Isothermal Entropy Change Research Articles

Nb-doping in (Mn,Fe)2(P,Ge) giant magnetocaloric materials

The influence of Nb substitutions on the structure, magnetoelastic transition, and magnetocaloric properties is investigated in MnFe1−xNbxP0.76Ge0.24 (x = 0, 0.02, 0.04, 0.6) compounds. Powder XRD show that the compounds crystallize in the hexagonal Fe2P-type crystal structure (space group P6̄2 m) and contain minor amount of MnO impurity phase. At a low applied magnetic field (<1.6 T), the isothermal entropy change of the Nb-substituted MnFe0.98Nb0.02P0.76Ge0.24 sample is higher than that of the parent or more substituted samples, reaching a maximum of 14.0 J kg−1 K−1. Nb substitutions are found to efficiently reduce the thermal hysteresis in Ge-based Fe2P magnetocaloric materials; for instance, only 6 at. % of Nb for Fe substitution reduces the thermal hysteresis from 7.6 to 2.6 K while preserving giant magnetocaloric effect. Direct ΔTad measurements confirm the cyclic character of the giant magnetocaloric effect for x = 0.02.

Thermodynamics and entropic inference of nanoscale magnetic structures in Gd

A bulk gadolinium (Gd) single crystal exhibits virtually zero remnant magnetization, a common trait among soft uniaxial ferromagnets. This characteristic is reflected in our magnetometry data showing virtually hysteresis free isothermal magnetization loops with large saturation magnetization. The absence of hysteresis allows to model the measured easy axis magnetization as a function of temperature and applied magnetic field, rather than a relation, which permits the application of Maxwell relations from equilibrium thermodynamics. Demagnetization effects broaden the isothermal first-order transition from negative to positive magnetization. By analyzing magnetization data within the coexistence regime, we deduce the isothermal entropy change and the field-induced heat capacity change. Comparing the numerically inferred heat capacity with relaxation calorimetric data confirms the applicability of the Maxwell relation. Analysis of the entropy in the mixed phase region suggests the presence of hitherto unresolved nanoscale magnetic structures in the demagnetized state of Gd. To support this prediction, Monte Carlo simulations of a 3D Ising model with dipolar interactions are performed. Analyzing the cluster size statistics and magnetization from the model provides strong qualitative support of our analytic approach.

Anisotropic magnetic critical behavior of van der Waals room-temperature ferromagnet Fe5GeTe2 identified by magnetocaloric measurements

Due to the room-temperature Curie temperature and large saturation moment, Fe5GeTe2 is considered a highly attractive van der Waals ferromagnet. Studying its magnetic critical behavior can provide valuable information about its magnetic phase transition. Notably, compared with the conventional methods for studying magnetic critical behavior, such as the modified Arrott plot, scaling analysis based on isothermal magnetic entropy change ΔSM(T,H) has advantages in dealing with the complex magnetic system Fe5GeTe2. However, studies on its magnetic critical behavior based on this method remain completely lacking. Here, we investigate the magnetic critical behavior of Fe5GeTe2 based on its ΔSM(T,H). Through scaling analysis of its ΔSM(T,H), two sets of reliable critical exponents β, δ, and γ are obtained, which are 0.320(8), 7.99(1), and 2.24(2) for H//ab and 0.494(2), 4.28(4), and 1.62(3) for H//c. The significant difference between H//ab and H//c indicates strong anisotropy in its magnetic critical behavior. Furthermore, the fact that the obtained critical exponents for both H//ab and H//c cannot be simply described by a single universality class reveals a crossover of magnetic interactions in Fe5GeTe2.

Direct and indirect assessment of the elastocaloric properties of cast NiMnTi alloys

The study of the elastocaloric effect of NiMnTi alloys is a key topic for developing materials for sustainable and efficient solid-state cooling and heating applications. In the current work, the mechanical behavior of two arc melted and heat treated NiMnTi alloys with 15 and 18 at% of Ti has been investigated. The effects of heat treatments and operating conditions on the mechanical and caloric properties of the alloys have been assessed through calorimetric analysis, isothermal stress-strain compressive measurements, and adiabatic tests. To evaluate the caloric performance of the NiMnTi alloys, both experimental and theoretical adiabatic temperature changes have been identified, and the isothermal entropy change involved in the stress-induced martensitic transformation has been computed from the discrete integration of the stress-strain curves. The NiMnTi alloys treated at 900 °C exhibited better caloric performance than those treated at 1000°C. Specifically, the sample that achieved the highest experimental positive and negative ΔT values (10.2 °C and −12.6 °C, respectively) was the NiMnTi alloy with 15 at% of Ti and heat treated at 900 °C. This study provides a detailed analysis of the physical properties, functionality, and caloric properties of polycrystalline NiMnTi fabricated through a melting process, considering its potential use in solid-state cooling and heat pumping technologies.

Large and reversible elastocaloric effect induced by low stress in a Ga-doped Ni-Mn-Ti alloy

The Solid-state cooling based on caloric effects is considered a potential alternative to conventional refrigeration technology, which uses ozone-depleting gases. Several shape memory alloys have attracted attention for solid-state cooling since they present a high reversibility of the caloric effect, which depends mainly on thermal hysteresis and sensitivity to the applied field. In the present work, a study substitution of Mn by Ga in the Ni50Mn34Ti16 alloy led to diminished thermal hysteresis in the martensitic transformation by 6 K. The elastocaloric effect, thermal and microstructure properties of a polycrystalline Ni50Mn32Ti16Ga2 alloy have been characterized. The elastocaloric effect was obtained indirectly from the length change as a function of temperature at constant stress. An isothermal entropy change (ΔSISO) of 23.0 J kg−1 K−1 during heating and 22.0 J kg−1 K−1 during cooling was observed for applied stress of 160 MPa. In addition, the ΔSISO is reversible for a temperature span between 287 and 319 K, reaching a maximum of 20.5 J kg−1 K−1 at 299 K. The thermal hysteresis changed slightly while the applied stress increased up to 160 MPa since the sensitivity of the martensitic transformation temperatures to stress was 0.150 K M/MPa during cooling and 0.160 K/MPa during heating. The X-ray diffraction analysis revealed a mixture of B2-type cubic austenite, 5 M modulated martensite, and a second intermetallic phase identified as Ni3Ti. All these results were obtained around room temperature.

Magnetocrystalline anisotropy energy of AYbO3 (A= Sr, Ba and Ca) perovskite oxides

First-principles calculations and Monte Carlo simulations were employed to explore the effect of magnetic anisotropy on magnetocaloric properties and the magnetocrystalline anisotropy energy (MAE) of AYbO3 (A = Sr, Ba and Ca) perovskite oxides. The equilibrium lattice constant a0 is calculated AYbO3 (A = Sr, Ba and Ca). The elastic constants and related mechanical quantities such as bulk modulus B, Zener anisotropy factor A and Cauchy pressure Cp were calculated. CaYbO3 and SrYbO3 exhibit the half-metallic characteristic. While BaYbO3 exhibits semiconductor character. Sr, Ba and Ca atoms located in the A site affected the values of magnetocrystalline anisotropy energy (MAE) so changing the sign of the magnetic anisotropy constant in SrYbO3. The magnetic anisotropy shows that the easy axis is along the axis [100] for SrYbO3, CaYbO3 and BaYbO3 respectively. The critical temperature around Tc = 65 K using the MCS method. The isothermal magnetic entropy change was calculated for BaYbO3 under different magnetic fields and magnetic anisotropy. The highest isothermal entropy change obtained at H = 5T and D = 0 is 5.51 J/kg.K. While it’s equal to 8.10 J/kg.T at H = 5T and D =5.

On the nonmagnetic atom disorder influence in the magnetic and magnetocaloric properties of the Er2Cu0.92Si2.76 off-stoichiometric single-phase compound

This paper presents and discusses material aspects, as well as the magnetic and magnetocaloric properties of nominal Er2Cu0.92Si2.76 alloy synthesized by arc melting. The stoichiometric alloy, prepared with a precise 2(Er):1(Cu):3(Si) ratio, adopts dual symmetries manifesting AlB2-type hexagonal and ThSi2-type tetragonal structures. It was observed that stoichiometric deviations by reducing both Cu/Si concentrations imply a decrease in the tetragonal phase fraction. An 8 % reduction in Cu and Si contents leads to the formation of an off-stoichiometric single-phase: the Er2Cu0.92Si2.76 phase - the focus of this investigation. Notably, Er2Cu0.92Si2.76 showed a uniform grain microstructure without porous and with a homogeneous elemental composition. On the other hand, a long-range antiferromagnetic ordering around 5.6 K was noticed, with a weak influence of magnetic frustration likely arising from nonmagnetic atomic disorder and vacancy at the Cu/Si site. An external magnetic field (greater than 10 kOe) induces ferromagnetic-like ordering, with a saturation tendency at ≈ 50 kOe, corresponding to half the expected magnetic moment for fully ordered Er3+ ions. Er2Cu0.92Si2.76 exhibits large magnetocaloric properties, with isothermal entropy change and relative cooling power values reaching 14.6 J/kgK and 312 J/kg, respectively, for a magnetic field change of 50 kOe. The absence of magnetic and thermal hysteresis is also an attractive prospect for this material aimed at magnetic refrigeration applications at low temperatures.

A point of view of the elastocaloric effect associated with 7M and 5M modulated martensite in Ni–Mn–Ga alloys

Solid-state refrigeration has emerged as the most promising alternative to conventional refrigeration technology. However, for this technology to be applicable, the caloric effects produced in the alloys must be highly reversible. In this context, we compare the elastocaloric effect of two Ni–Mn–Ga alloys with different types of modulated martensite. The elastocaloric effect, quantified as the isothermal entropy change (ΔSela), was investigated in Ni50Mn28Ga22 and Ni50Mn30Ga20 alloys with 5M and 7M modulated martensite, respectively. Maximum ΔSela values obtained were 1.91 J kg−1 K−1 during cooling and 1.83 J kg−1 K−1 during heating in martensite 5M and 0.19 J kg−1 K−1 during cooling and 0.26 J kg−1 K−1 during heating in martensite 7M, for a constant applied stress of 10 MPa. However, although the 7M modulated martensite exhibited a lower ΔSela, its reversibility was higher. Therefore, our results could be useful for selecting a good material to be used in solid-state refrigeration.

Giant magnetocaloric effect of Ni-Co-Mn-Ti all-d Heusler alloys in high magnetic fields

Ni-Co-Mn-Ti all-d Heusler alloys are attracting considerable attention for solid-state caloric cooling applications due to their promising combination of excellent caloric and mechanical properties. Here, we report on the maximum attainable magnetocaloric effect in Ni37Co13Mn34.5Ti15.5, which shows a first-order magnetostructural martensitic transformation around room temperature. Heat capacity measurements reveal a giant transition entropy change of 43.5J(kgK)−1 and are utilized to estimate the magnetocaloric effect as well as the magnetic fields required to saturate it in isothermal and adiabatic conditions. Confirming the results based on this approach, we achieve maximum isothermal entropy changes and directly measured adiabatic temperature changes of 37.8J(kgK)−1 and −20.2K, respectively. Thus, the herein reported maximum attainable magnetocaloric effect outperforms classical Ni-Mn-based Heusler alloys, such as Ni(-Co)-Mn-In. Especially the saturated adiabatic temperature change surpasses all previously published values of magnetic field-induced first-order phase transitions measured around room temperature in pulsed magnetic fields in recent years. Thereby, we demonstrate that Ni(-Co)-Mn-Ti Heusler alloys are particularly suitable for the application of sufficiently large external stimuli to fully induce the phase transition and exploit their intrinsically large caloric effect.

Enhanced magnetocaloric effect in a frustrated spin-5/2 Ising hexagonal bipyramid

We study the thermodynamic properties of a frustrated Ising spin-5/2 hexagonal bipyramidal cluster. By employing exact enumeration, we investigate the ground-state phase diagram and the roles of thermal fluctuations and external magnetic field on the behavior of magnetization and entropy. The magnetocaloric effect of this nanosystem is also investigated by computing the isentropes in the temperature-magnetic field plane and the isothermal entropy change. The ground state exhibits magnetization plateaus at 1/4 and 1/2 of the saturation magnetization and a large degeneracy can be found at specific strengths of exchange couplings and magnetic field. The large ground-state degeneracy leads to an enhanced magnetocaloric effect manifested in the isentropic curves. More importantly, a significant reduction of temperature can be achieved by the isentropic removal of a very weak external field for a wide range of exchange couplings. Therefore, our results indicate that the frustrated spin-5/2 Ising hexagonal bipyramidal cluster is a nanostructure with potential application in efficient magnetic cooling.

Structural, Magnetic, and Magneto-Thermal Properties of Rare Earth Intermetallic GdRhIn.

We study the structural, magnetic, and magneto-thermal properties of the GdRhIn compound. The room-temperature X-ray diffraction measurements show a hexagonal crystal structure. Temperature and field dependence of magnetization suggest two magnetic transitions-antiferromagnetic to ferromagnetic at 16 K and ferromagnetic to paramagnetic at 34 K. The heat capacity measurements confirm both the magnetic transitions in GdRhIn. The magnetization data were used to calculate isothermal magnetic entropy change and refrigerant capacity in GdRhIn, which was found to be 10.3 J/Kg-K for the field change of 70 kOe and 282 J/Kg for the field change of 50 kOe, respectively. The large magnetocaloric effect in GdRhIn suggests that the material could be used for magnetic refrigeration at low temperatures.

Magnetostructural transformation and magnetocaloric response in Mn(Fe)NiSi(Al) alloys

An efficient magnetocaloric refrigerant must have certain characteristics such as a sharp transition near the desired working temperature, a large cyclic magnetocaloric response, and the use of raw materials with reduced costs and low environmental consequences. In this sense, this work focuses on a series of rare-earth- and Co-free Mn0.5Fe0.5NiSi1-xAlx alloys (x = 0.0525, 0.060, 0.0685) with promising magnetocaloric properties. The alloys were synthesized using combined arc melting and induction melting techniques, as this synthesis route provides improved control on the sample composition and homogeneity. We investigated how the heat treatment temperature and Al content affect the magnetostructural and magnetocaloric properties of the alloys. On the one hand, it is found that annealing at 1173 K for 7 days leads to a sharp magnetostructural transformation with no traces of impurities for the alloy with x = 0.0525. Under these conditions, a large isothermal entropy change of –11.5 J kg−1 K−1 for 1 T is obtained near room temperature, significantly improving the value of the as-cast sample. On the other hand, following this optimal heat treatment, the influence of Al content is studied: upon increasing the Al concentration the magnetostructural transformation shifts to lower temperatures, ranging from 320 K for x = 0.0525 to 220 K for x = 0.0685 (measured upon heating).

An in-depth comparison of dielectric, ferroelectric, piezoelectric, energy storage, electrocaloric, and piezocatalytic properties of Ba0.92Ca0.08Zr0.09Ti0.91O3 and Ba0.92Ca0.08Sn0.09Ti0.91O3 piezoceramics

The futuristic technology demands materials exhibiting multifunctional properties. Keeping this in mind, an in-depth investigation and comparison of the dielectric, ferroelectric, piezoelectric, energy storage, electrocaloric, and piezocatalytic properties have been carried out on Ba0.92Ca0.08Zr0.09Ti0.91O3 (BCZT) and Ba0.92Ca0.08Sn0.09Ti0.91O3 (BCST) lead-free compounds synthesized through the conventional solid state reaction method. Further, the Rietveld refinement against the XRD patterns affirmed the coexistence of orthorhombic (Amm2) and tetragonal (P4mm) phases at room temperature for both compounds. While BCZT compound demonstrated remarkable dielectric, piezoelectric, and piezocatalytic characteristics with maximum dielectric constant () ≈ 13304 near the Curie temperature ( ≈ 105 °C), converse piezoelectric coefficient ≈ 609 pm V-1 , power density () ≈ 89.71 KW cm-3 at room temperature, and 82.7 % Rhodamine B (RhB) dye degradation capability; on the other hand, BCST compound exceled in superior energy storage density () ≈ 191.8 mJ cm-3 with efficiency η ≈ 73.26% near room temperature and enhanced electrocaloric properties with adiabatic temperature change ≈ 0.980 K, isothermal entropy change ≈ 1.15 J Kg-1K-1 and a higher electrocaloric responsivity ) ≈ 0.49 K mm kV-1. These distinctions arise from the difference in ionic radii, lattice distortion, and the electronic configurations of the dopant ions, leading to a greater polarization change and a lower coercive electric field in the BCST ceramic. The outcome of the present study provides valuable insights for selecting a dopant tailored compound for specific application and underscores the potential of such ceramics in energy storage, piezocatalysis, and the development of advanced solid-state cooling devices for future applications.

Establishing universality in entropy change and critical behavior for magnetocaloric effect in Si-substituted Mn50Ni40Sn10 inverse Heusler alloy

Magnetic refrigeration working based on the magnetocaloric effect can be the perfect replacement of the conventional gas compression-based refrigeration technology and reduces its harmful effects on the environment. The boundary between a first-order and a second-order phase transition would be where the perfect magnetocaloric material would be found. Therefore, establishing the sequence of phase transitions clearly is essential for the characterization of other phase change materials and for applied magnetocaloric research. A quantitative fingerprint of second-order thermomagnetic phase transitions is reported here in Si-substituted high content Mn-based inverse Heusler alloy systems, which are found to be crystallized in cubic structures. The second-order nature of the phase transition has been confirmed from the Arrott plot analysis and a correlation between magnetocaloric effect and local exponent is established. Using the Arrott plot, the critical exponents are evaluated employing different techniques such as modified Arrott plot, Kouvel–Fisher method, and critical isotherm. Their values are found to be in great agreement with each other and follow the mean-field model signifying the presence of long-range ordering in the materials. The high value of isothermal magnetic entropy change and the reversibility justifies the suitability of the reported materials in the practical application as magnetic refrigerants.

Structure, Magnetic and Magnetocaloric Properties of Attempted P Substitutions in LaFe<sub>11.5</sub>Si<sub>1.5</sub> Giant Magnetocaloric Material

Recent theoretical and experimental studies suggested that P can enter the structure of La(Fe,Si)13 alloys and lead to a significant enhancement of the isothermal entropy change, one of the two main quantities characterizing the magnetocaloric effect. Here, we report a systematic study of P for Si substitutions in La(Fe,Si)13 alloys. Eight LaFe11.5Si1.5-xPx polycrystalline bulk samples with 0 ≤ x ≤ 0.2 were prepared by arc-melting followed by heat treatment. Powder x-ray diffraction and SEM/EDX analyses show that the α-Fe secondary phase progressively increases with the increase in P substitutions and that a La-rich LaP secondary phase appears. We therefore found that P does not actually enter the main La(Fe,Si)13 phase. Magnetization and DSC measurements confirm this interpretation as the Curie temperatures of La(Fe,Si,P)13 alloys are nearly insensitive to P for Si substitutions and the latent heat of the first-order ferromagnetic transition decreases with the increase in nominal P substitutions. Our work put into questions former reports of the literature on P addition in La(Fe,Si)13 and highlights the particularly complex synthesis of these alloys.

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.

Structural and magnetic properties of Mn1.25Fe0.7P0.5Si0.5 alloys prepared by spark plasma sintering using raw materials milled with different lengths of time

Spark plasma sintering (SPS) is an effective method for synthesizing the Fe2P-type alloys due to its fast heating rate and short sintering time. Raw materials in the form of powder are sintered and pressed simultaneously in the preparation. Therefore, the particle size of the powder would have important influences on the structural and magnetic properties of the Fe2P-type alloys. In this work, the structure and magnetic properties of Mn1.25Fe0.7P0.5Si0.5 alloys prepared by SPS using raw materials milled with different lengths of time were investigated. We found that the particle size of the raw materials reduced mainly within milling time of 5 h and only slight difference existed when the milling time exceeded 5 h. Obvious secondary phase of (Mn,Fe)3Si was observed in the alloy prepared with milling time of 1 h. Other alloys prepared with milling time of 2 h, 5 h and 10 h showed similar magnetic properties. Giant isothermal entropy change values of 7.4 ∼ 9.6 J ⋅ kg−1 ⋅ K−1 for a magnetic field change of 1.5 T were obtained near room temperature for the alloys. The results demonstrate that a milling time longer than 2 h is required to obtain such magnetocaloric Mn-Fe-P-Si alloys with excellent magnetic properties using SPS method and the obtained alloys are promising candidates for magnetic refrigeration near room temperature.

Layered composite magnetic refrigerants for hydrogen liquefaction

Many exciting effects resulting from the coupling of magnetic sublattice with a magnetic field, may be exposed by changing the field. One such phenomenon is the magnetocaloric effect, which is characterized by the absorption or emission of heat in response to changes in the external magnetic field. Magnetic refrigeration based on the magnetocaloric effect has emerged as an attractive alternative to conventional cooling technology that relies on gas compression and expansion. It is not only more efficient and environmentally friendly, but it can also be implemented across a broad temperature range, from ultra-low to a few hundred Kelvin temperatures. One of the areas where magnetic cooling can have a significant impact is hydrogen liquefaction. Hydrogen is one of the most promising candidates for clean energy sources, but it must be liquefied to facilitate storage and transportation, which requires cooling it down to ∼20 K. The ideal magnetic refrigerant should exhibit consistent magnetocaloric properties across the entire operating temperature range of a cooler. This paper presents novel three-layer composite magnetic refrigerants that provide a uniform magnetocaloric response over a 30 K temperature range. The selected initial HoNi2, DyNi2, and TbNi2 magnetic intermetallic compounds with a Laves phase structure exhibit large magnetocaloric properties in the temperature range of 13–37 K. The composite composition of 23.56 wt% HoNi2 + 18.21 wt% DyNi2 + 58.23 wt% TbNi2 optimized for 2 T magnetic field change, was determined through numerical approach. The composites are manufactured using spark plasma sintering (SPS) and innovative high-isostatic-pressure (HIP) synthesis. The results of isothermal entropy change derived from magnetization data for a 2 T magnetic field change indicated 4.7 J/kgK (47.2 mJ/cm3K) and 4.4 J/kgK (44.2 mJ/cm3K) in the temperature range of approx. 13–42 K for composites after SPS and SPS + HIP processes, respectively. Despite the sample subjected to HIP showing slightly lower results, the entropy change is nearly uniform over the 30 K temperature range due to enhanced atomic diffusion between neighboring compounds, as confirmed by microscopic studies. Such a uniform magnetocaloric response in ∼30-K temperature range has never been observed before in layered refrigerants. By utilizing the innovative high-isostatic-pressure synthesis technique, we have paved the way to high-performing magnetic composite materials that can be used in cryogenic magnetic coolers operating over a broad temperature span, expanding the possibilities of what can be achieved and laying the foundation for cost-effective, clean hydrogen energy.

Impact of magnetic, atomic and microstructural ordering on the magnetocaloric performance of powdered NiCoMnSn metamagnetic shape memory ribbons

Co-doped NiMnSn Heusler-type metamagnetic shape memory alloys (MMSMAs) are promising materials for the next-generation solid-state refrigeration systems due to their excellent magnetocaloric performance around the martensitic transformation, which is easily tuneable by slight changes in the alloy composition. An improvement in the thermal efficiency of active magnetic regenerator devices, a key element in magnetocaloric cooling systems, arises by obtaining powdered magnetocaloric alloys that meet technical requirements for their implementation as a feedstock material in the additive manufacturing of 3D-printed heat exchangers. In the present work, powders of Mn-rich NiCoMnSn Heusler-type MMSMAs were obtained from their ribbon form avoiding or minimizing residual stresses, the number of defects and disorder in the crystal lattice and microstructure. Since atomic order and crystallographic structure are crucial in the transformation and magnetic properties of these alloys, a complementary structural analysis of the powders after different heat treatments was performed by powder neutron diffraction. The results show that the cubic austenitic phase of the non-heat-treated melt-spun powder exhibits a highly stressed structure, which leads to an incomplete martensitic transformation and, therefore, to the coexistence of martensitic and austenitic phases at low temperatures. The magnetic structure of the austenite phase was also determined by neutron powder diffraction, obtaining a ferromagnetic coupling between 4a and 4b Wyckoff positions in the samples analysed. It was found that a heat treatment facilitates the martensitic transformation and enables the formation of a pure martensitic phase.The observed changes in the magnetocaloric performance of the powders have been understood in terms of the differently stressed structures and their impact on the martensitic transformation. A fully completed structural transformation leads to a significant increase of the magnetisation change across the martensitic transformation and, consequently, to high values of both a magnetic field induced isothermal entropy change and refrigeration capacity.

The effect of metal/non-metal ratio on the microstructure and magnetic properties of (MnFe)x(P0.5Si0.5) microwires

The effects of metal/non-metal ratio (M/NM ​= ​x: 1) on the microstructure and magnetocaloric properties of promising melt-extracted Mn–Fe–P–Si microwires with short heat treatment have been investigated here. More Fe2P principal phase, which is considered favorite for magnetocaloric effect (MCE), should achieve at low M/NM ratio and the fraction of Fe2P phase increased with the reduction of x. Meanwhile, the (Mn, Fe)3Si impurity phase is formed for x ​= ​2.00–1.90 whereas change to (Mn, Fe)5Si3 structure for x ​= ​1.85. It's worth noting that a metal deficiency resulted in the thermal hysteresis (Thys) and the magnetic hysteresis loss (Wy) decreased by ∼40 ​%., The magnetic transition temperature (Ttran), peak value of isothermal magnetic entropy change (−ΔSisopeak), refrigerant capacity (RC) and effective refrigerant capacity (RCE) first increased then decreased with the decrease of x, and reached the maximums at x ​= ​1.90, i.e., 370 ​K, 26.0 ​J ​kg−1 ​K−1, 367.4 and 339.8 ​J ​kg−1, respectively. Therefore, the customizable microstructure and magnetic properties of the melt-extracted (MnFe)x(P0.5Si0.5) microwires will be achievable effectively by tuning M/NM ratio x, and optimized Mn–Fe–P–Si compounds with novel thermomagnetic properties will be obtained.