Structural and magnetic properties with reversible magnetocaloric effect in PrSr1–xPbxMn2O6 (0.1 ≤ x ≤ 0.3) double perovskite manganite structures

In this study, the effect of Pb substitution with Sr on structural, magnetic and magnetocaloric properties in Pr(Sr_1–xPb_x)Mn₂O₆ (0.1?≤?x?≤?0.3) double perovskite manganite compounds prepared by the sol–gel method has been reported. Crystallinity has been worked by X-ray diffraction measurement and the Rietveld refinement method, which shows all samples are in monoclinic structure with P21/n space group without impurity phase. Scanning electron microscope analysis shows that all samples constituted from square-shaped grains with distinct grain boundaries, and energy-dispersive X-ray spectroscopy analysis shows that all expected elements are detected and there is no impurity element in the sample. All samples exhibit a magnetic phase transition from ferromagnetic to paramagnetic phase at T_C determined as 274, 282 and 286?K for x?=?0.1 (PSPM-1), 0.2 (PSPM-2) and 0.3 (PSPM-3) in the PrSr_1–xPb_xMn₂O₆ structure, respectively. Isothermal magnetisation curves near T_C are used to determine the nature of the magnetic phase transition and the magnetic entropy change values (?ΔS_M), which are found as 2.78, 3.11 and 3.52?J?kg^?1?K^?1 for PSPM-1, PSPM-2 and PSPM-3 samples, respectively. Relative cooling power (RCP) values of the samples are also determined as 241, 311 and 246?J?kg^?1 for PSPM-1, PSPM-2 and PSPM-3 samples, respectively. ΔS_M and RCP values are also calculated by using the Landau theory and the results are in accordance with the experimental results, which indicate the role of electron condensation energy and magneto-elastic coupling in magnetic entropy change value of the samples.     

Magnetism and the magnetocaloric effect in PrMn1.6Fe0.4Ge2

The magnetic and magnetocaloric properties of PrMn_1.6Fe_0.4Ge₂around the ferromagnetic transitions T _C ^inter ~ 230 K and T _C ^Pr ~ 30 K have been investigated by magnetisation, ⁵⁷Fe Mössbauer spectroscopy and electron paramagnetic resonance (EPR) measurements over the temperature range 5–300 K. The broad peaks in magnetic entropy around T_C ^inter (intralayer antiferromagnetism of the Mn sublattice to canted ferromagnetism) and T_C ^Pr (onset of ferromagnetic order of Pr sublattice in addition to ferromagnetically ordered Mn sublattice) are typical of second order transitions with maximum entropy values of -ΔS _M ~ 2.0 J/kg K and -ΔS _M ~ 2.2 J/kg K respectively for ΔB = 0–6 T. The EPR signal around T = 48 K of g value g ~ 0.8 is consistent with paramagnetic free ion Pr^3?+?. Below T_C ^Pr ~ 30 K the g value increases steadily to g ~ 2.5 at 8 K as saturation of the Pr^3?+? ion is approached. The EPR measurements indicate additional effects in this system below T ~ 20 K with the appearance of EPR signals of low g value g ~ 0.6.     

Magnetocaloric and magnetoresistance properties in Co-based (Co0.402Fe0.201Ni0.067B0.227Si0.053Nb0.05)100−xCux (x=0–1) glassy alloys

The magnetocaloric and magnetoresistance properties of amorphous Co-based (Co_0.402Fe_0.201Ni_0.067B_0.227Si_0.053Nb_0.05)_100?xCu_x (x = 0, 0.5, 0.75 and 1) ribbons were investigated. Cu additions changed the crystallisation temperature (T_x) and the Curie temperature (T_C). The saturation magnetisation (M_s) and coercivity (H_c) for alloys were in the range of 65.51–38.49 emu/g and 1.99–6.84 A/m, respectively. Under an applied magnetic field change of 2.2 T, the (?ΔS_M)^max for (Co_0.402Fe_0.201Ni_0.067B_0.227Si_0.053Nb_0.05)_100?xCu_x with x = 0, 0.5, 0.75 and 1 are 0.77, 0.71, 0.89 and 0.67 Jkg^?1 K^?1, respectively. The values of refrigeration capacity (RC) for the as-spun glassy alloys are comparable with those of previously studied Fe-based metallic glasses such as Fe₈₀B₁₀Zr₉Cu₁, (Fe_0.76B_0.24)₉₆Nb₄ and Fe₈₂Ni₂Zr₆B₁₀. In addition, the maximum magnetoresistance (MR) values for (Co_0.402Fe_0.201Ni_0.067B_0.227Si_0.053Nb_0.05)_100?xCu_x with x = 0, 0.5, 0.75 and 1 are found to be 110, 38, 23 and 1% around the Curie temperatures under an applied magnetic field change of 1 T, respectively. With good RC, negligible hysteresis due to very low coercivity values and large magnetoresistance, these Co-based amorphous alloys can be used as the high temperature magnetic refrigerants and multifunctional applications working in the temperature range of 450–600 K.     

Magnetic transition and large reversible magnetocaloric effect in EuCu1.75P2 compound

The magnetocaloric effect(MCE) in EuCu1.75P2 compound is studied by the magnetization and heat capacity measurements.Magnetization and modified Arrott plots indicate that the compound undergoes a second-order phase transition at TC ～ 51 K.A large reversible MCE is observed around TC.The values of maximum magnetic entropy change(-△SxMma) reach 5.6 J·kg-1·K-1 and 13.3 J·kg-1·K-1 for the field change of 2 T and 7 T,respectively,with no obvious hysteresis loss in the vicinity of Curie temperature.The corresponding maximum adiabatic temperature changes(△Tadmax) are evaluated to be 2.1 K and 5.0 K.The magnetic transition and the origin of large MCE in EuCu1.75P2 are also discussed.     

Double magnetic entropy change peaks and high refrigerant capacity in Gd1-xHoxNi compounds in the melt-spun form

Gd_1-xHo_xNi melt-spun ribbons were fabricated by a single-roller melt spinning method. All the compounds crystallize in an orthorhombic CrB-type structure. The Curie temperature (T_C) was tuned between 46 and 99 K by varying the concentration of Gd and Ho. A spin reorientation (SRO) transition is observed around 13 K. Different from T_C, the SRO transition temperature is almost invariable for all compounds. Two peaks of magnetic entropy change (ΔS_M) were found. One at the higher temperature range was originated from the paramagnet-ferromagnet phase transition and the other at the lower temperature range was caused by the SRO transition. The maximum of ΔS_M around T_C is almost same. The other maximum of ΔS_M around SRO transition, however, had significantly positive relationship with x. It reached a maximum about 8.2 J kg⁻¹ K⁻¹ for x = 0.8. Thus double large ΔS_M peaks were obtained in Gd_1-xHo_xNi melt-spun ribbons with the high Ho concentration. And the refrigerant capacity power reached a maximum of 622 J kg⁻¹ for x = 0.6. Gd_1-xHo_xNi ribbons could be good candidate for magnetic refrigerant working in the low temperature especially near the liquid nitrogen temperature range.     

Magnetic transitions in LaFe13−x−yCoySix compounds

The magnetic properties of a set of LaFe_13?x?yCo_ySi_x compounds (x = 1.6 ? 2.6; y = 0, y = 1.0) have been investigated using magnetic measurements, thermal expansion, ⁵⁷Fe Mössbauer spectroscopy and high resolution neutron powder diffraction methods over the temperature range 10–300 K. The natures of the magnetic transitions in these LaFe_13?x?yCo_ySi_x compounds have been determined. The Curie temperatures of LaFe_13?xSi_x were found to increase with Si content from T_C = 219(5) K for Si content x = 1.6 to T_C = 250(5) K for x = 2.6. Substitution of Co for Fe in LaFe_10.4Si_2.6 resulted in a further enhancement of the magnetic ordering temperature to T_C = 281(5) K for the LaFe_9.4CoSi_2.6 compound. The nature of the magnetic transition at the Curie temperature changes from first order for LaFe_11.4Si_1.6 to second order for LaFe_10.4Si_2.6 and LaFe_9.4CoSi_2.6. The temperature dependences of the mean magnetic hyperfine field values lead to T_C values in good agreement with analyses of the magnetic measurements. The magnetic entropy change, ?ΔS_M, has been determined from the magnetization curves as functions of temperature and magnetic field (ΔB = 0 ? 5 T) by applying the standard Maxwell relation. In the case of LaFe_12.4Si_1.6 for example, the magnetic entropy change around T_C is determined to be -ΔS_M ～ 14.5 J kg^?1 K^?1 for a magnetic field change Δ B = 0 ? 5 T.     

双层钙钛矿La1.8Ca1.2Mn2O7磁性相关I-V非线性与电输运性质

采用传统固相反应法制备了双层钙钛矿结构锰氧化物La_1.8Ca_1.2Mn₂O₇陶瓷,并用X射线粉末衍射法,扫描电镜,HL5500PC Hall效应分析仪和综合物性测量系统(PPMS)对其磁、电性质进行了表征.结果表明:经过两次高温烧结可合成具有双层Sr₃Ti₂O₇型四方结构的La_1.8Ca_1.2Mn_2<     

Estimation of the magnetic entropy change by means of Landau theory and phenomenological model in La0.6Ca0.2 Sr0.2MnO3/Sb2O3 ceramic composites

In this paper, magnetocaloric properties of La_0.6Ca_0.2Sr_0.2MnO₃/Sb₂O₃ oxides have been investigated. The composite samples were prepared using the conventional solid-state reaction method. The second-order phase transition can be testified with the positive slope in Arrott plots. An excellent agreement has been found between the ?ΔS_M values estimated by Landau theory and those obtained using the classical Maxwell relation. The field dependence of the magnetic entropy change analysis shows a power law dependence,|ΔS_M|≈Hⁿ , with n(T_C) = 0.65. Moreover, the scaling analysis of magnetic entropy change exhibits that ΔS_M(T) curves collapse into a single universal curve, indicating that the observed paramagnetic to ferromagnetic phase transition is an authentic second-order phase transition. The maximum value of magnetic entropy change of composites is found to decrease slightly with the further increasing of Sb₂O₃ concentration. A phenomenological model was used to predict magnetocaloric properties of La_0.6Ca_0.2Sr_0.2MnO₃/Sb₂O₃ composites. The theoretical calculations are compared with the available experimental data.     

Investigation of magnetic transitions through ultrasonic measurements in double-layered CMR manganite La1.2Sr1.8Mn2O7

A polycrystalline, double-layered, colossal magnetoresistive manganite La_1.2Sr_1.8Mn₂O₇ is synthesized by sol-gel process and its magnetic and ultrasonic properties were investigated in the temperature range 80–300 K. The sample has Curie temperature at 124 K, where the sample exhibits a transition from paramagnetic insulator to ferromagnetic metallic state. The longitudinal sound velocity measurements show a significant hardening of sound velocity below T_C, which may be attributed to the coupling between ferromagnetic spins and longitudinal acoustic phonons. The magnetization and ultrasonic studies reveal the presence of secondary transition at ≈ 260 K in this sample. The present sound velocity measurement results confirm the reliability of ultrasonic investigations as an independent tool to probe magnetic transitions in manganites.     

Magnetic behaviour of Cu2FeGeSe4

Measurements of magnetic susceptibility χ as a function of temperature T and of magnetisation M as a function of applied magnetic field H at a number of fixed temperatures were made on polycrystalline samples of Cu₂FeGeSe₄. The χ versus T data show that an antiferromagnetic transition occurs at 20 K and that a second transition occurs at 8 K, indicating a transition to weak ferromagnetic form. The M versus H curves indicated that at all temperatures below 70 K bound magnetic polarons (BMP) occur, in the paramagnetic, antiferromagnetic and weak ferromagnetic ranges. Below 8 K, the M versus H curves exhibited magnetic hysteresis, and this is attributed to the interaction of the BMPs with tetragonally anisotropic matrix. The B versus H curves were well fitted by a Langevin-type of equation, and the variation of the fitting parameters determined as a function of temperature. These showed that above 20 K the total BMP magnetisation fell almost linearly with increasing temperature and effectively disappeared at 70 K. The number of BMPs remained practically constant with temperature having a mean value of 6.55×10¹⁸/cm³. The analysis gave a value of 213 μ_B for the average magnetic moment of a BMP, corresponding to 42.4 Fe atoms. Using a simple spherical model, this gives the radius of a BMP as 12.0 Å.     

DyNi2Mn—magnetisation and M?ssbauer spectroscopy

The physical properties of DyNi₂Mn doped with ⁵⁷Fe have been investigated by X-ray diffraction, magnetisation (10–300?K) and ⁵⁷Fe M?ssbauer spectroscopy measurements (5–300?K). DyNi₂Mn(⁵⁷Fe) crystallizes in the MgCu₂-type cubic structure (Fd^??3m space group). The ordering temperature is found to be T_C?=?99(2) K, much higher than those of DyNi₂ (～22?K) and DyMn₂ (～35?K). Analyses of isothermal M–H curves and the related Arrott plots confirm that the magnetic phase transition at T_C is second order. The magnetic entropy change around T_C is 4.0?J/kg?K for a magnetic field change of 0?T to 5?T. The spectra above T_C exhibit features consistent with quadrupolar effects while below T_C the spectra exhibit magnetic hyperfine splitting. The Debye temperature for DyNi₂Mn has been determined as θ_D?=?200(20) K from a fit to the variable temperature isomer shift IS(T).     

Pressure effect on magnetic and insulator–metal transition of La0.67Pb0.33Mn0.9Co0.1O2.97 ceramic

In our paper, we study the effect of hydrostatic pressure on the magnetic and insulator–metal transitions of La_0.67Pb_0.33Co_0.1Mn_0.9O_2.97 manganite. The insulator–metal transition does not coincide with the magnetic transition in this ceramic. The compound undergoes the paramagnetic–ferromagnetic phase transition at T _C=260 K and the insulator–metal transition at T _p=184?K at ambient pressure. We have found that both characteristic temperatures T _C and T _p increase with increasing hydrostatic pressure with Δ T _C/Δ p=11.2 K/GPa and Δ T _p/Δ p=20.1 K/GPa, respectively.     

Study of magnetotransport in double-layered La1.4Ca1.6Mn2O7 manganite: Presence of nano-ferromagnetic domains in paramagnetic matrix

Double-layered manganite La_1.4Ca_1.6Mn₂O₇ has been synthesized using the solid-state reaction method. It had a metal-to-insulator transition at temperature T_M1≈127 K. The temperature dependence of ac susceptibility showed a broad ferromagnetic transition. The two-dimensional (2D)-ferromagnetic ordering temperature (T_C2) was observed as ≈245 K. The temperature dependence of its low-field magnetoresistance has been studied. The low-field magnetoresistance of double-layered manganite, in the temperature regions between T_M1 and T_C2, has been found to follow 1/T⁵. The observed behaviour of temperature dependence of resistivity and low-field magnetoresistance has been explained in terms of two-phase model where ferromagnetic domains exist in the matrix of paramagnetic regions in which spin-dependent tunneling of charge carriers occurs between the ferromagnetic correlated regions. Based on the two-phase model, the dimension of these ferromagnetic domains inside the paramagnetic matrix has been estimated as ∼12 Å.     

Table-like magnetocaloric effect involving the enhancement of refrigerant capacity in (AMn0.9Ti0.1O3)1−x/(AMn0.85Ti0.15O3)x composite

In this paper, a table-like magnetocaloric effect (MCE) is obtained in the (AMn_0.9Ti_0.1O₃)_1?x/(AMn_0.85Ti_0.15O₃)_x composite system. A flattening behaviour of the entropy change is investigated from the isothermal magnetic entropy change versus temperature curves ?S(T) of AMn_0.9Ti_0.1O₃ and AMn_0.85Ti_0.15O₃ (A = La_0.57Nd_0.1Pb_0.33) manganite materials. Both compounds exhibit nearly some large MCE, 5.2–4.85 J kg^?1 K^?1, under an applied field of 0–5 T, around their respective Curie temperature (T_C) ranged from 279 to 299 K. The temperature dependence of the isothermal magnetic entropy change ΔS(T) is calculated for (AMn_0.9Ti_0.1O₃)_1?x/(AMn_0.85Ti_0.15O₃)_x composite with 0 ≤ x ≤ 1. The optimum ΔS(T) of the composite with x = 0.495 approaches a nearly constant value of ～3.63 J kg^?1 K^?1 in a field change of 0–5 T in a wide temperature span over 15 K, resulting in large refrigerant capacity value of ～172.3 J kg^?1. The (AMn_0.9Ti_0.1O₃)_0.505/(AMn_0.85Ti_0.15O₃)_0.495 composite system indicates that this approach can be used to design magnetic refrigerant materials with enhanced magnetocaloric response in a magnetic refrigerator performing an Ericsson cycle near room temperature.     

Synthesis and characterization of graphene nanoplatelet-La0.7Ca0.3MnO3 composites

ABSTRACT

La_0.7Ca_0.3MnO₃ perovskite and its composites with graphene nanoplatelet (GNP) were prepared using a wet chemical method. The structural, magnetic and magnetocaloric properties of La_0.7Ca_0.3MnO₃: GNP composites were investigated to determine the effect of GNPs. The results of XRD analysis show that the synthesised powders can be almost indexed to pure phase orthorhombic La_0.7Ca_0.3MnO_3. The magnetic measurements demonstrate that 0.7 and 1% GNP amounts cause an increase in the Curie temperature (T_C), and for larger amounts of GNP, the T_C monotonically decreases, except for the sample with 10% GNP. The results obtained from the Arrott plots show that the magnetic phase transition of the samples transforms from the first to second order with increasing GNP amount. The changes in the magnetocaloric properties are interpreted in terms of perovskite phase formations via structural analysis. The amounts of graphene nanoplatelets in the oxide powders are correlated with the observed magnetocaloric properties. The best magnetocaloric performance with the maximum magnetic entropy change of 3.99 Jkg^?1K^?1 and refrigeration capacity of 90 Jkg^?1 was obtained at a 2?T magnetic field.     

Pressure effect on magnetic properties of RCo12B6 (R=Y,Ce) compounds

The effect of pressure on magnetic properties of YCo₁₂B₆ and CeCo₁₂B₆ was studied in temperature range 5–300 K at pressures up to 9 kbar. The Curie temperature T_C and spontaneous magnetization M_S decrease with pressure for both compounds. The decrease can be attributed mostly to the volume dependence of both, the Co magnetic moment and the exchange interactions. The hybridization of the p–d states as a consequence of small distances between the Co and B atoms can be one reason of the relatively low pressure effects (ΔT_C/Δp=?0.39±0.02 K/kbar, d ln M_S/dp=?0.0013±0.0002 kbar^?1) in YCo₁₂B₆. Higher volume sensitivity of magnetic properties of CeCo₁₂B₆ in comparison with YCo₁₂B₆ can be attributed to the pressure induced changes of the Ce f- and Co d-states.     

Weak itinerant ferromagnetism and clean superconductivity in Y9Co7

We present low-temperature magnetic properties for a high-quality polycrystalline sample of the first ferromagnetic superconductor Y₉Co₇. The results of susceptibility and magnetization measurements show the coexistence of a weak itinerant ferromagnetic order with a Curie temperature of T_C ? 4.5 K and superconductivity below T_S = 3 K. Several electronic quantities and parameters characterizing the superconducting and normal state are calculated within the Ginzburg–Landau–Abrikosov–Gorkov theory. The data reveal clean limit superconductivity in Y₉Co₇ due to the good chemical purity and structural order of the specimen.     

GREAT MAGNETIC ENTROPY CHANGE IN La(Fe, M)13 (M=Si, Al) WITH Co DOPING

Very large magnetic entropy change Δ S_M, which originates from a fully reversible second-order transition at Curie temperature T_C, has been discovered in compounds La(Fe, Si)₁₃, La(Fe, Al)₁₃ and those with Co doping. The maximum change ΔS_M\approx19 J·kg^-1·K^-1, achieved in LaFe_11.4Si_1.6 at 209K upon a 5T magnetic field change, exceeds that of Gd by more than a factor of 2. The T_C of the Co-doped compounds shifts to higher temperatures. ΔS_M still has a considerable large magnitude near room temperature. The phenomena of very large ΔS_M, convenience of adjustment of T_C, and also thesuperiority of low cost, strongly suggest that the compounds La(Fe, M)₁₃ (M=Si, Al) with Co doping are suitable candidates for magnetic refrigerants at high temperatures.     

Effect of heat treatment procedure on magnetic and magnetocaloric properties of Ni43Mn46In11 melt spun ribbons

The effect of heat treatment on the structural, magnetic and magnetocaloric properties of Ni₄₃Mn₄₆In₁₁ melt-spun ribbons was systematically investigated using X-ray powder diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), magnetic force microscope (MFM) and magnetic measurements. From the XRD studies, tetragonal and cubic phases were detected at room temperature for as-spun, quenched and slow-cooled ribbons. Furthermore, it was observed, upon annealing martensite transition temperatures increased when compared to the as-spun ribbon. To avoid magnetic hysteresis losses in the vicinity of the structural transition region, the magnetic entropy changes-ΔS _m of the investigated ribbons were evaluated from temperature-dependent magnetisation-M(T) curves on cooling for different applied magnetic fields. The maximum ΔS _m value was found to be 6.79 J kg^?1 K^?1 for the quenched ribbon in the vicinity of structural transition region for a magnetic field change of 50 kOe.