Magnetic and Magnetodielectric Properties of Ho<Subscript>0.5</Subscript>Nd<Subscript>0.5</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The magnetic and magnetodielectric properties of Ho_0.5Nd_0.5Fe₃(BO₃)₄ ferroborate with the competing Ho–Fe and Nd–Fe exchange couplings have been experimentally and theoretically investigated. Step anomalies in the magnetization curves at the spin-reorientation transition induced by the magnetic field B ║ c have been found. The spontaneous spin-reorientation transition temperature T_SR ≈ 8 K has been refined. The measured magnetic properties and observed features are interpreted using a single theoretical approach based on the molecular field approximation and calculations within the crystal field model of the rare-earth ion. Interpretation of the experimental data includes determination of the crystal field parameters for Ho³⁺ and Nd³⁺ ions in Ho_0.5Nd_0.5Fe₃(BO₃)₄ and parameters of the Ho–Fe and Nd–Fe exchange couplings.     

Magnetic and magnetoelastic properties of the TbMnSi and Tb<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>MnSi compounds

X-ray diffraction studies showed substitution of nonmagnetic lanthanum for terbium in the TbMnSi polycrystalline compound to initiate a structural transition from a TiNiSi-type orthorhombic structure (for TbMnSi) to a CeFeSi-type tetragonal phase (for Tb_0.5La_0.5MnSi). Magnetic measurements (of the magnetization, magnetostriction, thermal expansion) performed on Tb_0.5La_0.5MnSi revealed a change in the character of magnetic ordering, the appearance of a ferromagnetic component in the Mn magnetic moment, a strong increase in magnetization as compared to TbMnSi, and the appearance of a spontaneous magnetic moment. Insertion of the lanthanum ion onto the rare-earth sublattice of TbMnSi brings about a change in the unit cell size and, hence, in the Mn-Mn, Mn-Si, and R-Mn interatomic distances, which causes, in turn, a change in the character of exchange interactions in Tb_0.5La_0.5MnSi and the formation of a complex magnetic structure.     

Insertion properties of LiFe<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>PO<Subscript>4</Subscript> electrode materials for Li-ion batteries

This paper addresses the synthesis structural and electrochemical properties of LiFe_0.5Mn_0.5PO₄ electrode materials for Li-ion batteries. The charge–discharge reaction of Li/LiPF₆-EC–DEC/LiFe_0.5Mn_0.5PO₄ cell carried out at the 1-C rate shows a capacity retention of 128 mAh/g. The local structure of the delithiated Li _x Fe_0.5Mn_0.5PO₄ phases have been studied by Fourier transform infrared spectroscopy and magnetometry. Spectral features indicate that the structure of the delithiated phase remains in the orthorhombic system. The compositional dependence of the magnetic moment is found to be in quantitative agreement with the theoretical value predicted for oxidation of M ²⁺ ions in the high spin state. Paper presented at the 11th Euro-Conference on Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 Sept. 2007     

Exciton and intracenter luminescence in Cd<Subscript>0.6</Subscript>Mn<Subscript>0.4</Subscript>Te/Cd<Subscript>0.5</Subscript>Mg<Subscript>0.5</Subscript>Te quantum-well structures

Exciton luminescence and intracenter luminescence (IL) of Mn²⁺ ions in Cd_0.6Mn_0.4Te/Cd_0.5Mg_0.5Te structures with quantum wells (QWs) 7, 13, and 26 monolayers thick were studied. It was established that in QWs the intensity of exciton luminescence with respect to that of IL is a few orders of magnitude higher than that in bulk crystals. The spectral position of manganese IL profile changes noticeably in going from a bulk crystal to a QW of the same composition. The nonexponential parts of the IL decay curves are determined by excitation migration and the cooperative upconversion process, whose contribution is high under strong excitation and efficient migration. At 77 K, the IL decay constant τ within the exponential region increases with decreasing QW thickness. The decay constant τ in a QW, unlike in a bulk Cd_0.5Mn_0.5Te crystal, decreases substantially under cooling from 77 to 4 K.     

Diode pumped Nd:Lu<Subscript>0.5</Subscript>Y<Subscript>0.5</Subscript>VO<Subscript>4</Subscript>-LBO red laser at 671 nm

We report a efficient compact red laser at 671 nm generation by intracavity frequency doubling of a continuous wave laser operation of a diode pumped Nd:Lu_0.5Y_0.5VO₄ laser on the ⁴ F _3/2-⁴ I _13/2 transition at 1342 nm. An LBO crystal, cut for critical type I phase matching at room temperature is used for second harmonic generation of the laser. At an absorbed pump power of 17.8 W, as high as 2.25 W of continuous wave output power at 671 nm is achieved with 10-mm-long LBO. The optical-to-optical conversion efficiency is up to 12.6%, and the fluctuation of the red output power was better than 3.6% in the given 30 min.     

Multifrequency electron spin resonance study of the charge-ordered manganite Nd<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript>

We performed multifrequency electron spin resonance (ESR) on the antiferromagnetic (T_N = 160 K) and charge-ordered (T _co = 250 K) insulating manganite Nd_0.5Ca_0.5MnO₃. Temperature (4–300 K) and frequency (9.4–285 GHz) dependence of the linewidth, intensity and position of the ESR line were studied. In the paramagnetic state we observe a single Lorentzian absorption line. For a given frequency, the ESR line position is temperature independent (close tog = 1.99). A strong linewidth broadening is observed below T_co. This indicates that there is no magnetic order in the temperature rangeT _cos>T >T _N but strong antiferromagnetic fluctuations are present. Below T_N, due to high-frequency and high-field ESR (up to 12 T) measurements, we were able to observe unexpected lines within the antiferromagnetic gap revealing the presence of a phase separation.     

Study on the spin-states of cobalt-based double-layer perovskite Sr<Subscript>2</Subscript>Y<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>Co<Subscript>2</Subscript>O<Subscript>7</Subscript>

The spin-states of cobalt based perovskite compounds depend sensitively on the valence state and local crystal environment of Co ions and the rich physical properties arise from strong coupling among charge, spin, and orbital degrees of freedom. While extensive studies have been carried out in the past, most of them concentrated on the isotropic compound LaCoO₃. In this paper, using the unrestricted Hartree-Fock approximation and the real-space recursion method, we have investigated the competition of various magnetically ordered spin-states of anisotropic double-layered perovskite Sr₂Y_0.5Ca_0.5Co₂O₇. The energy comparison among these states shows that the nearest-neighbor high-spin-intermediate-spin ferromagnetically ordered state is the relevant magnetic ground state of the compound. The magnetic structure and sizes of magnetic moments are consistent with the recent experimental observation.     

Electrical properties of a lead-free perovskite ceramic: (Na<Subscript>0.5</Subscript>Sb<Subscript>0.5</Subscript>)TiO<Subscript>3</Subscript>

Polycrystalline (Na_0.5Sb_0.5)TiO₃ was prepared using a high-temperature solid-state reaction method. An XRD analysis indicated the formation of a single-phase monoclinic structure. Complex impedance studies revealed the presence of grain boundary effects from 300 °C onwards. Also, the dielectric relaxation in the system was found to be of a non-Debye type. The ac conductivity data were used to evaluate the density of states at the Fermi level, the minimum hopping length and activation energy of the compound. The dc electrical and thermal conductivities of grain and grain boundary have been assessed. The correlated barrier hopping model was found to successfully explain the mechanism of charge transport in (Na_0.5Sb_0.5)TiO₃. PACS 72.20.Ee; 77.22.Ch; 77.22.Gm; 77.84.Dy; 81.05.Je     

Short-Range Order in Amorphous and Crystalline Ferroelectric Hf<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>O<Subscript>2</Subscript>

The microstructures of amorphous and polycrystalline ferroelectric Hf_0.5Zr_0.5O₂ films are studied by X-ray spectroscopy and ellipsometry. EXAFS spectra demonstrate that the amorphous film consists of an “incompletely mixed” solid solution of metallic oxides HfO₂ and ZrO₂. After rapid thermal annealing, the mixed Hf_0.5Zr_0.5O₂ oxide films have a more ordered polycrystalline structure, and individual Hf and Zr monoxide islands are formed in the films. These islands are several nanometers in size and have a structure that is similar to the monoclinic structure of HfO₂ and ZrO₂. The presence of the HfO₂ and ZrO₂ phases in the Hf_0.5Zr_0.5O₂ films is also detected by ellipsometry.     

Magnetostructural phase separation and giant isotope effect in R<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript>

Results of neutron diffraction studies of R_0.5Sr_0.5MnO₃ manganites (R = Sm, Nd_0.772Tb_0.228, and Nd_0.544Tb_0.456) performed to reveal the microscopic origins of the giant oxygen isotope effect recently discovered in Sm_0.5Sr_0.5MnO₃ are presented. It is shown that two crystalline phases differing in the type of Jahn-Teller distortions of oxygen octahedra and in the type of magnetic ordering coexist at low temperatures in all the studied compositions. A scenario for the observed phase transitions is suggested based on the diffraction data. It is found that the percolation transition from the metallic to insulating state in compositions with Sm upon substitution of ¹⁸O for ¹⁶O is associated with a sharp (from 65 to 13%) decrease in the volume of the ferromagnetic metallic phase.     

Oxygen isotope effect in chromium-doped Pr<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> manganites

The effect of oxygen isotope substitution on the properties of Pr_0.5Ca_0.5Mn_{1 ? x} Cr _x O₃ manganites (x = 0, 0.02, 0.05) have been studied. The introduction of chromium favors (i) the decomposition of a charge-ordered state and (ii) the appearance of a ferromagnetic metallic phase in Pr_0.5Ca_0.5Mn_{1 ? x} Cr _x ^16–18O₃. The isotope substitution ¹⁶O → ¹⁸O leads to a decrease in the content of the ferromagnetic phase, an increase in the charge-ordering transition temperature (T _CO), and a decrease in the ferromagnetic transition temperature (T _FM). The isotope mass exponent is evaluated.     

Microemulsion-mediated in-situ synthesis and magnetic characterization of polyaniline/Zn<Subscript>0.5</Subscript>Cu<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocomposite

Polyaniline/Zn_0.5Cu_0.5Fe₂O₄ nanocomposite was synthesized by a simple, general and inexpensive in-situ polymerization method in w/o microemulsion. The effects of polyaniline coating on the magnetic properties of Zn_0.5Cu_0.5Fe₂O₄ nanoparticles were investigated. The structural, morphological and magnetic properties of as-prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, scanning electron microscopy (SEM) and magnetic measurements. The morphology analysis confirmed that polyaniline was deposited on the porous surface of magnetic Zn_0.5Cu_0.5Fe₂O₄. It was shown that the saturation magnetization and coercivity of Zn_0.5Cu_0.5Fe₂O₄ decreased after polyaniline coating, which can be interpreted by the interparticle dipole–dipole interactions that contributed to magnetic anisotropy and changed the magnetic properties of the nanoparticles. PACS  74.25.Ha; 81.05.-t; 81.05.Lg     

Electronic Structure of Oxygen Vacancies in the Orthorhombic Noncentrosymmetric Phase Hf<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>O<Subscript>2</Subscript>

The electronic structure of stoichiometric and oxygen-depleted Hf_0.5Zr_0.5O₂ in the orthorhombic noncentrosymmetric phase has been studied by X-ray photoelectron spectroscopy and quantum-chemical simulation based on the density functional theory. It has been established that the ion-etching-induced peak in the photoelectron emission spectrum with the energy above the top of the o-Hf_0.5Zr_0.5O₂ valence band is due to oxygen vacancies. A method of estimating the density of oxygen vacancies from the comparison of the experimental and theoretical photoelectron spectra of the valence band has been proposed. It has been established that oxygen polyvacancies in o-Hf_0.5Zr_0.5O₂ are not formed: the energetically favorable spatial arrangement of oxygen vacancies in a crystal corresponds to noninteracting oxygen vacancies distant from each other.     

Optical properties of Cd<Subscript>0.6</Subscript>Mn<Subscript>0.4</Subscript>Te/Cd<Subscript>0.5</Subscript>Mg<Subscript>0.5</Subscript>Te quantum-well structures

Emission spectra of three Cd_0.6Mn_0.4Te/Cd_0.5Mg_0.5Te superlattices with Cd_0.6Mn_0.4Te quantum-well (QW) widths of 7, 13, and 26 monolayers, respectively, and the same thickness (46 monolayers) of the Cd_0.5Mg_0.5Te barriers have been studied. The QW width affects the shape and spectral position of the Mn²⁺ intracenter luminescence (IL) band as a result of the crystal field being dependent on the position of the manganese ion with respect to the interface. Measured in identical experimental conditions, the exciton luminescence as compared to the IL is substantially higher in intensity in a QW than in a bulk CdMnTe crystal. Some samples of superlattices and bulk crystals exhibit, in addition to the conventional IL band near 2.0 eV, a weaker band at about 1.45 eV. This band apparently derives from intracenter transitions in the Mn²⁺ ions in the regions where the crystal lattice has the rock-salt rather than the conventional zinc blende structure.     

Synthesis and lithium-ion storage performances of LiFe<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>PO<Subscript>4</Subscript>/C nanoplatelets and nanorods

Carbon-coated olivine-structured LiFe_0.5Co_0.5PO₄ solid solution was synthesized by a facile rheological phase method and applied as cathode materials of lithium-ion batteries. The nanostructure’s properties, such as morphology, component, and crystal structure for the samples, characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer, Emmett, and Teller (BET) determination, X-ray photoelectron spectroscopy (XPS), and the electrochemical performances were evaluated using constant current charge/discharge tests and electrochemical impedance spectroscopy (EIS). The results indicate that nanoplatelet- and nanorod-structured LiFe_0.5Co_0.5PO₄/C composites were separately obtained using stearic acid or polyethylene glycol 400 (PEG400) as carbon source, and the surfaces of particles for the two samples are ideally covered by full and uniform carbon layer, which is beneficial to improving the electrochemical behaviors. Electrochemical tests verify that the nanoplatelet LiFe_0.5Co_0.5PO₄/C shows a better capacity capability, delivering a discharge specific capacity of 133.8, 112.1, 98.3, and 74.4 mAh g^?1 at 0.1, 0.5, 1, and 5 C rate (1 C?=?150 mA g^?1); the corresponding cycle number is 5th, 11th, 15th, 20th, and 30th, respectively, whereas the nanorod one possesses more excellent cycling ability, with a discharge capacity of 83.3 mAh g^?1 and capacity retention of 86.9% still maintained after cycling for 100 cycles at 0.5 C. Results from the present study demonstrate that the LiFe_0.5Co_0.5PO₄ solid solution nanomaterials with favorable carbon coating effect combine the characteristics and advantage of LiFePO₄ and LiCoPO₄, thus displaying a tremendous potential as cathode of lithium-ion battery.     

Magnetic properties of the Nd<Subscript>0.5</Subscript>Gd<Subscript>0.5</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

The magnetic properties of the Nd_0.5Gd_0.5Fe₃(BO₃)₄ single crystal have been studied in principal crystallographic directions in magnetic fields to 90 kG in the temperature range 2–300 K; in addition, the heat capacity has been measured in the range 2–300 K. It has been found that, below the Néel temperature T _N = 32 K down to 2 K, the single crystal exhibits an easy-plane antiferromagnetic structure. A hysteresis has been detected during magnetization of the crystal in the easy plane in fields of 1.0–3.5 kG, and a singularity has been found in the temperature dependence of the magnetic susceptibility in the easy plane at a temperature of 11 K in fields B < 1 kG. It has been shown that the singularity is due to appearance of the hysteresis. The origin of the magnetic properties of the crystal near the hysteresis has been discussed.     

Current-voltage characteristics of polycrystalline (La<Subscript>0.5</Subscript>Eu<Subscript>0.5</Subscript>)<Subscript>0.7</Subscript>Pb<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at low temperatures

The current-voltage characteristics of the polycrystalline substituted lanthanum manganite (La_0.5Eu_0.5)_0.7Pb_0.3MnO₃ have been measured at temperatures close to the metal-insulator transition temperature and at low temperatures. In both cases, the current-voltage characteristics exhibit nonlinear properties that are strongly dependent on the strength of an applied magnetic field. The mechanisms responsible for the nonlinear properties at these temperatures are found to be different: near the metal-insulator transition, the current-voltage characteristics are determined by the phase layering inside granules, while at low temperatures, they are determined by tunneling of carriers through insulating interlayers of the granules.     

Pressure induced antiferromagnet-ferromagnet transition in La<Subscript>0.5</Subscript>Ba<Subscript>0.5</Subscript>CoO<Subscript>2.8</Subscript> cobaltite

The anion deficient cobaltite La_0.5Ba_0.5CoO_2.8 with theformal cobalt valence state close to 3+ has been studied as function of pressure up to6.5 GPa at different temperatures by neutron powder diffraction. At ambient pressure thecrystal structure of this compound has cubic symmetry (space group Pm3?m) and is found to become antiferromagnetic withT _N close to 250 K. Applied pressure inducesa gradual transition from the antiferromagnetic into a ferromagnetic state through a mixedmagnetic state. The transition is not accompanied by obvious changes in the macroscopiccrystal symmetry. It is suggested that the magnetic ground state strongly depends on theunit cell volume and that the transition is associated with a spin state crossover of thecobalt ions whereas the formal Co³⁺/Co⁴⁺ ratio is less importantthan expected following the double exchange scenario for the appearance offerromagnetism.     

High-temperature complex impedance and modulus spectroscopic studies of doped Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> ferroelectric ceramics

Lead-free Na_0.5Bi_0.5TiO₃ (NBT) and (1 ? x)Na_0.5Bi_0.5TiO₃ + xBaTiO₃ with x = 0.1 and 0.2 (where x = 0.1 and 0.2 are named as NBT1 and NBT2, respectively), (1 ? y)Na_0.5Bi_0.5TiO₃ + yBa_0.925Nd_0.05TiO₃ with y = 0.1 and 0.2 (where y = 0.1 and 0.2 are named as NBT3 and NBT4, respectively)-based relaxor ferroelectric ceramics were prepared using the sol-gel method. The crystal structure was investigated by X-ray diffraction (XRD) at room temperature (RT). The XRD patterns confirmed the presence of the rhombohedral phase in all the samples. The electrical properties of the present NBT-based samples were investigated by complex impedance and the modulus spectroscopy technique in the temperature range of RT–600 °C. The AC conductivity was found to increase with the substitution of Ba²⁺ ions to the NBT sample whereas it significantly decreased with the addition of Nd³⁺ ions. The more anion vacancies in Ba-added samples and the lower anion vacancies in Nd-added samples were found to be responsible for higher and lower conductivities, respectively.     

Structural and magnetic phase transformations in the BiFeO<Subscript>3</Subscript>-CaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Bi_{1 ? x} Ca _x Fe_{1 ? x/2}Nb _x/2O₃ system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe_0.5Nb_0.5O₃, the Fe³⁺ and Nb⁵⁺ ions are partially ordered and the unit cell is monoclinic (space group P2₁/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.