Electron spin-lattice relaxation of Yb3+ and Gd3+ ions in glasses

The electron spin-lattice relaxation rate (T ₁ ^?1) was measured in two glass samples: (i) a phosphate glass doped with 1 wt% Yb₂O₃ and (ii) a Li₂Si₄O₉ glass sample doped with 0.2 wt% Gd₂O₃. In the Yb³⁺-doped glass sample,T ₁ was measured by an electron-spin-echo technique from 4.2 to 6 K, by the modulation method from 10 to 26 K and by the EPR linewidth from 30 to 100 K. It was found thatT ₁ ^?1 αT ⁿ withn=9 in the range 4.2–6 K.n decreased gradually as the temperature was increased and tended towards 2 above 40 K. Over the entire temperature range 4.2–100 K,T ₁ ^?1 was fitted toAT+BT ⁹ J ₈ (Θ _D/T) (whereA andB are two temperature-independent constants,J ₈ is the well-known Van Vleck integral andΘ _D is the Debye temperature). The value ofΘ _D (=46.3±0.9 K) so determined is in good agreement with that of Stevens and Stapleton from theirT ₁ measurements in the range 1.5 to 7 K. In the Gd³⁺-doped glass, it was observed thatT ₁ ^?1 αT over the range 50–150 K. The data for Ye³⁺-doped glass sample were accounted for by assuming that the phonon modulation of the ligand field is the dominant mechanism, associated with a low Debye temperature in accordance with the published data obtained by using other techniques to study lattice dynamics. On the other hand, the data on the Gd³⁺-doped glass sample were explained to be predominantly due to a mechanism involving Two-Level-Systems (TLS)     

Temperature and concentration dependences of the spin-lattice relaxation rate in four borate glasses doped with Fe2O3

Spin-lattice relaxation time (T ₁) of Fe³⁺ ions has been measured in four borate glasses doped with 0.25, 0.5, 1 and 3 wt.% Fe₂O₃ by the modulation method over the temperature range 5–250 K. In the three less concentrated samples, it was observed thatT ₁ ^?1. αT at intermediate temperatures. This is explained in terms of a relaxation mechanism involving TLS (Two Level Systems). At higher temperatures, the temperature dependence ofT ₁ ^?1 is slowed down by cross-relaxation. In the most concentrated sample, the exchange interaction plays a dominant role leading to a very different relaxation-time behaviour, described well by the Bloembergen and Wang three-reservoir model.     

Emission characteristics of Dy3+ ions in lead antimony borate glasses

Glasses with the composition 30PbO–25Sb₂O₃–(45?x)B₂O₃–xDy₂O₃ for x=0 to 1 were prepared in steps of 0.2 by the melt-quenching method. Various physical parameters, viz., density, molar volume, and oxygen packing density, were evaluated. Optical absorption and luminescence spectra of all the glasses were recorded at room temperature. From the observed absorption edges optical band gap, the Urbach energies are calculated; the optical band gap is found to decrease with the concentration of Dy₂O₃. The Judd–Ofelt theory was applied to characterize the absorption and luminescence spectra of Dy³⁺ ions in these glasses. Following the luminescence spectra, various radiative properties, like transition probability A, branching ratio β and the radiative life time τ for different emission levels of Dy³⁺ ions, have been evaluated. The radiative lifetime for the ⁴F_9/2 multiplet has also been evaluated from the recorded life time decay curves, and the quantum efficiencies were estimated for all the glasses. The quantum efficiency is found to increase with the concentration of Dy₂O₃.     

Effect of Mo3+ ions on Nd3+ spin-lattice relaxation in Y3Al5O12

The content of Mo³⁺ ions in YAG:Nd garnet samples prepared by different technologies has been studied, and the spin-lattice relaxation rate of these ions at temperatures of 4–5 K measured. It is concluded is drawn that Mo³⁺ ions can play the part of rapidly relaxing centers mediating the Nd³⁺ spin-lattice relaxation at liquid-helium temperatures. This may account for a number of features in the spin-lattice relaxation of rare-earth ions in garnets, observed earlier at low temperatures. Fiz. Tverd. Tela (St. Petersburg) 40, 2026–2028 (November 1998)     

Nonequivalent positions of Fe3+ ions and relaxation effects in Mn-Zn-ferrites

A study is made of manganese-zinc ferrites Mn_1–xZn_xFe₂O₄ where x varies from 0.3–0.9. It is established that the variation of the form of the spectra with variation of x is of a relaxation nature. In this system the nonequivalent nature of the neighborhood of the ions has an effect on the relaxation processes and on the character of the magnetic ordering.Translated from Izvestiya VUZ, Fizika, No. 3, pp. 77–81, March, 1973.     

Field dependence of spin-lattice relaxation of Nd3+ ions in Y3Al5O12 crystals

Our studies involve measuring spin-lattice relaxation times for Nd³⁺ ions in yttrium-aluminum garnets over the temperature range 4–50 K at 9.25 and 36.4 GHz for different orientations of the external magnetic field in relation to the crystallographic axes. The temperature dependence of the relaxation rate is described by T ₁ ⁻¹ =AT ⁿ+b exp(−Δ/kT), where n varies from sample to sample, with n=1 for “perfect” samples (i.e., with the longest relaxation times). Here Δ is approximately 130 cm⁻¹, which is the energy of the excited Kramers doublet of the neodymium ion closest to the ground state, and this makes it possible to interpret the second term in T ₁ ⁻¹ as the contribution of two-stage relaxation proceeding through the intermediate level Δ. A strong field dependence of these processes has been discovered: when the frequency was increased fourfold, the relaxation rate increased by a factor of 10. The effect is a specific manifestation of the degeneracy of the excited level, breaking of the symmetry of the crystalline field due to lattice defects, and the prevalence of deformations of a certain type in the spin-lattice interaction. Zh. éksp. Teor. Fiz. 111, 332–343 (January 1997)     

Multiphonon relaxation of rare earth ions in borate,phosphate, germanate and tellurite glasses

Non-radiative multiphonon relaxation rates were obtained for excited electronic states in borate, phosphate, germanate and tellurite glasses. The rates were calculated from the intensities of fluorescence in the visible range of the spectrum, the measured radiative transitions and the decay times of fluorescence. A functional dependence was found between the relaxation rates and the energy gaps of the rare earth ion. It was shown that by changing the glass host from the borate to tellurite matrix, an increase in visible fluorescence was achieved. This was especially notable in Er³⁺, where the increase of fluorescence from germanate to tellurite was by a factor of 15.     

Investigation of Fe+3 and Fe+2 properties in calcium aluminoborate glasses

Two types of silicon detectors are reported (ion implantation and surface barrier detector) for the detection of neutrons in high gamma-background. The purpose is to find an optimal design of silicon detector combined with the neutron converter, which is efficient enough to neutron flux and with low sensitivity to %-rays. The experiments show that for fabrication of effective Si detectors for neutrons (using the (n,!) reactions) in the presence of an intensive %-field one must optimise the resistivity of the Si substrate and the active area of the detector, at which the detector has a maximum efficiency to the charged particles and is relatively insensitive to the %-rays.     

ESR and spin-lattice relaxation of Nd3+ in silicates of various structures

ESR studies have been made on Nd³⁺ in certain orthosilicate and pyrosilicate single crystals. It is found that Nd³⁺ ions produce a type of activator center, and the principal values of the chief factors have been measured. The ESR spectra are compared with structural data on the crystals. The spin-lattice relaxation gives the positions of the nearest excited Stark level of Nd³⁺ in the silicates.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 35–38, July, 1982.We are indebted to A. M. Korovkin for providing the single crystals, and to A. M. Tkachuk and L. G. Morozova for providing the optical data on the silicates.     

Faraday effect and aggregation of paramagnetic ions in borate glasses

The magneto-optical Faraday effect and electron spin resonance (ESR) in a potassium-aluminum borate glass containing small amounts of impurities of Fe and Mn oxides have been investigated. The Faraday effect measurements and ESR data made it possible to reveal the formation of clusters of paramagnetic ions interacting via oxygen even in the stage of glass synthesis. Heat treatment leads to cluster coarsening and formation of nanoparticles.     

Nuclear spin-lattice relaxation in antiferromagnetic RbMnF3

The ⁸⁷Rb and ¹⁹F nuclear relaxation rates in antiferromagnetic RbMnF₃ are calculated as a function of temperature. The large difference in the relaxation rates of both nuclei as well as their temperature dependences are well accounted for over a considerable temperature range by a two-magnon process induced by the dipolar and “contact” parts of the hyperfine interaction.     

Rb87 spin-lattice relaxation in RbMnF3

Nuclear spin-lattice relaxation rate measurements of Rb⁸⁷ in the isotropic antiferromagnet RbMnF₃ have been performed from 27 to 500°K. Both the high temperature value of the relaxation rate and its temperature dependence are well explained by a theoretical calculation essentially based on Moriya's theory.     

EPR and spin-lattice relaxation of rare-earth ions in LiLuF4 monocrystals

The EPR and spin-lattice relaxation are studied of impurity rare-earth ions in LiLuF₄ crystals at liquid-helium temperatures. It is detected that paramagnetic relaxation of Er³⁺ ions is retarded by the effect of the phonon throat. The effect of resonance attenuation of the phonon throat is clarified in two-phonon resonance relaxation processes of Er³⁺ ions. The Debye temperature of the crystal is determined from an analysis of experimental results.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 24–27, February, 1988.     

Intensity parameters of Tm3+ and Er3+ in borate,phosphate and germanate glasses

Intensity parameters τ_λ of the Judd-Ofelt expression were obtained from oscillator strengths of the electronic transitions of Tm³⁺ and Er³⁺ in the visible and infrared part of the spectra. The parameters are interpreted by means of static and dynamic crystal field expansion. The τ_λ 's of Tm³⁺, Er³⁺ and Eu³⁺ are expressed as a function of the h (covalency parameter) parameter of the glass. It is concluded that vibrational interaction plays a dominant role in the origin of the forced electric dipole transitions of Tm³⁺ and Er³⁺ in glasses.     

Laser spectroscopy of Nd and Dy ions in lead borate glasses

The spectroscopic and laser properties of Nd³⁺ and Dy³⁺ ions in lead borate glass were studied. Luminescence spectra recorded in the near-infrared and visible ranges correspond to ⁴F_3/2-⁴I_J/2 (J=9, 11, 13) transitions of Nd³⁺ and ⁴F_9/2-⁶H_J/2 (J=11, 13, 15) transitions of Dy³⁺, respectively. Luminescence decay curves were analyzed as a function of activator concentration. Luminescence quenching is observed, which is due to Ln-Ln interaction increasing. Several spectroscopic parameters relevant to laser potential of Ln³⁺ ions (Ln=Nd, Dy) in lead borate glass were determined. The relatively large values of the quantum efficiency and the room-temperature emission cross-section for the ⁴F_3/2-⁴I_11/2 transition of Nd³⁺ at 1061 nm and the ⁴F_9/2-⁶H_13/2 transition of Dy³⁺ at 573 nm imply that Ln-doped lead borate glasses can be considered as promising solid-state materials for laser applications.     

EPR and dielectric relaxation of Fe3+ in KTaO3

EPR and the method of dielectric losses have been used to investigate Fe³⁺ centers of axial and orthorhombic symmetry in KTaO₃ single crystals. The EPR spectrum of orthorhombic-symmetry Fe³⁺ obtained in the 8-mm wavelength range at T=77 K is described by the spin-Hamiltonian with parameters g _x=1.98, g _y=2.01, g _z=2.00, D=0.43 cm⁻¹, and E=5.87×10^t-2 cm⁻¹. From the dielectric measurement data we have obtained the following parameters of the relaxation of the orthorhombic Fe³⁺ centers in KTaO₃: characteristic relaxation frequency τ ₀ ⁻¹ =2.33×10¹² Hz and activation energy E _a=0.044 eV. A model of the orthorhombic Fe³⁺ center in KTaO₃ is discussed within the framework of the kinetic parameters obtained. Fiz. Tverd. Tela (St. Petersburg) 39, 861–864 (May 1997)