Effect of dehydration on the ferrimagnetic resonance in [Cr(CN)<Subscript>6</Subscript>][Mn(<Emphasis Type="Italic">S</Emphasis>)-<Emphasis Type="Italic">pn</Emphasis>H(H<Subscript>2</Subscript>O)](H<Subscript>2</Subscript>O) single crystals

The effect of dehydration-induced structural transitions on the ferrimagnetic resonance spectra of the [Cr(CN)₆][Mn(S)-pnH(H₂O)](H₂O) chiral molecular magnet has been studied for three crystalline modifications. The differences in the anisotropy field and exchange interaction constants due to the change in the dimensionality of the magnetic ordering upon the phase transitions have been established. In the dehydrated amorphous phase, apart from the ferrimagnetic resonance, additional isotropic magnetic resonance lines corresponding to the spin-glass state have been revealed.     

Spin dynamics and ferromagnetic resonance in an [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 molecular magnetic

Ferrimagnetic resonance spectra of the [Mn{(R/S)-pn}]₂[Mn{(R/S)-pn}]₂(H₂O)][Cr(CN)₆]₂ molecular magnetics were examined. It was established that, within the high-temperature (paramagnetic) region (T > 53 K), the EPR spectrum features a single Lorentz-shaped peak, while, at temperatures below 53 K, in a three-dimensional ferrimagnetic state, this peak splits into several components, some of which correspond to different states of spins in the helical chains of atoms comprising the chiral crystalline structure.     

VUV spectroscopy of Ce<Superscript>3+</Superscript>-doped Na<Subscript>0.4</Subscript>Lu<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript> single crystals

The short-wave transmission spectrum of Na_0.4Lu_0.6F_2.2 with the visible/ultraviolet transmission edge of 8 eV was studied. Absorption spectra of the 4f—5d transitions of the Ce³⁺ ion in the region of 4–8 eV were studied in Ce³⁺-doped Na_0.4Lu_0.6F_2.2 single crystals. Luminescence spectra in the ultraviolet and visible spectral regions, luminescence decay kinetics and reflection and luminescence excitation spectra in the visible/ultraviolet and ultraviolet regions (4–20 eV) were investigated at helium and room temperatures.     

On the magnetic properties of Pr[Mn(CN)<Subscript>6</Subscript>]. 4H<Subscript>2</Subscript>O

A Prussian Blue type compound of the nominal composition Pr[Mn(CN)₆].4H₂O has been prepared. It is shown that the compound exhibits ferrimagnetism with a Curie temperature T_C=38.9 K. Observed magnetic relaxation displays a logarithmic behavior.     

Paramagnetic resonance of LaGaO<Subscript>3</Subscript>: Mn single crystals grown by floating zone melting

The EPR spectrum of Mn-doped lanthanum gallate single crystals grown by floating zone melting with optical heating has been studied. In contrast to the crystals grown according to the Czochralski method, no manganese is found in these crystals even after high-temperature annealing in air. The spectral characteristics of Fe³⁺ and Gd³⁺ centers in crystals prepared by various methods have been compared in the rhombohedral phase, and the fourth-rank nondiagonal parameters of the Fe³⁺ trigonal centers have been determined, as well.     

Magnetocaloric effect in (La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>)<Subscript>0.9</Subscript>Mn<Subscript>1.1</Subscript>O<Subscript>3</Subscript>

The results of investigations of the magnetization, susceptibility, and magnetic-field-induced changes in the entropy of polycrystalline manganite (La_0.6Ca_0.4)_0.9Mn_1.1O₃ near the magnetic phase transition have been presented. Magnetic measurements have been carried out at temperatures in the range from 210 to 310 K in magnetic fields of up to 9 T. The magnetocaloric effect has been revealed by measuring the magnetic-field dependences of magnetization. The magnitude of the magnetocaloric effect is compared with similar results obtained for other manganites.     

Nuclear spin resonance of <Superscript>53</Superscript>Cr in ferromagnetic CuCr<Subscript>2</Subscript>S<Subscript>4</Subscript>: Sb

The influence of variable valence on NSR spectra of ⁵³Cr nuclei in ferromagnetic CuCr_2?xSb_xS₄ (x = 0, 0.02, 0.07) at T = 77 K is considered. For quadrupole nuclei in locally anisotropic positions, the effects of variable valence result in averaging of not only the resonance frequency but also of the quadrupole and magnetic anisotropy constants. The significant difference between the experimental and calculated values of these constants indicates the important role of the intrinsic electronic contribution to the anisotropy of hyperfine fields of compounds containing Cr⁴⁺ ions. Additional lines caused by intrinsic and induced defects in the structure are observed in the spectra of doped and undoped compounds.     

Characteristics of thermoelastic martensitic transformations in heterophase ferromagnetic Ni<Subscript>54</Subscript>Fe<Subscript>19</Subscript>Ga<Subscript>27</Subscript> single crystals

Using single-crystal Ni ₅₄ Fe ₁₉ Ga ₂₇ (at.%) specimens, the effects of heat treatment, annealing at the temperatures within T = 473–1473 K for one hour followed by quenching into water, on the development of thermoelastic L2₁-L1₀ martensitic transformations is investigated. It is found out that using proper heat treatment modes one can control the temperature of L2₁-L1₀ martensitic transformations, the value of the temperature hysteresis, the critical stresses of twin boundary motion, and certain functional properties of single-crystal Ni-Fe-Ga. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 24–27, October, 2007.     

Structural transformations of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals with variations in temperature

Single crystals of the K₃H(SO₄)₂ compound are investigated using X-ray diffraction on Xcalibur S and Bruker diffractometers. The structure of the low-temperature monoclinic phase is refined (space group C2/c, z = 4, a = 14.698(1) Å, b = 5.683(1) Å, c = 9.783(1) Å, β = 103.01(1)°, T = 293 K, Bruker diffractometer), the structural phase transition is revealed, and the structure of the high-temperature trigonal phase is determined (space group R \(\bar 3\) m, z = 3, a = 5.73(1) Å,c = 21.51(1) Å,T = 458 K, Xcalibur diffractometer).     

Crystal structure and electronic states of Co and Gd ions in a Gd<Subscript>0.4</Subscript>Sr<Subscript>0.6</Subscript>CoO<Subscript>2.85</Subscript> single crystal

X-ray diffraction and X-ray absorption near edge structure (XANES) spectra have been measured at the Со K-edge and Gd L ₃-edge in GdCoO₃ and Gd_0.4Sr_0.6CoO_2.85 cobaltites. The effect of Sr substitution on the crystal structure and electronic and magnetic states of Co³⁺ ions in a Gd_0.4Sr_0.6CoO_2.85 single crystal has been analyzed. The XANES measurements at the Co K-edge have not showed a noticeable shift of the absorption edge with an increase in the concentration of Sr. This indicates that the effective valence of cobalt does not change. An increase in the intensity of absorption at the Gd L ₃-edge is due to an increase in the degree of hybridization of the Gd(5d) and O(2p) states. The effect of hole doping on the magnetic properties results in the appearance of the ferromagnetic component and in a significant increase in the magnetic moment.     

Coercive field enhancement in microstructured (La<Subscript>0.4</Subscript>Pr<Subscript>0.6</Subscript>)<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> thin films

The perovskite material (La_0.4Pr_0.6)_0.67Ca_0.33MnO₃ (LPCMO) has complex electronic and magnetic behavior based on phase competition between ferromagnetic metallic (FMM) and insulating phases with similar free energies. Experimental evidence has indicated that in-plane stress anisotropy influences these phases and can affect electronic and magnetic properties. Here we investigate the roles that both stress and shape anisotropies may play in controlling the coercive field of the material. LPCMO thin films of various thicknesses (20, 25, and 30 nm) were deposited on (110) NdGaO₃ (NGO) substrates using pulsed laser deposition and the coercive fields were measured. Photolithography was then used to fabricate microstructured arrays of LPCMO on the NGO substrates for each of the films. The coercive fields of these arrays of LPCMO were compared to the behavior of the corresponding unpatterned LPCMO thin films across a range of temperatures. Microstructure arrays for the thicker (25 and 30 nm) films showed a substantial increase in the coercive field after forming the arrays, whereas a thinner film (20 nm) showed almost no change in the coercive field. Stress anisotropy continues to play a dominant role in the behavior of LPCMO thin films and dimensionality of the magnetic domains also influences the results. The films show 2D behavior when film thickness approaches the size of the critical radius for single-to-multidomain transitions. Making thicker films allows for 3D behavior and a role for shape anisotropy to influence the coercive fields.     

Long-time relaxation phenomena in Pb<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>Sc<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> (PBSN-6) single crystals

Long-time polarization relaxation in the temperature range where PBSN-6 single crystals reside in the relaxor state was studied. An analysis of the time dependence of the permittivity ε′(t) performed at measuring frequencies from 1 Hz to 1 kHz in weak electric fields E₀ showed that the relaxation (or freezing) times derived by extrapolating relations of the type ε′(t) ～ log(t/t₀) and ε′(t) ～ exp{?[ln(t/t₀)]_β} range from 10⁸ to 10¹¹ min and depend substantially on the bias voltage applied to the sample. A study of the pattern of the dielectric response in moderate and strong infralow-frequency fields revealed that, after a sample was maintained under a bias lower than the coercive force, it no longer exhibited the additional anomalies in the amplitude dependences of the effective loss tangent tan_eff(E₀) than were observed in a thermally recuperated sample.     

Ionic conduction in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals doped with Cr and Mn ions

The electrical conductivity σ of crystals of lithium heptagermanate Li₂Ge₇O₁₅ doped with Cr and Mn is measured in an alternating-current field with a frequency of 1 kHz in the temperature range 300–700 K. It is found that doping strongly affects the electrical conductivity. It is established that the addition of 0.1 wt % Cr leads to an increase in the electrical conductivity σ by almost one order of magnitude, whereas the introduction of 0.03 wt % Mn substantially reduces the electrical conductivity along particular crystallographic directions. Data available on the incorporation of Cr and Mn impurity atoms into the lattice suggests that the electrical conductivity is determined by lithium ions hopping over interstitial positions along the structural channels.     

Photostimulated electron transfer and its action on paramagnetism of Sp<Subscript>3</Subscript>Cr(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The effect of irradiation by ultraviolet light on the effective magnetic moment of a paramagnetic single crystal based on photochrome spiropyran (Sp) and chromium oxalates Sp₃Cr(C₂O₄)₃ molecules is detected. It is shown that the deviation of the temperature dependence of the magnetic moment from the Curie law is caused not by the exchange interaction, but by electron redistribution between Cr³⁺ and Cr⁴⁺ ions and spiropyran molecules Sp⁰ and Sp⁺. Analysis of the angular dependence of EPR spectra makes it possible to determine the contribution of Cr³⁺ ions to the magnetic properties of the crystals and to determine the crystal field parameters D = 0.619 cm⁻¹ and E = 0.024 cm⁻¹. Irradiation of hydrated samples by ultraviolet light leads to intensity redistribution of EPR lines attributed to Cr³⁺ and Sp⁰. Thermally stimulated paramagnetism of triplet states of spiropyran ions Sp⁺ and the SpI salt is observed.     

Temperature dependence of phase separation and magnetic anisotropy by electron spin resonance in Pr<Subscript>0.6</Subscript> Ca<Subscript>0.4</Subscript> Mn<Subscript>0.9</Subscript> Ru<Subscript>0.1</Subscript>O<Subscript>3</Subscript>

Electron spin resonance (ESR) measurements have been performed on polycrystalline samples of Pr_0.6Ca_0.4Mn_1-xRu_xO₃ (x = 0, 0.1). The substitution of Ru in the Mn-site strengthens ferromagnetic interactions due to the double exchange between the Mn³⁺ and Mn⁴⁺ species and super-exchange between the Ru⁵⁺ and Mn³⁺ species. The temperature dependence of the ESR spectra indicates development of magnetic phase separation in Pr_0.6Ca_0.4Mn_0.9Ru_0.1O₃ in contrast with the un-doped sample.     

Electron transport and anisotropy of the upper critical magnetic field in Ba<Subscript>0.68</Subscript>K<Subscript>0.32</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> single crystals

Early work on the iron-arsenide compounds supported the view, that a reduced dimensionality might be a necessary prerequisite for high-T _c superconductivity. Later, however, it was found that the zero-temperature upper critical magnetic field, H _c2(0), for the 122 iron pnictides is in fact rather isotropic. Here, we report measurements of the temperature dependence of the electrical resistivity, ρ(T), in Ba_0.5K_0.5Fe₂As₂ and Ba_0.68K_0.32Fe₂As₂ single crystals in zero magnetic field and in Ba_0.68K_0.32Fe₂As₂ in static and pulsed magnetic fields up to 60 T. We find that the resistivity of both compounds in zero field is well described by an exponential term due to inter-sheet umklapp electron-phonon scattering between light electrons around the M point to heavy hole sheets at the Γ point in reciprocal space. From our data, we construct an H-T phase diagram for the inter-plane (H | c) and in-plane (H | ab) directions for Ba_0.68K_0.32Fe₂As₂. Contrary to published data for 122 underdoped FeAs compounds, we find that H _c2(T) is in fact anisotropic in optimally doped samples down to low temperatures. The anisotropy parameter, γ = H _c2 ^ab /H _c2 ^c , is about 2.2 at T _c . For both field orientations we find a concave curvature of the H _c2 lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism, we perfectly can describe H _c2 and its anisotropy.     

Study of spectroscopic characteristics of holmium-doped double sodium-yttrium fluoride crystals Na<Subscript>0.4</Subscript>Y<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript>:Ho<Superscript>3+</Superscript>

We have grown crystals Na_0.4Y_0.6F_2.2:Ho³⁺ (NYF:Ho³⁺) by the Bridgman-Stockbarger method. The optical spectra and luminescence kinetics of NYF:Ho³⁺ crystals have been studied. Based on the analysis of low-temperature absorption spectra, we determine the structure of the Stark splitting of holmium levels in NYF:Ho³⁺ crystals. From absorption spectra examined at T = 300 K, we calculate absorption cross-section spectra and oscillator strengths of transitions from the ground state of holmium to excited multiplets. We show that the absorption spectra of NYF:Ho³⁺ crystals consist of broad bands that lie in the UV, visible, and near-IR ranges. The most intense bands are observed in the visible range, they correspond to transitions ⁵ I ₈ → (⁵ F ₁, ⁵ G ₆) and ⁵ I ₈ → (⁵ F ₄, ⁵ S ₂), and their maximal absorption cross sections are σ_abs^max (λ = 450.3 nm) = 1.16 × 10⁻²⁰ cm² and σ_abs^max (λ = 535.1 nm) = 0.9 × 10⁻²⁰ cm². The intensity parameters Ω _t have been calculated by the Judd-Ofelt method taking into account 10, 12, and 20 transitions from the ⁵ I ₈ ground state to excited multiplets. We show that, with an increasing number of transitions taken into account in the calculation, the parameters Ω _t somewhat increase. For 20 transitions, we have obtained the following intensity parameters: Ω₂ = 0.97 × 10⁻²⁰, Ω₄ = 1.74 × 10⁻²⁰, and Ω₆ = 1.15 × 10⁻²⁰ cm². With these parameters, we have calculated the probabilities of radiative transitions, the radiative lifetimes, and the branching ratios. The rates of multiphoton nonradiative transitions have been estimated. The luminescence decay kinetics from excited holmium levels ⁵ F ₃ (⁵ F ₄, ⁵ S ₂) and ⁵ F ₅ have been studied upon selective excitation in the range of 490 nm, and the lifetimes of these levels have been experimentally determined. We find that the calculated and experimental rates of radiative and nonradiative relaxation from excited holmium levels agree well with each other. We show that, upon pumping in the range of 490 nm, the multiplet (⁵ F ₄, ⁵ S ₂) is populated as a result of the radiative and nonradiative excitation relaxation from the ⁵ F ₃ level, while the lower-lying ⁵ F ₅ level is populated due to direct radiative transitions ⁵ F _{3, 2} → ⁵ F ₅, obviating the cascade scheme ⁵ F ₃ → (⁵ F ₄, ⁵ S ₂) ↝ ⁵ F ₅. We conclude that NYF:Ho³⁺ crystals are processable; admit doping by holmium in high concentrations (up to 100%); and, with respect to all their radiative characteristics, can be considered as potential active media for solid-state continuously tunable lasers in the IR and visible ranges.     

EPR and optical absorption studies of Cu<Superscript>2+</Superscript> ions in [ZnPd(CN)<Subscript>4</Subscript>(C<Subscript>4</Subscript>H<Subscript>12</Subscript>N<Subscript>2</Subscript>O<Subscript>2</Subscript>)] single crystals

A Cu²⁺-doped single crystal of catena-trans-bis(N-(2-hydroxyethyl)-ethylenediamine) zinc(II)-tetra-m-cyanopaladate(II) [ZnPd(CN)₄(C₄H₁₂N₂O₂)] complex has been investigated by electron paramagnetic resonance (EPR) technique at room temperature. EPR spectra indicate that Cu²⁺ ions substitute for magnetically equivalent Zn²⁺ ions and form octahedral complexes in [ZnPd(CN)₄(C₄H₁₂N₂O₂)] hosts. The crystal field affecting the Cu²⁺ ion is nearly axial. The optical absorption studies show two bands at 322 nm (30864 cm⁻¹) and 634 nm (15337 cm⁻¹) which confirm the axial symmetry. The spin Hamiltonian parameters and the relevant wave function are determined.     

Defect characterization of Ga<Subscript>4</Subscript>Se<Subscript>3</Subscript>S layered single crystals by thermoluminescence

Trapping centres in undoped Ga ₄Se ₃S single crystals grown by Bridgman method were characterized for the first time by thermoluminescence (TL) measurements carried out in the low-temperature range of 15 ?300 K. After illuminating the sample with blue light (～470 nm) at 15 K, TL glow curve exhibited one peak around 74 K when measured with a heating rate of 0.4 K /s. The results of the various analysis methods were in good agreement about the presence of one trapping centre with an activation energy of 27 meV. Analysis of curve fitting method indicated that mixed order of kinetics dominates the trapping process. Heating rate dependence and distribution of the traps associated with the observed TL peak were also studied. The shift of peak maximum temperature from 74 to 113 K with increasing rate from 0.4 to 1.2 K /s was revealed. Distribution of traps was investigated using an experimental technique based on cleaning the centres giving emission at lower temperatures. Activation energies of the levels were observed to be increasing from 27 to 40 meV by rising the stopping temperature from 15 to 36 K.     

Anti-stokes luminescence of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed organic dye molecules and few-atom silver clusters

For microcrystals of Zn_0.6Cd_0.4S with adsorbed molecules of a number of organic dyes, we have observed sensitized anti-Stokes luminescence excited by radiation with wavelengths in the range 610–750 nm and flux density 10¹⁴–10¹⁵ photons/cm²·sec. The positions of the bands in the excitation spectra for such luminescence match those of the absorption spectra for the adsorbed dye molecules, which is evidence in favor of a cooperative mechanism for its appearance. We have shown that enhancement of the anti-Stokes luminescence is possible when silver atoms and few-atom clusters appear on the Zn_0.6Cd_0.4S surface in addition to the dye molecules. We hypothesize that its excitation in the latter case occurs as a result of two-photon optical transitions. These transitions occur sequentially, with transfer of an electron or the electronic excitation energy from the dye molecules to silver atoms and few-atom clusters adsorbed on the surface of Zn_0.6Cd_0.4S, creating deep localized states in the bandgap with photoionization energies 1.80–2.00 eV. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 617–621, September–October, 2007.