A field-sweep/field-lock system for superconducting magnets--Application to high-field EPR

We describe a field-lock/field-sweep system for the use in superconducting magnets. The system is based on a commercially available field mapping unit and a custom designed broad-band 1H NMR probe. The NMR signal of a small water sample is used in a feedback loop to set and control the magnetic field to high accuracy. The current instrumental configuration allows field sweeps of +/-0.4 T and a resolution of up to 10(-5) T (0.1 G) and the performance of the system is demonstrated in a high-field electron paramagnetic resonance (EPR) application. The system should also be of utility in other experiments requiring precise and reproducible sweeps of the magnetic field such as DNP, ENDOR or PELDOR.     

EPR studies of rhodium in LiH and LiD single crystals

EPR studies of rhodium in LiD and LiH single crystals reveal an axial rhodium complex with the unpaired electron in a 4d_x²?y² molecular orbital and with 5% of the spin density transferred to the planar ligands.     

A high-field EPR tour of radicals in photosystems I and II

High magnetic-field electron paramagnetic resonance (HFEPR) has been extensively applied to the study of photosynthetic reaction centers. HFEPR experiments have provided accurate measurements ofg-values of radicals in photosystems I and II. Theg-values not only reflect the structure of the radical, but also its immediate environment. Hence, valuable information about radicalprotein interactions can be obtained fromg-values. Data on tyrosine-, quinone-, pheophytin- and chlorophyll-based radicals are reviewed. Experiments dealing with spin pairs in photosystem I and II are examined. The spectrometer with which numerous experiments on photosystems have been carried out is described, as well as the 10 T magnet around which the spectrometer is based.     

Spectroscopic investigations of neodymium-doped BaLaGa3O7 single crystals

Neodymium doped BaLaGa₃O₇ obtained in single crystal form by the Czochralski method has been studied by both adsorption and emission spectroscopy. The experimental data have been used to determine the radiative transition probabilities, luminescence branching ratios and quantum efficiency of the luminescent state of Nd⁺³.     

Raman investigations of rare‐earth arsenate single crystals

Polarized Raman spectroscopy was used to investigate the room‐temperature phonon characteristics of a series of rare‐earth arsenate (REAsO₄, RE = Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, and Lu) single crystals. The Raman data were interpreted in a systematic manner based on the known tetragonal zircon structure of these compounds, and assignments and correlations were made for the observed bands. We found that the wavenumbers of the internal modes of the AsO₄ tetrahedron increased with increasing atomic number. This increase seems to be correlated to the contraction of the RE–O bond length. For three out of four lattice wavenumbers observed, this tendency was not nearly so marked as in the case of the internal mode wavenumber. Copyright © 2009 John Wiley & Sons, Ltd.     

X-band EPR study on benzo[c]cinnoline single crystals

The benzo[c]cinnoline compound has been crystallized in ethanol solution. These crystals have been irradiated with ⁶⁰Coγ-rays at room temperature. The crystals have been investigated by an X-band electron spin resonance (ESR) spectrometer two days after irradiation. Very strong ESR peaks have been observed at the temperatures between 130 and 430 K. However, a line broadening has been observed at 130 K. Also analyses found that EPR specra are temperature-dependent and show anisotropic behaviour.     

Luminescence and EPR spectroscopy of neutron-irradiated single crystals of magnesium aluminium spinel

Neutron irradiated single crystals of stoichiometric MgAl₂O₄ and MgAl₂O₄:Mn²⁺ (0.15 wt. %) were studied using the methods of luminescence spectroscopy and versions of electron paramagnetic resonance. In stoichiometric MgAl₂O₄:Mn²⁺ single crystals, the transition of a part of manganese impurity ions from tetrahedral to octahedral coordination, caused by irradiation with fast fission neutrons, is detected using photoluminescence spectra. This fact confirms the partial inversion of a normal spinel due to neutron-irradiation. Using pulse EPR technique it is shown that an electron in the field of oxygen vacancy (i.e. from neutron-irradiation induced F⁺ centres, which are spatially separated from each other) is in hyperfine interaction only with neighbouring ²⁷Al nuclei with nuclear spin 5/2.     

Crystal growth and conductivity investigations on BiVO4 single crystals

Some oxides with the scheelite structure are well known mixed ionic and electronic conductors. This structure family, with the general composition ABO₄, can be described as a layer structure derivative of the fluorite structure. The relations between the structural and the electrical transport properties of these materials have been investigated. The model system used in this work was BiVO₄, which shows both electronic and ionic conduction. Single crystals of bismuth vanadate (BiVO₄) were grown using the Czochralski technique. Single crystal boules up to 55 mm in diameter and up to 30 mm long were produced. The crystallographic orientation was determined by the Laue method, and they were cleaved along [001] planes and polished to optically perfect cubes with an edge length of 5 mm. Two-point ac impedance measurements in the temperature range between 550°C and 700°C were performed. The conductivities were measured as a function of the crystal orientation and showed strong anisotropy as expected from the crystal structure. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

EPR and optical studies of Cu doped ammonium dihydrogen phosphate single crystals

Electron paramagnetic resonance (EPR) spectra of Cu²⁺ ion in ammonium dihydrogen phosphate are studied at liquid nitrogen temperature (77 K). Four magnetically inequivalent Cu²⁺ sites in the lattice are identified. The angular variation spectra of the crystal in the three orthogonal planes indicate that the paramagnetic impurity, Cu²⁺ enters the lattice substitutionally in place of NH₄⁺ ions. The spin Hamiltonian parameters are determined with the fitting of spectra to rhombic symmetry crystalline field. The ground state wave function of Cu²⁺ ion is constructed and found to be predominantly |x²-y²〉. The cubic field parameter (Dq) and tetragonal parameters (Ds and Dt) are determined from optical spectra at room temperature. By correlating EPR and optical absorption spectra, the bonding coefficients are calculated and nature of bonding of metal ion with different ligands in the crystal is discussed.     

Wide-band EPR spectroscopy of YAlO3: Tm3+ single crystals

The YAlO₃: Tm³⁺ single crystal has been studied on a wide-band EPR spectrometer. The EPR spectra of Tm³⁺ ions in the frequency range of 90–160 GHz have been detected for the first time. It has been confirmed that thulium ions substitute the position of Y³⁺ in the crystal lattice. The detected spectra have been described with the use of a spin Hamiltonian with the effective spin S = 1/2. A comparative analysis of the orientation of the magnetic axes of the Tm³⁺ paramagnetic center with earlier data on other rare-earth ions has been performed.     

EPR and optical absorption study of Cr-doped ammonium oxalate monohydrate single crystals

The electron paramagnetic resonance (EPR) study of the Cr³⁺-doped ammonium oxalate monohydrate (AOM) single crystal is done at room temperature. Two magnetically inequivalent sites for chromium are observed. The hyperfine structure for Cr⁵³ isotope is also obtained. The spin Hamiltonian parameters are evaluated as: D=(309±2)×10⁻⁴ cm⁻¹, E=(103±2)×10⁻⁴ cm⁻¹, g=1.9820±0.0002, A=(161±2)×10⁻⁴ cm⁻¹ for site I and D=(309±2)×10⁻⁴ cm⁻¹, E=(103±2)×10⁻⁴ cm⁻¹, g=1.9791±0.0002, A=(160±2)×10⁻⁴ cm⁻¹ for site II, respectively. On the basis of EPR data the site symmetry of Cr³⁺ doped single crystal is discussed. The optical absorption spectra are recorded in 195-925 nm wavelength range at room temperature. The energy values of different orbital levels are determined. On the basis of EPR and optical data, the nature of bonding in the crystal is discussed. The values of different parameters are B=803, C=3531, D_q=2208 cm⁻¹, h=0.59 and k=0.21, where B and C are Racah parameters, D_q is crystal field parameter and h and k are nephelauxetic parameters, respectively.     

Anisotropic hyperfine structure in the EPR spectrum of impurity ions in single crystals

Expressions are obtained for the hyperfine splitting in EPR spectra of impurity ions with electron spin 1/2 and arbitrary nuclear spin for arbitrary anisotropy of the g-factor and hyperfine structure constants. These results generalize the previously obtained results of Weil for the case of weakly anisotropic g-factors and hyperfine structure constants. Fiz. Tverd. Tela (St. Petersburg) 41, 1026–1027 (June 1999)     

The dependence of EPR linewidth on concentration in Mn/MgO single crystals

The EPR spectra of Mn/MgO single crystals grown by electrofusion have been examined at 9 GHz for manganese concentration of between 840 and 2900 ppm. The values of the spin Hamiltonian parameters deduced agreed well with those previously reported by Low (1957, 1958) and by Matarrese and Kikuchi (1956). The variation of linewidth with manganese concentration was also examined and compared with the predictions of the de Biasi and Fernandes theory of dipolar broadening in magnetically dilute systems; the peak-to-peak linewidth is concentration dependent and is of the right order of magnitude to be explained adequately in terms of this model. The results indicate that clustering, effects are absent in these Mn/MgO single crystals.     

EPR and optical absorption study of Cr-doped tetramethyl ammonium cadmium chloride single crystals

EPR study of Cr³⁺-doped tetramethyl cadmium chloride (TMCC) single crystals is carried out at room temperature. The crystal field and spin-Hamiltonian parameters are evaluated from the resonance line positions of different lines observed in the EPR spectra. The g and D parameter values are found to be g=1.9741±0.0002 and D=553±2×10⁻⁴ cm⁻¹, respectively. EPR data indicate that the site symmetry of Cr³⁺ ion in the crystal is distorted octahedron. Cr³⁺ ions enter the lattice substitutionally replacing Cd²⁺ sites and bind to the neighboring extra Cd vacancies necessary for charge compensation. The optical absorption spectra are measured in 195–925 nm wavelength range at room temperature. From optical study the energy values of different orbital levels are estimated. Further, the bonding parameters are obtained by correlating optical and EPR data and the nature of bonding in the crystal is discussed. The values of Racah parameters (B and C), crystal field parameter (Dq) and nephelauxetic parameters (h and k) are obtained to be B=722, C=2845, Dq=2043 cm⁻¹, h=1.015 and k=0.21.     

Identification of nitrogen vacancies in an AlN single crystal: EPR and thermoluminescence investigations

The electronic structure of nitrogen vacancies in specially undoped aluminum nitride single crystals has been determined and the depth of the donor level of these vacancies in the band gap has been found to be ∼75 meV by combined electron paramagnetic resonance and thermoluminescence investigations.     

Spin quenching and EPR of Mn2+ in single crystals of nickel salts

An EPR study of Mn²⁺ in (NH₄)₂Ni(SeO₄)₂·6H₂O crystals has been reported for the first time at room temperature (≈300 K) and at the X-band. The observations regarding the anisotropic and field dependent linewidths of Mn²⁺ in nickel Tutton salts at room temperature are discussed in terms of the spin quenching ideas. Also, the g_z-shift in Mn²⁺-doped Ni-Tutton salts is attributed to the internal magnetic field at the site of the Mn²⁺ ion, caused by the moments induced on Ni²⁺ ions.