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This paper covers a detailed study of the sintering temperature (TS) effect on the electrical and magnetotransport properties of Pr0.67Sr0.33MnO3 nanoparticles prepared by sol–gel method. The temperature dependence of resistivity displays a metal–insulator transition with increasing temperature. The electrical resistivity is strongly affected by particle size. The resistivity minimum at low temperature is associated to electron–electron coulombic interactions. Magnetotransport analysis was carried out using a phenomenological percolation model for all our samples. The obtained results indicate that a percolation model is not suitable for samples presenting high magnetic disorder. The magnetoresistance (MR) can be modulated by particle size effect. We have recorded considerable MR values (−MR(T) = 34% at 10 K for a 1T field when TS = 850°C, −MR(T) = 33% at 290 K for a 2T field when TS = 1000°C and −MR(H) = 33% at 5 K for a 1T field when TS = 700°C). The considerable MR values indicate the possibility of using these samples for several technological applications.

In this paper, we have studied the effect of size reduction on charge ordering (CO) and magnetic phase separation in La0.5Ca0.5MnO3. The magnetic ground state at low temperature shifts from ferroma...

In this work, we report the effect of sintering temperature (TS) on the magnetic interactions and the critical behavior of Pr0.67Sr0.33MnO3 manganite prepared via sol–gel method (700 °C ≤ TS ≤ 1200 °C). The critical exponents study revealed that the 3D-Ising model is the best model to describe the magnetic interactions in the P1000 (TS = 1000 °C) and P1200 (TS = 1000 °C) samples. Consequently, a strong magnetic anisotropy characterizes these samples near the paramagnetic-ferromagnetic transition temperature, such anisotropy can be linked to the magnetic moment of Pr3+ ions (µ = 3.58 µB). For TS ≤ 850 °C, it becomes nearly impossible to speak about universality class due to the presence of great magnetic disorder induced by several factors like spin disordered shell in nanoparticles, magnetic frustration and Griffiths phase.

The effect of an external constant magnetic field on the dielectric properties and magnetodielectric (MD) coupling in La0.4Eu0.1Ca0.5MnO3 polycrystalline manganite at room temperature (RT) has been investigated. This sample exhibits negative colossal MD values under low applied magnetic fields (− 30% and − 45% under applied fields of 0.6 T and 1.2 T, respectively). The colossal MD values as well as the relatively low dielectric loss values suggest that the La0.4Eu0.1Ca0.5MnO3 sample can be a suitable␣candidate for technological applications at RT, like magnetic field sensing and gyrators.

We have studied in this work the electrical and magnetoresistive properties of polycrystalline La0.75Dy0.05Sr0.2MnO3. Magnetic study has shown the presence of a paramagnetic–ferromagnetic transition at Curie temperature TC = 225 K. Temperature-dependent resistivity shows two resistivity peaks. The magnetotransport in the metallic phase can be described by Zener’s polynomial law, while small polaron hopping mechanism is more suitable to describe resistivity evolution in the insulating phase. The resistivity upturn observed around 75 K was ascribed to electron–electron scattering process at low temperature. We have recorded important values of negative magnetoresistance (MR) around TC (MR (T) ~ 22% and MR (H) ~ 19%) for only 1 T applied magnetic field. This suggests the possibility of using our sample for magnetic field sensing and spintronics.

We have studied the contribution induced by a low magnetic field (0.6T and 1.2T) on the frequency dependent permittivity, impedance and conductivity recorded at room temperature for La0.4Gd0.1Ca0.5MnO3 polycrystalline compound. Strong magnetodielectric (MD) coupling has been detected at room temperature. Colossal value of negative MD (more than 50%) has been observed under 1.2T field. The MD response is found to be stable in the studied frequency range (up to 2 MHz) suggesting the possibility of using this compound for technological applications at room temperature. The significant MD coupling has been correlated with the presence of charge ordering transition near room temperature.

In this study, we have tried to simulate both temperature and magnetic field dependence of electrical resistivity for polycrystalline La0.75Ho0.05Sr0.2MnO3. The studied sample was elaborated by sol–gel method. The energy-dispersive X-ray confirms the presence of Ho element. The temperature dependence of resistivity in the whole temperature range (100–300 K) was successfully fitted by phenomenological percolation model, testifying the percolative nature of the electrical transition. The evolution of the volume fraction of the ferromagnetic phase is in agreement with the magnetic properties. The magnetic field dependence of resistivity follows the recently established universal behaviour of resistivity, which may confirm the validity of this model.

International audience; Magnetotransport properties of La0.5-xHoxCa0.5MnO3 (x=0.05 and 0.1) polycrystalline samples were investigated in order to study the effect of magnetic Ho 3+ ions on the stability of the charge ordering (CO) and magnetic phase coexistence in pristine La0.5Ca0.5MnO3. Our samples were synthesized by using sol-gel method. Temperature dependence of resistivity shows an insulating behavior with high resistivity values, confirming the presence of long-range CO. The application of 7 T magnetic field slightly reduces resistivity values and demonstrates relatively small magnetoresistance (MR) values (-MR(T) did not exceed 14 % under 7 T applied field). This observation indicates that the CO in the Ho-based specimens is strong, which indicates that Ho-substitution for La in La0.5Ca0.5MnO3 gives more strength to the CO.

The new perovskite compound La0.62Eu0.05Ba0.33Mn0.85Fe0.15O3 was successfully synthesized via the sol–gel process. X-ray diffraction revealed that our sample is pure and crystallizes in the orthorhombic structure with Pbnm space group. Electrical properties were performed using the complex impedance spectroscopy technique in the range of frequency (40–10 MHz) and at various temperatures (80–400 K). AC-conductivity results are well described by the universal Jonsher’s power law at low temperatures and the DC-conductivity data are well fitted by the small polaron hopping model at high temperatures. Impedance results show a negative temperature coefficient of resistance (NTCR) which discloses the semiconductor behaviour of the studied sample. Nyquist plots were well fitted by an equivalent circuit involving the contribution of grain and grain boundaries in the conduction process. Giant dielectric values, useful in electronic devices, were obtained. Moreover, a relaxation diffuse phase transition is attributed to a strong heterogeneity in A and B sites of the studied perovskite.

In this study, the critical behavior and magnetic and magnetocaloric properties in a polycrystalline Pr0.8Sr0.2MnO3 sample synthesized by solid-state technique were investigated in detail. By analyzing the temperature and the field dependence of magnetization, we have demonstrated that this compound exhibits a clear second-order magnetic phase transition around the Curie temperature estimated at TC = 161 K. Refined values of the critical exponents β, γ and δ determined by the modified Arrott plots, the critical isotherm and Kouvel–Fisher analysis show that our compound is described by the 3D Ising model. The reliability of these critical exponents was further confirmed using the universal scaling hypothesis. Under an applied magnetic field of 5 T, the calculated value of the maximum of the magnetic entropy change is estimated at 1.740 J K−1 kg−1 and the relative cooling power (RCP) turns out to be 134.46 J kg−1. Moreover, the critical exponents were evaluated from RCP results which confirm the correlation between the critical behavior and magnetocaloric results in the Pr0.8Sr0.2MnO3 compound.

In this research work, our central focus is upon the investigation of the effect of the substitution of non magnetic Ti ions on the critical behavior of Pr0.75Bi0.05Sr0.1Ba0.1Mn1-xTixO3 (x = 0, 0.02 and 0.04) manganites. All our compounds underwent a second-order paramagnetic to ferromagnetic transition as temperature decreases. Several techniques such as modified Arrott plots, Kouvel-Fisher analysis and critical isotherm method were used in order to determine the values of the critical exponents β, γ and δ as well as the accurate Curie temperature TC. The obtained values were found to be close to the 3D-Heisenberg model for the parent compound indicating a short-range inhomogeneous magnetic exchange coupling and to the mean-field model for doped samples describing long-range exchange interactions. We have also demonstrated that the universality class of the studied manganites was affected by Ti-doping. Besides, we highlighted that these parameters follow scaling equation with the magnetization data scaled into two different curves below and above TC confirming that the determined critical exponents and TC are reasonably accurate.

A detailed investigation of structural, electrical and dielectric properties of Pr0.75Bi0.05Sr0.1Ba0.1Mn0.98Ti0.02O3 manganite prepared by the sol-gel method was undertaken. The Rietveld refinement of X-ray diffraction pattern reveals that this compound is indexed in the orthorhombic structure with Pbnm space group. Structural properties are also analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy at room temperature. DC conductivity (σDC) data show the presence of a semiconductor-metal transition at TSM = 360 K. σDC is described by small polaron hopping model at high temperatures and variable range hopping model at low temperatures. AC conductivity results follow the universal Jonsher's power law at relatively low temperatures and Drude's model at high temperatures. The impedance plots display the contribution of both intra- and inter-granular contributions. Dielectric measurements exhibit highly remarkable dielectric permittivities useful in electronic devices. Dielectric losses are shown to be governed by the DC conduction mechanism and described by the Giuntini theory.

In this paper, magnetic phase coexistence and magnetotransport properties of Pr0.5–xBixSr0.5MnO3 (x ≤ 0.15) samples have been studied. Our specimens demonstrate complicated magnetic properties through the presence of several transitions as a function of temperature. With increase in bismuth content, a spin–glass–like–state appears at low temperatures and long–range charge ordering can be observed for x = 0.15. Magnetization and resistivity seems to be sensitive to magnetic field cycling, testifying the presence of training effect. Both, magnetic phase separation phenomenon and 6s2 lone pair electrons of Bi3+ ions, play a key role in the governance of the physical response of studied specimens.

This paper covers a detailed study of the structural, morphological, magnetic, and magnetocaloric properties of Pr0.67Sr0.33MnO3 compound prepared via sol–gel method with different sintering temperatures (700 °C ≤ TS ≤ 1200 °C). The structural study revealed that all samples crystallize in the orthorhombic structure with Pnma space group, with the presence of some impurities for TS ≤ 850 °C. With increasing sintering temperature TS, the Curie temperature shifts from 256 to 290 K. The evolution of the magnetic properties may be explained in terms of the core–shell model. The disordered magnetic behavior induces a spin-glass-like state at low temperature with the presence of Griffiths phase above Curie temperature. Magnetocaloric study indicated a relative cooling power of 97.78 J/kg under 2 T magnetic field at room temperature for the sample sintered at 1000 °C, indicating that this sample is a suitable candidate for magnetic refrigeration.

Orthorhombic Pr0.8Sr0.2MnO3 sample is prepared by solid-state reaction method and quenched in water. The occurrence of a second-order phase transition is confirmed by Arrott plots. The critical exponents determined using different models are in good agreement with the 3D-Ising model. The magnetocaloric results have shown an important maximum of the magnetic entropy change and relative cooling power evaluated at 2.14 J kg−1 K−1 and 101.52 J kg−1 under a field change of 2T. The DC conductance measurements revealed that this compound exhibited a semiconductor to metallic behavior at T SM = 400 K. Moreover, the AC results are described by the Jonscher power law. Concerning the electrical and dielectric properties, they disclosed the behavior-dependency of both temperature and frequency. As for the complex impedance measurements, they showed that the conduction mechanism was governed by grain boundaries. The dielectric results demonstrated important values of the real part of the permittivity, and the Pr0.8Sr0.2MnO3 material can be used for capacitor manufacturing.

In this work, we report the effect of Ti-doping on the structural, electrical and dielectric properties of Pr0.75Bi0.05Sr0.1Ba0.1Mn1−xTixO3 (x = 0 and 0.04) manganites synthesized by sol–gel route. X-ray structural analysis using Rietveld refinement technique reveals that our samples are single phase and crystallize in the orthorhombic system with Pbnm space group. These structures were characterized by impedance spectroscopy in the frequency range from 40 to 1 MHz and temperatures between 80 and 400 K. DC conductivity analysis shows that our samples disclose a semiconductor behaviour with a semiconductor–metal transition temperature TMS at 380 and 360 K for x = 0 and 0.04 respectively. This result shows that the TMS temperature can be controlled by varying the titanium content. AC conductivity (σAC) results show a considerable sensitivity to both temperature and frequency. The impedance plots were well described by an equivalent circuit model taking into account the contributions of semiconductor grains and insulating grain boundaries in the conduction mechanism. Furthermore, a non-Debye type of relaxation in our compounds was confirmed. In addition, we have demonstrated that the dielectric relaxation is hidden by the DC conduction mechanism.

Polycrystalline samples of La0.67-xEuxBa0.33Mn0.85Fe0.15O3 (x = 0.0, 0.1) manganites have been prepared using sol-gel route. Structural characterizations of our materials were performed by powder X-ray diffraction, Scanning Electron Microscopy and by Energy Dispersive X-ray spectroscopy at room temperature. Our samples are single phase and crystallize in the orthorhombic structure with the Pbnm space group. Electrical and dielectric properties of La0.67-xEuxBa0.33Mn0.85Fe0.15O3 (x = 0.0, 0.1) compounds have been studied using impedance spectroscopy technique in the temperature range from 80 to 400 K and the frequency range from 40 to 107 Hz. DC measurements show that the studied structures exhibit semiconductor (SC) behaviour in all temperature range of study. AC data analysis show a transition from SC to metallic behaviour at 160

The electrical and dielectric properties of Pr0.8Sr0.2MnO3 perovskite prepared by the solid-state method were investigated. The complex impedance results showed the contribution of grain and grain boundaries in the conduction mechanism. The dielectric data have confirmed the presence of interfacial polarization in the structure. Moreover, a dielectric transition fitted by Curie-Weiss law was observed. Besides, the observed relaxation diffuse phase transition is attributed to a strong heterogeneity introduced by the substitution of Pr by Sr ions. DC electrical measurements indicate that the structure exhibits a semiconductor behavior and the conductivity is governed by hopping process. The AC conductivity measurements show a clear semiconductor-metallic transition at high temperatures. The AC conductivity data can be divided into two parts, the first of which is a semiconductor one, described by the NSPT model, at low temperatures and the second is a metallic regime, fitted by the classical Drude model, at high temperatures.

In this work, we have investigated the effect of rare earth substitution in La0.75Ln0.05Sr0.2MnO3 (Ln˭La, Sm, Eu, Gd, Dy and Ho) manganites on the critical behavior near the magnetic phase transition temperature. Different techniques were used to determine the values of the critical exponents β, γ and δ as well as transition temperature TC. The reliability of the critical exponents values was confirmed by both Widom relation and scaling hypothesis. In the case of Ln˭La, Sm, Eu, Gd and Ho, the magnetic interactions were described by 3D-Ising model. However, the substitution by Dy ions changes the universality class from 3D-Ising to tricritical mean field, which can be ascribed to the intrinsic contribution of Dy3+ ions.

The electrical and dielectric properties of Pr0.5-xGdxSr0.5MnO3 (0 ≤ x ≤ 0.3) manganites prepared by the conventional sol-gel method have been studied in a wide range of temperature [80–320 K] and frequency [40–107 Hz] using impedance spectroscopy technique. DC conductivity (σDC) results show that for x = 0, 0.1 and 0.2, the samples present a semiconductor behavior in the temperature range of the study. For x = 0.3, a semiconductor-metal transition is observed at 280 K. σDC shows also a decrease in the conductivity with the increase of Gd doping. The calculated activation energies are Gd content dependent. AC conductivity results were analysed using Drude model in the metallic region and by the Jonsher power law in the semiconductor one. The Niquist plots for x = 0.2 and x = 0.3 obeys at Cole-Cole model. The impedance spectra were fitted by an adequate equivalent circuit involving two contributions attributed to grains and grain boundaries associated with an inductance. Such a result reveals a correlation between electrical and magnetic properties of Gd-doped manganites. In addition, grain-boundary resistance shows a major contribution at higher temperature. High dielectric constants are obtained in particular at low frequencies. Dielectric loss is analyzed through the Giuntini theory with which we have calculated the energy required for charge carriers to hope over the potential barriers.

In this study, we report the effect of Bi doping on the critical behavior and the magnetocaloric properties of Pr0.8−xBixSr0.2MnO3 (x = 0, 0.05 and 0.1) structures prepared by sol-gel method. Refined values of the critical exponents β, γ, and δ determined from the modified Arrott plots and Kouvel–Fisher method show that the parent sample is described by the tricritical mean field model whereas the 3D-Ising model is the best model describing doped samples. Consequently, a transition from long-range to short range FM interactions caused by the introduction of Bi in the parent sample can be identified. All samples reveal the presence of some ferromagnetic domains in the paramagnetic phase. In the vicinity of the Curie temperature TC and for an applied magnetic field of 5 T, the maximum of magnetic entropy change |ΔSM| decreases from 5.41 J K−1kg−1 for x = 0–3.11 J K−1kg−1 for x = 0.1, respectively. The |ΔSM| for doped samples exhibits a large broad variation with temperature around TC as compared with the parent sample. Such effect is substantially advantageous for magnetic refrigeration. Also, the magnetocaloric effect is investigated using Landau theory. A good agreement is obtained between this theory and experimental data, which suggests that electronic interactions and magnetoelastic coupling are at the origin of the magnetocaloric properties of our samples. In addition, we report that |ΔSM| curves follow the universal law confirming the second-ordered paramagnetic-ferromagnetic transition.

We have investigated the effect of charge ordering and phase separation on the electrical and magnetotransport properties of La0.4Eu0.1Ca0.5MnO3 polycrystalline sample. Temperature dependence of resistivity shows a metal–insulator transition at transition temperature Tρ. A hysteretic behavior is observed for zero field resistivity curves with Tρ = 128 K on cooling process and Tρ = 136 K on warming process. Zero field resistivity curves follow Zener polynomial law in the metallic phase with unusual n exponent value ∼9. Presence of resistivity minimum at low temperatures has been ascribed to the coulombic electron–electron scattering process. Resistivity modification due to the magnetic field cycling testifies the presence of the training effect. Magnetization and resistivity appear to be highly correlated. Magnetoresistive study reveals colossal values of negative magnetoresistance reaching about 75% at 132 K under only 2T applied field. Colossal values of magnetoresistance suggest the possibility of using this sample for magnetic field sensing and spintronic applications.

In this paper, we have investigated the electrical and magnetic response of a La0.4Gd0.1Ca0.5MnO3 polycrystalline sample. This sample seems to exhibit fascinating phenomena like charge ordering, magnetic phase separation, training effects and kinetic arrest. It also shows colossal values of negative magnetoresistance (∼91.7% at 96 K under 1 T applied magnetic field), which raises the possibility of using this sample for technological applications. We have also proposed, in this work, a new empirical model to describe the evolution of resistivity and magnetoresistance as a function of magnetic field. This model was successfully tested on the La0.4Gd0.1Ca0.5MnO3 sample in spite of its complicated magnetic behavior, which suggests the use of this model for other magnetic samples in order to check its validity.

The multiferroic HoCrO3 orthochromite compound was prepared using the sol-gel method. The optical properties of this ceramic were characterized by infrared (IR) and UV–Vis absorption spectroscopy. The IR spectrum shows a set of absorption peaks that can be assigned to the Cr–O and O–Cr–O vibrations. From UV–Vis absorption spectrometry, the reflectance spectrum and the Tauc model, reveal a direct and wide optical band gap evaluated at 3.34 eV and 3.206 eV respectively. The high value of the Urbach energy (528 ± 13) meV shows the presence of a high density of localized states. The extinction coefficient (k) was used to estimate the evolution of the refractive index n with the wavelength. Additionally, n obeys to Cauchy relation. Furthermore, the dispersion parameters were analyzed in the bases of Wemple and Di-Domenico model. The optical constants such as the skin depth and the optical conductivity were calculated and the results are discussed. From the UV–vis absorbance spectrum attributed to the d-d transitions of Cr3+ ion in HoCrO3, a theoretical crystal-field analysis is realized which leads to a good agreement between the calculated and the experimental energy levels. Furthermore, the electronic structure was compared with that obtained for Cr3+ ion in PrCrO3 ceramics. From Racah theoretical study, Tanabe Sugano Diagram and Photoluminescence measures, a transfer charge mechanism is proposed. The achieved results confirm the importance of our compound in optoelectronic device applications.

The aim of this paper is to investigate the structural, electrical and dielectric properties of Pr0.8-xBixSr0.2MnO3 (x = 0, 0.05 and 0.1) perovskite materials. Our samples were synthesized by sol-gel route and characterized by X-ray powder diffraction at room temperature. It has been demonstrated that they crystallize in the orthorhombic perovskite system with Pnma space group. As x varies from 0 to 0.1, the Mn O bond length and the unit cell volume increases. Consequently, the bandwidth describing the overlap between the Mn3d and O2p orbital is reduced and this affects physical properties. The studied samples were also characterized by complex impedance spectroscopy in a wide range of temperature and frequency. DC conductance results show that doping by Bi suppresses the metal behavior observed at low temperatures in the parent compound. The conductivity decreases when increasing the concentration of Bi due to the reduction of the specific free volume. AC conductivity measurements were split in two parts: The metallic region is well described by the classical Drude model and the semiconductor one was well fitted by the universal Jonscher power law. Results show that the conduction mechanism is described by non-overlapping small polaron tunneling (NSPT) model. Besides, the Cole-Cole plots which are fitted by adequate equivalent circuits show the contributions of grain and grain boundaries in the conduction mechanism. At high temperatures, the total conductivity tends to be governed by grains. Dielectric results show that the Pr0.75Bi0.05Sr0.2MnO3 sample has better dielectric properties than Pr0.7Bi0.1Sr0.2MnO3. Dielectric loss is analyzed by Giuntini theory and correlated with conduction losses. The energy required for carriers to jump over the potential barriers is determined.

The influence of the A-site cation size-disorder σ 2 on the structural and magnetic properties of the polycrystalline Pr 0.4 Re 0.1 Sr 0.5−y Ba y MnO 3 (Re = Pr, Sm, Eu, Gd, Dy and Ho) samples with a constant average ionic radius A > = 1.2445 A, is investigated. All samples were synthesized using the solid-state reaction. Rietveld refinement of the X-ray diffraction patterns shows that the substitution generates a structural transition from tetragonal to orthorhombic symmetry. The temperature dependence of magnetization indicates a weakening of ferromagnetism by the increase of σ 2 . The large difference between the field cooled and zero field cooled magnetization below Curie temperature suggests the presence of an inhomogeneous mixture of a ferromagnetic and antiferromagnetic domains at low temperature. The maximum of the magnetic entropy change decreases with increasing the mismatch effect. However, several peculiarities were observed in the magnetic behavior indicating that σ 2 is not the only factor controlling the physical properties; the nature and the magnetic moment of the substituent should be taken in consideration.

In this work, we are interested in the study of the critical behavior and the Landau theory of Pr 0.63 A 0.07 Sr 0.3 MnO 3 (A=Pr, Sm and Bi) compounds. The critical exponents of all our samples were studied using the magnetic field dependent isothermal magnetization around Curie temperature T C , based on various techniques. The validity of our critical exponents is confirmed by the scaling analysis. The study revealed that tricritical mean field model is the best for calculating the critical exponents for all our samples. The substitution of praseodymium by samarium and bismuth does not change the universality class. Using Landau theory, we have confirmed that the magnetic transition around T C is of second-order. An agreement was found between the (-ΔS) values estimated by Landau theory and those obtained using Maxwell relation for a magnetic field equal to 2 T, which shows the validity of this theory for all our compounds. The deviation obtained for bismuth and samarium based samples below T C can be attributed to the existence of some antiferromagnetic domains in the ferromagnetic phase.

Nanostructure Pr0.5Sr0.5MnO3 compounds were elaborated by mechanical milling in a planetary high energy ball mill at various milling times followed by high temperature sintering under air at 1400 °C for 20 h. The phase structure, the morphology, the magnetic and magnetocaloric properties of the powders were characterized by X-ray diffractometry, scanning electron microscopy and a vibrating sample magnetometer. Rietveld refinement of the X-ray diffraction patterns shows that all specimens crystallize in the tetragonal system with I4/mcm space group. Increasing the milling time up to 16 h, the average crystallites size decreases to the nanoscale (∼36 nm). During the intermediate stage of milling, significant changes occur in the morphology of the powder particles, due to the severe plastic deformation. Significant refinement in particle size was found evident at the final stage of milling (∼2 µm). From magnetic measurements, it was found that all samples present two magnetic transitions as a function of temperature. The Curie temperature TC decreases with increasing milling time. Moreover, it was revealed that the antiferromagnetic domains fractions highly dependent on crystallites sizes. A large magnetocaloric effect and an important value of the relative cooling power around Neel temperature was observed in all samples. These characteristics may be related to the first-ordered nature of this transition. Moreover, the magnetic entropy change and the relative cooling power were increased with decreasing crystallites sizes.

In this communication, we have used theoretical calculations in order to prepare a composite with an enhanced magnetocaloric effect from La0.4Re0.1Ca0.5MnO3 (Re = Dy, Gd, and Eu) samples. Although the obtained composites did not exhibit very important values of magnetic entropy change (− ΔS), they are active in a very wide temperature range. One of the composites is characterized by a plateau-like shape of − ΔS(T) curves. Several anomalies were found in the magnetic and the magnetocaloric properties of our composites. The origin of these anomalies is essentially attributed to the phase separation phenomenon characterizing all the La0.4Re0.1Ca0.5MnO3 samples.

Electrical and magnetoresistive response of phase–separated La 0.4 Dy 0.1 Ca 0.5 MnO 3 polycrystalline bulk sample has been investigated. The magnetic structure below 25 K is identified as a coexistence of ferromagnetic, strong antiferromagnetic and weak metastable antiferromagnetic domains. The irreversibility in the electrical resistivity with magnetic field cycling is associated to the training effect. The resistivity in the metallic range for μ 0 H ≤ 4 T follows Zener polynomial law with unusual values of n exponent due to the high spin fluctuations. Temperature dependence of magnetoresistance (MR) exhibits a plateau–like shape for μ 0 H ≤ 2 T. For MR(T), maximum MR ∼ 98.8% under 2 T field at 77 K and for MR(H), MR is about 90.5% under 1 T magnetic field at 75 K has been recorded. The colossal values of MR suggest the possibility of using our sample for technological applications. An enhancement of MR(H) at 75 K was observed in the field range 0–2 T due to magnetic field cycling. The spectacular behavior of resistivity for La 0.4 Dy 0.1 Ca 0.5 MnO 3 sample is essentially attributed to phase separation phenomenon.

In this work, we have investigated the magnetic and magnetocaloric properties of polycrystalline La0.75Bi0.05Sr0.2MnO3 prepared by sol–gel method. Structural analysis has shown that this compound crystallizes in the rhombohedral structure with R$$ \bar{3} $$c space group. The magnetic study revealed that our sample exhibits a ferromagnetic–paramagnetic transition at 300 K. A significant magnetocaloric effect in the vicinity of the room temperature was detected for this sample (for only 2T applied field, the maximum of magnetic entropy change |∆SMax| = 1.75 J/kg K and the relative cooling power RCP = 77.12 J/kg). The obtained results suggest the possibility of using this compound as a magnetic refrigerant.

In this work, we have investigated the magnetic and magnetocaloric properties of $$\hbox {Pr}_{0.8-x}\hbox {Bi}_{x}\hbox {Sr}_{0.2}\hbox {MnO}_{3}$$ ( $$x=0$$ , 0.05 and 0.1) polycrystalline manganites prepared by sol–gel route on the basis of a phenomenological model. Temperature dependence of magnetization indicates that all our samples exhibit a second order paramagnetic to ferromagnetic transition with a decrease in temperature. A correlation between experimental results and theoretical analysis based on a phenomenological model is investigated. The magnetic and magnetocaloric measurements are well simulated by this model. Under a magnetic applied field of 5 T, the theoretical absolute values of the maximum of magnetic entropy change $$\Delta S_{{\mathrm{Max}}}$$ are found to be equal to 5.33, 3.33 and $$2.97\,\hbox {J}\,\hbox {kg}^{-1}\,\hbox {K}^{-1}$$ for $$x=0$$ , 0.05 and 0.1 respectively. The relative cooling power and the specific heat capacity values are also estimated. The predicted results permit us to conclude that our compounds may be promising candidates for magnetic refrigeration at low temperatures.

In this study, we present an investigation of the critical behaviour of Pr0.5-xGdxSr0.5MnO3 (0 ≤ x ≤ 0.1) polycrystalline compounds synthesized by solid-state reaction method. Magnetic measurements performed around the Curie temperature TC revealed that our compounds exhibit a second order ferromagnetic (FM) to paramagnetic (PM) transition at TC. The obtained critical exponents are found to be close to the mean-field model, suggesting a long-range magnetic interactions for all samples. Then, the substitution of Pr by Gd does not affect the universality class. The phenomenological model proposed by Hamad is used to simulate not only the FM-PM transition, but also the two magnetic transitions present in these structures. Furthermore, thanks to this model, the magnetic entropy change and the relative cooling power can be predicted.

We studied the effects of the partial substitution (10%) of praseodymium by samarium and bismuth, on the structural, magnetic, magnetocaloric and electrical properties of the Pr 0.63 A 0.07 Sr 0.3 MnO 3 (A=Pr, Sm and Bi) manganites prepared using the solid state reaction. Refinement of the X-ray diffraction patterns shows that all our samples are single phase and crystallize in the orthorhombic structure with Pnma space group. Magnetic studies indicate that all the samples exhibit a ferromagnetic–paramagnetic transition with increasing temperature. Curie temperature T C decreases by substitution. M(H) curves indicate the presence of some antiferromagnetic domains in the substituted samples testifying the phase-separated nature of these samples. The magnetic entropy curves –Δ S ( T ) show a maximum in vicinity of T C . Important values of maximum of −Δ S are recorded for our compounds. For the parent compound, we found 4.59 J/kg K for an applied magnetic field of 2 T at T C =266 K which raises the possibility of using this compound as a magnetic refrigerant. The temperature dependence of the electrical resistivity ρ ( T ) indicates that all compounds exhibit a metal-insulator transition with increasing temperature. Electrical study suggests the presence of a correlation between electrical and magnetic properties.

The magnetic properties and the magnetocaloric effect have been studied in Pr 0.5−x Gd x Sr 0.5 MnO 3 (x = 0, 0.05 and 0.1) manganites prepared by the solid state reaction method. Magnetic measurements versus temperature in an applied magnetic field of 0.05 T show that the all samples exhibit a paramagnetic to ferromagnetic transition when temperature decreases. The x = 0.1 sample shows a clear transition, from the ferromagnetic state to the antiferromagnetic one at T N = 125 K. The Curie temperature decreases gradually with increasing gadolinium content. The absolute values of the maximum of magnetic entropy change |ΔS M | for an applied magnetic field of 2 T are equal to1.26, 1.25 and 1.02 J kg −1 K −1 for x = 0, 0.05 and 0.1, respectively. The decrease of |ΔS M | with Gd doping can be attributed to the enhancement of phase separation phenomenon. The magnetic entropy curves follow the universal law corresponding to the second-ordered paramagnetic-ferromagnetic transition.

We have reported in this work the study of correlation between electrical transport and magnetic properties of La 0.5 Ca 0.5 Mn 0.98 Bi 0.02 O 3 polycrystalline sample prepared by solid state method. Structural analysis reveals that presently studied compound crystallizes in the orthorhombic structure with Pnma space group. Temperature dependence of magnetization indicates that our studied compound undergoes a paramagnetic–ferromagnetic transition at Curie temperature T C =237 K. Magnetotransport analysis was successfully carried out using percolation model in the temperature range 40–300 K for magnetic field values up to 14 T. Strong correlation between electrical and magnetic properties was observed along with the absence of charge ordering inside the structure of our sample.

In this work, we investigate the structural, electrical and dielectric properties of HoCrO3, Ho0.9Gd0.1CrO3 and HoCr0.9Gd0.1O3 orthochromites successfully synthesized by the sol-gel process. Our samples were characterized by scanning electron microscopy, energy dispersive and X-ray diffraction (XRD) at room temperature. The XRD analysis shows that the samples crystallize in the orthorhombic system with the Pbnm space group. Depending on frequency in the range [40–107Hz] and temperature in the range [160–440K], the electrical properties were explored by impedance spectroscopy measurements. The obtained results demonstrate that the DC electrical conductivity decreases by doping HoCrO3 with Gd in A-site and increases if Gd occupies the B-site of the perovskite. The parent sample shows a semiconductor behavior until 400K where a transition to a metallic one appears. For doped samples, the transition temperature is estimated at 420K. The AC conductivity measurements can be described by the Jonscher power law and the conduction mechanism can be related to the cationic disorder. Electrical results reveal that the contribution of grains and grain boundaries in the conduction process differs according to the site of the doping by gadolinium. Furthermore, the impedance plots were well described by an adequate equivalent circuit taking into account the contributions of grains and grain boundaries. Colossal dielectric permittivities of the order of 105 F/m are obtained and were attributed to the Maxwell-Wagner interfacial effect. The dielectric losses can be described by Giuntini law and the dielectric relaxation phenomenon is shown to be obscured by the DC-conductivity process. The capacitance values are in the order of 10−2 μF for all studied compounds. These results can conduct the use of our compounds towards the electronic devices industry.

Ceramic Pr0.75Bi0.05Sr0.1Ba0.1Mn1-xTixO3 (x = 0.0, 0.02 and 0.04) samples were prepared by the conventional sol-gel route. For a magnetic applied field of 0.05T, the magnetic measurements indicated that the ferromagnetic-paramagnetic (FM-PM) transition temperature TC decreased from 130 to 98 K with increasing titanium doping level from x = 0.0 to 0.04 respectively. The magnetic properties of these compounds are discussed relying on the magnetization measurements and the susceptibility analysis. The undoped compound displays a Griffiths phase at TG = 285 K which confirms the existence of ferromagnetic entities within the paramagnetic domain far above Tc. At μ0H = 5T, the maximum value of the magnetic entropy change varies from 2.33 to 1.88 J kg−1K−1 for x varying from 0.00 to 0.04 respectively. The master curve behavior for the temperature dependence of ΔSM(T), determined for various applied magnetic fields, revealed the second-order FM-PM phase transition of our compounds. Accordingly, important values of the relative cooling power (RCP) for all investigated materials were obtained proving that they can be proposed as good candidates for magnetic refrigeration at low temperatures.

We investigate the effect of holmium substitution on the structural and magnetic proprieties of La0.5-xHoxCa0.5MnO3 (0 ≤ x ≤ 0.15) manganites synthesized by sol-gel method. The structural analysis shows that all the holmium-based samples are single phase and crystallize in the orthorhombic structure with Pnma space group. The magnetic characterization have shown that increasing Ho content leads to the weakening of ferromagnetic interaction in favor of charge ordering which changes its magnetic nature from antiferromagnetic to paramagnetic. In addition, a suppression of the spin-glass like state is induced by the increase in Ho concentration. All the studied compounds are characterized by magnetic phase separation phenomenon.

In the present communication, we propose a simple phenomenological model in order to describe the magnetic field dependence of negative magnetoresistance (MR) and resistivity isotherms. Our model is founded on the decomposition of MR into two distinct components: low field and high field MR. The model was successfully tested on several magnetic specimens and the evolution of the fitting parameters was discussed in detail. The present model works even for samples showing complicated behavior like charge ordering, magnetic phase separation and training effect. The validity of our proposed model indicates the presence of a universal behavior controlling the evolution of MR and resistivity isotherms as a function of the applied magnetic field. This model allows also the comparison of samples performances for magnetic field sensing and spintronics applications.

The PrCrO3 ceramic compound was prepared by the sol-gel route. The optical properties of this ceramic were characterized by UV–Vis absorption spectroscopy. By direct investigation of the UV absorption measurements and using the Tauc model, we have determined the direct optical band gap estimated between 3.26 and 3.20 eV. Furthermore, the Urbach energy, the optical extinction coefficient and the refractive index were calculated on the basis of absorbance measurements. The skin depth, the optical conductivity as well as the dispersion energy parameters as a function of the wavelength of the incident photon were determined and discussed. The emissions are attributed to the d-d transitions of Cr3+ ion in PrCrO3. A theoretical crystal-field analysis of the visible lines associated to the Cr3+(3d3) ion transition occupying an Oh site symmetry in PrCrO3 is realized which leads to a satisfactory correlation between experimental and calculated energy levels. The electronic structure was compared with the reported for Cr3+ ion in RECrO3 (RE = Er, Tm, Yb, Lu, Y) ceramics. The obtained results show that our compound may be proposed as a good candidate for opto-electronic devices.

The structural, optical, electrical and dielectric properties of PrCrO3 perovskite prepared through the sol-gel method were investigated in details. Our compound was characterized by X-ray powder diffraction (XRD) at room temperature. It has been demonstrated that it crystallizes in the orthorhombic system with Pnma space group. The infrared (IR) spectrum highlighted a set of absorption peaks that can be attributed to the O–Cr–O and Cr–O vibrations. The reflectance spectrum, determined by UV–Vis absorption spectrometry, provides the direct optical band gap Eg = 3.24eV of the studied structure. The electrical and dielectric study was carried out by impedance spectroscopy (IS) in a wide frequency (40–106Hz) and at a temperature varying from 160 to 440 K. DC conductivity measurements indicate that PrCrO3 discloses a semiconductor behavior with a semiconductor to metal transition estimated at TMS = 400 K. AC conductivity follows Jonscher's equation. Nyquist plots reveal the existence of two well separated relaxation contributions related to the grains at high frequencies and grain boundaries at low frequencies. Furthermore, colossal dielectric permittivities were detected, showing the importance of this compound in electronic devices.

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In this paper, we have investigated the electrical and magnetic response of a La0.4Gd0.1Ca0.5MnO3 polycrystalline sample. This sample seems to exhibit fascinating phenomena like charge ordering, magnetic phase separation, training effects and kinetic arrest. It also shows colossal values of negative magnetoresistance (∼91.7% at 96 K under 1 T applied magnetic field), which raises the possibility of using this sample for technological applications. We have also proposed, in this work, a new empirical model to describe the evolution of resistivity and magnetoresistance as a function of magnetic field. This model was successfully tested on the La0.4Gd0.1Ca0.5MnO3 sample in spite of its complicated magnetic behavior, which suggests the use of this model for other magnetic samples in order to check its validity.

Structural, magnetic, magnetocaloric, and electrical properties of Pr0.55Sr0.45−x Na x MnO3 (x = 0.05 and 0.1) have been investigated. Our samples were elaborated using the conventional ceramic method at high temperature. Rietveld refinement of the X-ray diffraction patterns shows that all our compounds are single phase and crystallize in the orthorhombic structure with Pbnm space group. Low field magnetic measurements indicate that all investigated samples exhibit a paramagnetic–ferromagnetic transition with decreasing temperature. This transition takes place at room temperature. A large magnetocaloric effect has been observed in all our compounds. The maximum value of the magnetic entropy change is found to be 3.87 J K−1 kg−1 for the sample with x = 0.05 under 5T. The values of relative cooling power (RCP) are 246.52 J kg−1 for x = 0.05 and 262.73 J kg−1 for x = 0.1, respectively, for an applied magnetic field of 5T. The electrical resistivity measurements reveal a metal–semiconductor transition with increasing temperature with the presence of a resistivity minimum at low temperature which is attributed to the electron–electron columbic interactions.

We have investigated the effect of gadolinium doping on the structural and electrical properties of Pr0.5−x Gd x Sr0.5MnO3(x=0.0−0.1) manganites. Our samples have been elaborated using the solid-state reaction, at high temperature. Rietveld refinement of the X-ray diffraction patterns has shown that all our samples are single phase and crystallize in the orthorhombic structure with Pbnm space group. Scanning electron microscopy (SEM) revealed that the grains are irregularly spherical-like shaped with mean sizes of 380, 450, and 460 nm for x=0,0.05, and 0.1, respectively. Increasing Gd content enhances electrical resistivity values. Extrinsic magnetoresistance is observed at low temperature.