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We report on Raman and photoluminescence spectroscopic studies on oxygen vacancy induced structural disorder in Gd2Ti2O7-d single crystals grown by optical floating zone technique under argon atmosphere. The oxygen vacancies in Gd2Ti2O7-d wafers decrease with thermal annealing in an air atmosphere. The full width at half maximum of X-ray diffraction rocking curve decreases from 245 to 157 arc-second and the optical transmittance increases from 23 to 87 % (at 1000 nm) upon post growth thermal annealing. Raman spectroscopic studies reveal a monotonic increase in intensity of O-Gd-O (Eg) and Ti-O (A1g) stretching modes with thermal annealing. Since these modes are associated with modulation of oxygen x parameter which is sensitive to Ti-O octahedron distortion, the increase in Raman intensity indicates an improvement in structural ordering of oxygen sub-lattice in Gd2Ti2O7-d. Moreover, the photoluminescence studies also corroborate the Raman analysis in terms of reduction of structural defects associated with oxygen vacancies as a function of thermal annealing. This study demonstrates the effectiveness of using Raman spectroscopy to probe the structural disorder in Gd2Ti2O7-d crystals., Comment: 16 pages, 6 figures, 1 table

Here, we report the synthesis and physical properties of Co2FeGe (CFG) Heusler alloy (HA) nanoparticles (NPs). The NPs of size 23 nm are prepared using the co-precipitation method. X-ray and selected area electron diffraction patterns have confirmed the cubic Heusler phase of the NPs with the A2-disorder. These NPs are soft ferromagnetic, and exhibit a high saturation magnetization (Ms) along with a very high Curie temperature (Tc) of 1060 K. The observed Tc value matches closely with the theoretically calculated one following a model provided by Wurmehl et al. [1]. The high Ms and Tc make the present system a potential candidate for magnetically activated nano-devices working at high temperatures. The near-integral value 5.9 mehoB/f.u. of Ms at low temperatures indicates that the half-metallic ferromagnetism is preserved even in the particles even on the 20 nm length scale. Additionally, we have facilitated the existing HA-NP preparation method, which can be used in synthesizing other HA-NPs. The first-principles density functional theory computations complement the experimental results., Comment: 26 pages, 8 figures, 3 tables

Spherical nanoparticles (NPs) of size 14 nm, made of intermetallic Fe2CoAl (FCA) Heusler alloy, are synthesized via the co-precipitation and thermal deoxidization method. X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns confirm that the present nanoalloy is crystallized in A2-disordered cubic Heusler structure. Magnetic field (H) and temperature (T) dependent magnetization (M) results reveal that the NPs are soft ferromagnetic (FM) with high saturation magnetization (Ms) and Curie temperature (Tc). Fe2CoAl nanoalloy does not follow the Slater Pauling (SP) rule, possibly because of the disorder present in the system. We also investigate its magnetic phase transition (MPT) and magnetocaloric (MC) properties. The peak value of the magnetic entropy change vs T curve at a magnetic field change of 20 kOe corresponds to about 2.65 J/kg-K, and the observed value of refrigeration capacity (RCP) is as large as 44 J/kg, suggesting a large heat conversion in magnetic refrigeration cycle. The Arrott plot and the nature of the universal curve accomplish that the FM to paramagnetic (PM) phase transition in Fe2CoAl nanoalloy is of second-order. The present study suggests that the Fe2CoAl nanoscale system is proficient, useful and a good candidate for the spintronics application and opens up a window for further research on full-Heusler based magnetic refrigerants., Comment: 25 pages, 9 figures, 2 tables

Recently d0 or intrinsic ferromagnetism was believed to provide an alternative pathway to transition metal induced ferromagnetism in oxide. In pursuit of augmenting the area of d0 ferromagnetism; we have undertaken to study the crystal structure and magnetic properties of Ag-doped ZrO2 compounds. Polycrystalline samples of Zr1-xAgxO2 (with x=0, 0.02, 0.04, 0.06 and 0.08) were prepared by solid-state reaction route. All the prepared compounds are found to crystallize in monoclinic symmetry of ZrO2. In our study, pure ZrO2 compound exhibits paramagnetic behavior. However, the Ag-doped ZrO2 compounds exhibit ferromagnetic to paramagnetic transition. The Curie temperature was found to increase from 28.7 K for x=0.02 to 173.2 K for x= 0.08 doped ZrO2. Thus, the introduction of Ag in ZrO2 induces ferromagnetism with a large ThetaC. The measurements of hysteresis curves indicate that Ag doped ZrO2 compounds exhibit hysteresis loops with a coercivity of around 1350 Oe. Moreover, increase in Ag concentration resulted increase in the value of saturation magnetization (MS); the maximum value of MS was recorded as 0.01 {\mu}B/Ag ion for x= 0.06 sample. The sintering of sample at high temperature (13500C) diminishes the ferromagnetism and it leads to paramagnetic behaviour., Comment: 19 pages

In a quest of enriching the area of d0 magnetism in oxide materials, we have undertaken to study Mg-doped TiO2 compounds. The Ti1-xMgxO2 (x=0, 0.02, 0.04 and 0.06) nanoparticles were prepared by solid-state reaction route. The X-ray diffractions (XRD) patterns of these samples indicate single phase of tetragonal rutile-structure of TiO2. The refinement of the XRD patterns reveals no change in the crystallographic lattice parameters in comparison to pure TiO2 upon Mg doping and it indicates that Mg2+ ions do not enter core grains and form core/shell structure. SEM observations reveal the uniform morphology with nanometric grains in the range of 150-200 nm. The measurement of magnetic properties of these compounds indicates that pure TiO2 and Ti0.98Mg0.02 compounds exhibit paramagnetic behavior and Ti0.96Mg0.04 compound exhibits ferromagnetic (FM) phase superimposed with the dominating paramagnetic phase. However, Ti0.94Mg0.06 compound exhibits ferromagnetic to paramagnetic transition with FM transition temperature of 180.2 K. The measurements of zero field and field cooled magnetization data indicate low temperature magnetic irreversibility for x=0.06 sample and it was attributed to the competing AFM (core) and the FM (shell) interactions. The measurement of hysteresis curves at various temperatures indicates domain wall pinning and an exchange-bias behavior., Comment: 14 pages

Recently, d0 ferromagnetic materials have been projected as one of the promising novel materials for spintronics applications. In this work, we have studied Li-doped ZnO compounds, i.e. Zn1-xLixO (x=0, 0.02, 0.04, and 0.06) samples, prepared by the solid-state reaction route method. From the study of crystal structure using X-ray diffraction (XRD) patterns, it is evident that the prepared materials have been formed in a single-phase of the hexagonal wurtzite structure. The refinement of the XRD patterns suggests that there are very small changes in the lattice parameters upon Li-incorporation in ZnO. The average crystallite size (SC), estimated from XRD patterns was found to be in the range of 35-50 nm. The microstructural study by scanning electron microscope reveals the uniform morphology of the grains of the order of 50-70 nm. The energy dispersive spectrum indicates that no unwanted ferromagnetic impurities have crept into the final prepared samples. The measurement of the temperature (T) variation of magnetization (M) with SQUID magnetometer indicates that undoped ZnO exhibits diamagnetic property but all Li-doped compounds exhibit room-temperature ferromagnetism and with a magnetic irreversibility behavior between zero-field cooled and field cooled M-T data. From the magnetization versus field measurements at 3 and 300 K, it is observed that Li-doped samples exhibit ferromagnetic loops with ultra-soft coercivity (~50 Oe) and with a maximum saturation magnetization of 0.10 emu/gm for x= 0.02 sample, which decreases with the increase in Li concentration., Comment: 16 pages

We report on the growth, structural and optical properties of YAlO3 single crystals grown by optical floating zone technique. Powder X-ray diffraction and Raman spectroscopic studies confirm the phase purity of the crystals. Raman analysis reveals that the intensity and line-width of Raman bands increase significantly with beta irradiation indicating the formation of structural defects in YAlO3 lattice. The optical properties are studied through UV-visible absorption, and photoluminescence emission and excitation spectroscopies under pre- and post- beta irradiation. The optical studies indicate the presence of Sm and Cr impurities by exhibiting characteristic emission lines in the orange red region. Further, a systematic study on the thermoluminescence (TL) characteristics of the crystal is also carried out at different doses of beta irradiation. The crystals exhibit a prominent TL glow peak at 239 C for less than 5 Gy doses while a weak second glow peak evolves at higher doses. Also, the crystals show a nearly linear dose response in the studied range from 0.1 to 10 Gy. The glow curve analysis reveals that the TL emission obeys the first order kinetics model. Based on the optical studies, the plausible mechanism for the TL glow curve is discussed in terms of the intrinsic defects and impurities that are present in the crystal., Comment: 21 pages, 9 figures; Accepted for publication in "Optical Materials"

This paper deals with the spherically symmetric self-gravitating star which is considered to be expansion free dissipative perfect fluids distribution. Some recent research reveals that expansion free dynamical star must be accelerating and dissipating. We adopted some conjectures to obtain the analytical solution for the dynamical model of such stars. Firstly, it has shown that density of dynamical star is homogeneous and \Lamada-dominated under quasi-static diffusion approximation. Secondly, the self-similar solution is also discussed to describe the dynamical model., Comment: Require comments

Density functional theory calculations within the generalized gradient approximation are employed to study the ground state of Co2FeAl. Various magnetic configurations are considered to find out its most stable phase. The ferromagnetic ground state of the Co2FeAl is energetically observed with an optimized lattice constant of 5.70 {\AA}. Thereafter, the system was subjected under uniform and non-uniform strains to see their effects on spin polarization (P) and half-metallicity. The effect of spin orbit coupling is considered in the present study. Half-metallicity (and 100 % P) is only retained under uniform strains started from 0 to +4%, and dropped rapidly from 90% to 16% for the negative strains started from -1% to -6%. We find that the present system is much sensitive under tetragonal distortions as half-metallicity (and 100% P) is preserved only for the cubic case. The main reason for the loss of half-metallicity is due to the shift of the bands with respect to the Fermi level. We also discuss the influence of these results on spintronics devices., Comment: 18 pages, 8 figures,4 tables

Co2FeAl (CFA) nanoparticles (NPs) of different sizes were synthesized by chemical route. The effect of the size of NPs upon the structure and magnetization compared to its bulk counterpart was investigated. The structure and composition were determined from X-ray diffraction (XRD) and electron microscopy. XRD analysis shows that the samples are having single (A2-type) disordered phase. Magnetization measurements suggest that the samples are soft ferromagnetic in nature with very low coercivity. Enhanced magnetic properties like saturation magnetization, coercive force, retentivity, and Curie-temperature are observed with a decrease in particle size. The effect of particle size on hysteresis losses is also discussed. The smallest particles of size 16 nm exhibited the highest saturation magnetization and transition temperature of 180.73 emu/g and 1261 K, respectively. The origin of enhancement in the magnetization of Co2FeAl nano-alloy is attributed to the strong Co-Co exchange interaction due to disorder present in the systems., Comment: 12 pages, 6 Figures

A giant magnetocaloric effect across the ferromagnetic (FM) to paramagnetic (PM) phase transition was observed in chemically synthesized Co2FeAl Heusler alloy nanoparticles with a mean diameter of 16 nm. In our previous report, we have observed a significant enhancement in its saturation magnetization (Ms) and Curie temperature (Tc) as compared with the bulk counterpart. Motivated from those results, here, we aim to explore its magnetocaloric properties near the Tc. The magnetic entropy change shows a positive anomaly at 1252 K. Magnetic entropy change increases linearly with the magnetic field, and a large value of ~15 J/Kg-K is detected under a moderate field of 14 kOe. It leads to a net relative cooling power of 89 J/Kg for the magnetic field change of 14 kOe. To confirm the nature of magnetic phase transition, a detailed study of its magnetization is performed. The Arrott plot and nature of the universal curve conclude that FM to PM phase transition in the present system is of second-order., Comment: 23 pages, 7 figures, one Table

The generalized gradient approximation (GGA) scheme in the first-principles calculations are used to study the effect of L21 and XA ordering on the phase stability, half-metallicity and magnetism of Co2FeAl (CFA) Heusler alloy. Various possible hypothetical structures: L21-I, L21-II, XA-I, and XA-II were prepared under the conventional L21 and inverse XA phases by altering the atomic occupancies at their Wyckoff sites. It is found that the XA-II phase of CFA is the most stable phase energetically among all the structures. The electronic structure calculations without U show the presence of half-metallic (HM) ground state only in L21-1 structure and the other structures are found to be metallic. However, the electronic structures of CFA are significantly modified in the presence of U, although the total magnetic moments per cell remained the same and consistent with the Slater-Pauling (SP) rule. The metallic ground states of CFA in L21-II and XA-II structures are converted into the half-metallic ground states in presence of U but remained the same (metallic) in XA-I structure. The results indicate that the electronic structures are not only dependent on the L21 and XA ordering of the atoms but also depend on the choice of U values. So experiments may only verify the superiority of GGA+U to GGA., Comment: 21 pages, 8 figures, 4 tables, Accepted for publication in JMMM 23 July, 2019

The physical properties of Fe2CoAl (FCA) Heusler alloy are systematically investigated using the first-principles calculations within generalized gradient approximation (GGA) and GGA+U. The influence of atomic ordering with respect to the Wyckoff sites on the phase stability, magnetism and half metallicity in both the conventional L21 and XA phases of FCA is focused in this study. Various possible hypothetical structures viz., L21, XA-I, and XA-II are prepared by altering atomic occupancies at their Wyckoff sites. At first, we have determined the stable phase of FCA considering various non-magnetic (or paramagnetic), ferromagnetic (FM) and antiferromagnetic (AFM) configurations. Out of these, the ferromagnetic (FM) XA-I structure is found to be energetically most stable. The total magnetic moments per cell are not in agreement with the Slater-Pauling (SP) rule in any phases; therefore, the half-metallicity is not observed in any configurations. However, FM ordered XA-I type FCA shows 78% spin polarization at EF. Interestingly, the results of XA-I type FCA are closely matched with the experimental results., Comment: 15 pages, 6 figures

The main purpose of this paper is to present the spherical characterization of Legendre curves in $3$-dimensional quasi-Sasakian pseudo-metric manifolds. Furthermore, null Legendre curves are also characterized in this class of manifold., Comment: 14 Pages and 4 Figures

Thermal spike model (TSM) is presently a widely accepted mechanism of swift heavy ion (SHI) - matter interaction. It provides explanation to various SHI induced effects including mixing across interfaces. The model involves electron-phonon (e-p) coupling to predict the evolution of lattice temperature with time. SHI mixing is considered to be a result of diffusion in transient molten state thus achieved. In this work, we assess this conception primarily via tuning the e-p coupling strength by taking a series Pd$_{1-x}$Ni$_x$ of a completely solid soluble binary, and then observing 100 MeV Au ion induced mixing across Pd$_{1-x}$Ni$_x$/Si interfaces. The extent of mixing has been parametrised by the irradiation induced change $\Delta \sigma^2$ in variances of Pd and Ni depth profiles derived from X-ray photoelectron spectroscopy. The $x$-dependence of $\Delta \sigma^2$ follows a curve that is concave upward with a prominent minimum. Theoretically, e-p coupling strength determined using density functional theory has been used to solve the equations appropriate to TSM, and then an equivalent quantity L$^2$ proportional to $\Delta \sigma^2$ has been calculated. L$^2$, however, increases monotonically with $x$ without any minimum, bringing out a convincing disparity between experiment and theory. Perhaps some mechanisms more than the TSM plus the transient molten state diffusion are operative, which can not be foreseen at this point of time.