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ErFe0.55Mn0.45O3 single crystal shows canted antiferromagnetic ordering below 365 K with weak ferromagnetic moment along c axis. On cooling, the magnetic moment is completely suppressed at a certain temperature due to Er and Fe/Mn sublattice compensation, below which negative magnetization is observed. The compensation and negative magnetization temperatures are much higher than those in other orthoferrite systems. There is a spin reorientation transition from Γ 4 ( G x , A y , F z ) to Γ 1 ( A x , G y , C z ) structure at 255 K along c axis which is not seen along a and b axes. The spin reorientation is tunable with applied magnetic field and is completely suppressed at sufficiently high magnetic field. The behavior is explained by spin configuration and net magnetization orientation of individual Er and Fe/Mn sublattices. The proposed model is further validated by measurements using ’training’ protocol.

Magnetic measurements on oriented single crystals of Ho 0.5 Dy 0.5 FeO 3 grown using optical floating zone furnace provide evidence for two spin reorientations of Fe 3 + magnetic sublattice viz., Γ 4 → Γ 1 → Γ 2 at temperatures T SR 1 = 49 K and T SR 2 = 26 K . As magnetic field along c axis increases, the sample resumes Γ 4 spin configuration and a large field is likely to disrupt the spin reorientation process. It is evident from isothermal magnetization along c axis that field induced Γ 1 → Γ 4 transition is feasible within the intermediate temperature range of T SR 1 - T SR 2 and can be triggered at a critical field. Such magnetic behavior is better understood as a consequence of anisotropic effective field, modified by the application of external magnetic field.

Despite numerous attempts of electron doping in different manganites (RMnO3, R = rare earth), successful reports are scarce in the literature till date. In this paper, we have synthesized a series of phase-pure electron doped multiferroic compound Sc1-xZrxMnO3 (x = 0, 0.05, 0.1, and 0.2) and evaluated the effect of doping on structural properties, oxidation states of cations, DC magnetization, heat capacity, resistivity, dielectric behaviour and ferroelectricity in the material. The presence of Zr4+ and mixed valence state of Mn comprising of Mn2+ and Mn3+ ions are confirmed using X-ray photoelectron spectroscopy. All these samples exhibit antiferromagnetic ordering; as Zr4+ content increases, antiferromagnetic ordering gradually diminishes while shifting to low temperatures. Additionally, ferromagnetic-like interaction develops in doped systems which gives rise to hysteresis in isothermal magnetization loops with greatly enhanced magnetization in comparison to pure antiferromagnetic nature of x = 0 i.e. ScMnO3. Interestingly, even with zero magnetic moment of Sc3+, Schottky-like anomaly is observed at 5 K in heat capacity data of samples with x = 0.1 and 0.2, a result that we attribute to the highly resistive nature of doped samples. Moreover, while measuring ferroelectric hysteresis loops, we observe a significant reduction of leakage current in doped sample (x = 0.2) compared to pure ScMnO3. Additionally, the compound x = 0.2 shows improved dielectric and ferroelectric behaviour. It is proposed that doping of Zr4+ compensates for the cation deficiency and consequently eliminates the inherent oxygen vacancies by charge compensation. (C) 2017 Elsevier B.V. All rights reserved.

Structural phase transition in half doped compound (CH3)(2)NH2]Mn0.5Ni0.5(HCOO)(3) is investigated. Differential scanning calorimetry data indicated this to be a distinct transition with enthalpy 465.6 J/mol. Powder X-ray diffraction at different temperature shows structural phase transition from trigonal R -3c to monoclinic Cc through co -existing mixed phase. Based on the experimental results, a phase diagram has been proposed. This phase transition is confirmed to be of first order nature by specific heat measurement. Entropy change and latent heat of the phase transition are also calculated from specific heat which are found to be 2.53 J/mol-K and 412 J/mol respectively.

(CH3)(2)NH2]Mn0.5Ni0.5(HCOO)(3) is prepared via solvothermal method. Phase purity and crystal structures are determined at room temperature using powder X-ray diffraction. Temperature variation of magnetization and isothermal field variation of magnetization indicate antiferromagnetic ordering with weak ferromagnetism. This is possibly induced by spin canting. Thermally activated relaxation is observed which is attributed to ordering of dimethylamine cation. This is associated with first order improper antiferroelectric transition. Temperature dependence of polarization and its spontaneity has been confirmed from pyroelectric current measurements. Effect of electric ordering is clearly reflected in impedance spectroscopy data. Two regions are distinct above and below electric ordering temperature which are demarcated by the magnitude of resistance and capacitance of equivalent circuit model and activation energy. Co-existence of magnetic and electric ordering at low temperature is a signature of multiferroic phase.

Implementing topological insulators as elementary units in quantum technologies requires a comprehensive understanding of the dephasing mechanisms governing the surface carriers in these materials, which impose a practical limit to the applicability of these materials in such technologies requiring phase coherent transport. To investigate this, we have performed magnetoresistance (MR) and conductance fluctuations (CF) measurements in both exfoliated and molecular beam epitaxy grown samples. The phase breaking length (${l}_{\ensuremath{\phi}}$) obtained from MR shows a saturation below sample dependent characteristic temperatures, consistent with that obtained from CF measurements. We have systematically eliminated several factors that may lead to such behavior of ${l}_{\ensuremath{\phi}}$ in the context of TIs, such as finite size effect, thermalization, spin-orbit coupling length, spin-flip scattering, and surface-bulk coupling. Our work indicates the need to identify an alternative source of dephasing that dominates at low $T$ in topological insulators, causing saturation in the phase breaking length and time.

It is well known that in double perovskites, the degree of cation ordering plays a very important role in determining the intrinsic magnetic properties. Therefore, in order to distinguish between the intrinsic properties and the disordered cation driven magnetic properties, we have grown ordered and disordered epitaxial thin films of Lu2MnNiO6 (LMNO) using two different cubic substrates (0 0 1)-LaAlO3 and (0 0 1)-SrTiO3 (LAO and STO respectively) by pulsed laser deposition (PLD) method. It is found that both LMNO films grow in monoclinic crystal symmetry and possess two out-of-plane domains (1 1 0) and (0 0 1). Moreover, various in-plane rotations of these domains give rise to a total of six in-plane twin variants. The films show dominant ferromagnetic nature. Due to good lattice matching with LAO, LMNO/LAO films are well-ordered and exhibit a sharp ferromagnetic to paramagnetic transition at TC = 38 K, whereas, the large lattice mismatch with STO resulted in disordered LMNO/STO films and TC is lowered to 34 K. The value of maximum magnetization (MS, at 70 kOe) in disordered LMNO/STO (MS = 2.4 μB/f.u.) is almost half of that in the ordered LMNO/LAO (MS = 4.0 μB/f.u.). Interestingly, the well-ordered LMNO/LAO films show memory effect and relaxation of magnetization in the entire temperature range below TC. This is in conformance to the intrinsic glassy magnetic nature of LMNO.

We have carried out dielectric and transport measurements in NdFe1-xMnxO3 (0 ≤ x ≤ 1) series of compounds and studied the variation of activation energy due to a change in Mn concentration. Despite similar ionic radii in Mn(3+) and Fe(3+), large variation is observed in the lattice parameters and a crossover from dynamic to static Jahn-Teller distortion is discernible. The Fe/Mn-O-Fe/Mn bond angle on the ab plane shows an anomalous change with doping. With an increase in the Mn content, the bond angle decreases until x = 0.6; beyond this, it starts rising until x = 0.8 and again falls after that. A similar trend is observed in activation energies estimated from both transport and dielectric relaxation by assuming a small polaron hopping (SPH) model. Impedance spectroscopy measurements delineate grain and grain boundary contributions separately both of which follow the SPH model. Frequency variation of the dielectric constant is in agreement with the modified Debye law from which relaxation dispersion is estimated.

A key feature of topological insulators (TI) is symplectic symmetry of the Hamiltonian which changes to unitary when time reversal symmetry is lifted and the topological phase transition occurs. However, such a crossover has never been explicitly observed, by directly probing the symmetry class of the Hamiltonian. In this report, we have probed the symmetry class of topological insulators by measuring the mesoscopic conductance fluctuations in the TI Bi$_{1.6}$Sb$_{0:4}$Te$_2$Se, which shows an exact factor of two reduction on application of a magnetic field due to crossover from symplectic to unitary symmetry classes. The reduction provides an unambiguous proof that the fluctuations arise from the universal conductance fluctuations (UCF), due to quantum interference and persists from T = 22 mK to 4.2 K. We have also compared the phase breaking length (l$_\phi$) extracted from both magneto-conductivity and UCF which agree well within a factor of two in the entire temperature and gate voltage range. Our experiment confirms UCF as the major source of fluctuations in mesoscopic disordered topological insulators, and the intrinsic preservation of time reversal symmetry in these systems., Comment: 6 pages, 4 figures

Magnetization reversal and glassy phase in Tb0.7Sr0.3MnO3 are reported. Polycrystalline samples of Tb0.7Sr0.3MnO3 are prepared by solid-state synthesis. This material crystallizes in orthorhombic Pbnm space group with lattice parameters: a = 5.471(1)A, b = 7.608(2)A, and c = 5.398(2)A. Field-cooled/zero field-cooled magnetization measurements show two magnetic transitions, one at 40 K and another at 8 K. The transition at 40 K is due to magnetic ordering of Mn while that at 8 K arises due to Tb ordering. Low field-cooled measurements reveal magnetization reversal at low temperature. The temperature at which this occurs increases with decrease in field strength. The observed magnetization reversal is attributed to the competition between interacting Mn (3 d) and Tb (4 f) magnetic sublattices which makes the system ferrimagnetic. The glassy magnetic behavior below 50 K and absence of long-range order in Tb0.7Sr0.3MnO3 is confirmed through ac susceptibility studies.

Double perovskite R2NiMnO6 (R=Pr, Nd, Tb, Ho and Y) composites are prepared via solid state synthesis or nitrate route. Isothermal magnetic entropy change ( Δ S M ( T , H ) ) and relative cooling power (RCP) for all systems are calculated and compared. All of them possess relatively high values of Δ S M ( T , H ) and RCP as compared to many perovskite and double perovskite systems reported previously. Ho2NiMnO6 has the highest value for | Δ S M ( T , H ) | and RCP as 8.4 JKg − 1 K − 1 at 70 kOe and 59.18 J Kg−1 at 20 kOe respectively. This study highlights the potential of magnetocaloric refrigerant materials at low temperature.

Magnetic and dielectric properties of the double perovskite Ho2NiMnO6 are reported. The compound is synthesized by nitrate route and is found to crystallize in monoclinic P2(1)/n space group. Lattice parameters obtained by refining powder x-ray diffraction data are; a = 5.218(2)angstrom, b = 5.543(2)angstrom, c = 7.480(3)angstrom and the monoclinic angle i beta = 90.18 degrees(4). A phase transition is observed at T-C = 86 K in the temperature-dependent magnetization curve, M(T). The inverse magnetic susceptibility, (1/chi(T)) fits reasonably well with modified Curie-Weiss law by incorporating the paramagnetic response of Ho3+. 1/chi(T) manifests as an upward deviation from ideal Curie-Weiss behaviour well above the ferromagnetic transition. Signs of inherent Griffiths phase pertaining to the Ni/Mn subsystem are visible when one subtracts the Ho3+ paramagnetic contribution from total susceptibility and does the power-law analysis. The magnetic hysteresis at 2 K gives the maximum value of magnetization M-max approximate to 15 mu(B)/f. u. at 50 kOe. Field-derivative of magnetization at 2 K shows discontinuities which indicates the existence of metamagnetic transitions in this compound. This needs to be probed further. Out of the two dielectric relaxations observed, the one at low temperature may be attributed to phononic frequencies and that at higher temperature may be due to Maxwell-Wagner relaxation. A correlation between magnetic and lattice degrees of freedom is plausible since the anomaly in dielectric constant coincides with T-C.

We report the detailed experimental characteristics of LuMn0.5Fe0.5O3 synthesized by the wet chemical method and proclaim it as a new member of the multiferroic family. The compound stabilizes in P63cm crystal symmetry. It exhibits a spin re-orientation transition at TSR and an antiferromagnetic transition at TN. In addition, our magnetization vs. temperature data reveals an extra broad maximum close to room temperature; unseen in earlier studies. By invoking the compatible nature of the magnetic exchange path in P63cm symmetry, we have argued that the origin lies in the intraplane short-range spin ordering. Heat capacity is measured and analysed to elucidate the magnetic entropy. Though long-range antiferromagnetic ordering vanishes at TN ∼ 103 K, we find the experimental magnetic entropy calculated till 200 K is less by a significant amount from the value of theoretical spin randomization magnetic entropy; further supporting the existence of spin ordering beyond TN and even above 200 K. While the specif...

Single crystals of Tb0.5Sr0.5MnO3 were grown in an optical float zone furnace and their magnetic and thermodynamic properties were studied. Temperature dependent DC magnetization measurements at different fields show strong irreversibility below the magnetic anomaly at 44 K. The upward deviation from ideal CW behavior well above the transition temperature and its field independent nature are signatures of non-Griffiths phase. The origin non-Griffiths phase owe to competition between the antiferromagnetic and ferromagnetic Mn3+ Mn4+ interactions mediated through intervening oxygen. Further, 44 K transition is confirmed as a magnetic glassy transition. The estimated dynamical spin flip time (tau(o)=2.11(3) x10(-14) s) and zv(9.3(2)) values fall into the range of typical spin-glass systems. Detailed memory and temperature cycling relaxation measurements were performed and support the Hierarchical relmtation model. Low-temperature specific heat data displays a linear term, identifying the glassy magnetic phase contribution.

Multiferroic materials are characterized by simultaneous magnetic and ferroelectric ordering making them good candidates for magneto-electrical applications. We conducted thermal expansion and magnetostriction measurements in magnetic fields up to 14 T on perovskitic GdMnO3 by highresolution capacitive dilatometry in an effort to determine all longitudinal and transversal components of the magnetostriction tensor. Below the ordering temperature T (N) = 42 K, i.e., within the different complex (incommensurate or complex) antiferromagnetic phases, lattice distortions of up to 100 ppm have been found. Although no change of the lattice symmetry occurs, the measurements reveal strong magneto-structural phenomena, especially in the incommensurate sinusoidal antiferromagnetic phase. A strong anisotropy of the magnetoelastic properties was found, in good agreement with the type and propagation vector of the magnetic structure. We demonstrate that our capacitive dilatometry can detect lattice expansion effects and changes of the dielectric permittivity simultaneously because the sample is housed inside the capacitor. A separation of both effects is possible by shielding the sample. Dielectric transitions could be detected by this method and compared to the critical values of H and T in the magnetic phase diagram. Dielectric changes measured at 1 kHz excitation frequency are detected in GdMnO3 at about 180 K, and between 10 K and 25 K in the canted antiferromagnetic structure which is characterized by a complex magnetic order on both the Gd- and Mn-sites.

We report on the single crystal growth of 50% Sr and Y doped multiferroic DyMnO3 using optical floating zone technique. A comparison of the effect of growth ambience and of chemical substitution on the crystal structure of DyMnO3 is attempted. It is observed that DyMnO3 adopts Pm3m cubic structure with 50% Sr doping whereas with 50% Y doping, the crystal structure is hexagonal P6(3)cm. Orthorhombic Pnma structure is adopted by DyMnO3 when grown in air, whereas hexagonal P6(3)cm structure is obtained when grown under the ambience of argon. The structural polymorphism is discussed in terms of difference in ionic sizes of Sr, Y and Dy, comparable Gibbs free energies and coordination schemes of surrounding oxygens for hexagonal and orthorhombic structures of DyMnO3. (C) 2012 Elsevier B.V. All rights reserved.

Lithium L-Ascorbate dihydrate (LLA) is a new metal organic nonlinear optical crystal belonging to the saccharide family. Single crystals of LLA were grown from aqueous solution. Solubility of the crystal has a positive temperature coefficient facilitating growth by slow cooling. Rietveld refinement was used to confirm the phase formation. The crystal has prismatic habit with (010), (001) and (10-1) prominent faces. Thermal analysis shows that the crystal is stable up to 102 degrees C. Transmission spectrum of the crystal extends from 302 nm to 1600 nm. Dielectric spectroscopic analysis revealed Cole Cole behaviour and prominent piezoelectric resonance peaks were observed in the range of 100-200 kHz. Second harmonic generation (SHG) conversion efficiency of up to 2.56 times that of a phase matched KDP crystal was achieved when the (010) plate of LLA single crystal was rotated about the +ve c axis, by 9.4 degrees in the clockwise direction. We also observed SHG conical sections which were attributed to noncollinear phase matching. The observation of the third conical section suggests very high birefringence and large nonlinear coefficients. A detailed study of surface laser damage showed that the crystal has high multiple damage thresholds of 9.7 GW cm(-2) and 42 GW cm(-2) at 1064 nm and 532 nm radiation respectively. (C) 2012 Elsevier B.V. All rights reserved.

Single crystals of lithium D-isoascorbate monohydrate (LDAM), (C6H7O6Li center dot H2O), are grown by a solution growth method. The crystal structure of LDAM is solved using single crystal X-ray diffraction. The space group is orthorhombic P2(1)2(1)2(1) with four formula units per unit cell and lattice parameters a = 7.7836(3) angstrom, b = 8.7456(3) angstrom, and c = 11.0368(4) angstrom. Solubility of the material in water is determined thermogravimetrically and found to have a positive temperature coefficient of solubility. Large optical quality single crystals are subsequently grown from aqueous solution by a slow cooling method. The crystal has a bulky prismatic habit and among the prominent faces the c face appears as the only principal morphological face. The crystal exhibits a (010) cleavage. Dielectric spectroscopy reveals a nearly Debye type Cole-Cole behavior with anisotropy in relaxation. Optical transmission range is found to be from 300 to 1400 nm. The principal refractive indices of this biaxial crystal, measured using Brewster's angle method, at wavelengths 405, 543, and 632.8 nm, show high dispersion. The crystal is negative biaxial with 2V(z) = 107.8 degrees (405 nm) and belongs to the Hobden class 3. Theoretically generated type 1 and type 2 second order phase matching curves match very well with the experimental results. The second-order nonlinear coefficient d(14) was determined to be 7 x 10(-13) m/V. For the optimum phase matching direction (type 2), the second-order effective nonlinear coefficient and the walk off angle are determined to be 0.84 times d(14) and 3.5 degrees respectively. The crystal possesses high multiple surface damage thresholds of 18 GW/cm(2) and 8 GW/cm(2) at laser wavelengths 1064 and 532 nm, respectively.

Critical behavior of double perovskite Y2NiMnO6 near the second-order ferromagnetic transition is studied. Scaling exponents calculated frommodified Arrot plots are confirmed by Kouvel-Fisher method and satisfy the Widom's scaling relation. The exponents do not follow any conventional theoretical models.beta values areconsistent with 3D-Ising model while delta conformsto TCMF and gamma valueclosely relates to the 3D-Heisenberg model. Critical exponents are compared with similar R2NiMnO6 double perovskites which shows that a decrease in size of R ion changes exponents from mean-field to the 3D-Ising model.

Here, we report the dielectric, impedance and transport studies of non-charge-ordered magnetic glass, Tb0.5Sr0.5MnO3 single crystals. The temperature-and frequency-dependent real (epsilon') and imaginary (epsilon'' or tan delta) parts of the dielectric constant display large frequency dispersion. The colossal dielectric constant observed (approximate to 3000) above 100K is considered extrinsic. The activation energy of thermally activated relaxation is calculated using the Arrhenius law. Interestingly, two relaxation regions, each with different activation energies (E-a) are clearly evident, one occurring above and the other below the glassy magnetic transition temperature (T-g = 44K). E-a relates to the electron hopping between Mn3+ and Mn4+ ions and the origin of dielectric dispersion. Ea in the glassy region is lower than that in paramagnetic region due to a lower energy spent in hopping between frozen spins. Bulk capacitance and resistivity derived from impedance measurements reveal anomalies around T-g. Electrical transport data between 60 and 300K shows insulating behavior and the calculated E-a is in good agreement with the value obtained from dielectric measurements. Although, these results cannot be interpreted in terms of magneto-electric coupling, the correlation observed between magnetic and electronic states of the system is significant. Copyright (C) EPLA, 2016

Highly non-linear I–V characteristics and apparent colossal electro-resistance were observed in non-charge ordered manganite Tb 0.5 Sr 0.5 MnO 3 single crystal in low temperature transport measurements. Significant changes were noticed in top surface temperature of the sample as compared to its base while passing current at low temperature. By analyzing these variations, we realize that the change in surface temperature ( Δ T sur ) is too small to have caused by the strong negative differential resistance. A more accurate estimation of change in the sample temperature was made by back-calculating the sample temperature from the temperature variation of resistance (R–T) data ( Δ T cal ) , which was found to be higher than Δ T sur . This result indicates that there are large thermal gradients across the sample. The experimentally derived Δ T cal is validated with the help of a simple theoretical model and estimation of Joule heating. Pulse measurements realize substantial reduction in Joule heating. With decrease in sample thickness, Joule heating effect is found to be reduced. Our studies reveal that Joule heating plays a major role in the nonlinear electrical response of Tb0.5Sr0.5MnO3. By careful management of the duty cycle and pulse current I–V measurements, Joule heating can be mitigated to a large extent.

A new double perovskite Ho2FeMnO6 was grown by nitrate route. Temperature dependent dielectric response was recorded at different frequencies. Relaxor-like-behavior is observed whose activation energy was calculated using Arrhenius equation. The dispersion is very small at room temperature. The Nyquist plot over a broad frequency range at room temperature provides evidence for the presence of three relaxations from sample electrode interface, grain boundary and grain. The corresponding values of resistance and capacitance were calculated from the equivalent circuit model analysis of the Nyquist plot.

The double perovskite Y2NiMnO6 displays ferromagnetic transition at T-c approximate to 81 K. The ferromagnetic order at low temperature is confirmed by the saturation value of magnetization (Ms) and also validated by the refined ordered magnetic moment values extracted from neutron powder diffraction data at 10 K. This way, the dominant Mn4+ and Ni2+ cationic ordering is confirmed. The cation-ordered P2(1)/n nuclear structure is revealed by neutron powder diffraction studies at 300 and 10 K. Analysis of the frequency-dependent dielectric constant and equivalent circuit analysis of impedance data take into account the bulk contribution to the total dielectric constant. This reveals an anomaly which coincides with the ferromagnetic transition temperature (T-c). Pyrocurrent measurements register a current flow with onset near T-c and a peak at 57 K that shifts with temperature ramp rate. The extrinsic nature of the observed pyrocurrent is established by employing a special protocol measurement. It is realized that the origin is due to reorientation of electric dipoles created by the free charge carriers and not by spontaneous electric polarization at variance with recently reported magnetism-driven ferroelectricity in this material.

We present an extensive study on magnetic and transport properties of La(0.85)Sr(0.15)CoO(3) single crystals grown by a float zone method to address the issue of phase separation versus spin-glass (SG) behavior. The dc magnetization study reveals a kink in field-cooled magnetization, and the peak in the zero-field-cooling curve shifts to lower temperature at modest dc fields, indicating the SG magnetic phase. The ac susceptibility study exhibits a considerable frequency-dependent peak shift (similar to 4 K) and a time-dependent memory effect below the freezing temperature. In addition, the characteristic time scale tau(0) estimated from the frequency-dependent ac susceptibility measurement is found to be similar to 10(-13) s, which matches well with typical values observed in canonical SG systems. The transport relaxation study evidently demonstrates the time-dependent glassy phenomena. In essence, all our experimental results corroborate the existence of SG behavior in La(0.85)Sr(0.15)CoO(3) single crystals.

The electronic structure of double perovskite Pr2MnNiO6 is studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 2+ and 4+ states respectively. Using charge transfer multiplet analysis of Ni and Mn 2p XPS spectra, we find charge transfer energies {\Delta} of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni2+ and Mn4+ reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the atomic values of 8.00 and 3.00 respectively thereby indicating the covalent nature of the system. The O 1s edge absorption spectra reveal a band gap of 0.9 eV which is comparable to the value obtained from first principle calculations for U-J >= 2 eV. The density of states clearly reveal a strong p-d type charge transfer character of the system, with band gap proportional to average charge transfer energy of Ni2+ and Mn4+ ions., Comment: 18 pages, 9 figures

The magnetic behavior of La0.85Sr0.15CoO3 single crystals and polycrystals has been subjected to a controversial debate for the last several years; while some groups show evidence for phase separation, others show spin-glass (SG) behavior. Here, we present a comprehensive investigation of the structural, ac susceptibility, and dc magnetization properties of La0.85Sr0.15CoO3 single crystals grown by float zone method. The structural analysis of XRD data by Rietveld refinement reveals the single crystallographic phase. The ac susceptibility results exhibit a frequency dependent peak shift (∼2 K) and time-dependent memory effect below the freezing temperature. The characteristic time scale τo calculated from this peak shift is found to be of ∼10−13 s which matches very well with typical values observed for a SG system. Further, the peak shift of the zero-field-cooling curves to lower temperature at higher dc fields is well described by the well-known de Almeida Thouless line, a characteristic of SG behavior. Thus, all our experimental findings confirm the existence of SG behavior in La0.85Sr0.15CoO3 single crystals.

The emergence of exchange bias effect in Fe3O4 thin films has been since attributed to the presence of anti phase boundary (APB) growth defects despite lack of direct experimental evidence. In the present report, APB induced anomalous weak ferromagnetism and exchange bias property of single-phase antiferromagnetic (AFM) system LuMn0.5Fe0.5O3 (LMFO) is discussed and Fe-57 Mossbauer spectroscopy and high resolution transmission electron microscopy (HRTEM) measurements are used to probe the origin of the observed effect. In addition to the sextet component corresponding to the long range AFM ordering, the measured Mossbauer spectra reveal the presence of a small component (10%-12%) near zero velocity with unusually small internal field. This indicates the presence of APB defects. From micro structural investigations using HRTEM, presence of APB type defects and dislocations are confirmed. In addition to the exchange bias effect, upon field cooling, hysteresis loop exhibits large vertical shift due to strong pinning effect of the APB. Finally we further annealed the optimally sintered sample LMFO and studied the evolution of defects, and their influence on weak ferromagnetism and exchange bias properties. Our present experimental findings may pave the way in creating new functionalities in materials using APB-type growth defects.

Cesium hydrogen l-malate monohydrate, CsH(C4H4O5)·H2O, is a new chiral open-framework semi-organic crystalline material with a second-harmonic generation efficiency one order of magnitude greater than KDP. Single crystals of this new material have been grown by the conventional slow cooling technique from aqueous solution. Grown crystals display both platy and prismatic morphologies depending on the imposed supersaturation. Hardness values measured using Vickers hardness indenter show considerable anisotropy. The resistivity behavior at room temperature and above, places the crystal between an ionic conductor and a dielectric. The single-crystal SHG efficiency estimated through Maker fringes experiment gives deff which is 4.24 times that of KDP. Single and multiple shot experiments performed on the grown crystals for the fundamental and second harmonic of pulsed Nd:YAG laser (1064 and 532 nm) show that it exhibits a high laser damage threshold which is a favorable property for nonlinear optical applications.

We report on the magnetic and electrical properties of Pr0.6Sr0.4MnO3 single crystals. This compound undergoes a continuous paramagnetic-ferromagnetic transition with a Curie temperature T C∼301 K and a first-order structural transition at T S∼64 K. At T S, the magnetic susceptibility exhibits an abrupt jump, and a corresponding small hump is seen in the resistivity. The critical behavior of the static magnetization and the temperature dependence of the resistivity are consistent with the behavior expected for a nearly isotropic ferromagnet with short-range exchange belonging to the Heisenberg universality class. The magnetization (M–H) curves below T S are anomalous in that the virgin curve lies outside the subsequent M–H loops. The hysteretic structural transition at T S as well as the irreversible magnetization processes below T S can be explained by phase separation between a high-temperature orthorhombic and a low-temperature monoclinic ferromagnetic phase.

Electron paramagnetic resonance (EPR) studies are carried out on single crystals of Pr 1 - x Pb x MnO 3 , for x = 0.23 and 0.30. The crystal with x = 0.23 undergoes a transition from paramagnetic insulator to ferromagnetic insulator with Curie temperature T C ∼ 167 K . The crystal with x = 0.30 undergoes a transition from paramagnetic insulator to ferromagnetic metal with T C ∼ 200 K . The EPR analysis is carried out in the paramagnetic region for both the crystals. The EPR signals are fitted to the Dysonian equation. Linewidth, intensity and asymmetry parameters are plotted as a function of temperature down to T C . The intensity shows a simple Curie–Weiss behaviour. The spin relaxation takes place through a “bottleneck” mechanism according to which linewidth depends only on strong exchange interaction between Mn 3 + and Mn 4 + ions. Around 240 K a secondary low field EPR signal is observed for both compositions which persists till around 200 K indicating a possible phase separation.

Slow intrinsic fluctuations of resistance, also known as the flicker noise or 1/f-noise, in the surface transport of strong topological insulators (TIs) is a poorly understood phenomenon. Here, we have systematically explored the 1/f-noise in field-effect transistors (FET) of mechanically exfoliated Bi1.6Sb0.4Te2Se TI films when transport occurs predominantly via the surface states. We find that the slow kinetics of the charge disorder within the bulk of the TI induces mobility fluctuations at the surface, providing a new source of intrinsic 1/f-noise that is unique to bulk TI systems. At small channel thickness, the noise magnitude can be extremely small, corresponding to the phenomenological Hooge parameter gamma(H) as low as approximate to 10(-4), but it increases rapidly when channel thickness exceeds similar to 1 mu m. From the temperature (T)-dependence of noise, which displayed sharp peaks at characteristic values of T, we identified generation-recombination processes from interband transitions within the TI bulk as the dominant source of the mobility fluctuations in surface transport. Our experiment not only establishes an intrinsic microscopic origin of noise in TI surface channels, but also reveals a unique spectroscopic information on the impurity bands that can be useful in bulk TI systems in general.

Unusual behavior of reentrant spin-glass (RSG) compound Lu2MnNiO6 has been investigated by magnetometry and neutron diffraction. The system possesses a ferromagnetic (FM) ordering below 40 K and undergoes a RSG transition at 20 K. Additionally, Lu2MnNiO6 retains memory effect above the glassy transition till spins sustain ordering. A novel critical behavior with unusual critical exponents (beta =similar to 0.241 and gamma similar to 1.142) is observed that indicates a canting in the spin structure below the ferromagnetic transition (T-C). A comprehensive analysis of temperature-dependent neutron diffraction data and first-principles calculations divulge that a structural distortion induced by an octahedral tilting results in a canted spin structure below T-C.

We investigated the evolution of the temperature-composition phase diagram of Fe$_{1+y}$Te upon Se substitution. In particular, the effect of Se substitution on the two-step, coupled magneto-structural transition in Fe$_{1+y}$Te single crystals is investigated. To this end, the nominal Fe excess was kept at $y$ = 0.12. For low Se concentrations, the two magneto-structural transitions displayed a tendency to merge. In spite of the high Fe-content, superconductivity emerges for Se concentrations, $x \geq$ 0.1. We present a temperature-composition phase diagram to demonstrate the interplay of structure, magnetism, and superconductivity in these ternary Fe chalcogenides., 7 figures

Ferrimagnetism and metamagnetic features tunable by composition are observed in the magnetic response of Nd1-xYxMnO3, for x=0.1-0.5. For all values of x in the series, the compound crystallizes in orthorhombic Pbnm space group similar to NdMnO3. Magnetization studies reveal a phase transition of the Mn-sublattice below T-N(Mn) approximate to 80 K for all compositions, which, decreases up on diluting the Nd-site with Yttrium. For x=0.35, ferrimagnetism is observed. At 5 K, metamagnetic transition is observed for all compositions x < 0.4. The evolution of magnetic ground states and appearance of ferrimagnetism in Nd1-xYxMnO3 can be accounted for by invoking the scenario of magnetic phase separation. The high frequency electron paramagnetic resonance measurements on x=0.4 sample, which is close to the critical composition for phase separation, revealed complex temperature dependent lineshapes clearly supporting the assumption of magnetic phase separation. (C) 2014 Elsevier B.V. All rights reserved.

Magnetocaloric (MC) properties of GdMnO3 single crystals are investigated using magnetic and magneto-thermal measurements. GdMnO3 exhibits a giant MC effect (isothermal change in magnetic entropy (-Delta S-M) similar to 31 J (kg K)(-1) at 7 K and adiabatic change in temperature similar to 10 K at 19 K for magnetic field variation 0-80 kOe). Complex interactions between 3d and 4f magnetic sublattices influence MC properties. The rare-earth antiferromagnetic ordering induces an inverse MC effect (positive Delta S-M) along `a' and `c' axes whereas it's not seen along the `b' axis, revealing complex anisotropic magnetic ordering. The antiferromagnetic ordering possibly changes to ferromagnetic ordering at higher fields.

Iron-based layered chalcogenides are interesting because of their structural magnetic and superconducting properties. Single crystals of the parent binary chalcogenides, Fe1+y Te, and intercalated ternary chalcogenides, K0.8Fe2Se2, are grown and investigated in detail. Single crystals are grown by modified horizontal Bridgman method. Fe1+y Te demonstrates an antiferromagnetic (AFM) transition at T (N) =67 K which is identified as a magnetostructural transition. By varying the concentration of excess Fe, we have tuned T (N) over a range of temperature from 67 to 57 K. The superconducting properties of K0.8Fe2Se2 crystals are explored by magnetization measurements. A superconducting transition is observed at T (C) =31 K. The lower critical field of K0.8Fe2Se2 is estimated from field variation of magnetization measurements.

Orthorhombic single crystals of TbMn0.5Fe0.5O3 are found to exhibit spin-reorientation, magnetization reversal and weak ferromagnetism. Strong anisotropy effects are evident in the temperature dependent magnetization measurements along the three crystallographic axes a, b and c. A broad magnetic transition is visible at T_N (Fe/Mn) = 286 K due to paramagnetic to AxGyCz ordering. A sharp transition is observed at T_SR (Fe/Mn) = 28 K, which is pronounced along c axis in the form of a sharp jump in magnetization where the spins reorient to GxAyFz configuration. The negative magnetization observed below TSR Fe/Mn along c axis is explained in terms of domain wall pinning. A component of weak ferromagnetism is observed in field-scans along c-axis but below 28 K. Field-induced steps-like transitions are observed in hysteresis measurement along b axis below 28 K. It is noted that no sign of Tb-order is discernible down to 2 K. TbMn0.5Fe0.5O3 could be highlighted as a potential candidate to evaluate its magneto-dielectric effects across the magnetic transitions., Comment: Accepted in J. Appl. Phys

Static magnetization for single crystals of insulating Nd0.85Pb0.15MnO3 and marginally conducting Nd0.70Pb0.30MnO3 has been studied around the ferromagnetic to paramagnetic transition temperature T-C. Results of measurements carried out in the critical range vertical bar(T - T-C)/T-C vertical bar

Rare-earth manganites with divalent doping have attracted attention because of their interesting physical phenomena. Manganites, doped with divalent lead, are grown from solvent PbO–$PbF_2$. Single crystals of $R_{1-x}_Pb_xMnO_3$ and mixed rare-earth $(R_y R_{1-y})_{1-x}Pb_xMnO_3$, where R, R =La, Nd and Pr are grown with different values of x using eutectic composition of $PbO: PbF_2$ as solvent. Growth temperature can then be significantly reduced. Optimized charge-to-flux ratio 1:6 yields large crystals with dimension up to $4 \times 3 \times 2 mm^3$. A variety of surface features such as hillocks, micro and macro step patterns, dendrites as well as clear featureless surfaces are observed on as-grown crystals. Clear surfaces reveal that growth occurred predominantly by two-dimensional layer growth mechanism. The hillocks might have originated at the site of screw dislocation, which suggests the growth mechanism at low supersaturation. Hopper growth observed on a few crystals is suggestive of unstable growth. Optimum growth parameters are employed to achieve better yield and good quality crystals.

The structure of $Nd_{1-x}Pb_xMnO_3$ crystal for x = 0.25 is determined at room temperature by single-crystal x-ray diffraction. The structural refinement reveals that the crystal is tetragonal with space group P4/mmm, Z = 4 and R of 8.3%. The lattice parameters are a = 7.7652(1) $\AA$, c = 3.884(1) $\AA$ and $\alpha = \beta = \gamma = 90^o$. The structural analysis is then extended to x = 0.38. It is noticed that substitution of Pb at the Nd site results in structural phase change from tetragonal (x = 0.25) to cubic (x = 0.38). These changes are attributed to the progressive removal of inter-octahedral tilting and minimization of the octahedral distortion leading to a higher symmetry as doping concentration increases. While the unit cell volume of tetragonal structure (P4/mmm) is comparable to that of parent $NdMnO_3$ (Pnma), the volume of cubic unit cell $(Pm \bar{3}m)$ is doubled. Electron diffraction patterns support these results and rule out the possibility of twinning. Changes in transport properties as a function of temperature at different doping levels are in accordance with the observed structural changes. It is observed that $T_{\theta}$ and $T_{MI}$ increase with x.

Transport and magnetic properties of flux-grown Nd1−xPbxMnO3 single crystals (x=0.15–0.5) are studied in the temperature range 300–77 K and 280–2 K, respectively. Magnetization measurements with a superconducting quantum interference device confirm a paramagnetic to ferromagnetic transition around 110, 121, 150, 160, and 178 K for x=0.15, 0.2, 0.3, 0.4, and 0.5, respectively. Four probe resistivity measurements at low temperatures show a monotonic increase for x=0.15 which represents a ferromagnetic insulating (FMI) phase. For Nd0.8Pb0.2MnO3 there is a slope change present in the resistivity profile at 127 K where metal to insulator transition (MI) sets in. For x=0.3 this MI transition is more prominent. However, both these samples have FMI phase at low temperature. When the concentration of lead increases (x>0.3) the sample displays a clear insulator to metal transition with a low temperature ferromagnetic metallic phase. On the basis of these measurements we have predicted the phase diagram of Nd1−xPbxMnO3. Magnetization measurements by a vibration sample magnetometer point out the appreciable differences between zero field cooled and field cooled profiles below the ferromagnetic to paramagnetic transition temperature for all x. These are indicative of magnetic frustration.

Single crystals of Nd 1− x Pb x MnO 3 with various dopant concentrations ( x =0.15, 0.2, 0.3, 0.4 and 0.5) are grown by high temperature solution growth technique using PbO–PbF 2 flux. Charge-to-flux ratio and PbO–PbF 2 composition are optimized to grow good quality crystals at lower temperature. When the PbO:PbF 2 ratio is close to the eutectic composition, crystal yield has improved. Powder X-ray diffraction studies reveal that the crystals exhibit tetragonal symmetry. Super-lattice structure obtained from transmission electron microscopic investigations suggests lattice doubling. Magnetization measurements show that the Curie temperature ( T C ) increases with dopant concentration. A second transition observed below 30 K is probably due to ordering of the Nd 3+ ions. The magnetization does not saturate up to a field of 50 kOe at 20 K suggesting a non-collinear (canting) ordering of Nd-spins.

The electronic structure of Nd1-xYxMnO3 (x-0-0.5) is studied using x-ray absorption near-edge structure (XANES) spectroscopy at the Mn K-edge along with the DFT-based LSDA+U and real space cluster calculations. The main edge of the spectra does not show any variation with doping. The pre-edge shows two distinct features which appear well-separated with doping. The intensity of the pre-edge decreases with doping. The theoretical XANES were calculated using real space multiple scattering methods which reproduces the entire experimental spectra at the main edge as well as the pre-edge. Density functional theory calculations are used to obtain the Mn 4p, Mn 3d and O 2p density of states. For x=0, the site-projected density of states at 1.7 eV above Fermi energy shows a singular peak of unoccupied e(g) (spin-up) states which is hybridized Mn 4p and O 2p states. For x=0.5, this feature develops at a higher energy and is highly delocalized and overlaps with the 3d spin-down states which changes the pre-edge intensity. The Mn 4p DOS for both compositions, show considerable difference between the individual p(x), p(y) and p(z)), states. For x=0.5, there is a considerable change in the 4p orbital polarization suggesting changes in the Jahn-Teller effect with doping. (C) 2013 Elsevier Ltd. All rights reserved.

Antisite disorder is observed to have significant impact on the magnetic properties of the double perovskite Y2CoMnO6 which has been recently identified as a multiferroic. A paramagnetic-ferromagnetic phase transition occurs in this material at T-c approximate to 75 K. At 2K, it displays a strong ferromagnetic hysteresis with a significant coercive field of H-c approximate to 15 kOe. Sharp steps are observed in the hysteresis curves recorded below 8K. In the temperature range 2K

A systematic study of Raman spectra on Yttrium doped NdMnO3 polycrystalline samples was undertaken to understand the lattice dynamics in this compound. Raman active phonons were analyzed and the observed peak were assigned to elucidate various phonon modes in the range (200 - 800) cm(-1). It was observed that at 325 cm(-1) phonon frequency shifts upward as much as upto 4 % with increase in Yttrium content. Lattice distortions manifest themselves by frequency shifts in both bending and tilt modes of MnO6 octahedra, resulting in increase of Raman band line-widths.

We report the single crystal growth of antimony doped Fe1+yTe and Fe1+yTe0.5Se0.5 (Fe1+ySbxTe1-x (x=0, 2%, 5%) and Fe1+yTe0.49Se0.49Sb0.02) by a modified horizontal Bridgman method. Growth parameters are optimized to obtain high quality single crystals. The antiferromagnetic (AFM) transition at T-N = 62.2 K which is a first order transition, shifts to lower temperature on doping in Fe1+yTe. Alternately when the chalcogen site of the ternary compound Fe1+yTe0.5Se0.5 is doped with Sb, superconductivity is preserved albeit the superconducting transition temperature (T-C) falls slightly and a concomitant reduction occurs in superconducting volume fraction. (C) 2013 Elsevier B.V. All rights reserved

We have grown single crystals of Gd0.5Sr0.5MnO3 (GSMO50) using optical float zone method. We report AC susceptibility measurements carried out on these single crystals at various frequencies in the range 42 to 10,000 Hz under the application of small AC magnetic field (∼170 mOe). The frequency dependence of the peak temperature follows a critical slowing down with exponent zν=1.13(4) as seen in the dynamical scaling analysis reported in the present paper. We observe that the glass-like phase in GSMO50 (∼ below 32 K) is very sluggish (spin flipping time τ0=4×10−6 sec).

Frequency-dependent and temperature-dependent dielectric measurements are performed on double perovskite Tb$_2$NiMnO$_6$. The real ($\epsilon_1$) and imaginary ($\epsilon_2$) parts of dielectric permittivity show three plateaus suggesting dielectric relaxation originating from bulk, grain boundaries and the sample-electrode interfaces respectively. The temperature and frequency variation of $\epsilon_1$ and $\epsilon_2$ are successfully simulated by a $RC$ circuit model. The complex plane of impedance, $Z'$-$Z"$, is simulated using a series network with a resistor $R$ and a constant phase element. Through the analysis of frequency-dependent dielectric constant using modified-Debye model, different relaxation regimes are identified. Temperature dependence of dc conductivity also presents a clear change in slope at, $T^*$. Interestingly, $T^*$ compares with the temperature at which an anomaly occurs in the phonon modes and the Griffiths temperature for this compound. The components $R$ and $C$ corresponding to the bulk and the parameter $\alpha$ from modified-Debye fit tend support to this hypothesis. Though these results cannot be interpreted as magnetoelectric coupling, the relationship between lattice and magnetism is marked., Comment: Accepted in Europhysics Letters

Good quality single crystals of copper metagermanite, CuGeO3, are grown by flux technique. Growth is carried out at relatively low temperatures by using Bi2O3 along with CuO in an optimal flux ratio. Besides rendering the procedure simple, lower growth temperature reduces growth defect concentration. Single crystals of Cu1 - xCoxGeO3 and CuGe1 - yGayO3 are grown by the same method for different values of x and y to investigate the influence of in-chain and off-chain doping on spin-Peierls (SP) transition. Change in color, morphology and surface features as a result of doping are briefly discussed. Spin-Peierls transition of these crystals is studied by susceptibility measurements on a commercial SQUID magnetometer. Cationic substitution resulted in reduction of spin-Peierls transition temperature (T-SP) of CuGeO3. Substitution of magnetic impurity cobalt in-chain site caused more pronounced effects such as suppression of SP phase.