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Undoped and Ca doped ZnO Wurtzite nanostructures were prepared via sol-gel co-precipitation. Heavy Ca doping causes structural deterioration due to larger ionic radius of Ca2+. Strain-induced defor...

Structural and magnetic phase-transitions in La1-xSrxMnO3+δ (x = 0.67 & 0.71) have been studied employing x-ray diffraction, neutron diffraction and transmission electron microscopy (TEM) in conjunction with electrical-transport and magnetic studies. These studies reveal a cubic to incommensurate (IC) charge-ordered (CO) tetragonal phase-transition in both the samples with k-vector varying with temperature and thus indicates charge density wave-like nature. Followed by magnetic transition from paramagnetic phase to incommensurate antiferromagnetic phase. The nearly equal expansion of the a,b axes in the ab plane and the contraction of the c-axis (i.e. type-I Jahn-Teller distortion (JTD)) indicate that the CO is accompanied by a planer ferro-ordering of 3 d x 2 − y 2 orbitals in the ab plan, which is different from haring-bone type ordering of 3 d 3 x 2 − r 2 / 3 d 3 y 2 − r 2 orbitals, commonly observed in narrow bandwidth CO manganites. For x = 0.71, a second phase-transition from type-I JTD-CO phase (c a) was observed. Through EDAX analysis in TEM, a nano-scale marginal composition difference has been observed to lead to nanoscale phase-coexistence. These studies have revealed first microscopic evidence of CO and nano-scale multiphase coexistence in La1-xSrxMnO3-δ with x = 0.67 & 0.71.

We report gapless quantum spin liquid behavior in the layered triangular ${\mathrm{Sr}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$ system. X-ray diffraction shows superlattice reflections associated with atomic site ordering into triangular Cu planes well separated by Sb planes. Muon spin relaxation measurements show that the $S=\frac{1}{2}$ moments at the magnetically active Cu sites remain dynamic down to 65 mK in spite of a large antiferromagnetic exchange scale evidenced by a large Curie-Weiss temperature ${\ensuremath{\theta}}_{\mathrm{CW}}\ensuremath{\simeq}\ensuremath{-}143\text{ }\text{ }\mathrm{K}$ as extracted from the bulk susceptibility. Specific heat measurements also show no sign of long-range order down to 0.35 K. The magnetic specific heat (${C}_{m}$) below 5 K reveals a ${C}_{m}=\ensuremath{\gamma}T+\ensuremath{\alpha}{T}^{2}$ behavior. The significant ${T}^{2}$ contribution to the magnetic specific heat invites a phenomenology in terms of the so-called Dirac spinon excitations with a linear dispersion. From the low-$T$ specific heat data, we estimate the dominant exchange scale to be $\ensuremath{\sim}36\text{ }\text{ }\mathrm{K}$ using a Dirac spin liquid ansatz which is not far from the values inferred from microscopic density functional theory calculations ($\ensuremath{\sim}45\text{ }\text{ }\mathrm{K}$) as well as high-temperature susceptibility analysis ($\ensuremath{\sim}70\text{ }\text{ }\mathrm{K}$). The linear specific heat coefficient is about $18\text{ }\text{ }\mathrm{mJ}/\mathrm{mol}\text{ }{\mathrm{K}}^{2}$ which is somewhat larger than for typical Fermi liquids.

Ni3V2O8 is an archetypical multiferroic material with a kagome staircase structure of S = 1 (Ni2+) spins whose complex interplay between spin ordering and ferroelectricity has been studied for more than a decade. Here, we report a new kagome staircase compound PbCu3TeO7 with Cu2+ (S = 1/2) spins that exhibits two Néel temperatures at TN1 = 36 K and TN2 = 24 K, and a magnetic field (H)-induced electric polarization (P) below TN2. Pyroelectric and magnetoelectric current measurements in magnetic fields up to 60 T reveal that for H||c of ~ 8.3 T, a spin-flop transition induces a transverse P||a with a magnitude of 15 µC/m2 below TN2. Furthermore, for a parallel configuration with P//H//a, two spin-flop transitions occur, the first at ~ 16 T with P//a of 14 µC/m2, and the second at ~ 38 T, where P disappears. Monte Carlo simulations based on 12 major exchange interactions uncover that a sinusoidal amplitude modulation of the spins occurs along the b-axis below TN1 and an incommensurate, proper screw-type spin order occurs in the ac-plane below TN2. The simulation results show that P//a under H//c stems from a spin-flop transition facilitating an ab-plane-type spiral order, while the two successive spin-flop transitions for H//a result in spiral spin orders in the ab- and bc-planes. Based on the experimental and theoretical results, we establish field-induced magnetic and electric phase diagrams for the two H directions, demonstrating that the distorted kagome staircase structure with competing intra–interlayer interactions and lifted frustration creates a plethora of different noncollinear spin textures of S = 1/2 spins that in turn induce electric polarization.

Fine-tuning the charge distribution in Ba 2 FeMoO 6 obtained via “isovalent” substitution at the A-site (i.e., Ba) is expected to bring about changes in the physical properties of the system that can be manipulated in magnetic refrigerants. With this motivation, the phase formation, crystal structure, microstructure, magnetic and magnetocaloric properties of the Ba 2−x Sr x FeMoO 6 (0≤x≤0.4) samples fabricated by solid state reaction method have been investigated. The X-ray diffraction analysis confirmed the formation of cubic structure with Fm 3 m space group in all the fabricated samples. The magnetization measurements and Arrott analysis revealed a second order of ferromagnetic phase transition in all the samples. An increase in magnetization and Curie temperature (T C ) was observed with the increase in Sr-content that was attributed to the increased orbital hybridization and exchange interaction between Fe and Mo ions. The magnitude of the maximum magnetic entropy change at the Curie temperature and the relative cooling power were observed to slightly decrease with the increased Sr doping. The excellent magnetocaloric features and convenient adjustment of Curie temperature make these materials useful for magnetic refrigeration in a wide range of temperature.

The phase diagram of La 0.25 Sr 0.75 MnO 3.01 has been investigated by performing temperature dependent powder x-ray diffraction, transmission electron microscopy (TEM), and resistivity and magnetization measurements. It undergoes competing structural phase-transitions from charge disordered cubic to charge-ordered (CO) tetragonal phase having compressed Jahn-Teller (JT) distorted octahedra (c/a 1) at ∼250 K. The three phases coexist from 250 to 210 K. The nano-scale (10–20 nm) coexistence of CO and twin tetragonal phases sustains down to 80 K, with twin tetragonal phase fraction increasing substantially on cooling below 210 K. In TEM diffraction data, the CO phase shows well defined super-lattice spots with ∼8x8-fold CO modulation along [110] type direction of the perovskite lattice and typical CO micro-structure with lattice fringes. The twin microstructure resulting in twin spots can also be observed for the elongated JT-distorted tetragonal phase. The compressed and elongated distortions of the octahedra for CO and twin tetragonal phases implies ferro orbital-ordering of 3d x 2 -y 2 and 3d 3z 2 -r 2 orbitals in the ab -plane and along c -axis, respectively. During heating, the twin tetragonal to CO phase transformation was fully reversible with a wide hysteresis and the CO modulation was found to flip its direction from [110] to [10 1 ¯ ] & [01 1 ¯ ] in the same domain. Thus, our results reveal the nano-scale structural phase separation due to competitive nature of CO and twin phase over wide temperature range.

The influence of magnetic field on the crystal structure of colossal magnetoresistance (CMR) material La0.5Sr0.5MnO3+delta has been investigated. Low temperature high-magnetic field powder X-ray diffraction (XRD) measurements were carried out on La0.5Sr0.5MnO3+delta in temperature range from 4.2 to 300 K and magnetic field range from 0 to 8 T. A first order structural phase transition from tetragonal I4/mcm to orthorhombic Fmmm phase, coupled with reported electronic and magnetic phase transition from ferromagnetic metal to antiferromagnetic insulator state has been started at similar to 200 K and completed at similar to 100 K. On the application of 8 T magnetic field this magneto-structural phase transition shifted to lower temperature (similar to 162 K) with phase coexistence regime down to 4.2 K. Isothermal X-ray diffraction measurements at 150 K infer the evidence of magnetic field driven first order structural phase transition from Fmmm to I4/mcm. These results consistent with the resistivity and magnetoresistance results; present the microscopic evidence of strong spin-lattice coupling and reveal the magnetic field driven structural phase transition as the origin of observed colossal magnetoresistance in this material. (C) 2016 Published by Elsevier B.V.

We have investigated the complex dielectric and impedance properties of magnetoelectric compound Fe2TiO5 (FTO) as a function of temperature (T) and frequency (f) to understand the grain (G) and grain boundary (Gb) contributions to its dielectric response. The temperature and frequency dependent dielectric permittivity data shows a sharp increase in permittivity above 200K accompanied with a frequency dependent peak in loss. At T less than 175K, only G contribution dominates even at lower frequency (nearly 100Hz), but for T greater than 175K, the Gb contribution starts appearing at low frequency. The value of critical frequency distinguishing these two contributions increases with increasing temperature. The observed non-Debye dielectric relaxation follows thermally activated process and is attributed to polaron hopping. Further the frequency dependence of ac conductivity follows the Jonscher power-law. The temperature dependency of critical exponent s shows that the correlated barrier hopping model is appropriate to define the conductivity mechanism of FTO in the studied temperature regime., 16 pages, 6 figures, Accepted in Journal of alloys and compounds

We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline Ba(TiO)Cu$_{4}$(PO$_{4}$)$_{4}$ (BTCPO) via x-ray diffraction, magnetic susceptibility, heat capacity, and $^{31}$P Nuclear Magnetic Resonance (NMR) measurements. BTCPO has a 2D layered structure with interlinked Cu$_{4}$O$_{12}$ tetramer units. A broad maximum is observed around 16.5 K in our magnetization data accompanied by a sharp anomaly around $T$ = 9.5 K in the heat capacity. An anomaly at $T$ = 9.5 K is also found in the temperature dependence of the $^{31}$P NMR spin-lattice relaxation rate $1/T_1$. A power law behavior for the heat capacity as well as for the $^{31}$P $1/T_1$ below the ordering temperature could be obtained. The $^{31}$P NMR lineshape is asymmetric and the NMR shift tracks the bulk spin-susceptibility. We estimated the isotropic and axial components of the hyperfine coupling tensor to be as the $A^{iso}_{hf} \backsimeq 6794$ $\rm{Oe/\mu_{B}}$ and $A^{ax}_{hf} \backsimeq 818$ $\rm{Oe/\mu_{B}}$, respectively., Comment: 8 pages, 3 tables, 13 figures

We have investigated the thermodynamic and local magnetic properties of the Mott insulating system Ag$_{3}$LiRu$_{2}$O$_{6}$ containing Ru$^{4+}$ (4$d$$^{4}$) for novel magnetism. The material crystallizes in a monoclinic $C2/m$ structure with RuO$_{6}$ octahedra forming an edge-shared two-dimensional honeycomb lattice with limited stacking order along the $c$-direction. The large negative Curie-Weiss temperature ($\theta_{CW}$ = -57 K) suggests antiferromagnetic interactions among Ru$^{4+}$ ions though magnetic susceptibility and heat capacity show no indication of magnetic long-range order down to 1.8 K and 0.4 K, respectively. $^{7}$Li nuclear magnetic resonance (NMR) shift follows the bulk susceptibility between 120-300 K and levels off below 120 K. Together with a power-law behavior in the temperature dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic spin correlations with gapless excitations. Electronic structure calculations suggest an $S = 1$ description of the Ru-moments and the possible importance of further neighbour interactions as also bi-quadratic and ring-exchange terms in determining the magnetic properties. Analysis of our $\mu$SR data indicates spin freezing below 5 K but the spins remain on the borderline between static and dynamic magnetism even at 20 mK.

We have synthesized the single phase polycrystalline sample of silver doped disordered Sm0.55Sr0.45−xAgxMnO3 using the conventional solid state method. They have been investigated by X-ray diffraction for phase evolution and their Rietveld refined X-ray diffraction pattern indicates an increase in lattice parameters and unit cell volume with Ag doping. The magnetization analysis shows the presence of a first order paramagnetic to ferromagnetic transition; the transition temperature and amount of thermal hysteresis decreases on Ag doping. The transport, magneto-transport results indicates decrease in metal to insulator transition, drastic increase in resistance and temperature coefficient of resistance (TCR %) with nearly unaffected colossal magnetoresistance effect which indicates that at least a part of silver is incorporated in the lattice. Scanning electron micrographs indicate that silver has also increased the grain growth sharpness and thus decrease the effect of grain boundary scattering. The significant value of TCR of ∼43% of our samples indicates their applied nature for bolometric application.

We report the existence of ferromagnetic correlations (FMC) in the paramagnetic (PM) matrix of cubic La1−xSrxMnO3−δ (x = 0.80, 0.85) well above its coupled structural, magnetic, and electronic phase transitions. The dc-magnetization versus temperature [M(T)] behavior under different magnetic fields (from 100 Oe to 70 kOe) shows the presence of short-range magnetic correlations up to (TFMC ∼) 365 K, far above the antiferromagnetic ordering temperatures (TN =) 260 and 238 K for x = 0.80 and 0.85, respectively. More importantly the observed short-range FMC survive even up to 70 kOe, which indicates their robust nature. The temperature region between TN and TFMC is dominated by the presence of correlated ferromagnetic (FM) entities within the PM matrix and stabilized due to A-site chemical disorder. Our results further illustrate that for the studied compositions, the oxygen off-stoichiometry does not have any significant effect on the nature and strength of these FM entities; however, FM interactions increase in the oxygen-deficient samples. These compositions are the unique examples, where the presence of FMC is observed in an undistorted basic cubic perovskite lattice well above TN and therefore are novel to understand the physics behind the colossal magnetoresistance effect.

Structural phase transition studies employing low-temperature transmission electron microscopy (LT-TEM) and low-temperature x-ray diffraction (LT-XRD) have been carried out on wide-band La0.2Sr0.8MnO3−δ (δ=0.01) manganite. These studies reveal clear evidence of cubic to tetragonal phase transition occurring below 260 K in La0.2Sr0.8MnO3−δ (δ=0.01). TEM observations depict that cubic to tetragonal phase-transition results in formation of nanotwinned microstructure of the tetragonal phase. Formation of nanotwins has been attributed to orbital-orientation phase-separation leading to minimization of strain. The magnetic and electron transport measurements reveal that the structural phase transition is coupled with a canted C-type antiferromagnetic and semi-conductor to insulating phase transition.

Geometrical frustration, arising from the unsatisfying magnetic bonds in peculiar magnetic materials, leads to the emergence of a variety of ground states ranging from exotic disordered (quantum spin liquid) to unusual magnetic ordered states. We have prepared and studied the magnetic properties of a novel quantum magnet PbCuTeO5, whose structure suggests that it has 2D distorted check-board lattice. A large antiferromagnetic Curie-Weiss temperature of -165 K and a spin freezing temperature Tf = 6 K are observed in the magnetic data. Our results suggest that PbCuTeO5 is a new frustrated quantum magnet with a large frustration parameter f = θCW/Tf > 27.

Comprehensive X-ray diffraction (XRD) studies have been performed at different temperature (T) (4.2-300 K) and magnetic field (H) (0-8 T) to understand the evolution of crystal structure of Nd0.49Sr0.51MnO3+delta (NSMO) under non ambient conditions. The T dependent XRD results show the abrupt change in the lattice parameters without any change in lattice symmetry at similar to 200 K, which is associated with the first order structural phase transition from ferromagnetic to antiferromagnetic phase. This phase transition is strongly H dependent and shifted to lower temperature (similar to 150 K) on the application of 8 T field with phase coexistence (high temperature phase similar to 18%), even down to 4.2 K. Isothermal XRD results at 150 K under different H clearly illustrate the H induced first order structural phase transition. The critical H at which this phase transformation starts is similar to 1 T, with rapid growth above 4 T with hysteretic nature during increasing and decreasing H. These results are supported with the resistivity and magnetoresistance results and affirm the strong spin-lattice coupling in NSMO. Our detail studies reveal the structural correlations to the observed colossal magnetoresistance and magnetocaloric effect in this material. (C) 2017 Elsevier B.V. All rights reserved.

We report a cost effective high power (500 W) ferroelectric loop study set up. It has a new type of simple compensation circuit for continuous compensation of stray linear capacitance and large value resistive losses of the sample. It can be operated at continuous variable frequency modes to observe the loop structure at different frequencies. The proposed system has a drive voltage of 8400 V peak to peak and can be modified to higher voltage and higher frequencies. Instead of a loop plotter, it has a data saving facility for binary or ASCII files such that further processing of the data by computer software such as origin, excel etc can be achieved. This high power set up is suitable for a highly lossy sample. Moreover, uncompensated and compensated loop studies and time dependent loop growth observation for ceramic BaTiO3 are discussed here in.

Recently, Kumar et al. [APL 104, 262905 (2014)] have reported quantum paraelectric glass state in SrCu3Ti4O12. Their analysis is based on the misconception about the effect of quantum fluctuations on paraelectric properties. There are very serious issues with their analysis of permittivity vs T data where authors tried to prove the presence of quantum paraelectricity in SrCu3Ti4O12. In the comment we have clearly affirm that the analysis presented by the authors do not prove the presence of quantum paraelectric behavior and there by the quantum paraelectric glass state. The quantum fluctuations and the frequency dependence of permittivity, showing relaxor like behavior cannot exist simultaneously., Comment: Accepted in Applied Physics Letters, 5 pages, 1 figures

Solid ZnSe quantum dots (QDs) have been prepared via chemical route. The QDs have been characterized by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Photoluminescence (PL) and Ultraviolet-Visible Spectroscopy (Uv-Vis). The QD sizes were found to vary from 2.5 to 9.5 nm. The XRD measurement reveals an increase in the interplanar spacing in QDs as compared to their bulk counterpart. This observation is further supported by Rietveld analysis which establishes the formation of single phase zinc blende ZnSe QDs and confirms 3.9% lattice expansion. Calculations based upon the thermodynamical theory yield 8.7% concentration of vacancies due to the lattice expansion. We observe various peaks in the PL spectra which may arise either due to the QD size variations or the defects due to the vacancies.

The compound ${\mathrm{Fe}}_{2}\mathrm{Ti}{\mathrm{O}}_{5}$ (FTO) is a well-known uniaxial anisotropic spin-glass insulator with two successive glassy freezing temperatures, i.e., transverse $({T}_{TF}=9\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ and longitudinal $({T}_{LF}=55\phantom{\rule{0.16em}{0ex}}\mathrm{K})$. In this article, we present the results of measurements of complex dielectric behavior, electric polarization as a function of temperature $(T)$, in addition to characterization by magnetic susceptibility and heat capacity, primarily to explore magnetoelectric (ME) coupling and multiglass properties in uniaxial anisotropic spin-cluster-glass FTO. The existence of two magnetic transitions is reflected in the isothermal magnetodielectric (MD) behavior in the sense that the sign of MD is different in the $T$ regime $Tl{T}_{TF}$ and $Tg{T}_{TF}$. The data in addition provide evidence for the glassy dynamics of electric dipoles; interestingly, this occurs at much higher temperature (\ensuremath{\sim}100--150 K) than ${T}_{LF}$, with high remnant polarization at 10 K $(\ensuremath{\sim}4000\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2})$ attributable to short-range magnetic correlations, thereby offering a route to attain ME coupling above 77 K.

Low-temperature XRD and dielectric studies of Sr1−xCaxTiO3 (SCT) (x=0.27 and 0.30) shows paraelectric (PE) P21212 to antiferroelectric (AFE) Pbcm phase-transitions. Analysis of the XRD data reveals that the phase transition appears due to anti-parallel alignment of the ionic dipoles formed by non-centrosymetric shift of the Ti+4 and O−2 ions in the lattice. Analogous to antiferromagnetic (AFM) interaction, AFE interaction is also expected to get suppressed under applied DC electric field. In-field dielectric studies reveal low-temperature shift of the PE-AFE transition temperature. A sufficiently high-field is expected to cause metastable ferroelectricity in it.

We report the occurrence of kinetic arrest of the first-order phase transition from R3c to Pbnm in supercooled La(x)MnO(3±δ) (x = 1 and 0.9, i.e. δ0.125). Structural studies have been done, employing low temperature transmission electron microscopy (LT-TEM) and low temperature x-ray diffraction (LT-XRD) techniques. No phase transformation was observed even in La(x)MnO(3±δ) aged for ~12 h at 98 K. The evidence of the occurrence of kinetic arrest was realized at low temperatures through in situ electron beam triggered nucleation and perpetual devitrification of the R3c phase into a Pbnm phase. It was clearly evidenced that the R3c structure of La(x)MnO(3±δ), below its ferromagnetic transition temperature, is metastable and prone to be transformed to a Pbnm orthorhombic structure following initiation by an electron beam trigger. The electron beam transformed Pbnm phase was found to transform back to the R3c phase through a first-order phase transition occurring close to the ferromagnetic to paramagnetic transition (T(c)) during heating. The glass-like kinetics of the arrested R3c phase has been investigated through resistance relaxation measurements, showing a decreasing logarithmic rate of decay of the arrested R3c phase towards the stable Pbnm phase with decreasing temperature, down to 5 K. On the basis of the correlations observed in the resistance-versus-temperature, magnetization-versus-temperature, magnetization-versus-field, resistance relaxation and LT-XRD measurements, the occurrence of kinetic arrest has been attributed to the suppression of Jahn-Teller distortion by double exchange across the insulator-metal transition.

Low-temperature (LT) XRD and magnetoresistance (MR) have been studied in the single phase polycrystalline Pr0.5Sr0.5MnO3 with space-group I4/mcm. The resistance-vs-temperature (R-T) variation at zero-field show a broad paramagnetic to ferromagnetic metallic (FMM) transition at Tc ∼ 240K and it further undergoes FMM to antiferromagnetic insulating (AFMI) transition at TN ∼ 90K with a broad hysteresis indicates a disorder broadened first order phase transition (FOPT). The zero-field cooled (ZFC) MR at 5K shows an open loop with virgin curve lying outside the envelope curve. This anomalous behavior vanishes with increasing temperature. This has been attributed to field-induced transformation of AFMI to FMM phase, which remains arrested down to zero magnetic field at 5K.

Using low-temperature (LT) XRD and resistively vs temperature data we show that the evolution of the ferromagnetic state LaMnO3±δ is accompanied by the structural distortion, which is evident from the change is the FWHM of (220) peak of R-3c phase at TC. The crystal structure gets distorted at TC and regains its original form when the system completely transforms to a homogenous ferromagnetic phase (FM). Our results thus give a direct evidence of strong magneto-elastic coupling in these systems.

Low- as well as high-temperature electron and x-ray diffraction studies have been carried out on a rare-earth free B-site disordered electron-doped manganite SrMn0.875.Mo0.125O3−δ in the temperature range of 83–637 K. These studies reveal the occurrence of strong charge ordering (CO) at room temperature in a pseudo-tetragonally distorted perovskite phase with space group Pmmm. A non-integral modulation vector of 8.95 times along [−110] indicates a charge density wave-type modulation. The CO phase with basic perovskite structure Pmmm transforms to a charge disorder cubic phase through a first-order phase transition at 355 K. Supporting temperature-dependent measurements of resistance and magnetization show metal–insulator and antiferromagnetic transitions across 355 K with a wide hysteresis ranging from 150 K to 365 K. The occurrence of pseudo-tetragonality of the basic perovskite lattice with c/a < 1 together with charge-ordered regions with 2-dimensional modulation have been analyzed as the coexistence of two CO phases with -type and orbital ordering.