Cryogenic magnetic properties and magnetocaloric performance in double perovskite Pr2NiMnO6 and Pr2CoMnO6 compounds

Double perovskite Pr₂NiMnO₆ (PNMO) and Pr₂CoMnO₆ (PCMO) ceramics have been synthesized and their crystal structure, microstructure, as well as cryogenic magnetic properties and magnetocaloric performance have been investigated. The crystal structure of PNMO and PCMO ceramics was determined to be monoclinic with space group of P21/n at room temperature. A ferromagnetic to paramagnetic (FM-PM) phase transition (second-order) occurred in PNMO and PCMO around 208 and 170?K, respectively. Significant reversible magnetocaloric effect (MCE) was found and the values of the maximum magnetic entropy change (-ΔS_M^max) were 3.15 and 3.91?J/kg-K under the magnetic field change (ΔH) of 0–7?T in PNMO and PCMO, respectively. The corresponding relative cooling power (RCP) values were 241.9 and 229.9?J/kg.     

Cryogenic magnetic properties and magnetocaloric effects (MCE) in B-site disordered RE2CuMnO6 (RE = Gd,Dy, Ho and Er) double perovskites (DP) compounds

In this paper, a detailed investigation with respect to the structural, cryogenic magnetic properties and magnetocaloric performances of RE₂CuMnO₆ (RE = Gd, Dy, Ho and Er) double perovskite (DP) compounds has been performed. All the RE₂CuMnO₆ compounds are confirmed to B-site disordered and crystallized in the GdFeO₃-type structure (Pnma space group, N 62, oP20). The magnetic transition temperatures (T_M) are found to be ~7.5 K for Gd₂CuMnO₆, ~12.1 K for Dy₂CuMnO₆, ~12.2 K for Ho₂CuMnO₆, and ~3.6 K for Er₂CuMnO₆, respectively. Moreover, for checking the magnetocaloric performances several vital parameters including -ΔS_M^max (peak value of magnetic entropy change, -ΔS_M), TEC (3) (temperature averaged -ΔS_M) and RCP (relative cooling powers) are evaluated to be 7.84, 7.73 J/kgK and 151.1 J/kg for Gd₂CuMnO₆, 5.69, 5.59 J/kgK and 180.9 J/kg for Dy₂CuMnO₆, 7.12, 7.05 J/kgK and 192.4 J/kg for Ho₂CuMnO₆, as well as 9.92, 9.60 J/kgK and 195.9 J/kg for Er₂CuMnO₆ under the magnetic field change ΔH = 5 T, respectively.     

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.     

Effect of sintering temperature on microstructure and magnetic properties of double perovskite Y2CoMnO6

Polycrystalline double perovskite Y₂CoMnO₆ oxides ceramics sintered at four different temperatures from 1000?°C to 1300?°C have been fabricated by conventional sol-gel method. All the Y₂CoMnO₆ compounds are single phase with monoclinic structure (P21/n space group). The mean grain size grows significantly large and the shape becomes regular obviously with increasing sintering temperature. The effect of sintering temperature on magnetic properties of Y₂CoMnO₆ compounds has been studied in detail. We found that the oxygen vacancies are introduced by sintering at high temperature has a certain influence on the magnetic properties. Moreover, the magnetic entropy changes (-?S_M) as well as relative cooling power (RCP) in the double perovskite Y₂CoMnO₆ oxides ceramics around paramagnetic to ferromagnetic transition were also investigated.     

Structure,magnetic properties and cryogenic magneto-caloric effect (MCE) in RE2FeAlO6 (RE = Gd,Dy, Ho) oxides

In this study, we present the crystal structure, magnetic properties, and cryogenic magneto-caloric effect (MCE) of RE₂FeAlO₆ (RE = Gd, Dy, Ho) oxides. The XRD refinement analysis suggests that all the RE₂FeAlO₆ oxides are crystallized in B-site disordered orthorhombic structure. The RE₂FeAlO₆ oxides exhibit large MCEs around T_C. The peak magnetic entropy change (-ΔS_M) and refrigeration capacity (RC) are 25.9 J/(kgK) and 240.1 J/kg for Gd₂FeAlO₆, 10.7 J/(kgK) and 274.9 J/kg for Dy₂FeAlO₆, 9.6 J/(kgK) and 249.6 J/kg for Ho₂FeAlO₆ under ΔH of 0–70 kOe, respectively. Notably, Gd₂FeAlO₆ exhibits promising magneto-caloric performance and therefore is a favorable candidate for cryogenic magnetic refrigeration.     

Structural,magnetic and cryogenic magneto-caloric properties in RE2FeCrO6 (RE = Er and Tm) compounds

In this work, polycrystalline Er₂FeCrO₆ (EFCO) and Tm₂FeCrO₆ (TFCO) oxides were fabricated via conventional sol-gel method, and studied with respect of crystal structure together with cryogenic magnetic and magneto-caloric properties. Both oxides are confirmed to exhibit B-site disordered hexagonal perovskite-type crystal structure. Two magnetic transitions around 11.7 and 5.7 K for EFCO, whereas only one transition around 10.5 K for TFCO, have been observed. Both oxides exhibit considerable cryogenic reversible magneto-caloric effects. The values of magnetic entropy change peak and relative cooling power (refrigerant capacity) with 0–5 T magnetic field change reach 11.95 J/kg-K and 215.8 (169.8) J/kg for EFCO, and 4.78 J/kg-K and 123.6 (97.8) J/kg for TFCO, respectively, indicating the present Er₂FeCrO₆ oxide is also considerable for cryogenic magnetic cooling application.     

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).     

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.     

Study on the critical behavior and magnetocaloric effect of RCrO3 (R = Gd,Dy)

In this study, we aim to examine the critical behavior and magnetocaloric effect of RCrO₃ (R = Gd, Dy) polycrystal synthesized by solid-state sintering. As per the heat capacity fitting curves, GdCrO₃ and DyCrO₃ fit the 3D-XY and 3D-Ising models, respectively. The analysis of heat capacity curves, field cooling (FC), and zero-field cooling (ZFC) curves reveals two magnetic phase transitions in the samples, with T_N^Cr = 174 K, T_N^Gd = 3.7 K and T_N^Cr = 152 K, T_N^Dy = 3.5 K for GdCrO₃ and DyCrO₃, respectively. Under a 50 kOe magnetic field, the maximum magnetic entropy change (−ΔS_m^max) and relative cooling efficiency (RCP) of the sample are 39.9 J/kg K, 373.4 J/kg and 15.9 J/kg K, 276 J/kg for the two samples, respectively. We believe that the significant magnetocaloric effect of GdCrO₃ is associated with its critical behavior. This provides an additional reference for the origin of the large magnetocaloric effect of materials.     

Review of the research on oxides in low-temperature magnetic refrigeration

The discovery of the giant room temperature magnetocaloric effect (MCE) has renewed a remarkable research interest in the field of magnetocaloric materials at low temperatures. Among the various magnetocaloric refrigerants, rare-earth transition-metal oxides are one of the most promising candidates for low-temperature applications. In this study, the magnetic properties of rare-earth transition-metal oxides and their research status are reviewed. Three different systems are introduced in the review: (1) Manganates including RMnO₃, RMn₂O₅ and R₂Mn₂O₇ series; (2) Chromates including RCrO₃ and RCrO₄ series; (3) Vanadates including RVO₃ and RVO₄ series. The strong correlation of magnetocaloric effect of materials with crystal structure and magnetic phase transition is summarized. The origin of large MCE and potential applications of these materials are discussed. The purpose of the review is to provide a new idea for the selection, search and design of magnetic materials with enhanced MCE, and to promote the development of low-temperature magnetic refrigeration technology.     

Electronic transition,ferroelectric and thermoelectric properties of bismuth pyrostannate Bi2(Sn0.85Cr0.15)2O7

The ferroelectric properties of bismuth pyrostannate Bi₂(Sn_0.85Cr_0.15)₂O₇ in the high-temperature region are established. The linear thermal expansion coefficient, electrical resistance, impedance, I?V characteristics, capacitance, loss-angle tangent, charge, and thermopower of the investigated material are measured in the temperature range of 300?700 K at frequencies of 10²?10⁶ Hz. Anomalies of the thermal expansion coefficient and hodograph spectrum variation in the region of polymorphic phase transitions are observed. The high resistance and change of the hopping conductivity for the tunnel-emission are found. The hysteresis in the electric field dependence of polarization is established. The change in the thermopower sign with temperature is revealed. The obtained experimental data are explained in the framework of the model of migration polarization by charged chromium ions.     

Magnetocaloric effect and magnetic properties of the isovalent Sr2+ substituted Ba2FeMoO6 double perovskite

Fine-tuning the charge distribution in Ba₂FeMoO₆ obtained via “isovalent” substitution at the A-site (i.e., Ba) is expected to bring about changes in the physical properties of the system that can be manipulated in magnetic refrigerants. With this motivation, the phase formation, crystal structure, microstructure, magnetic and magnetocaloric properties of the Ba_2−xSr_xFeMoO₆ (0≤x≤0.4) samples fabricated by solid state reaction method have been investigated. The X-ray diffraction analysis confirmed the formation of cubic structure with Fm3m space group in all the fabricated samples. The magnetization measurements and Arrott analysis revealed a second order of ferromagnetic phase transition in all the samples. An increase in magnetization and Curie temperature (T_C) was observed with the increase in Sr-content that was attributed to the increased orbital hybridization and exchange interaction between Fe and Mo ions. The magnitude of the maximum magnetic entropy change at the Curie temperature and the relative cooling power were observed to slightly decrease with the increased Sr doping. The excellent magnetocaloric features and convenient adjustment of Curie temperature make these materials useful for magnetic refrigeration in a wide range of temperature.     

Electrical properties of (Bi2O3)0.75(RE2O3)0.25 ceramics (RE=Dy, Y, Ho, Er and Yb)

Five kinds of rare earth stabilized bismuth oxide ceramics, (Bi₂O₃)_0.75(RE₂O₃)_0.25 (RE=Dy, Y, Ho, Er and Yb), were synthesized by sintering a mixture of Bi₂O₃ and RE₂O₃ at 900–1100 °C and their electrical properties were investigated. The bulk density and the lattice constant linearly increased with an increase in the atomic weight of RE and the ionic radius of RE³⁺, respectively. The electrical conductivity at 300 °C slightly increased with the increasing ionic radius of RE³⁺, while at 500 and 700 °C, it was constant regardless of the ionic radius of RE³⁺. The migration activation energy and the association activation energy showed a maximum value and a minimum value at RE=Er, respectively.     

Microstructure evolution and magnetic properties of Eu doped CuFeO2 multiferroic ceramics studied by positron annihilation

A series of multiferroic ceramics CuFe_1-xEu_xO₂ (x?=?0–0.10) are prepared by traditional solid-state reaction. The effects of Eu doping on the microstructure, vacancy-type defects, and magnetic properties of CuFeO₂ ceramics are investigated systematically by means of X-ray diffraction, Raman spectroscopy, scanning electron microscope, positron annihilation lifetime and physical property measurement system. The results show that no phase transition occurs in the entire range of doping content (x?=?0–0.10), but the single phase structure is damaged by high Eu content (x?=?0.04–0.10). Positron annihilation measurements indicate that the local electron density and the vacancy-type defect concentration increase gradually with the increase in Eu content from 0 to 0.08. Furthermore, abnormal changes in lifetime parameters can be found in x?=?0.10 sample induced by the existence of impurity phase in the system. The magnetic measurements reveal that all the samples exhibit two successive magnetic transitions at T?=?15 and 11?K. In x?=?0.02 sample, the coexistence of ferromagnetism and antiferromagnetism can be found, and a maximum saturation magnetization of 11.548?emu/g at 5?kOe is achieved. The possible reasons for the above observations are discussed in detail.     

Synthesis and magnetic properties of monodisperse CoFe2O4 nanoparticles coated by SiO2

The monodisperse CoFe₂O₄ nanoparticles were synthesized by a modified chemical coprecipitation method. Coating SiO₂ on the surface of the CoFe₂O₄ nanoparticles was carried out to keep single domain particles non-interacting with cubic magnetocrystalline anisotropy. The Curie temperatures (T_c) of the monodisperse CoFe₂O₄ nanoparticles can be accurately measured because the SiO₂ shells prevented the aggregation and growth of nanoparticles at high temperature. The magnetic properties of the CoFe₂O₄@SiO₂ nanoparticles with core-shell structure in a wide temperature range (300～950?K) were investigated. It is remarkable that the coercive field (H_c) of CoFe₂O₄ nanoparticles increased from about 760?Oe to 1806?Oe after being coated with SiO₂, which increased by 137.6% compared to the uncoated samples at 300?K. The saturation magnetization (M_s) of the CoFe₂O₄@SiO₂ nanoparticles is 34.59?emu/g, which is about 52% of the naked CoFe₂O₄ nanoparticles value (66.51?emu/g) at 300?K. The hysteresis loops of the CoFe₂O₄@SiO₂ nanoparticles showed an orderly magnetic behavior at high temperature, such as the M_s, remanence magnetization (M_r) and H_c decreased as temperature increasing, being equal to zero near T_c. This is a good indication that the CoFe₂O₄@SiO₂ nanoparticles are suitable for a wide variety of technological applications at high temperature.     

Synthesis and magnetic properties of CoAlFe2O4 nanoparticles

Nanosized particles of CoAl_xFe_2-xO₄, where 0?≤?x?≤?2, were synthesized by the sol–gel combustion method and the magnetic properties of these compounds were investigated. According to X-ray diffractograms, the samples are single phase and the crystallite size is between 7 and 25?nm. The room temperature saturation magnetization of the samples was estimated from the cation distribution and ferromagnetic resonance spectra were used to determine the magnetocrystalline anisotropy. The results show that the saturation magnetization and the magnetocrystalline anisotropy vary over a wide range, from maxima of M_s =?0.42?MA/m and K?=?0.39?kJ/m³ for x?=?1.0 to minima of almost zero for x?≈?1.4, a result that could be useful for practical applications of these materials.     

Microstructure and microwave dielectric properties of Al2O3 added Li2ZnTi3O8 ceramics

Recently, the rapid development of advanced communication systems increasingly strongly demands high-performance microwave dielectric ceramics in microwave circuits. Among them, Li₂ZnTi₃O₈ ceramics have been one of the most widely investigated species, due to its high quality factor, moderate firing conditions and low cost. However, the dielectric constants of the already reported Li₂ZnTi₃O₈ ceramics are fixed in a narrow range, limiting their wider applications. To adjust the dielectric constant of the Li₂ZnTi₃O₈ based ceramics, in this work Li₂ZnTi₃O₈ ceramics added with different amounts of Al₂O₃ (0–8?wt%) were prepared by conventional solid-state reaction. The microstructure and microwave dielectric properties of the samples were investigated. Due to the addition of Al₂O₃, the sintering temperature of the ceramics would be increased somewhat. Some Al³⁺ ions could substitute for Ti⁴⁺ ions in Li₂ZnTi₃O₈, and the added Al₂O₃ would react with ZnO to produce a ZnAl₂O₄ phase accompanying with the formation of TiO₂ phase, which would inhibit the growth of Li₂ZnTi₃O₈ grains. The dielectric constant of the finally obtained ceramics would be reduced from 26.2 to 17.9, although the quality factors of the obtained ceramics would decrease somewhat and the temperature coefficient of resonant frequency would deviate further from zero.     

Characterization and magnetic coercivity of nanostructured (Fe50Co50)100-XVX= 0,2,4 powders containing a small amount of Co3V intermetallic obtained by mechanical alloying

(Fe₅₀Co₅₀)_100−XV_X = 0,2,4 alloy powders were prepared by mechanical alloying. The milling times were 4 h, 8 h, 16 h, 24 h, 36 h, 55 h and 125 h, respectively. Structural, micro-structural and magnetic studies were carried out by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and a Vibration Sample Magnetometer (VSM). The XRD results showed that the inter-metallic compound (Co₃V) appears during milling and affects the coercivity, lattice parameter and micro-strain. Crystallite size decreases and reaches approximately 10 nm at 125 h. The coercivity increases during the milling and reaches a maximum at 55 h and then decreases slightly.     

Evolution of microstructure and electrical properties of Aurivillius phase (CaBi4Ti4O15)1-x(Bi4Ti3O12)x ceramics

(CaBi₄Ti₄O₁₅)_1-x(Bi₄Ti₃O₁₂)_x (CBT-xBIT) Aurivillius phase ceramics were synthesized by the conventional solid reaction method. The evolution of the structure and the electrical properties of CBT-xBIT ceramics were systematically investigated. Due to the enhanced spontaneous polarization induced by internal stresses on the Bi₂O₂ layers in the CBT-xBIT structure, the optimal piezoelectric coefficient (d₃₃ ~ 13?pC/N) was obtained in the ceramics with x?=?0.3 while exhibiting a relatively good thermal stability in the temperature range of 20–700?°C. The dc resistivity (ρ_dc) of the CBT-xBIT ceramics exhibited a higher value (≥?10⁹ Ω?cm) at room temperature, and the tan δ value of CBT-xBIT (x= 0, 0.1 and 0.3) within the temperature range of 20–500?°C maintained stability as a result of the domain structure and point defect concentration in the ceramics. In addition, a distinctive double dielectric peak anomaly was observed in the ε_r-T curves of the CBT-xBIT (x= 0.3, 0.5 and 0.7) ceramics, and it plays a remarkable role in the thermal stability of the piezoelectricity of CBT-xBIT ceramics. As a result, such research can benefit high temperature practical piezoelectric devices.