in 4f shell. The ground state of the ³H₆ is nonmagnetic with Γ₂ singlet, avoiding the Kramers doublet. The energy separation between Γ₂ and the first excited state Γ⁽²⁾₅ triplet is evaluated to be 68.0 K. The whole energy interval Δ between Γ₂ and the highest state Γ₁ in ³H₆ is estimated to be 339.5 K.     

Anomalies of the specific heat near the quantum critical point in Tm<Subscript>0.74</Subscript>Yb<Subscript>0.26</Subscript>B<Subscript>12</Subscript>

The behavior of the specific heat near the quantum critical point x ∼ 0.3 in the Tm_{1 − x} Yb _x B₁₂ system has been studied. Detailed measurements have been performed on high-quality single-crystalline Tm_0.74Yb_0.26B₁₂ samples within a wide temperature range of 1.9–300 K in a magnetic field up to 9 T. The temperature dependence of the magnetic contribution to the specific heat has a logarithmic divergence of the form C/T ∼ lnT at T < 4 K, which can be attributed to the quantum critical behavior regime suppressed by the external magnetic field. The Schottky anomaly of the magnetic contribution to the specific heat in Tm_0.74Yb_0.26B₁₂ has been analyzed.     

Magnetic, thermal and hyperfine behaviours of Tm3+ in TmPO4, YPO4 and LuPO4: a comparative study

Single crystals of thulium phosphates (TmPO₄) are grown and the principal magnetic susceptibility perpendicular to the symmetry axis ‘c’ of the crystal $$\left( {{\text{ $ \chi $ }}_ \bot } \right)$$ and the magnetic anisotropy $$\Delta {\text{ $ \chi $ }}\left( {{\text{ = $ \chi $ }}_\parallel - {\text{ $ \chi $ }}_ \bot } \right)$$ are measured in the temperature range 300–13.5 K and 300–80 K, respectively. Though $${\text{ $ \chi $ }}_ \bot $$ increases rapidly with the decrease of temperature, $${\text{ $ \chi $ }}_\parallel $$ increases very slowly with the lowering of temperature. The tetragonal crystal structure of TmPO₄ is similar to that of Tm³⁺ in YPO₄ and LuPO₄ and in all the cases the non-Kramers Tm³⁺ ion occupies a site of D_2d symmetry. Our observed magnetic data on TmPO₄ are analyzed using crystal field analysis where the Hamiltonian includes the atomic free ion and crystal field (one-electron) interaction term. The computed and measured paramagnetic susceptibilities of TmPO₄ agree very well in the temperature range of our study. The magnetic behaviours of Tm³⁺ in YPO₄ and LuPO₄ are also studied and the results are compared with that of TmPO₄. In all the cases the natures of thermal variations of average susceptibilities and magnetic anisotropies are mostly governed by the perpendicular susceptibilities as their values are higher and rapidly increase with the lowering of temperature compared to parallel susceptibilities. The computed nuclear quadruple splittings, electronic heat capacities of Tm³⁺ in three different hosts give some interesting results.     

Analysis of the ground configuration of Tm3+ in YVO4

An analysis has been made of the optical spectrum of Tm³⁺ in YVO₄ reported by K. D. Knoll. The approach taken was to describe the Tm³⁺ free ion as completely as possible and then to describe the effects of the crystal field on these levels. Parameters were obtained that gave an r.m.s. deviatioa of 10.6 cm^?1 in a least squares fit. In Knolls' earlier analysis, which did not describe the free ion adequately, several sets of parameters were given that are consistent with different portions of the spectrum. The results computed with our parameters gave a new interpretation of the ³F₄ and ³H₅ multiplets and gave g_∥ factors in good agreement with the measurements. The efiects of the hyperfine interaction and the spin-crystal field interaction on the energy levels resulted in displacements of smaller than 0.1 cm^?1.     

Thermal and optical properties of Tm: Li6Gd(BO3)3 crystal: A potential candidate for 1.83 μm lasers

Single crystal of Tm³⁺: Li₆Gd (BO₃)₃ was grown by the Czochralski method. The heat capacity was measured from 308 to 673 K. The absorption spectra of the crystal in three mutually perpendicular arbitrary directions were measured at room temperature. Based on the Judd-Ofelt theory and the spectra measured in three mutually perpendicular directions, the intensity parameters Ω_t (t=2, 4, 6), the line strengths, the oscillator strengths, the radiative rates, radiative lifetimes and fluorescent branching ratios were calculated. We calculated the emission cross-section by the reciprocity method and also obtained the gain cross-section.     

Hyperfeinstrukturaufspaltung von rückstoßfreien Gammalinien

The temperature dependence of the hyperfine splitting of the 8.42 keVγ-ray of Tm¹⁶⁹ in oxide crystals was investigated using the Mößbauer-effect. The quadrupole splitting of the first excited state in Tm¹⁶⁹ showed a pronounced increase with decreasing temperature. This manifests a contribution of various Stark levels, which are occupied according to their Boltzmann factors, to an average field gradient at the nucleus. It is shown in a model calculation for the temperature dependence of the field gradient that the data can be explained using a maximum splitting of about 400 cm^?1 of the electronic groundstate by the crystal field. The emission pattern of the Er₂O₃-source is broadened below 20° K. This is possibly produced by an interaction with a residual magnetic field from the surrounding Er-ions.     

Influence of the heat treatment on the nucleation of silver nanoparticles in Tm3+ doped PbO-GeO2 glasses

Nucleation of silver nanoparticles (NPs) in Tm³⁺ doped PbO-GeO₂ (PGO) glass is reported. The influence of the heat treatment on the nucleation of silver NPs is studied by means of transmission electron microscopy and optical spectroscopy. Two heat treatment procedures were applied in order to compare their performance. Observation of infrared-to-visible frequency upconversion (UC) luminescence of Tm³⁺ ions is reported and correlated with the heat-treatment procedure. Enhancement of the UC emission for samples heat treated during various time intervals is attributed to the increased local field in the vicinity of the NPs. Quenching of the UC signal was also observed and correlated with the growth of NPs amount and size.     

Low temperature heat capacities and thermal properties of DyAl2, ErAl2 and LuAl2

The heat capacities of the compounds DyAl₂, ErAl₂ and LuAl₂ were measured in an adiabatic calorimeter from approximately 5 to 300 K. The compounds DyAl₂ and ErAl₂ show C_P anomalies at 58.0 and 10.2 K, respectively, which are attributed to the destruction of magnetic order. In order to separate the crystal field and magnetic contributions from the measured heat capacities, it was necessary to evaluate the lattice heat capacity. The lattice term, C_L was obtained from the C_P data of LuAl₂ by a method of interpolation which gave values of C_L for an arbitrary R Al₂ compound. Using this “interpolated lattice blank”, excess entropies associated with the crystal field and magnetic terms were computed throughout the series. These values are quite close to R In (2J + 1). The results also indicate that, for the compounds studied, the degeneracy of the lowest ground state is completely lifted. In addition, the magnetic contribution to the heat capacity of the magnetically ordered R A1₂ phases was found to exhibit an exponential dependence below the temperature corresponding to the spin wave energy gap and a T^{$\text{3}\text{2}$} dependence above this temperature.Detailed calculations were performed to characterize the influence of cubic crystal field in ErAl₂ on the ⁴I_{$\text{15}\text{2}$} ground state multiplet of the Er³⁺ ion. It is concluded that the magnetic ordering in ErAl₂ takes place within the Γ₈³ quartet state. Smoothed values of heat capacity, entropy and related thermodynamic functions are tabulated.     

Paramagnetic fluctuations in Yb3Ga5O12 investigated with 172Yb PAC probe

¹⁷²Yb PAC measurements were carried out on Yb₃Ga₅O₁₂ at temperatures ranging from 14 to 1100 K. The time dependent hyperfine interaction is clearly evident below room temperature. It is attributed to spin fluctuations of Yb ion in the ground state of the ²F_7/2 multiplet. Above 500 K, the temperature dependence of the electric field gradient is shown to result from a varying population of the crystal electric field levels. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic properties of TmxDy1−xFeO3 single crystals

The magnetic behaviour of Tm_xDy_1?xFeO₃ (x = 0.3; 0.5; 0.7; 1.0) single crystals in the temperature range 90-4.2° K were investigated. The transition from the weak ferromagnetic to antiferromagnetic state was observed at about 9° K for Tm_0.3Dy_0.7FeO₃ single crystal. The reorientation of weak ferromagnetic moment from c- to a-axis was observed for Tm_0.7Dy_0.3FeO₃ single crystal at 35–65° K. The magnetic structure change of iron and rare-earth ions took place when external magnetic field was applied. The thulium and disprosium ion interaction does not essentially influence on the single crystal magnetic properties of the substituted compounds in a low temperature range.     

Spezifische Wärme der Nitride Seltener Erden

The specific heat of rare earth nitrides has been measured at temperatures between 1.7 ans 270°K by means of an adiabatic calorimeter. The various terms contributing to the total specific heat have been separated. The total splitting of the ground-state multiplet of the trivalent RE-ions in the octahedral crystal field has been determined. The experimental values agree reasonably well with those calculated with the point charge model. The spontaneous magnetization and the magnetic specific heat below the ordering temperature are calculated with the molecular field theory by taking into account all the 2J+1 energy levels of the ground-state multiplet and including exchange interaction and crystal field splitting. Comparision is made with the experimental results, and the values obtained for the exchange energies are listed.     

Examination of the various crystal field theories of d3 ions in trigonal fields

Zero-field splitting and g-anisotropy of the ground multiplet of HfS₂(V²⁺) have been measured by electron paramagnetic resonance. The unusually large g-anisotropy and its relationship to the zero-field splitting alow a comparison between various crystal field theories. We find that the experimental results are in disagreement with the simple crystal field theory of Van Vleck for the ground multiplet of d³ ions in trigonally distroted octahedral environments, but in reasonable agreement with the more elaborate but straightforward crystal field theory of Macfarlane. We thus conclude that it is unnecessary to resort to an anisotropic spin-orbit coupling as suggested by Sugano and Tanabe or to odd-parity crystal field terms as suggested by Artman and Murphy.     

Magnetic contributions to the low-temperature specific heat of the ferromagnetic insulator Pr 0.8 Ca 0.2 MnO 3

The Pr _1-x Ca_xMnO₃ system exhibits a ferromagnetic insulating state for the composition range x ? 0.25. A metallic ferromagnetic state is never realized because of the low hole concentration and the very small averaged A-site cation radius. In the present study, the nature of the magnetic excitations at low temperature has been investigated by specific heat measurements on a Pr _0.8 Ca_0.2MnO₃ single crystal. The decrease of the specific heat under magnetic field is qualitatively consistent with a suppression of ferromagnetic spin waves in a magnetic field. However, at low temperature, the qualitative agreement with the ferromagnetic spin waves picture is poor. It appears that the large reduction of the specific heat due to the spin waves is compensated by a Schottky-like contribution possibly arising from a Zeeman splitting of the ground state multiplet of the Pr³⁺ ions. Received 21 May 2001 and Received in final form 14 December 2001