Large Reversible Magnetic Entropy Change Research Articles

Influence of Chemical Substitution and Light Element Insertion on the Magnetic Properties of Nanocrystalline Pr2Co7 Compound

It is well recognized that intermetallics based on rare-earth (R) and transition metals (T) display numerous interesting magnetic properties, leading to potential applications in different fields. The latest progress regarding magnetic properties and the magnetocaloric effect (MCE) in the nanostructured Pr2Co7 compound, as well as its carbides and hydrides, is reviewed in this paper. Some of this progress reveals remarkable MCE performance, which makes it attractive in the field of magnetic refrigeration at high temperatures. With the purpose of understanding the magnetic and magnetocaloric characteristics of these compounds, the crystal structure, microstructure, and magnetism are also brought into focus. The Pr2Co7 compound has interesting magnetic properties, such as a high Curie temperature TC and uniaxial magnetocrystalline anisotropy. It crystallizes in a hexagonal structure (2:7 H) of the Ce2Ni7 type and is stable at relatively low temperatures (Ta ≤ 1023 K), or it has a rhombohedral structure (2:7 R) of the Gd2Co7 type and is stable at high temperatures (Ta ≥ 1223 K). Studies of the magnetocaloric properties of the nanocrystalline Pr2Co7 compound have shown the existence of a large reversible magnetic entropy change (ΔSM) with a second-order magnetic transition. After its substitution, we showed that nanocrystalline Pr2Co7−xFex compounds that were annealed at Ta = 973 K crystallized in the 2:7 H structure similarly to the parent compound. The extended X-ray absorption fine-structure (EXAFS) spectra adjustments showed that Fe atoms preferably occupy the 12k site for x ≤ 1. The study of the magnetic properties of nanocrystalline Pr2Co7−xFex compounds revealed an increase in TC of about 26% for x = 0.5, as well as an improvement in the coercivity, Hc (12 kOe), and the (BH)max (9 MGOe) product. On the other hand, the insertion of C atoms into the Pr2Co7 cell led to a marked improvement in the TC value of 21.6%. The best magnetic properties were found for the Pr2Co7C0.25 compound annealed at 973 K, Hc = 10.3 kOe, and (BH)max = 11.5 MGOe. We studied the microstructure, hydrogenation, and magnetic properties of nanocrystalline Pr2Co7Hx hydrides. The crystal structure of the Pr2Co7 compound transformed from a hexagonal (P63/mmc) into an orthorhombic (Pbcn) and monoclinic (C2/c) structure during hydrogenation. The absorption of H by the Pr2Co7 compound led to an increase in the TC value from 600 K at x = 0 to 691 K at x = 3.75. The Pr2Co7H0.25 hydride had optimal magnetic properties: Hc = 6.1 KOe, (BH)max = 5.8 MGOe, and TC = 607 K. We tailored the mean field theory (MFT) and random magnetic anisotropy (RMA) methods to investigate the magnetic moments, exchange interactions, and magnetic anisotropy properties. The relationship between the microstructure and magnetic properties is discussed. The obtained results provide a fundamental reference for adapting the magnetic properties of the Pr2Co7, Pr2Co6.5Fe0.5, Pr2Co7C0.25, and Pr2Co7H0.25 compounds for potential permanent nanomagnets, high-density magnetic recording, and magnetic refrigeration applications.

Large reversible magnetic entropy change of R3Ni6Al2 (R = Dy, Ho and Er) compounds

Large magnetocaloric effect (MCE) and magnetic properties in R3Ni6Al2 (R = Dy, Ho and Er) compounds were systematically investigated. Large magnetic moment and low magnetic transition temperature play a decisive role in the magnetocaloric performance for R3Ni6Al2 (R = Dy, Ho and Er) compounds. Ho3Ni6Al2 compound shows the largest magnetic entropy change with the value of 19.7 J/kg K at ~20 K under field change of 0–5 T because of the large magnetic moment and moderate transition temperature. For Dy3Ni6Al2 compound, it exhibits the largest refrigeration capacity with the value of 394.5 J/kg due to the successive magnetic transitions. The Er3Ni6Al2 compound has a large MCE at ultra-low temperatures around 4 K. Furthermore, the second-order phase transition in R3Ni6Al2 (R = Dy, Ho, Er) compounds were confirmed according to Arrott plots, rescaled universal magnetic entropy change curves and mean field theory with negligible thermal and magnetic hysteresis. The large reversible MCE with low working temperatures indicates that R3Ni6Al2 compounds are potential excellent MCE materials for low-temperature magnetic refrigeration.

Giant reversible magnetocaloric effect in MnNiGe-based materials: Minimizing thermal hysteresis via crystallographic compatibility modulation

MnMX (M = Co or Ni, X = Si or Ge) alloys with strong magnetostructural coupling exhibit giant magnetic entropy change and are currently extensively studied. However, large thermal hysteresis results in serious irreversibility of the magnetocaloric effect in this well-known system. In this work, we report a low thermal hysteresis and large reversible magnetocaloric effect in a MnNiGe-based system. The introduction of Fe into both Ni and Mn sites can establish stable magnetostructural transitions from paramagnetic hexagonal to ferromagnetic orthorhombic phases. Fascinatingly, a low thermal hysteresis of 5.2 K is achieved in Mn0.9Fe0.2Ni0.9Ge alloy with a large magnetization difference of 62.1 A m2/kg between the two phases. These optimized parameters lead to a partially reversible phase transformation under a magnetic stimulus and bring about a large reversible magnetic entropy change of −18.6 Jkg−1K−1 under the field variation of 0–5 T, which is the largest value reported in MnMX system up to now. Moreover, this low-hysteresis magnetostructural transformation and large reversible magnetocaloric effect can be tuned by doping with Si in a wide temperature range covering room temperature. We also introduce geometrically nonlinear theory to discuss the origin of low hysteresis in MnMX alloys. A strong relation is found between thermal hysteresis and the change of c axis in the orthorhombic structure during the transition. Our work greatly develops the potential of MnMX alloys as magnetocaloric materials and is meaningful to seek or design a MnMX system with low thermal hysteresis.

Large reversible magnetocaloric effect in a Ni-Co-Mn-In magnetic shape memory alloy

Reversibility of the magnetocaloric effect in materials with first-order magnetostructural transformation is of vital significance for practical magnetic refrigeration applications. Here, we report a large reversible magnetocaloric effect in a Ni49.8Co1.2Mn33.5In15.5 magnetic shape memory alloy. A large reversible magnetic entropy change of 14.6 J/(kg K) and a broad operating temperature window of 18 K under 5 T were simultaneously achieved, correlated with the low thermal hysteresis (∼8 K) and large magnetic-field-induced shift of transformation temperatures (4.9 K/T) that lead to a narrow magnetic hysteresis (1.1 T) and small average magnetic hysteresis loss (48.4 J/kg under 5 T) as well. Furthermore, a large reversible effective refrigeration capacity (76.6 J/kg under 5 T) was obtained, as a result of the large reversible magnetic entropy change, broad operating temperature window, and small magnetic hysteresis loss. The large reversible magnetic entropy change and large reversible effective refrigeration capacity are important for improving the magnetocaloric performance, and the small magnetic hysteresis loss is beneficial to reducing energy dissipation during magnetic field cycle in potential applications.

High-temperature solvothermal synthesis and magnetic properties of nearly monodisperse CdCr2S4 nanocrystals

Magnetic spinel CdCr2S4 nanocrystals have been synthesized using a high-temperature solvothermal method, which exhibit a large reversible magnetic entropy change over a wide temperature span.

Giant reversible magnetocaloric effect in flower-like β-Co(OH)2 hierarchical superstructures self-assembled by nanosheets

A facile hydrothermal strategy is proposed to synthesize flower-like β-Co(OH)2 hierarchical microspherical superstructures with a diameter of 0.5-1.5 μm, which are self-assembled by β-Co(OH)2 nanosheets with the average thickness ranging between 20 and 40 nm. The magnetocaloric effect associated with magnetic phase transitions in Co(OH)2 superstructures has been investigated. A sign change in the magnetocaloric effect is induced by a magnetic field, which is related to a filed-induced transition from the antiferromagnetic to the ferromagnetic state below the Neel temperature. The large reversible magnetic-entropy change -ΔSm (13.4 J/kg K at 15 K for a field change of 5 T) indicates that flower-like Co(OH)2 superstructures is a potential candidate for application in magnetic refrigeration in the low-temperature range.

Abnormal magnetic behaviors and large magnetocaloric effect in MnPS3 nanoparticles

A nanostructured honeycomb lattice consisting of MnPS3 nanoparticles synthesized via the ion-exchange technique was found to have restacked molecular layers stabilized by H2O insertion between the layers. Susceptibility (χ) and heat capacity measurements showed the absence of long range magnetic ordering, at least down to 2 K. However, the χ data showed that the system possesses a high effective Curie temperature, suggesting that the system is in a high spin lattice disordered state. Evaluation of the magnetocaloric effect indicates that the system has a large reversible magnetic-entropy change (−ΔSm) of 6.8 and 12.8 J/kg K and an adiabatic temperature change (ΔTad) of 3.8 K and 8 K at 2.85 K for magnetic field changes of 3 T and 9 T, respectively.

A sign of field-induced first order magnetic state transition and giant reversible magnetocaloric effect in cobalt hydroxide nanosheets

We report the synthesis of β−Co(OH)2 nanosheets using microwave assisted hydrothermal and conventional chemical reaction methods. A magnetic transition at an onset temperature, T1∼96 K, and a sign of antiferromagnetic state transition at the Néel temperature, TN=9∼10 K, can be determined from M-T curves and M-H curves magnetic properties investigation. It is found that a sign of field-induced first order transition below 9 K, act as a role of reversing the magnetocaloric effect from negative to positive value. The large reversible magnetic-entropy change (ΔSM) of 17 J/kg K around 11 K for a field change of 5 T indicates that this material is useful for refrigeration applications at low temperatures.

Large reversible magnetocaloric effect of light rare-earth intermetallic compound Pr 5Si 3

The structure and magnetic properties of Pr 5Si 3 compound have been investigated. A large reversible magnetic entropy change was observed at about 47 K accompanied with a second-order magnetic transition from the ferromagnetic to paramagnetic state. The maximum values of −Δ S M are 11.6 and 6.0 J kg −1 K −1 at 47 K, corresponding to the applied magnetic field changes of 50 and 20 kOe, respectively. The study on the magnetocaloric effect of the light rare-earth based compound, may open a field in searching for new magnetic refrigerant materials.

Tunable magnetocaloric effect around hydrogen liquefaction temperature in Tb 1−xY xCoC 2 compounds

The magnetic properties and magnetocaloric effect of Tb 1− x Y x CoC 2 ( x=0, 0.1, 0.2, 0.3, 0.4) compounds have been investigated systematically. All the compounds undergo second-order transitions from paramagnetic to ferromagnetic states without thermal and magnetic hysteresis. With increasing Y content from 0 to 0.4, the Curie temperatures decrease nearly linearly from 28 to 18 K. The nature of the second-order phase transitions can be confirmed by Arrott plots. For Tb 0.6Y 0.4CoC 2 compound, the maximum value of the magnetic entropy change –Δ S M at 20 K is 9.35 J kg −1 K −1 for an external field change of 5 T (5.14 J kg −1 K −1 for 2 T). The large reversible magnetic entropy change makes Tb 0.6Y 0.4CoC 2 compound an attractive candidate for the application at hydrogen liquefaction temperature.

Beneficial effect of minor Al substitution on the magnetocaloric effect of Mn 1 − xAl xAs

Magnetocaloric effect near room temperature is enhanced by minor Al substitution in Mn 1 − x Al x As compounds ( x = 0, 0.015, 0.03). The Curie temperature and magnetic entropy change can be tuned by Al concentration. Under a field change of 2 T, which is below the critical field of metamagnetic transition and easy to reach in applications, a large reversible magnetic entropy change 28.5 J kg − 1 K − 1 , and a huge magnetic entropy change 65.7 J kg − 1 K − 1 under a field change of 5 T is obtained at x = 0.015 near room temperature. Compared with that of MnAs mother alloy, it is concluded that minor Al substitution is beneficial for enhancing the magnetic entropy in Mn 1 − x Al x As alloys.

Giant reversible magnetocaloric effect in cobalt hydroxide nanoparticles

The magnetocaloric effect associated with magnetic phase transitions in β-Co(OH)2 nanoparticles has been investigated. A sign change in the magnetocaloric effect is induced by a magnetic field, which is related to a field-induced transition from the antiferromagnetic to the ferromagnetic state below the Néel temperature. The large reversible magnetic-entropy change −ΔSm (20.9 J/kg K at 15 K for a field change of 7 T) indicates that β-Co(OH)2 is a potential candidate for application in magnetic refrigeration in the low-temperature range.