Large ΔSM Research Articles

Enhanced Magnetocaloric Properties of the (MnNi)0.6Si0.62(FeCo)0.4Ge0.38 High-Entropy Alloy Obtained by Co Substitution.

In order to improve the magnetocaloric properties of MnNiSi-based alloys, a new type of high-entropy magnetocaloric alloy was constructed. In this work, Mn0.6Ni1-xSi0.62Fe0.4CoxGe0.38 (x = 0.4, 0.45, and 0.5) are found to exhibit magnetostructural first-order phase transitions from high-temperature Ni2In-type phases to low-temperature TiNiSi-type phases so that the alloys can achieve giant magnetocaloric effects. We investigate why chexagonal/ahexagonal (chexa/ahexa) gradually increases upon Co substitution, while phase transition temperature (Ttr) and isothermal magnetic entropy change (ΔSM) tend to gradually decrease. In particular, the x = 0.4 alloy with remarkable magnetocaloric properties is obtained by tuning Co/Ni, which shows a giant entropy change of 48.5 J∙kg-1K-1 at 309 K for 5 T and an adiabatic temperature change (ΔTad) of 8.6 K at 306.5 K. Moreover, the x = 0.55 HEA shows great hardness and compressive strength with values of 552 HV2 and 267 MPa, respectively, indicating that the mechanical properties undergo an effective enhancement. The large ΔSM and ΔTad may enable the MnNiSi-based HEAs to become a potential commercialized magnetocaloric material.

Enhancing magnetocaloric properties of NiMnGa-based alloys via Dy micro-alloying and pseudoelastic cyclic training

Considerable interest to improve magnetic entropy change (ΔSm) and broaden working temperature interval (WTI) of NiMnGa ferromagnetic shape memory alloys (FESMAs) was stimulated by their applications as promising candidate materials for solid-state refrigeration. In the present study, we presented an approach to enhance the magnetocaloric properties of polycrystalline NiMnGa FESMAs via combining Dy micro-alloying and pseudoelastic cyclic training. The introduction of Dy elements established stable magneto-structural coupling transformation from the paramagnetic austenite to ferromagnetic martensite, accompanied by a large ΔSm [−16.42 J/(kg K)] and a widened WTI (∼15.98 K). Fascinatingly, it was demonstrated that the internal strain fields at phase interface between matrix and DyNi4Ga precipitates could assist the phase transformation nucleation, which significantly reduced the hysteresis loss from 20.84 J/kg of Ni54Mn25Ga21 alloy to 8.14 J/kg of Ni54Mn25Ga20.7Dy0.3 alloy. More importantly, the subsequent pseudoelastic cyclic training produced a strong ⟨110⟩NM preferred crystallographic orientation, which facilitated the magnetic alignment along easy magnetization axis. Consequently, the giant ΔSm value up to −24.25 J/(kg K) and effective refrigeration capacity RCeff of 198.77 J/kg were further achieved in the trained Ni54Mn25Ga20.7Dy0.3 alloy under an external magnetic-field change of 5.0 T.

Impact of strain on the magnetocaloric effect of oxide heterostructures

We report a detailed study of the magnetocaloric effect in heterostructures of La2NiMnO6 and La2/3Sr1/3MnO3. The shape, width and magnitude of the temperature dependence of the magnetic entropy change (ΔSm) for these multilayer samples are affected by their layout with a clear impact from epitaxial strains on each layer. A large ΔSm over a wide temperature range which goes beyond room temperature is observed in all samples. We observe a temperature-independent table-top-like ΔSm over a temperature range as large as 100K in the trilayer samples. La2NiMnO6 double perovskite with multiple magnetic phase transitions sensitive to strain is the key to tuning and shaping the temperature dependence of the magnetic entropy changes to suit the requirements of several cooling cycles.

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.

Large magnetocaloric effect in DO3-type Fe3Al alloys

We have synthesized DO3-structured Fe3Al binary alloys and studied the magneto-caloric properties. The samples are prepared using a vacuum arc melting furnace and annealed in a vacuum. X-ray diffraction studies confirm ordered cubic DO3 crystallographic phase formation. We have employed Rietveld analysis, to obtain the refined lattiace parameter. The magnetic properties are determined from thermo-magnetic data. The sample exhibited ferromagnetic nature below 879 K marking the transition temperature (Tc). We have derived the change in the magnetic entropy (ΔSM) and calculated the refrigeration capacity (RC) of Fe3Al alloys. This is the first report of relatively large ΔSM recorded over a wide temperature range in binary alloys.

Magnetocaloric properties of LaFe11.4Si1.6 based amorphous alloys

The LaFe13-xSix type magnetic refrigeration material exhibits a large ΔSM but in a narrow temperature range about 180–220 K, which limits its application. In order to elevate the magnetic transition temperature, the LaFe11.4Si1.6 alloy is alloyed with B and Nb to prepare amorphous alloys (LaFe11.4Si1.6)(96-x)/14BxNb4 (x = 14.8, 19, 23.2). Under a magnetic field of 5 T, its maximum magnetic entropy change is 2.67 Jkg−1K−1, but the temperature range can exceed 180 K, which yields a refrigeration capacity (RC) of about 417 Jkg-1 comparable to that of crystalline LaFe13-xSix materials. It is intriguing that this amorphous alloy is a good soft magnetic material with higher saturation magnetization and lower coercivity compared to the crystalline material. It is more interesting that, after minor alloying of B and Nb, this amorphous alloy has a higher TC of about 500 K compared to the crystalline LaFe13-xSix. Our works show amorphizing the crystalline LaFe11.4Si1.6 alloy imparts it a wider magnetic transition temperature range, thus increasing the RC value.

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

Gd1-xHoxNi 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 (TC) 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 TC, the SRO transition temperature is almost invariable for all compounds. Two peaks of magnetic entropy change (ΔSM) 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 ΔSM around TC is almost same. The other maximum of ΔSM 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 ΔSM peaks were obtained in Gd1-xHoxNi 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. Gd1-xHoxNi ribbons could be good candidate for magnetic refrigerant working in the low temperature especially near the liquid nitrogen temperature range.

Magnetic properties and large magnetocaloric effects of GdPd intermetallic compound

With the intention to explore excellent magnetocaloric materials, the intermetallic compound GdPd was synthesized by arc melting and heat treatment. The microstructure, magnetic and magnetocaloric properties of the intermetallic compound of GdPd were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and the physical property measurement system (PPMS). A large reversible magnetocaloric effect is observed in GdPd accompanied by a second order magnetic phase transition from paramagnetism to ferromagnetism at ∼39 K. The paramagnetic Curie temperature (θp) and the effective magnetic moment (μeff) are determined to be 34.7 K and 8.12 μB/Gd, respectively. The maximum entropy change (|ΔSMMax|) and the relative cooling power (RCP) under a field change of 5 T are estimated to be 20.14 J/(kg·K) and 433 J/kg, respectively. The giant reversible magnetocaloric effects (both the large ΔSM and the high RCP) together with the absence of thermal and field hysteresis make the GdPd compound an attractive candidate for low-temperature magnetic refrigeration.

Magnetocaloric behavior of Mn rich Ni46Cu2Mn43In11 alloy

In this work, we studied the magnetic entropy change (ΔSM) across the martensite transformation (MT) in Mn-rich Ni46Cu2Mn43ln11 alloy. This compound undergoes a MT and a magnetic phase transition around the temperatures (TM=) 272 K and (TCA=) 325 K, respectively. A large field induced shift (=0.28 K/kOe) of the MT temperatures is observed. An application of magnetic field (H =) of 50 kOe causes a large ΔSM of 20 J/kg-K and -4.4 J/kg-K around TM and TCA, respectively. We also found that the change in magnetic field induced isothermal ΔSM(H)T is mainly depends on the induced austenite phase fraction by the applied magnetic field at that temperature. Possible reasons for the observed behaviours are comprehensively discussed.

Effects of crystallization treatment on the structure and magnetic properties of Gd65Fe25Zn10 alloy ribbons for magnetic refrigeration

The structure and magnetic properties of Gd65Fe25Zn10 alloy ribbons prepared by melt-spinning followed by crystallization annealing were investigated. In the as-spun ribbons, there is a small amount of α-Gd nanocrystalline existed in the amorphous matrix. After crystallization treatment, besides the pre-precipitated α-Gd phase, GdFe2 and GdZn crystalline phases precipitated in turn in the amorphous matrix. The Curie temperature (TC) of the fully amorphous matrix is ∼250 K. Owing to α-Gd nanocrystalline precipitated in the amorphous matrix, a second order magnetic transition occurs near 288 K for both the as-spun and the as-annealed Gd65Fe25Zn10 ribbons. For a field change of 2 T, the maximum magnetic entropy change (−ΔSM)max for the as-spun and as-annealed ribbons are 1.56–2.31 and 3.83–4.06 J/(kg·K), while the values of refrigeration capacity (RCFWHM) are 224.0–387.0 and 149.5–158.0 J/kg, respectively. The as-spun Gd65Fe25Zn10 ribbons display relatively low (−ΔSM)max but the largest RCFWHM value due to the wide (−ΔSM) versus T curve peak. With negligible thermal/magnetic hysteresis losses, relative large (‒ΔSM)max and its peak temperature near room temperature, the Gd65Fe25Zn10 alloy ribbons can be used as the potential magnetic refrigeration material working at room temperature.

A comparative study of critical phenomena and magnetocaloric properties of ferromagnetic ternary alloys

Magnetic and magnetocaloric properties, as well as the phase diagrams of a ferromagnetic ternary alloy system have been studied. A detailed comparison of two different methods, namely the effective field theory (EFT), and Monte Carlo (MC) simulations has been provided. Our numerical data show that the general qualitative picture presented by two methods are in a good agreement with each other. In terms of the magnetocaloric properties, our results yield that it is possible to design magnetic materials with a variety of working temperatures and magnetocaloric properties (such as large ΔSM and q values) by manipulating the magnetic phase transition via tuning the compositional factor (i.e. the mixing ratio of sublattice ions). The observed magnetocaloric effect has been found to be a direct one with ΔSM < 0 associated with a second order phase transition.

Large magnetic refrigerant capacity of HoFe1−xCoxAl (x = 0, 0.3) compounds

Magnetic and magnetocaloric properties of HoFe1−xCoxAl (x = 0, 0.3) were investigated. Both HoFeAl and HoFe0.7Co0.3Al undergo a second-order ferromagnetic (FIM) to paramagnetic (PM) transition at Curie temperatures (TC) of 87 and 82 K, respectively. The magnetocaloric effect is improved by the introduction of Co in HoFeAl compound. For a field change from 0 to 5 T, the maximum values of magnetic entropy change (−ΔSM) are 7.0 J·kg−1·K−1 for HoFeAl and 8.6 J·kg−1·K−1 for HoFe0.7Co0.3Al. In addition, the refrigerant capacity (RC) is enhanced largely from 416.2 J·kg−1 for HoFeAl to 561.9 J·kg−1 for HoFe0.7Co0.3Al. This large RC is attributed to the large ΔSM and the wide temperature span of ΔSM peak in HoFe0.7Co0.3Al compound. The physical mechanism of improvement in magnetocaloric effect by Co substitution in HoFeAl was also discussed in detail.

Magnetocaloric effect over a wide temperature range in GdxHo3−xAl2 compounds with successive magnetic transitions

The GdxHo3−xAl2 (x = 0.5–2.5) compounds undergo two successive magnetic transitions: a second-order magnetic transition from paramagnetism (PM) to ferromagnetism (FM) around TC followed by a FM to ferrimagnetism (FIM)-like transition at Tt. The TC increases linearly with the increase of Gd content due to the de Gennes factor of Gd being larger than that of Ho, while the Tt remains nearly constant around 45 K independent of the variation of Gd content. The working temperature range is expanded by substituting Ho with Gd atoms due to the higher transition temperature of Gd3Al2 and the different composition dependence of successive magnetic transitions, and thus leading to an enhanced refrigerant capacity (RC). For a relatively low field change of 2 T, a giant RC value of 272 J/kg with a wide working temperature range of ∼80 K is obtained for x = 1.0 compound due to the partial overlap of successive ΔSM peaks. This is probably the largest RC value so far reported in the temperature range of liquid natural gas (111 K). Consequently, the large ΔSM and giant RC suggest that GdxHo3−xAl2 compounds could be good candidates of magnetic refrigerants in a wide temperature range.

Large low field magnetocaloric effect in first-order phase transition compound TlFe3Te3 with low-level hysteresis.

Magnetic refrigeration based on the magnetocaloric effect (MCE) is an environment-friendly, high-efficiency technology. It has been believed that a large MCE can be realized in the materials with a first-order magnetic transition (FOMT). Here, we found that TlFe3Te3 is a ferromagnetic metal with a first-order magnetic transition occurring at Curie temperature TC = 220 K. The maximum values of magnetic entropy change (Δ) along the crystallographic c-axis, estimated from the magnetization data, reach to 5.9 J kg−1K−1 and 7.0 J kg−1 K−1 for the magnetic field changes, ΔH = 0–1 T and 0–2 T, respectively, which is significantly larger than that of MCE materials with a second-order magnetic transition (SOMT). Besides the large ΔSM, the low-level both thermal and field hysteresis make TlFe3Te3 compound an attractive candidate for magnetic refrigeration. Our findings should inspire the exploration of high performance new MCE materials.

Ericsson-like giant magnetocaloric effect in GdCrO4–ErCrO4 composite oxides near liquid hydrogen temperature

In this paper, magnetic and magnetocaloric properties of RCrO4 (R=Gd and Er) oxides have been investigated. It is found that GdCrO4 and ErCrO4 exhibit a second-order magnetic transition from ferromagnetic to paramagnetic states at their Curie temperatures of 22.0K and 14.9K, respectively. The maximal magnetic entropy change (ΔSM) for a field variation of 0–5T have reached 22.8Jkg−1K−1 for GdCrO4 and 20.5Jkg−1K−1 for ErCrO4. The collective magnetic contributions from R3+ and Cr5+ ions lead to the large ΔSM. For the same field variation, table-like ΔSM of ~19.5Jkg−1K−1 and a large refrigerant capacity (RC) of 117Jkg−1 have been achieved for the composite material formed by GdCrO4 and ErCrO4 with the mass ratio of 1:1 from 16.0K to 22.0K. Since the liquid hydrogen temperature (20.3K) locates within this range, large reversible magnetocaloric effect, high RC as well as the low cost make GdCrO4–ErCrO4 composite material a potential candidate for magnetic refrigerant for the storage of liquid hydrogen.

Enhanced magnetic refrigeration properties in Mn-rich Ni-Mn-Sn ribbons by optimal annealing.

The influence of annealing time on temperature range of martensitic phase transition (ΔTA-M), thermal hysteresis (ΔThys), magnetic hysteresis loss (ΔMhys), magnetic entropy change (ΔSM) and relative refrigeration capacity (RC) of the Mn-rich Ni43Mn46Sn11 melt spun ribbons have been systematically studied. By optimal annealing, an extremely large ΔSM of 43.2 J.kg−1K−1 and a maximum RC of 221.0 J.kg−1 could be obtained respectively in a field change of 5 T. Both ΔTA-M and ΔThys decreases after annealing, while ΔMhys and ΔSM first dramatically increase to a maximum then degenerates as increase of annealing time. A large effective cooling capacity (RCeff) of 115.4 J.kg−1 was achieved in 60 min annealed ribbons, which increased 75% compared with that unannealed ribbons. The evolution of magnetic properties and magnetocaloric effect has been discussed and proved by atomic ordering degree, microstructure and composition analysis.

Magnetothermopower, magnetic entropy change, and magnetoresistance in ferromagnetic cobaltites: R0.7Sr0.3CoO3 (R = La and Gd)

We compare the impact effect of external magnetic fields on the magnetic entropy (Sm), electrical resistivity, and thermopower in two metallic oxides R0.7Sr0.3CoO3 that become ferromagnetic below TC = 240 K and 117 K for R = La3+(4f0) and Gd3+(4f7), respectively. While Gd0.7Sr0.3CoO3 shows unusually high values of negative magnetothermopower (≈−35.5%), magnetic entropy change (ΔSm ≈ −12 J/kg K), and negative magnetoresistance (≈−3.7%) at 10 K for a field change of 5 T, these effects are absent in the La sample at 10 K. We suggest that the large ΔSm arises from the field-induced suppression of spin fluctuations associated with localized 4f7 moments of Gd3+ ions, but magnetic interaction among Co ions dictates magnetoresistance and magnetothermopower. Magnetization data indicate that while Co3+ ions adopt intermediate spin state (S = 1) in the La compound, major fraction of them transforms into low spin state (S = 0) in the Gd sample. Hence, the Gd sample is microscopically heterogeneous with a mixture of ferromagnetic metallic and paramagnetic metallic clusters. Spin dependent scattering at the surface of ferromagnetic clusters may be the cause for the coexistence of magnetothermopower and magnetoresistance in this compound. A close correlation between the magnetoresistance and the magnetothermopower is reported.

Effect of particle size reduction on the magnetic phase transition and the magnetocaloric properties in ferromagnetic insulator La0.9Sr0.1MnO3 nanoparticles

The effect of the particle size reduction on ferromagnetic (FM) phase transition and magnetocaloric properties of the perovskite manganite La0.9Sr0.1MnO3 was studied. Indeed, it has been proven that the maximum of entropy change (−ΔSMmax) and the relative cooling power (RCP) are strongly affected by the particle size. The measured entropy change (ΔSM) was analyzed using Landau theory. The obtained results have been explained in terms of core–shell model. Moreover, H12 (La0.9Sr0.1MnO3 annealed at 1200°C) having large RCP and ΔSM values could be an interesting cooling material.

Magnetic properties and low-temperature large magnetocaloric effect in the antiferromagnetic HoCu0.33Ge2 and ErCu0.25Ge2 compounds

Magnetic properties and magnetocaloric effect (MCE) of HoCu0.33Ge2 and ErCu0.25Ge2 compounds have been investigated. The compounds were determined to be antiferromagnetic with the Néel temperatures TN=9K and 3.9K, respectively. The critical transition magnetic fields for the metamagnetic transition from antiferromagnetic to ferromagnetic state below TN were determined to be 10kOe for HoCu0.33Ge2 at 5K and 6kOe for ErCu0.25Ge2 at 2K. Large MCE with the maximal values of magnetic entropy changes (ΔSM) as −10.2J/kgK at 10.5K were found in HoCu0.33Ge2 for field changes of 0–70kOe and −10.5J/kgK at 5.5K in ErCu0.25Ge2 for field changes of 0–50kOe, respectively. The large ΔSM around TN as well as no hysteresis loss made RCuxGe2 competitive candidates as low temperature magnetic refrigerant.

Influence of Ce addition on the structural, magnetic, and magnetocaloric properties in La0.7−xCexSr0.3MnO3 (0≤x≤0.3) ceramic compound

We report improvement in the magnetocaloric properties of Ce-doped lanthanum manganites. Polycrystalline La0.7−xCexSr0.30MnO3 (0≤x≥0.3) samples were prepared using the conventional solid-state reaction method with phase purity and structure confirmed using X-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal second order ferromagnetic transition in parent sample and as well as in doped sample with Curie temperature decreasing progressively with increasing Ce-concentration from ~370K for x=0.0 to 310K for x=0.30. Magnetic entropy change (ΔSM) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field dependent magnetization curves. A large ΔSM associated with the ferromagnetic–paramagnetic transition in La0.7−xCexSr0.30MnO3 samples has been observed. The value of ΔSM was found to increase with Ce-doping up to x=0.15 and the highest value of 2.12Jkg−1K−1 (at ΔH=2T) was observed for La0.55Ce0.15Sr0.30MnO3 sample near the Curie temperature of 356K. Also, improved relative cooling power of ~122Jkg−1 was observed for the same sample. Due to the large magnetic entropy change and high Curie temperature, the La0.55Ce0.15Sr0.30MnO3 sample is suggested to be used as potential magnetic refrigerants for magnetic refrigeration technology above room temperature.