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• Gadolinium doping in BHF results in modification of magnetic properties. • "Law of Approach to Saturation" gives the magnetocrystalline anisotropy constant. • Para-ferrimagnetic transition is observed around 700 K. • Maximum entropy change and RCP are found to be 0.72 J/kgK and 27.5 J/kg, respectively, near Curie temperature. • Possibility to use the Gd doped BHF at high temperature as a cooling agent. Gd doped barium hexaferrite (BaFe 12−x Gd x O 19 , x = 0.0–0.7) has been synthesized by the sol-gel method to explore its magnetic and MCE (magnetocaloric effect) properties. The materials crystallize to hexagonal magnetoplumbite phase. Average particle size decreases with the increase in Gd concentration in barium hexaferrite (BHF). The coercive field increases from 3.2 to 4.8 kOe, and saturation magnetization decreases from 68.21 to 54.23 emu/g with the increase in Gd concentration from x = 0.0 to x = 0.7. These large changes in magnetic parameters reveal the effect of Gd concentration in BHF. The saturation magnetization monotonously reduces with an increase in Gd concentration in BHF due to a decrease in average particle sizes. The saturation magnetization is found to be higher at a lower temperature (60 K) compared to that of room temperature (300 K). It is due to a reduction in thermal energy at low temperature which is smaller compared to the magnetic Gibbs free energy at low temperature. Hence, the magnetic spins are freezing along the applied magnetic field direction at the low temperature. Also, the magnetocrystalline anisotropy constant (obtained by the "Law of Approach to Saturation method") is found to be more at low temperature compared to that of room temperature due to an increase in the strength of spin-orbit coupling with the decrease in temperature (i.e. thermal energy). The M-T curves and M-H hysteresis loops reveal paramagnetic to ferromagnetic transition at the Curie temperature. The maximum entropy change was found to be in the range of 0.12–0.72 J/kgK in a window of the applied magnetic field of 0.5–3 T, and the corresponding RCP max was found to be 2.5–27.5 J/kg. The present study opens a window to explore the MCE on BHF based material. [ABSTRACT FROM AUTHOR]

Highlights: [•] Secondary phase formation has been controlled for Bi0.9Ca0.1Fe0.9Ti0.1O3 [•] Co-substitution of Ca & Ti at Bi & Fe sites significantly increases MS and HC [•] Compositional driven structural transition from rhombohedral to orthorhombic symmetry. [•] The leakage current has reduced significantly for Ca2+ and Ti4+ co-substituted BiFeO3. [•] Small band gap of co-substituted BiFeO3 may be useful for optoelectronic devices. [Copyright &y& Elsevier]

Ferroelectric perovskites materials are the most promising materials because of their multifunctional applications as well as the ongoing fundamental research on them. The present study investigates the correlation between the crystal structure and various physical properties such as dielectric, ferroelectric, and optical properties of Sr2+ modified Sodium bismuth titanate Na 0.5 Bi 0.5−x Sr x TiO 3 (x = 0.00, 0.02, 0.04, and 0.06) synthesized by solid-state reaction method. Rietveld refinement of the X-ray diffraction pattern reveals that the system undergoes a crystal symmetry transition from a two-phase crystal symmetries system (rhombohedral (R3c) and tetragonal (P4bm)) to a three-phase crystal symmetries system (rhombohedral (R3c), tetragonal (P4bm) and Cubic (Pm 3 ̅ m)) with the rise in Sr2+ concentration in Na 0.5 Bi 0.5−x Sr x TiO 3 which is the foremost reason behind the variation of physical properties of the system. The dielectric constant and maximum polarization increase with the Sr2+ concentration up to 2% and it decreases with the further increase in Sr concentration due to the distortion in crystal structure induced by the introduction of cubic phase (Paraelectric) in the system. The maximum value of the dielectric constant is estimated to be 3629 at a low frequency on the Na 0.50 Bi 0.48 Sr 0.02 TiO 3 system. The optical band gap (E g) of Na 0.5 Bi 0.5−x Sr x TiO 3 (x = 0.00, 0.02, 0.04, and 0.06) ceramics are found to vary between 3.202 eV to 3.172 eV which indicates the semiconducting nature of prepared samples but urbach energy shows an opposite behaviour i.e. increases with Sr concentration. [Display omitted] • Crystal symmetry transition occurs from R3c + P4bm to R3c + P4bm + Pm 3 ̅ m. • Na 0.50 Bi 0.48 Sr 0.02 TiO 3 exhibits Maximum polarization of 7.05 μC/cm2. • The highest dielectric constant is found to be 3629 for Na 0.50 Bi 0.48 Sr 0.02 TiO 3. • Na 0.50 Bi 0.48 Sr 0.02 TiO 3 ceramics exhibits a low optical band gap of 3.138 eV. • Sr2+ modified NBT can be utilized in various technological applications. [ABSTRACT FROM AUTHOR]

Structural, optical, temperature dependent magnetic and electrical properties have been investigated for the nanocrystalline NiFe 1.97 RE 0.03 O 4 (RE = La3+ and Sm3+) compound. Without any signs of a secondary REFeO 3 phase, the Rietveld refinement reveals a single-phase spinel structure possessing cubic symmetry for current samples. The decrease in lattice constant has been observed as a result of RE ions substitution. The TEM micrographs reveal nanosized particles with an average size of 35 ± 3 nm and 32 ± 3 nm for La3+ and Sm3+ substituted samples. EDX spectra of both samples show good compositional homogeneity. HRTEM micrographs of both samples show well resolved lattice fringes and their inter-planar spacing matches with that obtained from the Rietveld analysis of the XRD patterns. The concentration of Fe2+ and Fe3+ ions has been estimated from the XPS spectra analysis and it is found to be ∼ 22% and 78% for NiFe 1.97 La 0.03 O 4 and, 20% and 80% for NiFe 1.97 Sm 0.03 O 4 compound. The optical energy band gap for rare earth substituted samples is found to be more than that of pure nickel ferrite. The value of saturation magnetization (M s) and magneto-crystalline anisotropy constant (K 1) estimated from the "Law of Approach to Saturation" equation for rare earth substituted samples are found to be less than that of pure nickel ferrite. However, an enhanced value of coercivity has been observed with RE substitution. ZFC (zero field cooling) and FC (field cooling) DC magnetization curves measured at 100 Oe in the temperature range of 60–400 K reveal a combination of weak and intermediate forms of magnetic interaction between the particles for both samples. Temperature dependent analysis of saturation magnetization and coercivity employing modified Bloch's law and Kneller's relation supports the nanomagnetic behavior of both samples. Under an AC magnetic field of 14.92 kA/m and frequency 337 kHz, the SAR and ILP values have been found to be ∼ 331 W/g and 4.22 nHm2/kg for NiFe 1.97 La 0.03 O 4 and, ∼ 241 W/g and 3.21 nHm2/kg for NiFe 1.97 Sm 0.03 O 4. The dielectric relaxation data have been analyzed at various temperatures ranging from 40 to 300 0C over the frequency range 100 Hz-1 MHz using electrical impedance spectroscopy. The dielectric constant for rare earth substituted samples is found to be more and their AC conductivity is observed to be less than that of pure nickel ferrite. The analysis of temperature dependent AC conductivity employing Jonscher's power law suggests that the charge carrier's conduction mechanism of both samples follows the small polaron hopping mechanism below 200 0C, thereafter; it follows the correlated barrier hopping mechanism. The activation energy estimated by imaginary impedance spectra is found to be 0.411 eV and 0.398 eV for NiFe 1.97 La 0.03 O 4 and NiFe 1.97 Sm 0.03 O 4 which is more than that of pure nickel ferrite. The Cole-Cole plots at different temperatures suggest the presence of non-Debye-type relaxation. The modeling of Cole-Cole plots with equivalent circuits for both samples confirms the contribution of both grain and grain boundary in electrical conduction. The estimated stretching exponential factor by fitting the modified KWW (Kohlrausch-Williams-Watts) equation to the imaginary modulus curve reveals relatively more dipole-dipole interaction in NiFe 1.97 Sm 0.03 O 4 than that of the La3+ substituted sample. [Display omitted] • Samples crystallized to single phase cubic spinel structure with F d 3 − m space group. • ZFC-FC analysis reveals intermediate type magnetic interaction between particles. • SAR is estimated by initial slope, Box-Lucas and modified Newton cooling approach. • Nyquist plots reveal non-Debye type dielectric relaxation and NTCR type behavior. • Presence of considerable induced dipole-dipole interaction in current samples. [ABSTRACT FROM AUTHOR]

Abstract: Zn2+ ion substituted nanocrystalline cobalt ferrite materials with the chemical formula CoZn x Fe2− x O4 for x =0.0, 0.1, 0.15, 0.2, 0.25 and 0.3 have been synthesised by standard citrate precursor method. The crystal structure and phase purity have been studied by powder X-ray diffraction (XRD) method by employing Rietveld refinement technique. The cation distribution between the tetrahedral site (A-site) and octahedral site (B-site) has been determined by Rietveld analysis. The distribution of cations changes with the Zn2+ concentration. Average crystallite size and lattice constant increase with the annealing temperature. The lattice constants decrease with the increasing Zn concentration. The vibrational modes of the octahedral and tetrahedral metal complex in the sample have been carried out using Fourier transform infrared spectroscopy (FT-IR). The FT-IR spectra of the sample have been studied in the wave number range of 380–800cm−1 and it shows the presence of absorption bands which are assigned to tetrahedral and octahedral metal complexes. The elemental analysis has been carried out using energy dispersive spectroscopy (EDS) with the help of Field Emission Scanning Electron Microscope (FE-SEM) and the results reveal that the elements are as per the stoichiometric ratio in all the samples. The Magnetic hysteresis loop measurements were carried out at room temperature using a vibrating sample magnetometer (VSM) over a field range of ±2T. Annealing at 200°C, superparamagnetic nature has been observed for compositions x =0.15–0.3. The magnetic data in saturation have been analysed by employing the law of approach (LA) technique. The saturation magnetisation increases with the Zn for x ⩽0.15 and then decreases. The coercivity decreases with the increasing Zn concentration for 600°C annealed samples. The magnetocrystalline anisotropy constants are found to increase for x ⩽0.15 and then decrease with the Zn concentration. [Copyright &y& Elsevier]

Nickel-Manganese-Gallium-based Heusler alloys have been extensively studied for magnetic cooling applications with different compositions. However, the large hysteresis and narrow range of working temperatures associated with the first-order phase transition make limit their practical application. In this direction, the magnetocaloric effect was analyzed for three different off-stoichiometric compositions of Ni 50+y+z Mn 25+x-y Ga 25‐x‐z alloys, where x, y, and z vary once at a time. Ni 50 Mn 27 Ga 23 (NMG 1) and Ni 54 Mn 21 Ga 25 (NMG 2) samples show large magnetocaloric effects across the second-order ferromagnetic to paramagnetic phase transition. Another sample Ni 54 Mn 25 Ga 21 (NMG 3) undergoes coupled magneto-structural transition near room temperature with very low hysteresis loss (⁓4 J/kg) and a very high sensitivity of the martensite transition temperature (⁓3.6 K/T) with the field. By cyclic cooling protocol reliable magnetic entropy changes are found to be maxima of − 1.98 ± 0.09, − 1.90 ± 0.11 and − 3.53 ± 0.14 J/kg-K for 3 T field change with large effective RCP values of 235.8 ± 0.1, 161.5 ± 0.1 and 144.7 ± 0.3 J/kg for NMG 1, NMG 2 and NMG 3, respectively. The presence of inter-martensite transition along with the magnetic transition or magneto-structural phase transition widens the range of working temperature which effectively enhances the magnetic cooling performance of these materials. The T C and T M are tuned in Ni-Ga-Mn-based alloys by changing the chemical composition with considering the electron concentrations. • Large magnetocaloric effect on Ni 50+y+z Mn 25+x-y Ga 25‐x‐z Heusler alloys. • Hysteresis free large MCE on Ni 50 Mn 27 Ga 23 and Ni 54 Mn 21 Ga 25. • Reliable MCE by cyclic cooling across magneto-structural transition on Ni 54 Mn 25 Ga 21. • Low hysteresis loss and high sensitivity of the martensite transition temperature. • Wide working temperature due to the presence of inter-martensite transition. [ABSTRACT FROM AUTHOR]

(1-x)BiFe 0.80 Ti 0.20 O 3 –(x)Co 0.5 Ni 0.5 Fe 2 O 4 multiferroic composites (x = 0.0, 0.3, 0.5, 0.7 and 1.0) was synthesized by the tartaric acid modified sol-gel method. The Rietveld refinements of all XRD patterns have been carried out to obtain the phase percentages of perovskite BiFe 0.80 Ti 0.20 O 3 and spinel Co 0.5 Ni 0.5 Fe 2 O 4 . A change in the lattice parameters of the BFTO and CNFO phase in the composites have been observed, which could be due to strain at the interface of BFTO and CNFO. A change in the peak positions in the Raman spectra of BFTO and CNFO in the composite has been observed and it is well correlated with the XRD results. A significant increase in the magnetization has been observed due to incorporation of Co 0.5 Ni 0.5 Fe 2 O 4 phase in the composite. Vegard's law was used to calculate the theoretical value of magnetization for all the composites and compared with the experimental value. Excess value of magnetization observed in the experimental data of composites as compared to the theoretically calculated value and, explained on the basis of interfacial strain effect. An enhancement in the dielectric properties has been observed in the composite system. A correlation between the structure, magnetic and dielectric properties has been reported by assuming strain at the interface of BFTO and CNFO phase. [ABSTRACT FROM AUTHOR]

Magnetic materials with reversible magnetocaloric effect across their respective first-order magneto-structural transitions (MST) have vital significance for further advancement towards the ultimate goal of practical magnetic cooling applications. In this present study, the reversibility of the MST is investigated by analyzing the isofield curves under different minor and major thermal hysteresis loops and the isothermal curves under different magnetic field cycles. The reversibility of the magnetocaloric effect (MCE) depends upon the width of the nucleation energy barrier existing across the MST. Minor hysteresis loops are found to be minimizing the energy loss by preventing the nucleation of the martensite phase and can be easily returned to the austenite phase immediately while switching from cooling to heating. A large reversible isothermal magnetic entropy change Δ S M of −14.4 J kg−1 K−1 nearly at 322 K is observed for the (Mn 0.45 Fe 0.55) (Ni 0.6 Co 0.4) Si alloy under a low magnetic field change of 3 T. The observed large reversible MCE, very low magnetic field-induced hysteresis loss, and large reversible RC effe ∼ 155 J kg−1 will be beneficial for reducing the energy loss during several magnetic field cycles in practical cooling applications which makes this alloy promising for the development of efficient magnetic refrigerants. • Largest reversible Δ S M ∼ -14.4 Jkg−1K−1 is observed in Mn-Ni-Si based alloys for Δ H ∼ 3 T. • Minor hysteresis loops minimize the negative impact of thermal hysteresis on MCE. • Overlapping nature of successive M-H cycles confirm the reversibility of MCE. • Large sensitivity (2.75 KT−1) of MST helps to obtain a large Δ S tr ∼ 38.2 Jkg−1K−1. [ABSTRACT FROM AUTHOR]

Researchers have been making persistent efforts in recent years to find novel and suitable oxide materials with room temperature d0 ferromagnetic characteristics to be utilized in spintronics devices. The fundamental goal of this work is to investigate the effect of K/Mg co-doping on the structural, magnetic, optical, and transport (dielectric and Hall Effect) properties of Zn 0.94−y K 0.06 Mg y O compounds produced via solid-state process (y = 0, 0.02, 0.04, 0.06, 0.08). The X-ray diffraction pattern examination of these compounds revealed their hexagonal wurtzite structure. The Raman spectrum also displays the hexagonal wurtzite structure of ZnO without any lattice disorder. The elemental colour mapping demonstrates that all of the elements are dispersed uniformly throughout the compound. The concentration of K/Mg co-doping was increased, and this resulted in an improvement in the dielectric value and ac conductivity. Moreover, low dielectric loss in these compounds is observed and thus projecting it as a potential material to be utilized in high-frequency optoelectronic devices. The analyses of Nyquist plots show that higher Mg co-doping results in an increase of non-Debye kind relaxation and it led to a decrease in the grains and grain boundaries resistance. According to the analysis of dielectric data using the Maxwell-Wagner model, hopping of charge carriers are likely to be responsible for the electrical transport. As revealed from optical property measurement, the band gap was found to be slightly altered and the transmittance value increased from 82% for Zn 0.94 K 0.06 O to 86% for K/Mg co-doped ZnO compounds. The Hall Effect investigation clearly indicated p-type conductivity and enhanced carrier density concentration. Furthermore, it has been discovered that these substances exhibit ferromagnetism at room temperature. [Display omitted] • Zn 0.94 −y K 0.06 Mg y O compounds were synthesized by the solid-state reaction method. • Optical band-gap narrowing, p-type conductivity and carrier concentration enhancement was observed with the increase of Mg doping concentration. • The transmittance value increased from 82% for Zn 0.94 K 0.06 O to 86% for K/Mg co-doped ZnO compounds. • The dielectric constant and ac conductivity were found to increase with Mg-doping concentration. • Room temperature d 0 Ferromagnetism has been observed in all compounds. [ABSTRACT FROM AUTHOR]

The Na-doped TiO 2 compounds, i.e Ti 1 − x Na x O 2 0 ≤ x ≤ 0.12 were synthesized via the solid-state synthesis method. The formation of single rutile phase with tetragonal structure was identified through the structural study by recording the XRD patterns. The analyses of the XRD patterns further indicate that Na+ ions are likely being substituted at the interstitial site of Ti4+ ions. The microstructural study using SEM revealed the homogeneous morphology of all compounds, with the particle size in the range of 1–2 μ m. The room temperature magnetization versus field measurement reveals a ferromagnetic behavior for all Na-doped compounds with coercivity values ranging from 100 to 930 Oe. The saturation magnetization was observed to be maximum for Ti 0.97 Na 0.03 O 2 compound but thereafter it decreases. According to optical property measurements, the bandgap reduces from 2.94 to 2.71 eV as the concentration of Na doping increases. The maximum transmittance value 91% was noted for Ti 0.97 Na 0.03 O 2 compound. The Hall effect measurements indicate that the carrier concentration of all Na-doped TiO 2 ranged from 1.077 × 10 15 to 7.579 × 10 15 /cm3, and it grew monotonically with the increase of Na-doping concentration. The value of the ac conductivity and dielectric constant were observed to enhance with increasing Na-doping concentration; indicating that transport occurs via hopping of the charge carrier. Further, the dielectric properties observation reveals lossless nature in the produced sample, which suggests that this material could be employed in high-frequency optoelectronic devices. [Display omitted] • Room temperature d 0 Ferromagnetism has been observed in Na-doped rutile TiO 2 compounds. • Hysteresis loops with a maximum saturation magnetization of 0.038 µ B per Na-ion was observed for 3% Na-doped compound. • Optical band-gap narrowing and carrier concentration enhancement was observed with the increase of Na doping concentration. • Very high optical transmittance value i.e., 83% (for TiO 2) to 91% (for 3% Na-doped TiO 2) was observed. • Zero dielectric loss was observed for all Na-doped compounds. • The dielectric constant and ac conductivity were found to increase with Na-doping. • These materials can be used for potential application for spintronics and optoelectronics devices. [ABSTRACT FROM AUTHOR]

Ni 2.14 Mnd 0.55 Sb 1.3 Heusler alloy has been prepared by the conventional arc melting method with vacuum-sealed annealing. The sample has been crystallized to Fm 3 ¯ m space group in L2 1 phase, and it is the parent crystal structure of the Ni-based Full Heusler alloy. It has been found that the sample has the Curie temperature (para to ferromagnetic transition temperature) at 290K. It was determined from magnetization ∼ temperature plot (M ∼ T) as well as by employing the Arrott plot method. Also, critical exponent analysis by employing the Arrott plot and Kouvel-Fisher methods confirm that the sample (Ni 2.14 Mn 0.55 Sb 1.31) obeys mean field theory with long range spin ordering. Analysis of temperature dependent resistivity data interestingly reveals the phonon-phonon and magnon-manon scattering dominance above and below the Curie temperature, respectively. The electrical and magnetic properties investigation endorse that there is a correlation between magnetic and electrical properties of the above sample. The magnetocaloric effect (MCE) analysis is carried out by using the Maxwell's equations in isothermal magnetization data. It is interesting to note that the sample exhibits maximum MCE near the room temperature (∼290K), i.e., the magnetic Curie temperature. Hence the present alloy may be a potential material to be used for magnetic refrigeration application. • Para to ferromagnetic transition temperature (Curie temperature, T C) is at 290 K, which is near room temperature. • There is semi-metallic behaviour at 60 K–130 K and at 300 K–400 K, there is metallic behaviour. • There is metal to semimetal transition with reasonable RCP value near about 290 K, which is a magnetic Curie temperature. [ABSTRACT FROM AUTHOR]