We report an unexplored type-II multiferroic order in NiMn2O4, exhibiting strong linear magnetoelectric coupling above liquid-nitrogen (LN) temperature. The compound becomes ferroelectric at ∼100 K, coinciding with ferrimagnetic ordering, with a polarization value of ∼535 μC/m2 for a poling field of 5 kV/cm. At LN temperature, the polarization value increases linearly (∼21%) with a magnetic field up to 30 kOe. Rietveld refinement of neutron diffraction patterns reveals a ferrimagnetic model with antiparallel moments at tetrahedral and octahedral sites, as well as a canting of octahedral moment persisting up to ∼100 K. Low-temperature synchrotron diffraction confirms a step-like oxygen displacement during multiferroic ordering, suggesting that the Dzyaloshinskii–Moriya interaction polarizes the intervening oxygen atoms through magnetostriction, providing a microscopic mechanism for spontaneous electric polarization in this linear magnetoelectric multiferroic compound.

We investigate exchange bias (EB) effect in two members with x = 0.4 and 0.6 of multiferroic Cu1-xCoxCr2O4 series, which are close to the interface between two structures at room temperature and represent with structures having I4(1)/amd and Fd3 (m) over bar space groups, respectively. Both the compounds exhibit EB effect having EB field (H-E) of 244 and 870 Oe at 2 K for x = 0.4 and 0.6, respectively with a cooling field of 10 kOe, which decrease with increasing temperature and disappear near their ferrimagnetic ordering temperatures (T-N) close to 100 K. We note that higher H-E correlates the higher coercivity (H-C), which is found highest for x = 0.6 in the entire series of compounds. Our low temperature synchrotron diffraction studies for x = 0.6 confirm magnetoelastic as well as electroelastic coupling observed close to T-N and ferroelectric ordering temperature (T-FE). A structural transition to a polar Ima2 structure from I4(1)/amd is observed close to 154 K, which coincides with T-FE. Below T-FE, thermal variation of structural distortion defined by (a - b)/(a + b) demonstrates significant magnetoelastic coupling. More importantly, the distortion parameter is found significantly higher than the rest member of the series and provides a hint for the highest corecivity in the entire series and correlates the higher EB effect. Current study demonstrates a correlation between structural distortion and EB effect and provides a clue of improving EB effect through the crystal structural engineering.

The compounds, NiSb2O6 (NSO) and MnSb2O6 (MSO) attract the community for the quasi one-dimensional and layered structure composed of Ni2+ and Mn2+, which orders antiferromagnetically at T (N) = 6.7 and 12 K, respectively. Here, we report the Griffiths-like phase much above T (N) in the range of 37-85 K and 25-80 K for NSO and MSO, respectively. The dc magnetization results indicate the Griffiths-like phase, following the modified Curie-Weiss law. The magneto-capacitive responses for both the compounds show anomalies at the onset of the Griffiths-like phase. Intriguingly, the low temperature synchrotron diffraction results are conclusive for determining the singularities for both the compounds. Interplay between the low-dimensionality, magnetic frustration, and magneto-elastic coupling correlates the observed short range ordered state, which is suggested as a Griffiths-like phase, above T (N) for both the compounds.

The rhombohedral distortion-driven occurrence of the spontaneous electric polarization at a reasonably high temperature $(T)$ and an exchange bias (EB) effect below the antiferromagnetic (AFM) N\'eel temperature $({T}_{N})$ are revealed in ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$. We observe the magnetic memory effect, suggesting a cooperative glassy magnetic state below ${T}_{N}$. The phase separation between the long-range AFM and the glassy magnetic component leads to the EB effect below ${T}_{N}$. The synchrotron diffraction studies confirm a structural transition to a polar $R3m$ structure from the cubic spinel structure, involving a strong rhombohedral distortion. This distortion correlates with the occurrence of the spontaneous electric polarization $(P)$ below $\ensuremath{\sim}110$ K $({T}_{\text{FE}1})$, which is accompanied by a short-range order (SRO). The $P$-value enhances further due to an additional rhombohedral distortion below ${T}_{N}$. A considerable magnetoelectric (ME) coupling is detected below ${T}_{\text{FE}1}$. The increase of $P$ is $\ensuremath{\sim}7.2$% for 5 T at the liquid nitrogen temperature. There has been a fundamental interest among the community in the unique result of the occurrence of ferroelectric (FE) order coexisting with SRO and having a significant ME coupling, which is accompanied by a strong rhombohedral structural distortion analogous to that observed in ${\mathrm{BiFeO}}_{3}$.

We report thermoelectric and electrical transport properties of Bi1.8Sb0.2Te3-ySey by tuning y. In contrast to the reported p-type conductivity of the end compounds with y = 0 and 3, a dominant n-type conduction mechanism is observed for y = 1.5 from the Hall measurement. Intriguingly, the magneto-Seebeck consequence is enhanced up to ∼ 20 times for y = 1.5 compared to the end members. The reasonable value of magnetoresistance with an anisotropic character with respect to the direction of the magnetic field is observed at low temperature, which decreases with increasing temperature. The density of state at the Fermi level near room temperature correlates high Seebeck coefficient as well as magneto-Seebeck effect. High magneto-Seebeck effect at room temperature is promising for the application.

In this work we have studied the effect of network structure on the transport properties of Li2O-P2O5-MoO3 mixed former glasses. We have used Fourier Transform Infrared (FTIR) and Raman spectroscopy to study the network structure. We have measured ac conductivity and dielectric spectra for a wide range of temperature and frequency. We have observed that the conductivity increases and the activation energy decreases with the increase of Li2O content in the composition. The time temperature superposition has been verified using the scaling formalism of the conductivity spectra. We have calculated microscopic lengths of ion dynamics such as the characteristic mean square displacement of mobile ions and the spatial extent of localized motion within the framework of the linear response theory and correlated them with the ion transport properties. Also we have given a qualitative description for the correlation of the ionic transport with the relative strengths of structural units.

We report a ferroelectric order at ~ 98 K for NiFe2O4, which carries an inverse spinel structure with a centrosymmetric Fd3m structure at room temperature. The value of spontaneous electric polarization is considerably high as ~ 0.29 {\mu}C/cm2 for 5 kV/cm poling field. The electric polarization decreases considerably (~ 17 %) around liquid nitrogen temperature upon application of 50 kOe field, proposing a significant magnetoelectric coupling. The synchrotron diffraction studies confirm a structural transition at ~ 98 K to a noncentrosymmetric structure of P4122 space group. The occurrence of polar order is associated with an ordered occupancy of Ni and Fe atoms at the octahedral sites of the P4122 structure, instead of random occupancies at the octahedral site of the inverse spinel structure. The results propose that NiFe2O4 is a new type-II multiferroic material.

We observe a ferroelectric (FE) order in an unexplored CuCr2O4 with a reasonably high value of the FE Curie temperature (T-FE) at 170 K, which is also much higher than the magnetic ordering temperature. The systematic substitution of Jahn-Teller (J-T) active divalent Cu ion by a non-Jahn-Teller active divalent Co ion causes a systematic shift of T-FE from 170 K for x = 0 to 146 K for x = 0.8 in Cu1-xCoxCr2O4. The values of electric polarization vary from 0.0665 mu C cm(-2) to 0.1704 mu C cm(-2), which is maximum for x = 0.6, associated with the highest value of the coercivity. The synchrotron diffraction studies of the compounds with x = 0.2 and 0.8 confirm that a structural transition to a polar Ima2 space group from the tetragonally distorted I4(1)/amd structure gives rise to the ferroelectricity. In all the members of Cu1-xCoxCr2O4 series, the T-FE is observed at much higher temperature than the corresponding magnetic ordering temperatures (T-N). These results are in contrast to that of the reported results of T-FE < T-N for the end member with x = 1 or CoCr2O4, where the J-T active Cu2+ is absent. We propose that the J-T distortion in the entire series with 0 <= x <= 0.8 holds the key, where interplay between the J-T distortion driven orbital order and the structural distortion correlates tuning of the T-FE in Cu1-xCoxCr2O4.

We report a characteristic spin-glass-like behavior in Sr3NiSb2O9, as confirmed from the low-field dc magnetometry and ac susceptibility measurements. The memory effects in the dc magnetometry exhibit similar characteristic features of the spin-glass systems. The ac susceptibility measurements provide that the frequency dependent peak-shift follows the conventional Vogel-Fulcher and dynamical scaling laws with the characteristic relaxation time to be similar to 10(-14) s. This relaxation time fits in the recommended range of similar to 10(-12) - 10(-14) s for the classical spin-glasses. Low temperature synchrotron diffraction studies point to the strong magnetoelastic coupling close to the spin-glass-like transition. The transition is found to be associated with the step-like anomalous lattice contraction. Possible origin of magnetic frustration leading to the spin-glass ground state is discussed by correlating the microstructural results, as obtained from the analysis of synchrotron diffraction studies. (C) 2018 Elsevier B.V. All rights reserved.

In the present work we have studied transport properties of lithium ions in 0.3Li(2)O-0.7[xTeO(2)-1-x)P2O5] glasses, where x=0.5, 0.6, 0.7. We have measured acconductivity for a wide range offrequency and temperature. The real part of the conductivity spectra has been analyzed by the power law in Almond-West formalism. The dc conductivity has been obtained from the complex impedance plots. We have found that dc conductivity increases and activation energy decreases on increase of TeO2 for a particular Li2O content. We have also found that the dc conductivity and crossover frequency obey Arrhenius relation. The time temperature superposition has been verified using the scaling formalism of the conductivity spectra. We have found that the conductivity isotherms scaled to a single master curve with suitable scaling parameters for a particular composition at different temperatures. However the scaling to a single master curve fails for different compositions at a particular temperature.

In the present work we have studied dynamics of Li+ ions in xLi(2)O-(1-x)[0.4TeO(2)-0.6P(2)O(5)] glasses, where 0.30 <= x <= 0.45. We have measured ac conductivity for a wide range of frequency and temperature. The real part of the conductivity spectra has been analyzed using Almond-West power law formalism. We have observed that the dc conductivity increases and the activation energy decreases with the increase of Li2O content. We have also observed that the d.c. conductivity and crossover frequency obey Arrhenius relation. The time-temperature superposition has been verified using the scaling of conductivity spectra. We have calculated microscopic lengths of ion dynamics, such as the characteristic mean square displacement of mobile ions and the spatial extent of localized motion, within the framework of linear response theory. We have observed that both the lengths decrease with the increase in Li2O content. Besides, we have also investigated the Fourier transform infrared (FTIR) spectroscopy to study the structural aspect of the glassy network, and have established a simple correlation between ion dynamics and network structure.

In the present work we have studied ion transport and their correlation with network structure of Li2O-P2O5 glasses. We have measured the ac conductivity of the glasses for wide ranges of temperature and frequency. The conductivity shows a crossover from dc to dispersive behavior with the increase in frequency. We have observed that the dc conductivity and the crossover frequency obey Arrhenius relation with same activation energy. We have calculated characteristic lengths of ion transport, such as the characteristic mean square displacement of mobile ions and the spatial extent of localized motion, within the framework of linear response theory. We have observed that both the length scales decrease with the increase in Li2O content in the glasses. We have also investigated FTIR and Raman spectra to study the structure of glassy network and established a simple correlation between transport properties and network structure of the glasses.

We report significant correlation of the multiferroic order and ferroelectric polarization to the orthorhombic structural distortion for Ca1-xLaxBaCo4O7 (x <= 0.05). Analysis of the synchrotron diffraction studies reveal that La doping increases considerable structural distortion, which is associated with the increase of multiferroic ordering temperature and electric polarization. Intriguingly, the value of polarization increases remarkably to approximate to 385 mu C/m(2) (x = 0.05) from approximate to 150 mu C/m(2) (x = 0) for a 3 kV/cm poling field. Synchrotron diffraction studies in magnetic field provides an important clue, where structural distortion provides more impact on the polarization value than the contribution from the change in unit cell volume. Geometric magnetic frustration holds the key for the occurrence of the structural distortions, around which multiferroic ordering takes place for CaBaCo4O7. Our work thus highlights crystal structural distortion as a rich playground for tuning multiferroic order as well as polarization value.