Temperature-dependent Brillouin scattering studies of surface acoustic modes in Nd0.5Sr0.5MnO3

Brillouin scattering experiments are carried out to study the surface acoustic waves in Nd_0.5Sr_0.5MnO₃ as a function of temperature in the range of 40-300 K covering the metal-insulator and charge-ordering phase transitions. The surface modes include surface Rayleigh wave, pseudo-surface acoustic wave (PSAW) and high velocity PSAW. The observed softening of the sound velocities for the surface modes below paramagnetic to ferromagnetic transition, T_c is related to the softening of the C₄₄ elastic constant. The subsequent hardening of the sound velocity below the charge ordering transition temperature T_co is attributed to the coupling of the acoustic phonon to the charge ordered state via long range ordering of the strong Jahn-Teller (JT) distortion.     

Surface magnetic disorder in nanostructured Ni0.5Zn0.5Fe2O4 particles

Nanostructured ferroxide particles with initial formula Ni_0.5Zn_0.5Fe₂O₄ are investigated. The aim was to explore the monodomain and the superparamagnetic states of the ferrospinel and the impact of the surface magnetic disorder on the magnetization processes. Mössbauer spectroscopy (MöS) demonstrated that the ion distribution follows the general formula (Zn_0.5Fe_0.5)_A[Ni_0.5Fe_1.5]_BO₄, where A is the tetrahedral and B, the octahedral sublattice. MöS in an external magnetic field (5 T) at 4.2 K shows non-collinearity of the sublattices’ magnetic moments and deviations in the hyperfine magnetic field that could be related to a canting effect. Magnetic measurements were applied to characterize the temperature behavior of the magnetic properties and the a.c. complex magnetic susceptibility.     

Synthesis and dielectric characteristics of La0.5Bi0.5MnO3 ceramics

La_0.5Bi_0.5MnO₃ ceramics with a single phase were prepared by a solid-state reaction method, and their dielectric properties were characterized. Two dielectric relaxations with a giant dielectric constant were identified in the temperature range from 125 to 350 K. The electron hopping between Mn³⁺ and Mn⁴⁺ was found to be the origin of the dielectric relaxation at low temperatures (125–200 K) with an activation energy of 0.18 eV. The high temperature (200–350 K) dielectric relaxation can be attributed to the conduction.     

Evidence of glassy ferromagnetic phase and kinetic arrest of electronic phase in Sm0.35Pr0.15Sr0.5MnO3 manganites

The effect of doping of rare earth Pr³⁺ ion as a replacement of Sm³⁺ in Sm_0.5Sr_0.5MnO₃ is investigated. Temperature dependent dc and ac magnetic susceptibility, resistivity, magnetoresistance measurements on chemically synthesized (Sm_0.5−xPr_x)Sr_0.5MnO₃ show various unusual features with doping level x=0.15. The frequency independent ferromagnetic to paramagnetic transition at higher temperature (∼191 K) followed by a frequency dependent reentrant magnetic transition at lower temperature (∼31 K) has been observed. The nature of this frequency dependent reentrant magnetic transition is described by a critical slowing down model of spin glasses. From non-linear ac susceptibility measurements it has been confirmed that the finite size ferromagnetic clusters are formed as a consequence of intrinsic phase separation, and undergo spin glass-like freezing below a certain temperature. There is an unusual observation of a 2nd harmonic peak in the non-linear ac susceptibility around this reentrant magnetic transition at low temperature (∼31 K). Arrott plots at 10 and 30 K confirm the existence of glassy ferromagnetism below this low temperature reentrant transition. Electronic- and magneto-transport measurements show a strong magnetic field—temperature history dependence and strong irreversibility with respect to the sweeping of magnetic field. These results are attributed to the effect of phase separation and kinetic arrest of the electronic phase in this phase separated manganite at low temperatures.     

Charge ordering and structural transitions in Pr0.5Sr0.41Ca0.09MnO3

The structural and magnetic transitions in Pr_0.5Sr_0.41Ca_0.09MnO₃ have been investigated by neutron diffraction and electron microscopy. Two structural transitions, Imma to I4/mcm and I4/mcm to Pmmn, are observed by decreasing the temperature. Two magnetic transitions, from a paramagnetic insulating to a ferromagnetic metallic and from a ferromagnetic metallic to an antiferromagnetic insulating states at T_C=250 K and T_N=180 K, respectively, are also observed. The structures of these three forms have been determined from neutron powder diffraction data. The first important result concerns the low temperature antiferromagnetic CE type and charge ordered structure, which has been refined in the Pmmn space group, without any constraint. This structure is completely long range ordered, with two Mn-sites, Mn³⁺ in tetragonally elongated octahedra, and Mn⁴⁺, off-centered in nearly regular octahedra. The second important point concerns the abrupt character of the structural transition from the I4/mcm to the Pmmn structure, without any appearance of incommensurability. The magnetic and transport properties of this compound are compared with those of Pr_0.5Sr_0.5MnO₃.     

Impedance study of giant dielectric permittivity in BaFe0.5Nb0.5O3 perovskite ceramic

Single-phase BaFe_0.5Nb_0.5O₃ (BFN) ceramics were prepared by solid-state reaction method and were characterized by X-ray Diffraction (XRD) technique. Then, impedance spectroscopy measurements were conducted in a frequency range from 100 Hz to 1 MHz and in a temperature range from 293 to 600 K. Relaxation phenomena of non-Debye type have been observed in the BFN ceramics, as confirmed by the Cole–Cole plots. The higher values of ε′ at the lower frequencies are explained on the basis of the Maxwell–Wagner (MW) polarization model. Complex impedance analysis enables us to separate the contributions from grains and grain boundaries of our samples. We found that at higher temperatures grain boundary resistance is higher than grain resistance, irrespective of composition.     

Structure and electrical properties of Bi0.5(Na, K)0.5TiO3−BiGaO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x − y)Bi_0.5Na_0.5TiO₃−xBi_0.5K_0.5TiO₃− yBiGaO₃ have been fabricated by an ordinary sintering technique, and their structure and electrical properties and depolarization temperature have been studied. The results of X-ray diffraction reveal that Bi_0.5K_0.5TiO₃ and BiGaO₃ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure. An obvious change in microstructure with increasing concentration of Bi_0.5K_0.5TiO₃ and BiGaO₃ was observed. The piezoelectric constant d₃₃ and the electromechanical coupling factor k_p of the ceramics attain maximum values of 165 pC/N and 0.346 at y = 0.01(x = 0.18) and x = 0.21(y = 0.01), respectively. The temperature dependence of dielectric constant indicates an obvious relaxor characteristic with strong frequency dependence of dielectric constant. The depolarization temperature decreased with increasing content of BiGaO₃ and first decreases and then increases with increasing amount of Bi_0.5K_0.5TiO₃.     

Ferroelectric relaxor behaviour in Pb(Fe0.5Ta0.5)O3

The relaxor ferroelectric lead iron tantalate, Pb(Fe_0.5Ta_0.5)O₃ (PFT) is synthesized by Coulombite precursor method. The X-ray diffraction pattern of the sample at room temperature shows a cubic phase. The field dependence of dielectric response is measured in a frequency range 0.1 kHz — 1 MHz and in a temperature range from 173–373 K. The temperature dependence of permittivity (ɛ′) shows broad maxima at various frequencies. The frequency dependence of the permittivity maximum temperature (T _m ) has been modelled using Vogel-Fulcher relation.      

Low-temperature synthesis and growth of superparamagnetic Zn0.5Ni0.5Fe2O4 nanosized particles

Mono-disperse spinel Ni_0.5Zn_0.5Fe₂O₄ nanosized particles have been synthesized via a hydrothermal method at low temperature. X-ray diffraction (XRD), transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) analysis indicated that the synthesized nanocrystals were of pure cubic spinel structure with the size about 6-20 nm. The activation energy of grain growth is 35.06 kJ/mol experimented by the Arrhenius equation. A primary experimental model was put forward to shed light on the growth mechanism of crystallined spinel Ni-Zn ferrite nanosized particles under hydrothermal conditions. The magnetic measurements shows that the prepared Ni_0.5Zn_0.5Fe₂O₄ nanoparticle possess good superparamagnetic behavior.     

Photoelectronic and electrical properties of Tl2InGaS4 layered crystals

Tl₂InGaS₄ layered crystals are studied through the dark electrical conductivity, space charge limited current and illumination- and temperature-dependent photoconductivity measurements in the temperature regions of 220-350 K, 300-400 K and 200-350 K, respectively. The space charge limited current measurements revealed the existence of a single discrete trapping level located at 0.44 eV. The dark electrical conductivity showed the existence of two energy levels of 0.32 eV and 0.60 eV being dominant above and below 300 K, respectively. The photoconductivity measurements reflected the existence of two other energy levels located at 0.28 eV and 0.19 eV at high and low temperatures, respectively. The photocurrent is observed to increase with increasing temperature up to a maximum temperature of 330 K. The illumination dependence of photoconductivity is found to exhibit supralinear recombination in all the studied temperature ranges. The change in recombination mechanism is attributed to exchange in the behavior of sensitizing and recombination centers.     

相分离Nd0.5Ca0.5MnO3体系的再入型自旋玻璃行为和电荷有序

研究了Nd_0.5Ca_0.5MnO₃体系的结构和输运特性. 结构 分析表明，在300K下，体系表现为O′型正交结构并存在典型的Jahn-Teller畸变.在8 T磁场 下，体系出现顺磁绝缘-铁磁金属的转变，庞磁电阻效应发生. 磁测量发现，样品的奈尔温 度T_N和电荷有序转变温度T_CO分别在150和240K左右，在41K左右出 现典型再入型自旋玻璃行为，同时观察到了负的磁化率异常. 结果表明，Nd关键词： 庞磁电阻 自旋玻璃态 负磁化强度 电荷有序     

Raman spectroscopy of RuSr2(Eu1.5Ce0.5)Cu2O10 magneto-superconductor

Raman spectroscopy studies are reported for the RuSr₂Eu_1.5Ce_0.5Cu₂O₁₀ (Ru-1222) compound at various temperatures of 300, 250, 200 and 90 K. Three distinct vibrational bands: the first at 110, 140, and 160 cm⁻¹, the second at 295 and 347 cm⁻¹, and third one at 651 cm⁻¹ are seen in Raman spectra of the compound at room temperature. These bands are attached to the Cu atoms’ c-direction, the Ru atoms’ ab-plane stretching and Ru atoms’ c-direction anti-stretching modes. Below 200 K, an extra vibrational mode is also seen at 260 cm⁻¹. Also, with a decrease in temperature, though the Cu vibrational modes remain intact, the Ru atoms’ ab-plane stretching (295 cm⁻¹) and c-direction anti-stretching (651 cm⁻¹) modes shift gradually to higher wave number positions. The frequencies of modes at 260 and 651 cm⁻¹ showed anomalous softening and line-width broadening below 100 K that corroborates well with the spin ordering seen in susceptibility studies. The studied compound is a ferromagnetic superconductor with magnetic ordering of the Ru spins at 200 K and superconductivity below 30 K. A magnetic and electrical transport characterization of the compound is also presented briefly.     

Mössbauer spectroscopy as a probe of magnetic states in La0.5Ca0.5FeO3−δ perovskite

Charge disproportionation in La_0.5Ca_0.5FeO_3−δ perovskite has been detected by zero-field Mössbauer spectra from 20 K to room temperature. On the basis of the parameters of center shifts and hyperfine fields, Mössbauer spectra identified that the iron ionic states are Fe³⁺ and Fe⁵⁺ below 150 K, Fe³⁺, Fe⁴⁺ and Fe⁵⁺ in the intermediate temperature region, as well as Fe³⁺ and Fe⁴⁺ above 220 K. At low temperatures, the system exhibits a cluster-glass-like state resulting from competition between antiferromagnetic interaction of Fe³⁺–Fe³⁺ and ferromagnetic interaction of Fe³⁺–Fe⁵⁺.     

The unusual magnetic and electronic properties of Gd0.5Ba0.5CoO2.9

Magnetic and electrical properties of well-characterized Gd_0.5Ba_0.5CoO_2.9 have been studied carefully in order to compare them with those of other analogous cobaltates of the type Ln_0.5A_0.5CoO₃ (Ln=La, Nd and A=Sr, Ba) which are ferromagnetic. The results show that Gd_0.5Ba_0.5CoO_2.9, which has A-site cation ordering at room temperature, does not become a genuine ferromagnet at low temperatures, but the ferromagnetic interactions observed at 280 K give over to an antiferromagnetic (AFM) state on cooling to 230 K. The AFM state is rendered ferromagnetic on the application of high magnetic fields. The properties can be understood on the basis of phase separation induced by the large A-site cation-disorder, arising from the size mismatch.     

Neutron diffraction study of magnetic ordering in Tm0.5Eu0.5Se

A single crystal of the magnetic semiconductor Tm_0.5Eu_0.5Se was studied by means of neutron diffraction in the temperature range from 1.8 to 293 K. Long-range magnetic order is detected at temperatures below T_c = (18.5±1) K. The measured ferromagnetic moment component of (2.12±0.05) μ_B per rare-earth ion at saturation in zero external magnetic field indicates approximately antiparallel alignment of Tm moment and Eu spin (mutual angle 134°). The experimentally determined neutron magnetic form factor confirms the divalent state of both Tm and Eu in Tm_0.5Eu_0.5Se.     

Possible anisotropy gap in La1.35Sr1.65Mn2O7 and La1.5Sr0.5NiO4 detected through specific heat and magnetization measurements

Magnetization and specific heat measurements, as a function of temperature, were performed on single crystals of La_1.35Sr_1.65Mn₂O₇ and La_1.5Sr_0.5NiO₄, under different applied magnetic fields (H). The specific heat in La_1.35Sr_1.65Mn₂O₇ was decreased for H=9 T parallel to the crystal c axis, compared with H=0, possibly due to a suppression of spin-wave excitations (magnons) in that ferromagnetic bilayer structure. On the other hand, the applied magnetic field had no effect in the specific heat of the antiferromagnetic La_1.5Sr_0.5NiO₄. For H=9 T and below the temperature of 4 K the specific heat data, for each crystal, was well fitted by an exponential decay law. This allowed the calculation of energy gaps around 1 meV for both compounds, in close agreement with Δ=2μ_BH for an expected energy gap in the magnon spectrum. Detailed magnetization measurements showed monotonic variations below 4 K and a steep increase close to 2 K. Both magnetization and specific heat measurements suggest the existence of an anisotropy gap in the energy spectrum of La_1.35Sr_1.65Mn₂O₇ and La_1.5Sr_0.5NiO₄.     

The current-induced effect on the Jahn-Teller distortion in the La0.5Ca0.5MnO3 manganite

The influence of dc current on the resistivity ρ and the Young's modulus E of La_0.5Ca_0.5MnO₃ compound has been investigated by means of an in situ measuring method. At low temperatures, both the resistivity ρ and the relative modulus ΔE increase with the current. A relaxation behavior of ρ to the higher resistive state is observed at a fixed temperature and a constant current. After storing the sample for a few days, ρ decreases with the current, accompanying a slight drop of ΔE at low temperatures. Current-induced effects on ρ and ΔE are interpreted according to the current-induced interwinning of Mn³⁺O₆ octahedral distortion modes between Q₂- and Q₃-types, which is suggested to contribute to the variation of the resistivity.     

Electronic and vibrational Raman spectroscopy of Nd0.5Sr0.5MnO3 through the phase transitions

Raman scattering experiments have been carried out on single crystals of Nd_0.5Sr_0.5MnO₃ as a function of temperature in the range of 320–50 K, covering the paramagnetic insulator-ferromagnetic metal transition at 250 K and the charge-ordering antiferromagnetic transition at 150 K. The diffusive electronic Raman scattering response is seen in the paramagnetic phase which continue to exist even in the ferromagnetic phase, eventually disappearing below 150 K. We understand the existence of diffusive response in the ferromagnetic phase to the coexistence of the different electronic phases. The frequency and linewidth of the phonons across the transitions show significant changes, which cannot be accounted for only by anharmonic interactions.     

Li mobility in Li0.5 − xNaxLa0.5TiO3 perovskites (0 ≤ x ≤ 0.5): Influence of structural and compositional parameters

The dependence of Li mobility on structure and composition of Li_0.5 − xNa_xLa_0.5TiO₃ perovskites (0 ≤ x ≤  0.5) has been investigated by means of neutron diffraction, nuclear magnetic resonance and impedance spectroscopy. At 300 K, all samples display a rhombohedral superstructure (R-3c S.G.), where octahedra are out of phase tilted along [111] direction of the ideal cubic cell. The elimination of the octahedral tilting is responsible for the rhombohedral–cubic transformation, detected near 1000 K. In these perovskites, La and Na cations are randomly distributed in A sites, but Li ions are fourfold coordinated at unit cell faces of the cubic perovskite. Lithium conductivity, σ_300 K, decreases with the sodium content, decreasing from values typical of fast ionic conductors, 10^− 3 S/cm, to those of good insulators, 10^− 10 S/cm, when the interconnectivity between vacant A sites is lost (x > 0.3). In samples with x < 0.3, dc conductivity displays a non-Arrhenius behaviour, decreasing activation energy from ~ 0.37 to 0.25 eV when the sample is heated between 77 and 500 K. The temperature dependence of B_Li factors shows the existence of two regimes for Li motion. Below 373 K, Li ions remain partially located near square oxygen windows that connect contiguous A sites, but above 400 K, extended Li motions become dominant. The additional decrease of activation energy from 0.25 to 0.16 eV (low-temperature ⁷Li NMR value), should require the full elimination of octahedral tilting which is only produced above 1000 °C.     

First order paramagnetic-ferromagnetic phase transition in Tb doped La0.5Ca0.5MnO3 manganite

Effect of A-site disorder in half-doped La_0.5Ca_0.5MnO₃ created by the substitution of Tb at La site is studied through temperature dependent neutron diffraction, resistivity, and magnetization on La_0.375Tb_0.125Ca_0.5MnO₃ (LTC) to identify the evolution of phases as a function of temperature and magnetic field. Contrary to the parent system, the paramagnetic (PM) to ferromagnetic (FM) transition is found to be of first order but becomes observable only in fields above 1 T. Absence of this transition in zero and low fields along with the observation of a very intriguing and irreversible M-H curve at low temperature raises the question about the ground state of the system. It is identified from the specially designed measurement protocol that the ground state of the system becomes FM, contrary to the parent LCMO. However, this remains masked because of the hindrance to the transformation kinetics of the PM-FM first order transition in low fields. Significantly, though the thermal hysteresis of the first-order transition decreases with the measurement field, collapse of such a behavior to a critical point is not observed in LTC, unlike earlier observations in some other manganites.