Maximum Values Of Entropy Change Research Articles

Large magnetocaloric effect in a ternary Laves phase compoundHo2Rh3Ge

Structural and magnetic properties of an intermetallic Ho2Rh3Ge compound have been investigated in this paper. We report the observation of large magnetocaloric effect (MCE) in Ho2Rh3Ge from the characterization of magnetism isotherms. The Rietveld refinement of powder X-ray diffraction revealed that Ho2Rh3Ge crystallizes in a rhombohedral Laves phase structure (space group R3̄m, hR18). It exhibits ferromagnetic order below the Curie temperature TC = 13 K. Around TC, maximum value of entropy change −ΔSm = 25.4 J kg−1 K−1 was observed for a change of magnetic field 0−8 T. The obtained refrigeration capacity (relative cooling power) for a change of magnetic field from 0 8 T is 497(664) J.kg−1. The second–order magnetic phase transition in Ho2Rh3Ge is confirmed from Arrott plots, universal scaling plots of magnetic entropy changes, and also from the field dependence of the magnetic entropy change.

Synthesis of highly dense spark plasma sintered magnetocaloric Ni-Mn-Sn alloys from melt-spun ribbons

In this work, polycrystalline melt-spun ribbons of the magnetocaloric Ni50Mn37Sn13 Heusler alloy were used as precursor material in order to fabricate a nearly 98% dense piece by means of spark plasma sintering. SEM and XRD data show that consolidation was achieved through ribbons surfaces preserving the structural and microstructural properties. Structural transformation temperatures and magnetic properties for the as-sintered SPS sample remained similar to the precursor. A moderate shifting in the martensitic transformation to higher temperatures (8 K) for the sintered piece was observed. Furthermore, higher values of maximum entropy change were calculated for the sintered sample (10.1 Jkg−1K−1 at 5 T), which basically doubles that of the precursor ribbons.

Magnetocaloric properties of FM/AFM core/shell nanoparticles: a Monte Carlo simulation study

In the present work, magnetocaloric properties of FM/AFM core/shell nanoparticles have been studied by using detailed Monte Carlo simulations. Thermal variation of isothermal magnetic entropy change of the core, shell, interface part and total nanoparticle with a spherical shape for various magnetic fields strengths have been examined. Also, dependence of the isothermal magnetic entropy change on the size and geometry of the nanoparticle has been analysed by considering the variation of the shape of the nanoparticle from oblate and prolate to a spherical one. Moreover, for the same shape evolution, cooling capacity (q) and maximum value of the entropy change have been presented as a function of applied magnetic field. Maximum value of the isothermal magnetic entropy change has been shown to take larger values as the applied magnetic field increases. Our numerical results suggest a linear relationship as $$q \sim bh_{max}$$ between cooling capacity and magnetic field which is in accordance with the recent studies. A scaling behaviour as $$|\Delta S_{M}|_{max}\sim h_{max}^c$$ has also been obtained between maximum value of the magnetic entropy change and magnetic field.

Magnetic properties and near-room-temperature large magnetocaloric effect in (La1-xBix)0.67Ba0.33MnO3 (x = 0 - 0.3) ceramics

The magnetic and magnetocaloric properties of (La1-xBix)0.67Ba0.33MnO3 (for x = 0 - 0.3) ceramics are reported. Magnetization measurements reveal the onset of a ferromagnetic ordering (TC) in the range 336 K- 229 K which shifts towards low temperatures with increasing x. Magnetocaloric effect (MCE) measurements yield the maximum value of entropy change (-ΔSMmax) around TC that decreases from ∼6.19 J/ Kg/K for x = 0 to ∼4.79 J/ Kg/K for x = 0.3 for a field change of 9 T. Remarkable enhancement in the relative cooling power (RCP) is observed with the value increasing from ∼400.45 J/Kg for x = 0 to ∼472.81 J/Kg for x = 0.3. The substantial increase in the magnetocaloric properties of (La1-xBix)0.67Ba0.33MnO3 compounds compared to other compounds reported in the literature makes them potential candidates to be used for magnetic refrigeration purposes.

The structure, magnetic and magnetocaloric effect of misch-metal (MM)2Fe17−xAlx (x = 0–2) compounds

In this study, the structural, magnetic and magnetocaloric effects (MCE) of misch-metal MM2Fe17−xAlx (0 ≤ x ≤ 2) compounds are investigated. The XRD patterns indicate that all the samples crystallize primarily in a rhombohedral Th2Zn17 type crystal structure coexisting with a small amount of precipitation phase α-Fe. The lattice parameters and the Curie temperature (TC) increase linearly with the increase of Al content. This allows the MM2Fe17−xAlx compounds to achieve magnetic refrigeration over the entire room temperature range. The maximum values of entropy changes (ΔSM) for MM2Fe17−xAlx (x = 0, 0.5, 1 and 1.5) compounds with a magnetic field variation of 0–2 T and 0–5 T are determined to be −2.1 J Kg-1 K−1, −1.3 J Kg-1 K−1, −1.9 J Kg-1 K−1, 1.9 J Kg-1 K−1 and −4.0 J Kg-1 K−1, −2.7 J Kg-1 K−1, −3.5 J Kg-1 K−1, −3.5 J Kg-1 K−1, respectively. The relative cooling power (RCP) for all the studied compounds is larger than 90 and 270 Jkg−1 while the full-width half maxima (δTFWHM) is larger than 60 K and 100 K with the field changes of 0–2 T and 0–5 T, respectively. These results indicate that MM2Fe17−xAlx (0 ≤ x ≤ 2) compounds could have higher magnetocaloric cost-performance comparable to the room-temperature magnetic refrigeration materials R2Fe17 (R = Pr, Nd) and Gd with the second-order phase transition (SOPT).

Magnetocaloric effect and the change from first- to second-order magnetic phase transition in Pr0.7CaxSr0.3-xMnO3 polycrystalline compounds

The work presents the magnetocaloric effect and critical behavior of monoclinic polycrystalline Pr0.7CaxSr0.3-xMnO3 prepared by solid state reaction. The ferromagnetic-paramagnetic phase transition temperature decreases from 267, 202, 138 to 126 K with increasing doping concentration from x=0 to 0.3. The maximum values of entropy change appear near the transition phase points and were found to be 6.4, 7.2, 3.9 and 4.0 J.kg-1.K-1 with 5 Tesla of field change for x=0-0.3, respectively. However, the shift to the higher temperature side with the increasing field change was observed clearly in sample x=0.1. Besides, the considering magnetic phase transition of samples based on Banerjee’s criterion showed a first-order phase transition in Pr0.7Ca0.1Sr0.2MnO3 (x=0.1) while samples with x=0, x=0.2 and 0.3 exhibited a second-order phase transition.

Influence of spin fluctuations on the magnetocaloric behavior of Gd2Co3Al9 compound

Arc melted Gd2Co3Al9 compound crystallizes in orthorhombic Y2Co3Ga9 structure type (space group Cmcm, No. 63) as single phase. The magnetization M(T), specific heat C(T) and the resistivity ρ(T) measurements reveal an antiferromagnetic transition at 9.6 K. The temperature dependence of magnetic entropy change, -ΔSM(T), calculated from the M(B) data, gives a maximum value of entropy change -ΔSMMax=6.3J/kg.K for a field change of 0–7 T in a broad temperature range of ΔT=32K above the magnetic ordering temperature. B2 dependence of magnetic field dependent magnetocaloric effect (MCE) data in the paramagnetic regime indicates the role of applied magnetic field in suppressing the spin fluctuations. The interesting observation in this compound is that the presence of spin fluctuation smears out the MCE peak at the transition temperature and drags it to higher temperatures as a broad peak.

Magnetocaloric effect investigation in the ferromagnetic Eu2CuSi3 compound

Here, it is reported the study of the structural, magnetic properties and magnetocaloric effect (MCE) of the Eu2CuSi3 compound. Polycrystalline Eu2CuSi3 compound with AlB2-type derived hexagonal structure was successfully synthesized by arc-melting method. Magnetic measurements indicate a second-order ferromagnetic (FM) phase transition below 40 K. The effective magnetic moment of Eu is reduced compared with divalent free ion, indicating a compensation of Eu magnetic moment. Spin reorientation induce negative magnetocaloric effect below Curie temperature. The values of full width half-maximum of −ΔSM peak (δTFWHM = 35 K) and maximum value of entropy change (−ΔSMmax∼9.5 J/kg K) for μ0ΔH = 6 T strongly contribute to improve the relative cooling power (RCP∼332 J/kg) of Eu2CuSi3 compound. As observed, δTFWHM value is sensibly wider than the usual values observed for first and second order magnetic phase transition compounds. Then, −ΔSMmax and δTFWHM values as well as the neglected magnetic hysteresis that were obtained for Eu2CuSi3 have shown features as a desirable second order magnetic phase transition compound for low temperature magnetic refrigeration.

Influences of Eu2O3 characters and sulfurization conditions on the preparation of EuS and its large magnetocaloric effect

In this study, the effects of Eu2O3 characters and sulfurization conditions on the preparation of EuS were researched. Single-phase Eu3S4 and EuS powders can be obtained via CS2 gas sulfurization of spherical Eu2O3 powders with larger specific surface area and small grain size at 500 °C for longer than 0.5 h and 800 °C for 8 h, respectively. Moreover, EuS powders can be fabricated from self-prepared needle Eu2O3 powders at 750 °C for 8 h. However, single phase EuS was generated from oval Eu2O3 powder with large grain size at 900 °C for 2 h. The higher sulfurization temperature and shorter sulfurization time accelerated the formation of high purity EuS. The synthetics have some reunion and the shapes of synthesized EuS are similar. Specific surface area of synthetics lessened with the rising of sulfurization temperature and time. Magnetization curves of polycrystalline EuS bulk are similar with those of single crystal EuS and the maximum values of entropy change are 16.76 J/Kg/K and 28.4 J/Kg/K for the field changes of 2 T and 5 T, respectively.

The residual entropy of Shastry-Sutherland lattice of rare-earth tetraborides

The experimental temperature dependence of thulium tetraboride specific heat Cp(T) and other heat capacity data for other RE-tetraborides were investigated in the 2–300 K temperature interval. Anomalies of Cp(T) dependence of TmB4 at TN1 = 9.6 K and TN2 = 11.4 K due to the antiferromagnetic ordering have been revealed. The ground state of the ion Tm3+ was confirmed as a doublet. The diffuse anomaly of Cp(T) is attributed to the Schottky contribution to the specific heat of the tetraboride, which is caused by the influence of the crystal electric field (CEF). The presence of the residual (zero-point) entropy S0 of the magnetic moments system of Tm3+ ions, due to frustration of the Shastry-Sutherland lattice, is detected. As a measure of the frustration of the system, a new characteristic of frustrated systems, the entropy frustration figure of merit, the value of which depends on the zero-point entropy of the system, is introduced: fS = S0/ΔSm max, where ΔSm max is the maximum value of entropy change of the boride magnetic subsystem. For TmB4 and tetraborides of Gd, Tb, Dy, Ho, and Er, fs values determined from thermal measurements, and f = Өc-w/TN (Өc-w denotes the Curie-Weiss temperature; TN is Neel temperature), calculated according to the magnetic measurement data, are practically identical.

Electrical and morphological properties of magnetocaloric nano ZnNi ferrite

A series of Zn1–xNixFe2O4 nano ferrite (with x=0, 0.2, 0.4, 0.6, 0.8, and 1) compositions were synthesized using the combustion technique. The powder samples were characterized by XRD. The X-ray analysis showed that the samples were single phase spinel cubic structure. The AC resistivity decreases by increasing the frequency from 1kHz to 10kHz. As the frequency of the applied field increases the hopping of charge carrier also increase, thereby decreasing the resistivity. A shift in dielectric maximum is observed toward higher temperature with increasing the Ni content from 536K to 560K at 1kHz. The HRTEM (high resolution TEM) images of four compositions have lattice spacing which confirms the crystalline nature of the samples. The surface morphology SEM of the sample consists of some grains with relatively homogenies distribution with an average size varying from 0.85 to 0.92μm. The values for entropy change in this work are still small but are significally higher than the values that have been reported for iron oxide nanoparticle. The magnetic entropy change was calculated from measurements of M (H, T) where H is the magnetic field and T is the temperature. The maximum value of entropy change (∆S) obtained near Curie temperature which makes these material candidates for magnetocaloric applications.

Giant magnetocaloric effect in ferromagnetic superconductor RuSr2GdCu2O8

We have investigated the magnetic and magnetocaloric properties of ferromagnetic superconductor Ru(Sr1–xLax)2GdCu2O8 (x = 0–0.10) by magnetization and heat capacity measurements. RuSr2GdCu2O8 shows onset of superconductivity at around 46 K, which coexists with the weak ferromagnetic ordering of the RuO2 moments and the antiferromagnetic ordering of the Gd moments. Both superconducting RuSr2GdCu2O8 and non-superconducting Ru(Sr0.9La0.1)2GdCu2O8 compounds exhibit a large magnetic entropy change due to the field-induced metamagnetic transition. The maximum values of entropy change, adiabatic temperature change, and refrigerant capacity are ∼15.5 J kg−1 K−1, ∼14 K, and ∼125 J kg−1, respectively, for a field change of 0–7 T for both the compounds. These magnetocaloric parameters also have reasonably good values for a moderate field change (2–3 T), therefore, fulfilling the necessary conditions for refrigeration in the low-temperature region.

Structural and magnetocaloric properties of nano Zn ferrite doped with Ni under hydrothermal conditions

A series of Zn1−x Ni x Fe2O4 nano ferrite (with x = 0 , 0.2, 0.4, 0.6, 0.8, 1) compositions were synthesized using the hydrothermal technique. The powder samples were characterized by XRD, VSM, and FTIR. The X-ray analysis showed that the samples had a single phase spinel cubic structure. The lattice constant decreased with increasing Ni content from 8.373 to 8.279 A. The FTIR spectra of the samples in the range 200–5000 cm−1 were reported. The hysteresis loops of the samples were performed at room temperature, the coercivity (H c), saturation magnetization (M s), experimental magnetic moment (μ exp) and retentivity (M r) were determined. The magnetic entropy change was calculated from measurements of M(H,T) where H is the magnetic field and T is the temperature. The maximum value of entropy change (ΔS) increased as the applied field increased, which makes these material candidates for magnetocaloric effect.

Theoretical Work in Magnetocaloric Effect of LaFe13−xSi x Compounds

In this paper, the magnetocaloric effect (MCE) of LaFe13−xSix compounds with 1.2 ≤ x ≤ 2.2 has been investigated. For this purpose, the magnetization dependence on the temperature and magnetic field were measured. Magnetic entropy change (−ΔSM) allowing estimation of the MCE was determined based on thermodynamic Maxwell’s relation. The experimental results show that TC increases with the Si content, whereas the magnetic entropy variation decreases. A large magnetic entropy change has been observed. The maximum \((-\Delta \mathrm {S}_{\mathrm {M}}^{\text {max}})\) of LaFe10.8Si2.2 occurring close to TC = 240 K is about 2.3 Jkg−1 K−1 for an applied field change of 0–2 T. In addition, a magnetocaloric effect of LaFe13−xSix compounds (x = 1.2 and 1.6) has been also carried out using phenomenological model. Dependence of the magnetization on temperature variation for LaFe13−xSix compounds (x = 1.2 and 1.6) in different applied magnetic fields was simulated. The values of maximum entropy change, full width at half maximum, and relative cooling power (RCP) for the LaFe11.8Si1.2 and LaFe11.4Si1.6 compounds in different applied magnetic fields were calculated.

3d-4f spin interaction and field-induced metamagnetism in RCrO4 (R = Ho, Gd, Lu) compounds

We observe that the zircon-type RCrO4 (R = Ho, Gd, Lu) compounds exhibit complicated magnetic properties and large magnetic entropy change due to the strong competition between ferromagnetic and antiferromagnetic interactions. For a field change of 7 T, the maximum values of entropy change and refrigerant capacity reach 28 J kg−1 K−1 and 740 J kg−1, respectively, for GdCrO4 whereas the corresponding values for HoCrO4 are 29 J kg−1 K−1 and 550 J kg−1. For GdCrO4 compound, the magnetic entropy change is quite large even at low temperatures well below the ferromagnetic transition.

Inverse magnetocaloric effect of epitaxial Ni-Mn-Sn thin films

Epitaxial Ni-Mn-Sn thin films of 200 nm thickness were prepared by magnetron sputtering and deposited onto MgO(100) substrate. They reveal an inverse magnetocaloric effect with a martensitic phase transition around 260 K. The resulting magnetocaloric properties of these films have been determined performing magnetization measurements in the temperature range between 10 and 330 K applying different magnetic fields. The maximum values of entropy change and relative cooling power are 1.6 J kg−1 K−1 and 36.5 J kg−1 for cooling and 1.5 J kg−1 K−1 and 33.9 J kg−1 for heating in μ0ΔH=1 T, respectively. These data are comparable with bulk values of Ni-Mn-Sn Heusler alloys.

Influence of Dy addition on the magnetocaloric effect of La0.67Ca0.33Mn0.9V0.1O3 ceramics

The influence of partial substitution of La by Dy on the magnetocaloric response of (La1−xDyx)0.67Ca0.33Mn0.9V0.1O3, where x=0.03, 0.15 and 0.25 is studied. Rietveld refinement of X-ray diffraction pattern using GSAS method shows that the compounds adopt the orthorhombic structure with Pnma space group. The systematic change in lattice parameters and magnetic phase transition indicates the substitution effect of Dy. From the magnetization isotherms at different temperatures, magnetic entropy change close to their respective transition temperatures (TC) has been evaluated. The maximum value of entropy change near TC is found to be about 4.8J/kgK at 187.5K for LCMVDy0.03, 2.45J/kgK at 107.5K for LCMVDy0.15 and 2.15J/kgK at 92.5K for LCMVDy0.25 at 4T. Dy addition produces a reduction in TC and in magnitude of the magnetic entropy change. Even though the entropy change decreases with increasing Dy substitution the refrigerant temperature range, ΔT, is found to be 10K for LCMVDy0.03, 31K for LCMVDy0.15 and 35K for LCMVDy0.25 compounds [90%] at 4T. The field dependence of the magnetic entropy change is also analyzed showing the power law dependence, ΔSM∞Hn where n=0.75(2) for LCMVDy0.03, n=0.80(4) for LCMVDy0.15 and n=0.92(8) for LCMVDy0.25 compounds at their respective transition temperatures. The relative cooling power and its field dependance are also analyzed.

Magnetocaloric effect in La 1− xSr xCoO 3 (0.05⩽ x⩽0.40)

We report studies of magnetocaloric effect in lanthanum cobaltate doped with different Sr-concentrations, La 1− x Sr x CoO 3 (0.05⩽ x⩽0.4). The study has revealed that La 0.6Sr 0.4CoO 3, which exhibits a moderately large value of maximum entropy change of about 1.45 J/kg/K in 1.5 T DC-field around its Curie temperature (of 235 K), can be used as an active magnetic refrigerant (AMR) material at ∼235 K.