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.     

Beneficial effect of Mn-substitution on magnetic and magnetocaloric properties of La0.5Sr0.5CoO3 ceramics

In this paper, we present studies of the influence of B-site substitution of Mn (5% and 10%) on magnetic and magnetocaloric properties of La_0.5Sr_0.5CoO₃. Polycrystalline samples of La_0.5Sr_0.5Co_1-xMn_xO₃ (x = 0, 0.05, 0.1) are synthesized via solid state reaction route. Ferromagnetism is observed in all the compounds with enhancement in magnetisation and simultaneous effect of lowering of T_C from 243 K to 235 K through increase in Mn content from 0 to 10%. Ferromagnetic hysteresis is noticed in all cases with significant rise in the value of remanent magnetisation (M_r) from 4180 emu/mol for x = 0–5750 emu/mol for x = 0.1. Magnetocaloric effect (MCE) is observed with peaks in temperature dependent magnetic entropy change curves (-ΔS_M ~ T) appearing at T_C. However, the peaks undergo shift to higher temperatures for higher field changes due to possible augmented short range interactions among ferromagnetic clusters taking place above T_C that effectively modifies T_C to a larger value. Both $-\Delta {\text{S}}_{\text{M}}^{\text{max}}$and RCP (Relative Cooling Power) record increase with values lying in the ranges 2.64–2.74 J/kg/K and 159.48–177.66 J/kg respectively for ΔH = 6 T corresponding to Mn content x = 0–0.1. Such remarkable features predict their potential application in magnetic refrigeration technology. Moreover, these materials are quite exciting from basic physics point of view.     

Effects of synthesis route on the structural,magnetic and magnetocaloric properties of Pr0.8K0.2MnO3

Pr_0.8K_0.2MnO₃ ceramics are prepared by solid-state reaction and Pechini sol–gel method. The influence of the powder synthesis method on the structural, morphological, magnetic and magnetocaloric properties of the samples are investigated. The X-ray diffraction pattern shows reflections typical of the perovskite structure with orthorhombic symmetry. The experimental results reveal that both samples undergo a second-order phase transition. The maximum magnetic entropy changes, deduced from the M–µ₀H measurements, are 3.77 and 7.23 J/kg K under the magnetic field change of 5 T for Pr_0.8K_0.2MnO₃ synthesized by using solid-state reaction and sol–gel methods respectively. The field dependence of magnetic entropy change is analyzed showing the power law dependence, ΔS(T,µ₀H)=a(T)(μ₀H)^n(T,H) at the Curie temperature. Using the scaling laws of ∆S, the experimental ∆S collapse onto a universal curve for both ceramics. These results suggest that the physical properties of our samples are strongly depended on synthesis techniques. It is found that Pechini sol–gel method is more efficient and stable to obtain ceramic materials with good magnetic properties. Consequently, a substantial increase of the ferromagnetic to paramagnetic transition temperature and an enhancement of magnetocaloric properties are observed in the sol-gel made sample making it more suitable for magnetic refrigeration applications.     

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.     

Structural,magnetic and magnetocaloric properties of the rare earth (RE) molybdate RE2MoO6 (RE = Dy,Tb and Gd) oxides

Reported here is a systematical investigation on the crystal structure, magnetic properties and magnetocaloric (MC) effect of three rare earth (RE) molybdate RE₂MoO₆ (RE = Dy, Tb and Gd) oxides. The X-ray powder diffraction and the morphology as examined using the scanning electron microscope indicate phase-pure and polycrystalline nature of these oxides. The temperature (2–100 K) and magnetic field (up to 5 T) dependence of the magnetic measurements determine the magnetic phase transition (MPT) and MC properties. All the present RE₂MoO₆ oxides crystallize in a monoclinic structure belonging to C2/c space group with the antiferromagnetic ordering at low temperature. Moreover, the RE₂MoO₆ oxides hold reasonable values of MC parameters including the maximum isothermal magnetic entropy change/temperature-averaged entropy change (2 K lift) and relative cooling power values have been evaluated with the magnetic change of 0–5 T, yielding 17.22 (17.08) J/kgK and 277.67 J/kg for Dy₂MoO₆, 17.03 (16.83) J/kgK and 261.12 J/kg for Tb₂MoO₆, as well as 27.68 (26.69) J/kgK and 228.14 J/kg for Gd₂MoO₆, respectively. These acceptable MC parameters make the present RE₂MoO₆ oxides potential candidates for cryogenic magnetic refrigeration (MR).     

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.     

Dependence on sintering temperature of structure,optical and magnetic properties of La0.625Ca0.315Sr0.06MnO3 perovskite nanoparticles

In this study, La_0.625Ca_0.315Sr_0.06MnO₃ (LCSMO) nanoparticles were prepared by facile sol-gel method at low crystallization temperatures. Various test methods were used to characterize structure, optical and magnetic properties of LCSMO nanoparticles. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) suggested complete crystallization of LCSMO nanoparticles sintered at 700 °C. In addition, unit cell volume and grain size increased with sintering temperature. Besides, X-ray photoemission spectroscopy (XPS) fitting results of Mn2p core level peaks confirmed the increase in Mn³⁺ ion concentration with sintering temperature, mainly attributed to formation of more oxygen vacancies. Raman microscopy and Fourier transform infrared spectrometry (FTIR) jointly depicted the existence of Mn–O bond, indicating that sintering temperature definitely impacted vibration mode of Mn–O and affected both crystal structure and performance. UV–vis optical band gap width of LCSMO nanoparticles sintered at 700 °C, 1000 °C, and 1500 °C decreased from 1.2 to 0.75 eV as sintering temperature increased, suggesting the semiconducting properties of nanoscale LCSMO particles. Magnetization dependent temperature (M-T) and magnetic field (M-H) measurements revealed degradation in magnetic properties of the specimens with temperature. Overall, LCSMO nanoparticles sintered at different sintering temperatures provided novel insights into properties of rare earth doped perovskite manganites.     

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.     

Study of magnetic and magnetocaloric effect of REMnO3(RE=Dy,Eu)manganites

We have systematically investigated the magnetic and magnetocaloric effect (MCE) of REMnO₃ (RE = Dy, Eu) manganites sintered by solid-state reaction method. The Neel temperature (T_N), the maximum magnetic entropy change (-ΔS_m^max) and the relative cooling power (RCP) of the sample were obtained by measuring the magnetization and heat capacity. Under the magnetic field change of 50 kOe, large -ΔS_m^max of 8.93J/kg K and 9.31J/kg K are achieved around the T_N of 14 K and 50 K for DyMnO₃ and EuMnO₃, respectively. It is proved by the slope of the Arrott plot and the universal phenomenological curve that the sample undergoes second order phase transition. The critical behavior of antiferromagnetic to paramagnetic phase transition is studied by modifying Arrott plot, which shows that the spin order of the sample decreases at the phase transition temperature and tends to change into shortrange order. The large magnetic entropy change calculated by Landau theory and experimental data indicates that the sample has a large magnetocaloric effect and has potential application in low temperature magnetic refrigeration materials.     

Structural and magnetic properties of Co/Al2O3 cermet synthesized by mechanical ball milling

Cobalt nanoparticles in the alumina matrix were synthesized using high energy mechanical ball milling of Co₃O₄ and Al powders mixture. The effect of ball mill time of 1 up to 12 h on the phase formation and crystalline lattice of the samples was investigated by the fitting of the X-ray diffraction patterns with Fullprof software and Rietveld method. The results show that 6 h milling of the primary powders yields a nanocomposite of Co/Al₂O₃ cermet. The formation of Co/Al₂O₃ nanocomposite was confirmed by a morphological study using scanning electron microscopy and transmission electron microscopy. The prepared nanocomposite by 12 h ball mill time has ferromagnetic properties with a high saturation magnetization value of 118 emu/g. Also, using Henkel plot analysis, it was shown that there are strong dipole-dipole magnetic interactions between the prepared cobalt nanoparticles in the Al₂O₃ matrix.     

Preparation and absorbing properties of flaky GR/Fe95Si1B2P0.5Cu1.5 composite powders

In this study, flaky Fe₉₅Si₁B₂P_0.5Cu_1.5 powders with a particle size of 48–75?μm were compounded with graphene (GR) by high-energy ball milling to prepare GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite powders. The electromagnetic parameters of GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite powders in the frequency band of 0.3–8.5?GHz were determined by a vector network analyser. The influence of milling time and graphene content on the electromagnetic parameters of GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite powders was studied. The results showed that with the increase of graphene content, magnetic permeability and dielectric constant increase simultaneously. The 1.0?wt% GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite powders prepared by milling for 24?h exhibited the best absorbing property. In the frequency band of 0.3–8.5?GHz, for 1.0?wt% GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite, the dielectric constant was 182.4–18.2, and the magnetic permeability was 7.3–1.0. The dielectric loss tangent was 3.40–6.60, the magnetic loss tangent was 0.48–3.46, and their sum was 5.28–8.60; therefore, 1.0?wt% GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite exhibited excellent absorbing properties.     

Structural,magnetic and field-driven abrupt magnetocaloric properties of La1.4-xSmxCa1.6Mn2O7 Ruddlesden-Popper manganites

Evolution in the structural, magnetic and magnetocaloric (MCE) properties for a series of samarium (Sm) doped La_1.4-xSm_xCa_1.6Mn₂O₇ (0.0 ≤ x ≤ 0.4) Ruddlesden-Popper manganites prepared by using the solid-state reaction method have been investigated. Structural refinement of XRD profiles established that the MnO₆ octahedral structural units were distorted upon Sm-substitution. The effect of Sm-concentration on the microstructure was also very well observed in terms of increased average grain size and absolute density. Discrepancy in MCE parameters by implementing the conventional isothermal curve method was appeared which was resolved with a separate calorimetric method. The isothermal entropy change was enhanced from 2.52 J/kgK for x = 0.0 sample up to 4.61 J/kgK and 4.2 J/kgK for x = 0.1 and x = 0.2 samples respectively. The temperature averaged entropy change (TEC) was also remarkably improved from 4.04 J/kgK up to 8.5 J/kgK and 7.77 J/kgK for the same compounds respectively.     

Correlation between micro/nano-structure,mechanical and tribological properties of copper-zirconia nanocomposites

Fine-grained copper (Cu) and copper-zirconia (Cu–ZrO₂) nanocomposites were produced by high-energy ball milling up to 20 h. Scan Electron Microscope (SEM), Transmission Electron Microscope (TEM), X-Ray Diffraction (XRD), microhardness, wear rate and coefficient of friction measurements were performed to investigate the correlation between micro/nano-structure changes of powder and consolidated samples and the properties of the produced nanocomposites. Cu and Cu–15%ZrO₂ nanocomposites with 49.3 and 24.4 nm crystal size, respectively, were produced after 20 h milling achieving 1.76- and 3-times larger hardness than the as received Cu. The wear rate of milled Cu was slightly decreased than the as received Cu, however, it was highly reduced for Cu–15%ZrO₂ nanocomposites reaching 10-times lower than the as received Cu. SEM, TEM and XRD analysis revealed that four main strengthening mechanisms lead to the great improvement of Cu–ZrO₂ nanocomposites properties. The major strength improvement occurred due to Orowan and dislocation strengthening mechanisms activated by the well dispersion of ZrO₂ nanoparticles in Cu matrix and their impedance to dislocation movement, respectively. Besides these two main strengthening mechanisms, work hardening and grain refinement acted as minor strengthening mechanisms for Cu–ZrO₂ nanocomposites while they are the main strengthening mechanisms of Cu samples.     

Effect of Fe doping on structure,magnetic and electrical properties La0.7Ca0.3Mn0.5Fe0.5O3 manganite

The crystal and magnetic structures of La_0.7Ca_0.3Mn_0.5Fe_0.5O₃ compound have been studied by neutron powder diffraction in the temperature range of 10–300 К. The magnetization and electrical resistivity measurements have been also performed in the temperature range of 5–300?K in magnetic fields up to 1?T. These experimental results indicate a formation of a complex magnetic state in which the long-range antiferromagnetic G-type phase coexists with the short-range ferromagnetic clusters. The electrical conductivity of La_0.7Ca_0.3Mn_0.5Fe_0.5O₃ demonstrates an anomalous temperature behavior suggesting a switching between different states. The origin of the unconventional magnetic state, the mechanisms of the electrical conductivity, and correlation between magnetic and transport properties in this manganite have been discussed.     

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     

Synthesis and multiferroism in mechanically processed BiFeO3–PbTiO3 ceramics

In this study, (1 − x)BiFeO₃–(x)PbTiO₃ multiferroic ceramics, with x = 0, 0.1, 0.2, 0.25, 0.3 and 0.4, were processed through high-energy ball milling followed by reactive sintering in air atmosphere. The optimization of the procedure for the preparation of highly-dense (1 − x)BiFeO₃–(x)PbTiO₃ ceramics was carefully investigated and structural/microstructural effects on ferroic properties were carefully addressed. Shrinkage dilatometric measurements revealed an expansion related to a sintering reaction that has occurred before densification. This sintering behaviour was highly PbTiO₃ concentration-dependent. The sintering mechanism was found to be directly related with the aliovalent substitution of Pb and Ti ions on A and B sites of the perovskite structure. The obtained ceramics were confirmed as ferroelectric ordered in ferroelectric characterizations. Remnant polarizations and coercive fields greatly dependent on grain size distribution and aliovalent substitutions were revealed. The magnetic hysteresis displayed a weak-ferromagnetic behaviour in all studied samples.     

Structural,magnetic and electrical properties of Y1-xScxFeO3 (x= 0, 0.5 & 1) nanoparticles synthesized by the sol-gel method

Nanoparticles exhibiting crystalline and single-phase characteristics with Y_1-xSc_xFeO₃ (x = 0, 0.5 & 1) nanoparticles synthesized by sol-gel method. The sizes of nanocrystals determined by X-Ray Diffractometry (XRD) were obtained between 30 and 39 nm. The shape of nanoparticles and the surface morphology of the samples were carried out using Field Emission Scanning Electron Microscopy (FE-SEM) and energy dispersive spectroscopy (EDS). The magnetic properties of Y_1-xSc_xFeO₃ samples were investigated by Vibrating Sample Magnetometry (VSM) and Field-Cooled (FC) and Zero-Field-Cooled (ZFC) measurements. The results showed antiferromagnetic and paramagnetic behaviors for these prepared perovskites. The electrical properties of the samples were investigated by measuring the polarization-electric loops, dielectric constant changes and dielectric loss through frequency and temperature. These measurements showed the T_N values of the samples to be 628 and 578 K for YFeO₃ and ScFeO₃ nanoparticles, respectively. In addition, diffuse reflectance spectroscopy analysis was used to calculate the band gap energy of the samples based on the Kubelka-Munk function. The achieved values showed the band gap energy of 2.97 and 3.07 eV for YFeO₃ and ScFeO₃.     

Synthesis and magnetic properties of complex oxides La0.67Sr0.33MnO3 nanowire arrays

As a new approach of melt-injection-decomposition method, it has been successfully adopted for the synthesis of the complex oxides La_0.67Sr_0.33MnO₃ nanowire arrays. X-ray diffraction studies confirmed the formation of perovskite manganite phase of the sample. Transmission electron microscope and scanning electron microscope characterizations showed a large quantity of one-dimensional nanowires is obtained and the nanowires are continuous, concrete, oriented and rather uniform with an average diameter of 170 nm and length of several tens of micrometers. Magnetic measurements exhibited good ferromagnetic properties at the temperature of 10 K and 300 K respectively. The transition temperature of the complex oxides La_0.67Sr_0.33MnO₃ nanowire arrays is about 350 K, which will endow their great potential applications in magnetoresistance, spintronics or sensor fields at room temperature.