Giant Magnetocaloric Effect Research Articles

Itinerant ferromagnetism and room temperature giant magnetocaloric effect in (Mn0.6Fe0.4) (Ni0.5Co0.5) Si shape memory alloy

In this article, the feasibility of obtaining a magneto-structural coupling over a wide working temperature range of about 127 K (from 206.5 K to 333.5 K) with negligible thermal hysteresis in combination with the structural, microstructural, magnetic, and magnetocaloric properties of a 50 % Co-doped (at the Ni site) polycrystalline (Mn0.6Fe0.4) (Ni0.5Co0.5) Si (MFNCS-0.5) magnetic shape memory alloy (MSMA) has been thoroughly investigated. Along with leaf-like microstructure, at room temperature, the system shows a mixed phase of TiNiSi type low-temperature high-magnetic (ferromagnetic) orthorhombic martensite and Ni2In type high-temperature low-magnetic (paramagnetic) hexagonal austenite, with the dominance of the second one. To estimate the magnetocaloric effect (MCE) across the phase transition, the isothermal magnetic measurement technique has been employed. The merging of heating and cooling iso-field thermo-magnetization (M-T) curves, the presence of negligible thermal hysteresis in the MH curves, universal magnetic entropy curves, a proposed phenomenological model (across the Curie temperature), and the positive slopes of the Arrott plot (Banerjee criterion) show that the first-order structural and magnetic transition are coupled along with a second-order ferromagnetic to paramagnetic (FM → PM) phase transition (SOPT). By extrapolating the Arrott plot to the H/M = 0 axis the spontaneous magnetization value very close to 0 K, Ms(0), is found to be 71.92 emu/g (∼1.83 µB/f.u.). The linear dependency of MS2T on T2 below SOPT and the RWR (Rhodes–Wolhfarth ratio) = 4.29 > 1 indicate the long-range itinerant ferromagnetism in this alloy. A giant magnetic entropy change (ΔSm) of 25.77 J kg−1 K−1 and a refrigerant capacity (RC) of 472.83 J kg−1 are obtained at 6 T field. This observed magnetic refrigerant's key characteristics, including a giant magnetocaloric effect (MCE) near room temperature (RT), negligible thermal hysteresis loss, a wide operating temperature range of 127 K, and the use of cost-effective, non-toxic constituent elements, make it a suitable and very promising member for applications in solid-state refrigeration technology near RT.

Giant magnetocaloric effect and second order phase transition in PrMnO3

Magnetocaloric effect and the order of magnetic phase transitions of polycrystalline PrMnO3 have been investigated through magnetization measurements. The amount of magnetic entropy-change has been calculated by Maxwell relation and the maximum value of -△SMmax was found to be 23.4 Jkg-1K−1 at 5 T applied magnetic field. Near Curie temperature (TC = 104 K), PrMnO3 exhibits a second order paramagnetic to ferromagnetic phase transition. The order of magnetic phase transition is determined with the help of Arrott plot and Landau temperature dependent coefficient b(T). A 3D magnetic entropy plot is also implemented to further evaluate the order of magnetic phase transition. The fact that this material has large value of magnetic entropy change pairedwith second order magnetic phase transitionat a low strength appliedmagnetic field indicates its potential suitability for use in magnetic refrigeration devices.

Superexchange antiferromagnetism and cryogenic magnetocaloric effect in scheelite-type GdTi0.5Mo0.5O4 compound

Scheelite-type LnLiF4 compounds have become promising cryogenic magnetic refrigerants due to their giant low-field magnetocaloric effect (MCE). This work presents an example of a scheelite-type rare-earth transition metal oxide GdTi0.5Mo0.5O4. The magnetic interactions, magnetic phase transition (MPT) and MCE of GdTi0.5Mo0.5O4 compound were investigated through magnetization measurements and a mean-field approach. A mean-field model was used to elucidate the role of primarily superexchange antiferromagnetic (AFM) coupling on the magnetocaloric performance in this compound. From the analysis of the magnetization dependence, an isotropic AFM exchange coupling with energy scale εex = JnnS2 ≈ 1.7 K was estimated. The magnetic susceptibility reveals the occurrence of MPT from paramagnetic (PM) to AFM at the Néel temperature (TN ≈ 1.3 K). GdTi0.5Mo0.5O4 exhibits a large magnetic entropy change up to 43.3 J/(kg·K) at 1.3 K for μ0ΔH = 0−7 T, which makes it a cryogenic magnetic refrigerant with potential applications. Furthermore, a mean-field model with superexchange AFM above 3 K can well predict the MCE observed in GdTi0.5Mo0.5O4.

Synthesis, structure and properties of RNiBN (R = rare-earth elements)

We report the structural and physical properties of new nitridoborates RNiBN (R = Nd, Sm, Gd, Tb, Dy, Ho, and Er). These compounds crystallize in a tetragonal LaNiBN-type structure with two-dimensional Ni2B2 and RN layers. The physical properties measurements indicate that they are antiferromagnetic metals. For R = Gd and Ho, a successive magnetic transition was observed below N é el temperature, possibly associated with spin re-orientation due to Ruderman-Kittel-Kasuya-Yosida interaction. A giant magnetocaloric effect was measured in the R = Er and Ho compounds, which is related to a field-induced first-order metamagnetic transition from antiferromagnetic to ferromagnetic state. Our results indicate that RNiBN (R = Er and Ho) may be promising candidates for low temperature magnetic refrigeration

Effect of Melt-Spinning Parameters on the Structure and Properties of Ni55.5Mn18.8Ga24Si1.7 Heusler Alloy Ribbons.

Ni-Mn-based Heusler alloys are known to demonstrate magnetic shape memory and giant magnetocaloric effect (MCE). These effects depend on the phases, crystallographic and magnetic phase transitions, and the crystallographic texture characteristics. These structural characteristics, in turn, are a function of the processing parameters. In the current work, Ni55.5Mn18.8Ga24Si1.7 Heusler alloy was processed by melt-spinning under a helium atmosphere. This process results in a fine microstructure. The ribbon that was produced with a narrower nozzle width, faster wheel speed, and higher cast temperature, indicating a faster cooling rate, had double the magnetic entropy change close to room temperature. However, the other ribbon demonstrated a large entropy change over a broader temperature range, extending its usability. The effect of the melt-spinning process parameters on the developing microstructure, crystallographic structure and texture, transformation temperatures, and the magnetic entropy change were studied to explain the difference in magnetocaloric behavior.

Magneto-structural transitions driven giant magnetocaloric effect in Ni42Mn43Cr4Sn11 Shape Memory Heusler Alloy near room temperature

In the current work, a Ni–Mn-X (X = Sn, In) type off-stoichiometric, polycrystalline magnetic shape memory alloy (MSMA) having nominal composition Ni42Mn43Cr4Sn11 (at. %) has been thoroughly investigated in terms of its crystal structure, microstructure, magnetic properties, and magnetocaloric effect (MCE). X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX), differential scanning calorimetry (DSC), and magnetic and magneto-transport measurements are used to characterize it. By offering a significant value of parameters related to MCE at the first-order martensite transition (FOMT) and the second-order magnetic transition (SOMT), the “Cr” doping at the “Ni” site enables considerable magnetic refrigeration to occur close to room temperature (RT). The alloy crystallises into coexisting cubic L21 austenite and tetragonal L10 martensite phases, among which the first one prevails as the leading phase at room temperature (RT) without exhibiting any geometric frustration. Lattice strain is enumerated using the Williamson-Hall method. A huge working temperature spans of 29.65 K and 26.16 K; the maximum magnetic entropy changes of +8.70 J kg−1.K−1 and −5.04 J kg−1.K−1 and the maximum refrigerant capacities (RC) of 284.24 J. Kg−1 and 234.10 J. Kg−1 across FOMT and SOMT, respectively at an applied magnetic field of 6 T are found to be most suitable for real technological applications. This MSMA offers a comparatively low cost of the constituent elements (Ni, Mn, Cr, and Sn), as well as exhibits giant MCE and RC across both FOMT and SOMT, making it an excellent candidate for magnetic refrigeration near RT. It also opens up new avenues for future research on novel Heusler alloys of this type.

Giant magnetocaloric effects of MnNiSi-based high-entropy alloys near room temperature

The materials combining high-entropy alloys with good mechanical properties and magnetic materials with a significant magnetocaloric effect are among the best candidates for magnetic refrigeration. For this purpose, the high-entropy alloys (MnNiSi)1-x(FeCoGe)x (x = 0.4, 0.42, 0.45, and 0.47) were designed in this study. The effects of the Si/Ge ratio on the magnetocaloric effect and mechanical properties were also investigated. The results show that high-entropy alloy (MnNi)0.6Si0.62(FeCo)0.4Ge0.38 possesses a Vickers hardness value of 580.6HV2 and compressive strength of 267 MPa with respect to a significant entropy change of 48.5 J·kg−1K−1 under a magnetic field of 5 T near the Curie temperature of 308.8 K. In order to clearly understand the origin of this excellent comprehensive performance, the influence mechanism of crystal structure and configurational entropy were explored. Furthermore, the first-principles calculations indicated that the magnetic properties mainly originate from Mn atoms located in 3d sites and are influenced by the interaction between the Mn and Fe atoms. Additionally, the total magnetic moment and electron density are taken into account to reveal their influence on magnetocaloric properties.

Calculation of magnetocaloric effect with regard for dependence of heat capacity on magnetic field

A specific heat of the magnetic solid exhibiting AFM–FM phase transition is computed using the Landau-type theory of phase transitions. The experimentally observed dependence of the specific heat value on the external magnetic field is modelled. It is shown, that this dependence has strong influence on the giant magnetocaloric effect (MCE), which is inherent to the solids exhibiting the phase transitions accompanied by the strong change of magnetization value: the disregard of this dependence leads to the noticeable overestimation of adiabatic temperature change, which is the practically important characteristic of MCE. The temperature change characterizing the giant MCE observed in Fe–Rh alloys is computed. The reasonable agreement between the available experimental data and obtained theoretical results is demonstrated.

Giant dielectric constant, magnetocaloric effect and spin-phonon coupling in EuTbCoMnO6 semiconductor

Transport, dielectric, magnetic and Raman studies of polycrystalline double perovskite EuTbCoMnO6 have been performed. Temperature-dependent resistivity follows the variable range and small polaron hopping mechanism. The temperature-dependent dielectric study shows the usual frequency-dependent step-like behavior. It also shows thermally activated relaxor peak in loss curves with giant dielectric constant near room temperature. The temperature-dependent magnetization demonstrates a second-order phase transition around 113 K. The critical behavior near the magnetic transition has been systematically investigated. The Widom scaling relation and magnetic entropy change calculation have verified the reliability of critical parameters. Maximum magnetic entropy change around the magnetic transition temperature shows the magnetocaloric effect. The estimated values of critical parameters are close to the value of the theoretical mean-field model. Temperature-dependent Raman study shows spin-phonon coupling in the system. The appearance of the magnetocaloric effect and colossal dielectric constant shows that these materials are interesting for real-life applications.

Layering strategies for active magnetocaloric regenerators using MnFePSi for heat pump applications

The development of affordable magnetocaloric materials (MCM) with a giant magnetocaloric effect (MCE) has brought magnetocaloric heat pumps a step closer to commercialization. The narrow temperature range in which these materials exhibit a large MCE demands the use of several materials with Curie temperatures covering the temperature span of the heat pump in a so-called layered active magnetocaloric regenerator (AMR). How to place these materials in the AMR in terms of distribution of Curie temperatures and thickness of each layer is still a topic of study. In this research we used a one dimensional numerical model to unveil potential benefits of either using a distribution of Curie temperatures that follows a sigmoidal shape or using thicker layers at the cold and hot ends of the AMR along with a linear distribution of Curie temperatures. We found that these AMRs are less sensitive to changes in the hot and cold reservoir temperatures compared to an AMR that uses just a linear distribution of Curie temperatures with uniform layer length, but only the one with thicker ends produces similar heating capacities and second law efficiencies. The heating capacity of the simulated AMR with a sigmoidal distribution of Curie temperatures varies only 5.6 % in a high utilization scenario, flow rate 37.5 g/s and a frequency of 0.75 Hz, when the hot side temperature changes from 308 K to 312 K and the temperature span is 18 K while the corresponding change is 8.7 % for the AMR with thicker end layers, and 37.9 % for the one with a linear distribution of Curie temperatures. For the considered geometry and operating conditions, the maximum heating capacities with temperature span 27 K in the high utilization scenario are 28.6 W, 23.0 W, and 28.5 W, whereas the corresponding second law efficiencies are 33.2%, 27.3 %, and 32.7% for the AMRs with linear distribution of Curie temperatures, sigmoid distribution, and linear distribution with thicker ends respectively.

Giant magnetocaloric effect in the Co-doped Tb5Si2Ge2 by establishing magnetostructural coupling

The magnetostructural coupling materials exhibit excellent magnetocaloric properties, making them ideal candidates for magnetic refrigerants. Tb5Si2Ge2 reveals diverse phase transition behaviors, but its structural and magnetic phase transitions are decoupled, which restricts its potential applications in magnetic refrigeration technology. Here, we demonstrate that upon the addition of Co into the Ge site of Tb5Si2Ge2, the compound can obtain a strong magnetostructural coupling between the structural and the magnetic transition, which can significantly improve the magnetocaloric properties. It is observed that the magnetic entropy change under a magnetic field change of 5 T is increased from − 18.30 Jkg−1K−1 to − 31.34 Jkg−1K−1 after adding the magnetic element Co into the compound. The temperature-averaged entropy change (TEC) at the working range of 3 K under 1 T is also improved 2.1 times, exhibiting an enhanced low-field magnetocaloric performance. In addition, theoretical calculations show that a large lattice entropy change of 21.5 Jkg−1K−1 can be achieved after the magnetostructural transformation in the Co-doped compound. The results demonstrate that Co doping can be used to construct a stable magnetostructural transition, transforming Tb5Si2Ge2 into an excellent magnetic refrigerant.

Anomalous magnetic entropy changes of bulk and powder Mn0.5Fe0.5-xNi1+xSi0.94Al0.06 intermetallic system

The multi-component MnTX (T = Ni, Co; X = p-block elements) intermetallic system have attracted intensive attention due to their tunable magneto-structural phase transition and associated giant magnetocaloric effect. Here, we report an experimental study on the magnetic, magnetocaloric, and electrical properties of alloys that belong to the MnTX family- Mn0.5Fe0.5-xNi1+xSi0.94Al0.06. For all values of x (0≤ x ≤0.10), the materials exhibited a first-order magneto-structural phase transition from a low-temperature ferromagnetic orthorhombic phase to a high-temperature paramagnetic hexagonal phase. The transition was accompanied by large magnetic entropy changes of up to –22 and −57 J kg−1K−1 at T = 322 K for field changes of 2 T and 5 T, respectively. Most noticeably, a sharp jump in electrical resistivity was observed at T = 320 K for x = 0.1, which confirmed that the first-order phase transition preserved the mechanical stability of the material. The alloy with x = 0.1 was further crushed into powder and the magnetic properties were compared to its bulk counterpart. The enhanced mechanical stability and nearly zero magnetic hysteresis loss along with the fact that the system is constituted of cheap, non-toxic elements make this system worthy of consideration for near-room temperature magnetic refrigeration applications.

Giant Magnetocaloric Effect in Magnets Down to the Monolayer Limit.

2D magnets can potentially revolutionize information technology, but their potential application to cooling technology and magnetocaloric effect (MCE) in a material down to the monolayer limit remain unexplored. Herein, it is revealed through multiscale calculations the existence of giant MCE and its strain tunability in monolayer magnets such as CrX3 (X=F,Cl,Br,I), CrAX (A=O,S,Se;X=F,Cl,Br,I), and Fe3 GeTe2 . The maximum adiabatic temperature change ( ), maximum isothermal magnetic entropy change, and specific cooling power in monolayer CrF3 are found as high as 11K, 35µJm-2 K-1 , and 3.5nWcm-2 under a magnetic field of 5T, respectively. A 2% biaxial and 5% a-axis uniaxial compressive strain can remarkably increase of CrCl3 and CrOF by 230% and 37% (up to 15.3 and 6.0K), respectively. It is found that large net magnetic moment per unit area favors improved MCE. These findings advocate the giant-MCE monolayer magnets, opening new opportunities for magnetic cooling atnanoscale.

Room temperature giant magnetocaloric effect in Pd doped FeRh and the effect of martensitic transition

FeRh 0.8 Pd 0.2 is known to exhibit near room-temperature first-order antiferromagnetic (AF)–ferromagnetic (FM) transition, typical of chemically ordered FeRh system. In addition, it is also reported to show martensitic transition at a lower temperature. In this work, the effect of sample history on transition temperature (Tt) and the magnetocaloric effect (MCE) have been studied. The experimentally determined MCE parameters across FM to AF transition induced either by isothermal magnetic field sweep or by temperature sweep showed a large isothermal change in entropy around room temperature, i.e., 14 J/kg K for 50 kOe magnetic field change, whereas MCE corresponding to AF–FM transition depends on cooling history. Our study shows that in the presence of martensite phase, the peak value of MCE is shifted to higher temperatures but with significantly reduced magnitude.

Role of Magnetism in Lattice Instability and Martensitic Transformation of Heusler Alloys

Heusler alloys are subject of considerable interest because they exhibit a martensitic transformation (MT), a shape-memory effect and a giant magnetocaloric effect. As it is commonly believed, the pronounced magnetoelastic coupling plays a crucial role; however, the effect of alloy composition on MT is still under discussion. To describe the features of MT in Ni0.75−xMnxGa0.25 Heusler alloys, the phenomenological model that consistently considers the magnetic and lattice degrees of freedom and their mutual interplay has been developed. The magnetic entropy contribution was estimated within the framework of the microscopic approach. The proposed model allows us to describe the dependence of the martensitic transformation start temperature Ms(x) on the Mn concentration x in reasonable agreement with the experiment.

Improving work temperature span and reversibility of magnetoelastic transition of (Mn,Fe)2(P,Si) alloys by Mg doping

Magnetocaloric materials with wide working temperature span and reversible magnetic transition are beneficial for the magnetic refrigerator. Besides of giant magnetocaloric effect, (Mn,Fe)2(P,Si) alloys exhibit significant thermal/magnetic hysteresis and a narrow working temperature span. In this work, the impact of Mg doping on the phase structure and magnetocaloric effect of Mn1.05Fe0.9P0.5Si0.5-xMgx alloys has been studied. The results show that all samples crystallize into Fe2P-type main phase with little impurity phase. After Mg doping, the lattice parameter a of the main phase increases from 6.027(0) Å to 6.049(0) Å and c decreases from 3.488(6) Å to 3.480(6) Å, which leads to the increase of Curie temperature. Density functional theory calculation results confirm that the Mg atom located on the 2c site can increase the covalent bond length, and then enhance the ferromagnetic coupling between atoms leading to the increase of Curie temperature. Moreover, the first-order magnetic phase transition of Mn1.05Fe0.9P0.5Si0.5-xMgx alloys is inhibited with the increase of Mg content, which results in the decrease of thermal/magnetic hysteresis and the increase of the effective working temperature span. For Mn1.05Fe0.9P0.5Si0.48Mg0.02 alloy, it has a Curie temperature of about room temperature, effective refrigeration capacity of 291.7 J·kg−1, working temperature window of 46 K and magnetic entropy change of 9.4 J·kg−1·K−1. The alloy can be an excellent candidate in room temperature magnetic refrigeration devices with a wide temperature span.

Critical Behavior of the Competing Magnetic Nature in the Antiperovskite Compound

Antiperovskite materials are deemed as a promising candidate for data storage and efficient energy conversion, exemplified by giant magnetoresistance (GMR) and magnetocaloric effect (MCE). In particular, the abundant magnetic configurations of antiperovskites endow them with a high degree of diversity in probing novel phenomena. Herein, we report the critical behavior of the antiperovskite Sn0.6NFe3.4 with an obvious second-order magnetic transition around Curie temperature (TC = 230 K). The obtained critical exponents (β, γ, and δ) are well consistent for different theoretical methods, which are also verified by the Widom scaling law and scaling equation. It is found that the critical exponent of β is much in accordance with the value of the mean-field model, indicating a long-range FM coupling in the system, whereas the other γ and δ are accessible to the three-dimensional (3D) Heisenberg model. Our work promotes the understanding of antiperovskite magnetism, providing insight for further fundamental research and resulting applications.

Giant low-field reversible magnetocaloric effect at liquid helium temperature of niobium and iron co-substituted EuTiO3 compounds

Giant magnetocaloric effect (MCE) materials in the liquid helium temperature region have attracted a lot of attention in the field of low-temperature magnetic refrigeration (MR). In this study, a series of niobium (Nb) and iron (Fe) co-substituted EuTiO3 perovskites with cubic structure (space group pm3m) was successfully fabricated, and their magnetic properties as well as cryogenic magnetocaloric effects were investigated in detail. As expected, the introduction of Nb and Fe can significantly modulate the magnetic phase transition and magnetocaloric effect of the EuTiO3 compounds. With increasing Fe concentration, two local minima corresponding to the AFM-FM magnetic phase transition near 5.0 K and FM-PM transition near 10 K with no hysteresis in the thermomagnetic curves are observed, which is attributed to an enhancement of FM coupling. At the same time, the gradually widened –ΔSM–T curves and the two peaks with a broad shoulder lead to considerable refrigeration capacity (RC). With the field change of ΔH = 2 T, the calculated values of −ΔSMmax for the EuTi0.9375–xNb0.0625FexO3 (x = 0.075, 0.1, 0.125, 0.15) compounds are 24.2, 17.6, 14.5 and 14.0 J/(kg·K), respectively. The corresponding RC values were calculated to be 144.6, 138.3, 151.2 and 159 J/kg, respectively. Especially, the values of −ΔSMmax for EuTi0.8625Nb0.0625Fe0.075O3 are 8.6 and 15.1 J/(kg·K) under low field changes of 0.5 and 1 T, respectively. The giant low-field reversible magnetocaloric effect makes them attractive candidates for magnetic refrigeration in the liquid helium temperature region.

Crystal electric field level scheme leading to giant magnetocaloric effect for hydrogen liquefaction

In recent years, magnetic refrigeration has attracted considerable attention for hydrogen liquefaction. Most materials used for magnetic refrigeration contain heavy rare earth ions with complex crystalline electric field energy splittings, whose effect on the magnetic entropy change ΔSM has not been systematically studied. In particular, the theoretical upper limits of ∣ΔSM∣ for general heavy earth cases are unknown. Here, we show that the crystalline electric field level schemes result in a large ΔSM for general heavy rare earth cases. We provide a specific example of the magnetic refrigeration material HoB2 using inelastic neutron scattering experiments combined with mean-field calculations with crystal field splitting and exchange interactions. The relationship between ΔSM and crystal field parameters presented in this study can be useful for developing compounds with a large ∣ΔSM∣ and advancing the design of magnetic refrigeration materials.

Influence of the particle size on a MnFe(P,Si,B) compound with giant magnetocaloric effect

How the microstructure affects first-order magnetic transitions (FOMT) in materials with giant magnetocaloric effect remains poorly understood. Here, we study the FOMT and giant magnetocaloric effect occurring near room temperature in MnFe0.95P0.575Si0.36B0.065 particles with sizes ranging from 300 μm down to less than 15 μm. While this materials system shows a volume preserving FOMT, large anisotropic lattice discontinuities make it particularly sensitive to particle size. Grinding and sieving may lead up to 80% difference on the isothermal entropy change (ΔS). Differential scanning calorimetric measurements reveal that the decrease in ΔS does not only originate from the broadening of the transition but also involves a sudden drop in latent heat when particles are reduced from 54 μm to 31 μm, a range corresponding to about the average grain size of the bulk (26 μm). Thermal hysteresis is the largest in large particles and decreases when reducing the particle size.