Samih Isber

EPR of the Kramers ions Er3+, Nd3+, Yb3+ and Ce3+ in Y(NO3)3.6H2O and Y2(SO4)3. 8H2O single crystals : Study of hyperfine transitions

Abstract X-band (∼9.45xa0GHz) electron paramagnetic resonance (EPR) spectra of the Kramers ions Er 3+ , Nd 3+ , Yb 3+ and Ce 3+ substituting by about ∼1% Y 3+ ions, in yttrium trinitate hexahydrate [Y(NO 3 ) 3 ·6H 2 O] and yttrium trisulphate octahydrate [Y 2 (SO 4 ) 3 ·8H 2 O] single crystals were recorded at liquid-helium temperatures. Hyperfine EPR transitions were observed for the isotopes of Er, Nd and Yb, except for the Ce 3+ ion which possesses no isotope with non-zero nuclear moment. The spectra indicate the presence of two magnetically- inequivalent R 3+ sites in unit cell in the Y 2 (SO 4 ) 3 ·8H 2 O crystal, and one site in Y(NO 3 ) 3 ·6H 2 O crystal. The values of the elements of the (anisotropic) g 2 and A 2 tensors were estimated using a rigorous least-squares fitting procedure fitting simultaneously all resonant EPR line positions observed for numerous orientations of the external magnetic field in three mutually perpendicular planes. The absolute signs of the elements of A -matrix and orientations of g - and A -matrices relative to the magnetic axes of the Gd 3+ ion, present naturally as impurity in these crystals, were determined.

Electron paramagnetic resonance of Eu2+ in the dilute magnetic semiconductor Pb1-xEuxSe (x=0.013) with a cubic structure

Detailed X-band (about 9.5 GHz) electron paramagnetic resonance (EPR) measurements were carried out at 295, 77 and 4.2 K on the Eu2+ ion in a Pb1-xEuxSe single crystal. The values of the spin-Hamiltonian parameters have been determined by a simultaneous fitting of all the EPR line positions recorded for various orientations of the external magnetic field in the [010] plane using a rigorous least-squares fitting procedure. The absolute sign of the zero-field splitting parameter b4 was determined to be positive from the relative intensities of lines at liquid-helium temperature (4.2 K). The excellent fit of the line positions to a cubic spin Hamiltonian confirms that the site symmetry for Eu2+ ions is, indeed, cubic in the Pb1-x,Eux,Se single crystal over the temperature range 4.2-295 K. The Dynosian shape of the EPR lines indicates the metallic nature of the Pb1-xEuxSe sample. The nearest-neighbour Eu2+-Eu2+ exchange interaction has been estimated to be 20.4 GHz (antiferromagnetic) from the exchange pair EPR spectrum at 4.2 K. The hyperfine interaction constants for the isotopes 151Eu and 153Eu have been estimated.

Electron paramagnetic resonance of Er3+ doped in YVO4: hyperfine parameters

Abstract X-band (∼9.60 GHz) electron paramagnetic resonance (EPR) spectra of the Er 3+ Kramers ion substituting for the Y 3+ ion in yttrium vanadate (YVO 4 ) single crystal were recorded at liquid-helium temperatures. Fine and hyperfine EPR transitions were observed for the 166 Er (zero nuclear magnetic moment) and 167 Er ( I =7/2) isotopes, respectively. The values of the elements of the anisotropic g 2 - and A 2 -tensors of the 167 Er 3+ ion, and those of the g 2 tensor for the 166 Er ion, were estimated. The admixture of crystal-field wave functions in the ground-state wave function of Er 3+ were determined using the experimental g -values.

The effect of the crystal field on magnetization steps due to exchange-coupled open and closed triplets in

The values of the magnetic fields at which magnetization steps (MSs) occur due to exchange-coupled open and closed triplets of ions (S = 7/2) are calculated for a single crystal, taking into account the crystal-field (CF)-interaction, which causes the isotropic separation between two consecutive MSs, , where is the value of the l th MS, that exists in the absence of the crystal field to become anisotropic with respect to the orientation of the external magnetic field . The contribution of the single-ion anisotropic CF by an exchange-coupled open triplet (OT) leads to an average separation, , 10% higher than when , and 5 and 8% lower than for and , respectively. As for the exchange-coupled closed triplet (CT), it was estimated that is 7.0% higher than when , and 3.4 and 4.1% lower than when and , respectively. Furthermore, the values of and were found to depend on l (l = 1,...,6 for OT, and l = 1,...,9 for CT), varying by up to about 40%.

Superconductivity in non-stoichiometric and tin-substituted La2CuO4: preparation, characterization, Mössbauer and microwave-absorption studies

Two samples of non-stoichiometric La2CuO4 were synthesized, one with La/Cu<2, and the other with 10% Sn substituting Cu. They were investigated by X-ray diffraction, Mossbauer spectroscopy, and microwave-absorption techniques. The microwave-absorption data indicated that they were both superconducting, with the transition temperatures Tc of 40.5 and 41.5 K, the one doped with Sn possessing the higher Tc. The Mossbauer spectra revealed that there exist two kinds of Sn(IV) atoms disordered with Cu. Their isomer shift, δ=−0.244(4) mm/s, is in agreement with Sn(IV) coordinated by oxygen. One site was characterized by a single Mossbauer line, being associated with a weakly distorted environment, wherein the Sn, coordinated more symmetrically, is surrounded by four Cu2+ ions. On the other hand, the other site, characterized by a Mossbauer doublet exhibited a quadrupole splitting Δ=1.07(2) mm/s, being associated with a highly distorted coordination, explained to be due to Sn occupying two adjacent cationic sites. To our knowledge, such a substitution for copper ions not resulting in a decrease of Tc has not been reported previously.

Electron paramagnetic resonance of Gd3+ in praseodymium triacetate monohydrate

Abstract X-band (∼9.6xa0GHz) electron paramagnetic resonance measurements were carried out on a Gd 3+ -doped praseodymium triacetate monohydrate, Pr(CH 3 COO) 3 ·H 2 O, single crystal in the temperature range 4.2–295xa0K. The spin-Hamiltonian parameters were estimated at room, liquid-nitrogen and liquid-helium temperatures by the use of a rigorous least-squares fitting technique. The absolute sign of the zero-field splitting parameter, b 0 2 , was determined to be negative by comparison of the intensities of the high- and low-field EPR lines at room and liquid-helium temperatures. The behaviour of EPR spectrum was studied as a function of temperature; the variation of the linewidth and line positions indicated a significant reordering of acetate and water molecules at about 240xa0K. The present Gd 3+ EPR results in praseodymium triacetate monohydrate are compared with those in the rare-earth triacetate tetrahydrate hosts with Sm, Nd, Er, Y, Yb and Dy ions.

An EPR study of the phase transitions and zero-field splitting parameters in (R = Er, Dy, Pr, Nd, Sm, Eu)

X-band ( GHz) electron paramagnetic resonance measurements were carried out on -doped and single crystals in the 4.2-295 K temperature range, and on -doped , , and single crystals in the 4.2-110 K range. room-temperature spin-Hamiltonian parameters were estimated in samples with R = Er, Dy. The following phase transitions undergone by the various host crystals were found to occur: three second-order phase transitions in at 260.3 K, 226.9 K and 65.4 K, and second-order and first-order phase transitions in at 228.1 and 68.8 K, respectively. In addition, there were observed first-order phase transitions in each of (at 72.1 K), (at 70.3 K) and (at 79.4 K), and two in (at 71.4 and 9.7 K) crystals, in the 4.2-110 K range, in addition to those reported previously in the 110-295 K range. Systematics of the various phase transitions occurring in (R = Pr, Nd, Sm, Er, Dy, Eu) single crystals in the temperature range 4.2-295 K, and those of zero-field splitting parameters at room temperature, have been deduced.