Ashish Kumar Mall

Temperature dependent electron paramagnetic resonance study on magnetoelectric YCrO3

We report temperature dependent electron paramagnetic resonance (EPR) studies on polycrystalline YCrO3 samples at X-band (9.46 GHz) in the temperature range of 120 K-298 K. The EPR spectra exhibit a single broad line across the whole temperature range, attributed to Cr3+ ions. The variation of EPR spectra parameters (line width, integrated intensity, and g-factor) as a function of temperature was analyzed to understand the nature of spin-dynamics in the paramagnetic region of YCrO3. A peak in the g-factor suggests the presence of a new phase within the paramagnetic state at an intermediate point of temperature T IP ~ 230 K, attributed to the onset of short range canted antiferromagnetic correlations in the material much above 140 K, Néel temperature (T N) of YCrO3. The EPR intensity increases with a decrease in temperature up to T N due to the renormalization of the magnetic moments arising from the appearance of canted antiferromagnetic correlations. Further, temperature dependent dielectric measurements also exhibit an anomaly at ~230 K suggesting the presence of magnetodielectric coupling in YCrO3, with a possibility towards a relatively high temperature magnetodielectric system.

Temperature and grain size effect on electrical properties of gallium ferrite polycrystalline ceramic

In this paper, we have report the effect of grain size/sintering temperature on structure and electrical properties of polycrystalline GaFeO3 (GFO) ceramic sample synthesized using solid state reaction method. The structure and morphology of the GFO were studied by means of X-ray diffraction (XRD) and scanning electron microscopic (SEM). The XRD results confirm that all the samples exhibits single phase orthorhombic structure which interprets the high temperature stability of GFO. It has been confirmed further using Energy dispersive x-ray (EDS) analysis. Microstructure investigation revealed an optimum microstructure, in terms of average grain size at the different sintering temperature (T ≥1300°C). The average grain size was found to increase the overall dielectric and leakage current of the GFO increased with increasing in sintering temperature. Variation of dielectric constant (e′) and dielectric loss (tanδ) with frequency at different sintering temperature showed dispersion in the low frequency range.

Temperature dependent Raman scattering in YCrO3

High quality polycrystalline YCrO3 samples were synthesized using solid-state-reaction method. The samples were subsequently characterized using X-ray diffraction and magnetometry. Further, temperature dependent Raman spectroscopy over a spectral range from 100 to 800 cm−1 was used to examine the variation of phonons as a function of temperature from 90 to 300 K. In the low temperature ferroelectric phase of YCrO3, the observed phonon spectra showed softening of some Raman modes below the magnetic ordering temperature (TN ∼ 142K), suggesting a coupling between the spin and phonon degrees of freedom.

Truncated hexagonal bi-pyramidal gallium ferrite nanocrystals: integration of structural details with visible-light photo-activity and self-cleaning properties

We report the growth of gallium ferrite (GaFeO3 or GFO) nanocrystals with a unique morphology, “truncated hexagonal bi-pyramid” (THBP), and with a novel orthorhombic P212121 symmetry under hydrothermal conditions in pure water without using any reducing or shape defining reagent. The growth conditions of such a structure are optimized through a detailed study using field emission scanning electron microscopy. The structural details of the above hitherto unreported phase of the material are studied using room temperature powder X-ray diffraction (XRD), Raman spectroscopy, high resolution transmission electron microscopy and ab initio electronic structure calculations. In light of the above structural characterization, a possible mechanism for the formation of the specific shape of hexagonal bi-pyramid is proposed. Our investigations on optical, photocatalytical and wetting characteristics of the as-grown nanocrystals demonstrate their visible light photocatalytic activity as well as a strong-hydrophilic character. The material is poised to establish itself as a new “self-cleaning” material.

Structural, magnetic and magnetoelectric properties of single phase La3+ and Er3+ co-doped Bi0.85−x La0.15Er x FeO3 (0 ≤ x ≤ 0.1) ceramics

A sol–gel reaction method is used to prepare La3+ and Er3+ co-doped ceramic Bi0.85−x La0.15Er x FeO3 (x = 0, 0.05, 0.07 and 0.10) equivalently denoted as BLEFO. Bi0.85−x La0.15Er x FeO3 exhibits the structural transition from rhombohedral to a triclinic structure with quite small triclinic distortion; as confirmed by the Rietveld refinement analysis. Field emission scanning electron micrographs reveal the presence of interconnected spherical grains. From transmission electron microscopy, the average crystal size was found to be 60–65 nm. Co-doping with La and Er enhanced the magnetic properties such as saturation magnetization and magnetic moment of BiFeO3 as confirmed by vibrating sample magnetometer at 300 K and as well as by superconducting quantum interference device (SQUID) analysis at 5 K. An unusual behavior has been observed in BLEFO at 300 K, because it shows the weak ferromagnetic behavior besides its antiferromagnetic nature at room temperature. An irreversibility effect and a large bifurcation have been observed below room temperature in BLEFO in SQUID measurement. The value of magnetoelectric (ME) coupling coefficient increases from 2.60 (x = 0) to 6.58 (x = 0.10) mV cm−1–Oe at 3500 Oe. A negative temperature coefficient of the resistance behavior analogous to a semiconductor is confirmed by the impedance analysis. The enhanced multiferroic properties are attributed to the enhanced ME interaction, which results from the La and Er substitution at A site of BiFeO3.

High temperature X-ray diffraction, Raman spectroscopy and dielectric studies on yttrium orthochromites

The structural, thermal and dielectric properties of YCrO3 ceramic prepared by solid state reaction method have been investigated by a combination of XRD, Raman spectroscopy and permittivity measurement. The X-ray diffraction spectra shows single phase orthorhombically distorted perovskite structure with Pnma symmetry over a wide range of temperature 300K to 1100K. Impedance spectroscopy study on the sample showed that the dielectric constant, tangent loss and ac conductivity with frequency increases on increasing the temperature. Dielectric measurement shows a relaxor like transition at about 460K. Non-Debye type relaxation is observed with activation energy of 0.25 eV extracted from ac conductivity at 11 kHz frequency. We believe that the oxygen ion vacancies play an important role in conduction processes in addition to polaron hopping at higher temperatures. Raman scattering measurements were performed over a wide temperature range from 300K to 600 K. The line width of the modes due to CrO6 bending and...

Temperature dependent x-ray diffraction and dielectric studies of multiferroic GaFeO3

Polycrystalline GaFeO3 (GFO) samples were synthesized by sol-gel method. The structural and dielectric properties of GaFeO3 ceramic have been investigated by a combination of XRD and permittivity measurement. The X-ray diffraction spectra shows single phase orthorhombically distorted perovskite structure with Pc21n symmetry over a wide range of temperature 300K to 600K, with no evidence of any phase transition. Refined lattice parameters (a, b, c and V) increases with increasing temperature. Temperature dependent dielectric properties were investigated in the frequency range from 100Hz–5MHz. Impedance spectroscopy study on the sample showed that the dielectric constant and ac conductivity with frequency increases on increasing the temperature. Cole-Cole plots suggest that the response from grain is dominant at low temperature whereas grain boundary response overcomes as temperature increases. The relaxation activation energy (calculated from Cole-Cole plots) value is found to be 0.32 eV for the grain boun...

Raman effect, structural and dielectric properties of sol-gel synthesized polycrystalline GaFe1-xZrxO3 (0≤x≤0.15)

Polycrystalline ceramic samples of Zirconium (Zr)-doped GaFeO3 (GaFe1-xZrxO3) were studied using powder X-ray diffraction, complex impedance spectroscopy and Raman spectroscopic measurements to understand the effect of Zr doping on the structural and dielectric properties. The samples with varying Zr content were prepared by Sol-Gel method. X-ray data analysis confirmed the formation of single phase material without formation of any secondary phases and all are crystallized in Pc21n orthorhombic symmetry. Rietveld refinement of the X-ray data suggested an increase in the lattice constants due to size effect and decreases on x = 0.15 due to the effect of change in interplanner spacing. Impedance studies on the samples showed that the dielectric constant increases while loss tangent decrease as the Zr content increases. Raman scattering on GaFe1-xZrxO3 (x = 0, 0.05, 0.10, & 0.15) used to understand the composition dependence on phonon modes at room temperature. On Zr doping, Raman modes frequencies shifts t...

Composition-dependent structural and Raman spectroscopic studies on Y1-xHoxCrO3 (0≤x≤0.1)

In this paper we report the synthesis and structural characterization of polycrystalline holmium doped YCrO3 samples prepared by solid state reaction method. X-ray diffraction studies confirm the formation of single phase pure materials. Increasing Holmium substitution in Y1-xHoxCrO3 (0≤x≤0.1) allows a quasi-continuous tuning of the lattice in this multiferroic chromite without any magnetic interference effects of rare-earth ions. The composition dependent Raman scattering studies at room temperature reveal decreasing Raman mode frequencies with increasing holmium content consistent with the X-ray data. Decreasing phonon frequency shifts with increasing holmium content occurs, depending on the average rare-earth ion radius determined by the concentration of Y+3 and Ho+3.