Room-temperature large magnetocaloric effect and critical behavior in La0.6Dy0.1Sr0.3MnO3

The effect of dysprosium incorporation in La_0.7Sr_0.3MnO₃ perovskite manganite on its magnetic properties, magnetocaloric effect and critical behavior was investigated. The temperature dependent magnetization data exhibit a sharp paramagnetic–ferromagnetic transition at T_C=307 K, which nature has been identified to be a second-order transition by the scaling laws for magnetocaloric effect. The maximum magnetic entropy change and the relative cooling power are found to be, respectively, 8.314 J/kg K and 187 J/kg for a 5 T magnetic field change without a hysteresis loss, making this material a promising candidate for magnetic refrigeration at room temperature. To study the critical behavior of the paramagnetic–ferromagnetic transition, some related critical exponents (β, γ, and δ) have been also calculated. The values of critical exponents indicate that the present phase transition does not belong to the common transition classes but shows some abnormal variation. We suggest that the induced lattice disordering and magnetic disordering due to Dysprosium incorporation are essential reasons for the presence of a large magnetocaloric effect and of an anomalous ferromagnetic phase transition in the present material     

Large magnetocaloric and magnetoresistance effects in metamagnetic Sm0.55(Sr0.5Ca0.5)0.45MnO3 manganite

The metamagnetic transition, magnetocaloric and magnetoresistance effects are investigated in polycrystalline Sm_0.55(Sr_0.5Ca_0.5)_0.45MnO₃ (SSCMO) manganite. A sharp magnetization jump at Curie temperature (T_C) 73.5 K with large thermal hysteresis is observed. Magnetic measurements and Arrott plots analysis indicate that the transition is first order in nature. Under a low magnetic field change of 1 T, the magnetic entropy change exhibits a peak value of $-\Delta S_M^{\max }(T)$=4.01 J/kg K with relative cooling power (RCP) value of 44.1 J/kg in the vicinity of T_C. The mechanism of charge conduction in insulator phase is polaron transport. Large negative magnetoresistance ratio with value of ~99% is obtained within a broad temperature range below metal insulator transition temperature under 1 T magnetic field. These results indicate the potential applications of SSCMO in magnetic refrigeration and spintronics devices.     

The effect of Co doping on the magnetic and magnetocaloric properties of Pr0.7Ca0.3Mn1−xCoxO3 manganites

In this work, we report the effect of low amount of cobalt doping at the Mn-site on the magnetic and magnetocaloric properties of Pr_0.7Ca_0.3Mn_1−xCo_xO₃ (0≤x≤0.1) powder samples. Our samples, elaborated using the solid–solid reaction method at high temperature, are single phase and crystallize in the orthorhombic system with Pnma space group. While the parent compound Pr_0.7Ca_0.3MnO₃ exhibits a charge order state at low temperature, the substituted samples with low amount of cobalt exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature T_C increases with Co content from 105 K for x=0 to 116 K for x=0.1. The maximum values of the magnetic entropy change $\left|\mathrm{\Delta }{S}_{\mathrm{M}}^{\max }\right|$ are found to be 0.8 J/kg K, 2.2 J/kg K, 3.1 J/kg K and 3.2 J/kg K in a magnetic field change of 5 T for x=0, 0.02, 0.05 and 0.1 respectively. The maximum value of the relative cooling power RCP is found to be 378.2 J/kg in the Pr_0.7Ca_0.3Mn_0.95Co_0.05O₃ at 5 T. This value of RCP is about 92% of that obtained in gadolinium metal, known as one of the most important materials for magnetic refrigeration, at the same magnetic field change of 5 T.     

Study of physical properties of cobalt substituted Pr0.7Ca0.3MnO3 ceramics

An investigation on single phase semiconducting polycrystalline Pr_0.7Ca_0.3MnO₃ and Pr_0.7Ca_0.3MnCo_0.1O₃ crystallized in the orthorhombic system with Pnma space group is reported. We found that σ_DC increases when introducing Co for T<110 K but for T>110 K, it decreases. Also, the contribution of hopping process in conduction mechanism was in agreement with the Jonscher law and Mott theory. Capacitance was extensively dependent on temperature and frequency. A dielectric transition was observed at T=150 K for the doped compound. The temperature dependence of dielectric permittivity is well described by Curie–Weiss law. The parameter of deviation from Curie–Weiss behavior to modified Curie–Weiss law is found to be ΔT_m=30 K. The substitution of Mn by Co was found to destroy the charge order state observed in the parent compound and to induce a ferromagnetic phase at low temperature. The cobalt-substituted sample exhibits a maximum value of magnetic entropy change |∆S^max|=3.2 J kg⁻¹ K⁻¹and a large relative cooling power with a maximum value of 301 J/kg under an applied field of 5 T. Technically, these large values make the prepared material very promising for magnetic refrigeration.     

Influence of Ce addition on the structural,magnetic, and magnetocaloric properties in La0.7−xCexSr0.3MnO3 (0≤x≤0.3) ceramic compound

We report improvement in the magnetocaloric properties of Ce-doped lanthanum manganites. Polycrystalline La_0.7−xCe_xSr_0.30MnO₃ (0≤x≥0.3) samples were prepared using the conventional solid-state reaction method with phase purity and structure confirmed using X-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal second order ferromagnetic transition in parent sample and as well as in doped sample with Curie temperature decreasing progressively with increasing Ce-concentration from ~370 K for x=0.0 to 310 K for x=0.30. Magnetic entropy change (ΔS_M) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field dependent magnetization curves. A large ΔS_M associated with the ferromagnetic–paramagnetic transition in La_0.7−xCe_xSr_0.30MnO₃ samples has been observed. The value of ΔS_M was found to increase with Ce-doping up to x=0.15 and the highest value of 2.12 J kg⁻¹ K⁻¹ (at ΔH=2 T) was observed for La_0.55Ce_0.15Sr_0.30MnO₃ sample near the Curie temperature of 356 K. Also, improved relative cooling power of ~122 J kg⁻¹ was observed for the same sample. Due to the large magnetic entropy change and high Curie temperature, the La_0.55Ce_0.15Sr_0.30MnO₃ sample is suggested to be used as potential magnetic refrigerants for magnetic refrigeration technology above room temperature.     

Multilayer 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 elements for electrocaloric cooling

Electrocaloric (EC) cooling elements in the form of multilayers (MLs) were prepared. The elements consist of five layers of the relaxor-ferroelectric 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃, about 60 μm thick, with internal platinum electrodes and exhibiting a dense, uniform microstructure with a grain size of 1.7 μm. The largest temperature change ΔT_EC of 2.26 K was achieved at an electric field (E) of 100 kV cm⁻¹ and at 105 °C, measured by a high-resolution calorimeter. These results agree well with the indirect measurements. The EC coefficient, ΔT_EC/ΔE, obtained for the MLs, is similar to the value obtained for bulk ceramics of the same composition. The ΔT_EC values above 2 K over a broad temperature range from 75 to 105 °C make the ML elements suitable candidates for EC cooling devices at significantly lower voltages than bulk ceramic plates with comparable dimensions and mass.     

Structure,magnetocaloric and critical properties of layered La2Sm0.4Sr0.6Mn2O7 perovskite

We report the structure, magnetocaloric effect, and critical phase transition in the manganite La₂Sm_0.4Sr_0.6Mn₂O₇ (LSSMO) synthesized by a sol-gel method. X-ray diffraction together with Rietveld refinement show that the sample crystallizes in a Sr₃Ti₂O₇-type tetragonal structure with a space group of I4/mmm. This compound undergoes a second-order ferromagnetic (FM) to paramagnetic phase transition at T_C=348 K and shows strong FM properties below the T_C. Based on the data of isothermal magnetization measured around the T_C and Maxwell's relation, we calculated the maximum magnetic entropy change (-ΔS_M^max) to be 4.69 J kg⁻¹ K⁻¹ and the relative cooling power to be 233.9 J kg⁻¹ for a μ₀ΔH=5 T magnetic field variation. These results indicate that LSSMO can be considered as a potential candidate material for application in magnetic refrigeration above room temperature. The critical behavior near the T_C was studied through the analysis of the magnetic field dependence of the magnetic entropy change and Widom's scaling relation. The exponent values estimated in this work are fairly close to those theoretically predicted by mean field theory (β=0.5, γ=1.0, and δ=3.0), revealing that long-range FM ordering exists in LSSMO. Scaling law theory also confirms the validity of the deduced critical exponents.     

The dependence of magnetic properties on temperature for rare earth ErCrO3 chromites

Multiferroic ErCrO₃ was synthesized and the detailed magnetic as well as ferroelectric properties were investigated. The dc magnetization shows that ErCrO₃ undergoes a antiferromagnetic ordering at T_N = 133 K due to the Cr³⁺–Cr³⁺ followed by weak ferromagnetic ordering. Around T_SR ≈ 22 K, ErCrO₃ exhibits a spin reorientation from Γ₄ to Γ₁. And the stability of the ferromagnetic Γ₄ phase increases with the applied magnetic field increasing. Furthermore, at lower temperature, it shows weak antiferromagnetic ordering of Er³⁺. We also present the low temperature polarization data for ErCrO₃ and find a remarkable decreasing of polarization around T_N = 133 K on increasing temperature, this effect might be due to the coupling between magnetic and ferroelectric order parameters, and the magnetic field suppresses the polarization which demonstrates convincingly the strong magnetoelectric (ME) coupling in ErCrO₃.     

Structure and magnetic behavior of Zn doped NdMnO3 manganite: Neutron diffraction study

Effect of Zn doping on the structural and magnetic properties of NdMnO₃ has been investigated by neutron diffraction and dc magnetic susceptibility measurements. The partial replacement of Mn³⁺ by Zn²⁺ results in the decrease in T_N. In the temperature dependent magnetization measurements, a broad hump and a sharp peak has been observed around ~ 50 K and 10 K respectively for both the samples. Thermal hysteresis in magnetization between cooling and heating runs indicate first-order phase transition. Magnetization measurements on NdMn_0.95Zn_0.05O₃ sample clearly show that, 5% Zn-doping in NdMnO₃ results in the suppression in magnetism which is evident from the weakening of Nd–Mn interaction below T_N, and resulting in antiferromagnetic coupling of Mn³⁺ ions along x-axis with Mn³⁺ moments oriented parallel or antiparallel to the x-component of Nd³⁺ moments.     

Effects of Sr-doping on the giant magnetocaloric effect of EuTiO3

The magnetic properties and magnetocaloric effect of Eu_1−xSr_xTiO₃ (x=0–0.1) compounds are investigated. With slight Sr-doping, the ferromagnetic (FM) coupling significantly increased and FM exchange is dominant in the delicate balance. A giant reversible magnetocaloric effect (MCE) and large refrigerant capacity (RC) for Eu_1−xSr_xTiO₃ compounds were observed. The values of −ΔS_M^max are evaluated to be around 10 J/kg K under a magnetic field change of 1 T and 21 J/kg K under a magnetic field change of 2 T, respectively. But, the values of RC are increased with the more Eu in EuTiO₃ to be substituted by Sr. Therefore, the giant reversible MCE and large RC make the Eu_1−xSr_xTiO₃ compound a good candidate for magnetic refrigerant working at low-temperature and low-field.     

Role of spin-glass phase for magnetoresistance enhancement in nickel substituted lanthanum calcium manganite

Effect of Ni substitution in lanthanum calcium manganite (LCMO) has been investigated for change in magnetoresistance (MR). Scanning electron microscopy images revealed decrease in grain size from 3.72 µm to 0.55 µm by Ni substitution. Maximum increase in MR has been found 28% at low temperature (100 K) for x=0.10, Ni substitution at Mn site. Metal insulator transition temperature has been decreased from 253.2 K for x=0.0–90 K for x=0.10. Above x=0.10, Ni substitution no metal-insulator transition temperature appeared due to the presence of porosity in the samples. Ni substitution lowered the magnetic transition temperature from 255 K for x=0.0–125 K for x=0.25. Lowering of irreversible temperature (T_irr) from 250 K for x=0.0–135.4 K for x=0.20 has been obtained by zero field cooled (ZFC) and field cooled (FC) measurements confirm reduction of ferromagnetic clusters and spin-glass phase like behavior due to Ni presence. The spin-glass phase presence allows spin-polarized tunneling even at low magnetic field, which ultimately results in enhancement of MR at low temperature. Core level X-ray photoelectron spectroscopy measurements confirm Ni²⁺ charge state of Ni ions and increase in Mn⁴⁺/Mn³⁺ ratio with increasing Ni content. Increase in resistivity and weakening of ferromagnetism with Ni substitution at Mn site has been observed due to the reduction in grain size and dilution of double exchange interaction.     

Magnetic field induced delocalization in multi-wall carbon nanotube-polystyrene composite at high fields

In well dispersed multi-wall carbon nanotube-polystyrene composite of 15 wt%, with room temperature conductivity of ∼5 S/cm and resistivity ratio [R_2 K/R_200 K] of ∼1.4, the temperature dependence of conductivity follows a power-law behavior. The conductivity increases with magnetic field for a wide range of temperature (2–200 K), and power-law fits to conductivity data show that localization length (ξ) increases with magnetic field, resulting in a large negative magnetoresistance (MR). At 50 T, the negative MR at 8 K is ∼13% and it shows a maximum at 90 K (∼25%). This unusually large negative MR indicates that the field is delocalizing the charge carriers even at higher temperatures, apart from the smaller weak localization contribution at T < 20 K. This field-induced delocalization mechanism of MR can provide insight into the intra and inter tube transport.     

Phase–composition and temperature dependence of electrocaloric effect in lead–free Bi0.5Na0.5TiO3–BaTiO3–(Sr0.7Bi0.2□0.1)TiO3 ceramics

The (0.94–x)Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–x(Sr_0.7Bi_0.2□_0.1)TiO₃ (BNT–BT–xSBT, 0 ≤ x ≤ 0.24) solid solution ceramics were synthesized via a conventional solid–state reaction method and the correlation of phase structure, piezoelectric, ferroelectric properties and electrocaloric effect (ECE) was investigated in detail. The ECE in lead–free BNT–BT–xSBT ceramics was measured directly using a home–made adiabatic calorimeter with maximum adiabatic temperature change ΔT = 0.4 K with x = 0.08 under the electric field E = 6 kV/mm at room temperature. The position of maximum ECE was found in the vicinity of nonergodic and ergodic phase boundary, where the maximum change in entropy occurs as a result of the field–induced phase transformation between the ergodic and long–range ferroelectric phase. Besides, the mechanism for the shift of ECE peak is discussed in detail. Finally, the temperature dependence of ECE for BNT–BT–xSBT (x = 0, 0.04 and 0.08) was also investigated. This work may present a guideline for designing BNT–based ferroelectric relaxor ceramics for EC cooling technologies.     

Structural,morphological, and magnetic properties of sol-gel derived La0.7Ca0.3MnO3 manganite nanoparticles

La_0.7Ca_0.3MnO₃ (LCMO) manganite nanoparticles are synthesized via a sol-gel route at different annealed temperatures. Their structural, morphological, and magnetic properties are investigated. The X-ray diffraction patterns coupled with electron diffraction confirm that all the LCMO samples are single phase and crystallize in the orthorhombic perovskite structure (Pnma space group). The morphology of the samples observed by TEM, reveals a spherical shape with an average grain size lower than 50 nm. The resolved lattice fringes in high-resolution TEM images also reveal the single crystalline nature of the LCMO nanoparticles. Magnetization measurements versus temperature under low magnetic field (0.01 T) show a paramagnetic - ferromagnetic transition for all the samples. The Curie temperature (T_c) is found to be decreased with increasing the annealed temperature. A bifurcation is observed in the zero field-cooled and field-cooled magnetizations, indicating a competition between ferromagnetic and antiferromagnetic interactions in the nanoparticles at low temperatures. Field-cooled hysteresis measurements suggest a cluster glasslike behavior of the nanoparticles. Room temperature and low temperature M - H loops demonstrate that all the samples exhibit ferromagnetic behavior at 5 K, whereas a paramagnetic behavior at room temperature. Resistivity behavior of the LCMO samples shows that they exhibit a metal - insulator transition. Magnetoresistance of ~ 50% at the field up to 8 T was observed at 2 K in the LSCO samples annealed at 600 °C.     

Enhanced electrical properties in lead-free NBT–BT ceramics by series ST substitution

Comprehensive electrical properties of 0.94(Na_1/2Bi_1/2)TiO₃–0.06BaTiO₃ lead-free ceramics by doping series SrTiO₃ were investigated. High piezoelectric constant of 205 pC/N and electromechanical coupling factor of 0.34 were obtained due to the forming of the rhombohedral–tetragonal morphotropic phase boundary at x=0.02–0.06. Very large recoverable strain of 0.34% was obtained at x=0.10 due to the coexistence of ferroelectric and relaxor pseudocubic phases. A large electrocaloric effect (ΔT_max=1.71 K and ΔT/ΔE=0.34 K mm kV⁻¹ at 50 kV cm⁻¹) which determined by indirect measurements method was obtained at 120 °C at x=0.02, which is significantly higher than that of lead-free ferroelectric ceramics reported so far. Moreover, lower operating temperatures of 50 °C and 30 °C were proposed when x=0.10 and 0.20 with ΔT_max=0.79 K and 0.6 K, respectively. These properties added together indicate a promising material for applications in cooling systems and actuators.     

Influence of the electrohydrodynamic process on the properties of dried button mushroom slices: A differential scanning calorimetry (DSC) study

In this paper, drying of button mushroom (Agaricus bisporus) slices with an innovative drying technique of hot air combined with Electrohydrodynamic (EHD) drying process was investigated at three electrode gaps (5, 6, and 7 cm) and voltage levels (17, 19, and 21 kV). The effects of different hot air combined with EHD drying treatments on the temperature of the mushroom slices, drying time, final color and protein denaturation features including enthalpy (ΔH), onset temperature (T_o), peak transition temperature (T_p), conclusion temperature (T_c), and temperature range (T_c–T_o) of endothermic peaks were systematically evaluated. In addition, water state changes in DSC cooling thermograms of dried mushroom slices were investigated. The results obtained by differential scanning calorimetry showed that the ΔH values in the DSC traces of the EHD-dried mushroom slices were reduced with a decrease in the electrode gap and an increase in the voltage. Specifically, among voltages of 21, 17, and 19 kV, a voltage of 21 kV resulted in the lowest ΔH and T_c–T_o values and the highest T_p and T_o values. This result indicated that voltage had a significant effect on these responses. Similarly, the DSC results showed a considerable effect of high electric field intensity on ΔH, T_c–T_o and T_p responses related to protein denaturation in comparison to low electric field intensity.     

Crystal structure,magnetic properties,and magnetocaloric effect in B-site disordered RE2CrMnO6 (RE = Ho and Er) perovskites

In this work, polycrystalline oxides viz., Ho₂CrMnO₆ (HCMO) and Er₂CrMnO₆ (ECMO), were prepared using the sol-gel process, and their crystal structure, magnetic properties, and magnetocaloric effects (MCEs) were studied. X-ray refinement results demonstrate that both oxides exhibited the B-site disordered perovskite type structure (Pbnm space group). A ferromagnetic-paramagnetic phase transition as well as large reversible MCEs was also observed at Curie temperatures (T_C) of ~6.1 and ~5.2 K for HCMO and ECMO, respectively. For a magnetic field change (ΔH) of 0–7 T, the maximum magnetic entropy change (-ΔS_M), temperature-averaged entropy change (TEC 3), and relative cooling power (RCP) were estimated to be 11.03 J/kgK, 11.02 J/kgK, and 322.7 J/kg for HCMO, and 12.94 J/kgK, 12.80 J/kgK and 277.5 J/kg for ECMO.     

Magnetocaloric properties of Gd2MoO6 prepared by a simple and fast method

The experimental study of the specific heat, magnetic susceptibility, and magnetization of Gd₂MoO₆ powder sample was performed in the temperature range between 0.4 K and 300 K in the magnetic fields up to 9 T. Powder sample was prepared via nonconventional mechanochemical/thermal process from powdered oxide precursors. Magnetic ion Gd³⁺ with spin S = 7/2 is responsible for the magnetic properties. The specific heat study in zero magnetic field revealed two anomalies; a phase transition to the ordered state at T_N1 ≈ 0.98 K and a smaller maximum at T_N2 ≈ 0.6 K. Relatively high magnetic density 6.85 g/cm³ of Gd₂MoO₆ predetermines this compound as potential magnetocaloric material with a high cooling performance at cryogenic temperatures. A large conventional magnetocaloric effect was found around 3 K with magnetic entropy change ?ΔS_max ≈ 44 J/(kg K) (300 mJ/(Kcm³)) for magnetic field change from 9 T down to zero with a refrigerant capacity of 464 J/kg. Maximal temperature change -ΔT_ad ≈ 23.8 K was found for initial temperature T_init ≈ 29.9 K for the mentioned change of magnetic field. Alternating susceptibility measurements revealed the presence of two relaxation channels in Gd₂MoO₆ existing in two different time scales ≈ 10^-3 s and ≈ 1 s.     

Structural,magnetic and magnetocaloric properties of CoFe2−xMoxO4 (0.0≤x≤0.3) ferrites

We have investigated structural, magnetic and magnetocaloric properties of CoFe_2-xMo_xO₄ (0.0≤x≤0.3) ferrites. Polycrystalline samples were prepared by the sol gel method and characterized by the powder X-ray diffraction and scanning electron microscopy. X-ray diffraction patterns show that all samples have a cubic spinel structure and the lattice parameter, a, decreases monotonically with increase in Mo concentration. Scanning electron micrographs indicate that most of the particles are in the range of 400–850 nm size. Magnetic measurements, performed by using a cryogen free vibrating sample magnetometer, show that these samples are soft ferromagnets in the measured temperature range. The saturation magnetization, M_s, values of Mo-doped samples are larger than the parent compound with a maximum value of ~106 emu/g for x=0.2 sample. The magnetic entropy change (−ΔS) increases with increase in applied magnetic field and shows a peak in the vicinity of blocking temperature. A maximum value of 0.56 J kg⁻¹ K⁻¹ at 5 T field has been observed for x=0.2 sample.