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Here, we present the results of an experimental and numerical study of controllable spin-wave propagation in the yttrium iron garnet stripe with an Fe-Rh slab. The transformation of spin-wave dispersion and transmission are observed by the focusing laser light on top of the Fe-Rh slab. It was shown that the geometry variation and design of the Fe-Rh slab significantly affect spin-wave propagation. The static internal magnetic field profile in the yttrium iron garnet is subject to position and the geometric parameters of the Fe-Rh slab. Additionally, the possibility of width mode filtering was shown for a surface magnetostatic spin wave by the means of micromagnetic simulation. The qualitative correspondence between the behavior of spin waves in the numerical simulation and microwave spectroscopy data was observed. The proposed structure can be used as a functional unit with the simultaneous frequency selective and spin-wave mode filtration regime in planar magnonic networks. [ABSTRACT FROM AUTHOR]

NiMnGa-based Heusler alloys are known for their shape-memory effect. However, in this study, the focus will be on the magneto-thermal (magnetocaloric) properties of rapidly solidified Ni50Mn28Ga22 ribbons melt spun at 1300 and 1600 RPM as well as annealed bulk specimens. The ribbons, both as-spun and annealed, and the annealed bulk specimens were tested in a SQUID to determine the magnetic properties at a field of 50 kOe with a temperature step of 3 K in the temperature range 355–385 K. The magnetic data from the isotherms were used to achieve Arrott plots. Second-order transitions were observed in the materials at the TC temperature. The values of magnetic entropy ΔSm were calculated from the magnetic data and these were further used to calculate the values of refrigeration capacity RC. The highest RC value of 273 J/kg was obtained for 1300NMG5800 where 1300, 800 and 5, represent the melt spinning rate in RPM, the annealing temperature in Celsius and the duration of annealing in hours, respectively. The values of ΔSm were similar for all the ribbons. To confirm the order of the magnetic transition, universal curves were plotted which led to the conclusion that the transformation at the Curie temperature is a second-order ferromagnetic to paramagnetic transition. [ABSTRACT FROM AUTHOR]

Multiferroic materials with strong coupling between different degrees of freedom are prone to exhibit giant multicaloric effects resulting from the application or removal of diverse external fields. These materials exhibit a synergic response to the combined action of two fields when the monocaloric effects are both conventional (or both inverse), while a non-synergic response occurs when one of the monocaloric effects is conventional and the other is inverse. In all cases, the multicaloric properties (isothermal entropy and adiabatic temperature changes) do not result from the simple addition of the corresponding monocaloric quantities because there is a contribution from the interplay between degrees of freedom (cross-coupling term). In this paper, we analyse in detail the contribution of the cross-coupling term to the multicaloric entropy values obtained for both synergic and non-synergic multicaloric materials. We first introduce basic thermodynamic concepts accounting for the multicaloric effects, and next the contribution from the cross-coupling term is illustrated via several model examples. We finally analyse the realistic situation for two prototype materials with synergic and non-synergic multicaloric effects.

Magnetocaloric hydrogen liquefaction could be a ‘game-changer’ for liquid hydrogen industry. Although heavy rare-earth based magnetocaloric materials show strong magnetocaloric effects in the temperature range required by hydrogen liquefaction (77–20 K), the high resource criticality of the heavy rare-earth elements is a major obstacle for upscaling this emerging liquefaction technology. In contrast, the higher abundances of the light rare-earth elements make their alloys highly appealing for magnetocaloric hydrogen liquefaction. Via a mean-field approach, it is demonstrated that tuning the Curie temperature ( T _C ) of an idealized light rare-earth based magnetocaloric material towards lower cryogenic temperatures leads to larger maximum magnetic and adiabatic temperature changes (Δ S _T and Δ T _ad ). Especially in the vicinity of the condensation point of hydrogen (20 K), Δ S _T and Δ T _ad of the optimized light rare-earth based material are predicted to show significantly large values. Following the mean-field approach and taking the chemical and physical similarities of the light rare-earth elements into consideration, a method of designing light rare-earth intermetallic compounds for hydrogen liquefaction is used: tuning T _C of a rare-earth alloy to approach 20 K by mixing light rare-earth elements with different de Gennes factors. By mixing Nd and Pr in Laves phase (Nd, Pr)Al _2 , and Pr and Ce in Laves phase (Pr, Ce)Al _2 , a fully light rare-earth intermetallic series with large magnetocaloric effects covering the temperature range required by hydrogen liquefaction is developed, demonstrating a competitive maximum effect compared to the heavy rare-earth compound DyAl _2 .

Magnetic refrigeration is a highly active field of research. The recent studies in materials and methods for hydrogen liquefaction and innovative techniques based on multicaloric materials have significantly expanded the scope of the field. For this reason, the proper characterization of materials is now more crucial than ever. This makes it necessary to determine the magnetocaloric and other physical properties under various stimuli such as magnetic fields and mechanical loads. In this work, we present an overview of the characterization techniques established at the Dresden High Magnetic Field Laboratory in recent years, which specializes in using pulsed magnetic fields. The short duration of magnetic-field pulses, lasting only some ten milliseconds, simplifies the process of ensuring adiabatic conditions for the determination of temperature changes, $\Delta T_{\mathrm{ad}}$ . The possibility to measure in the temperature range from 10 to 400 K allows us to study magnetocaloric materials for both room-temperature applications and gas liquefaction. With magnetic-field strengths of up to 50 T, almost every first-order material can be transformed completely. The high field-change rates allow us to observe dynamic effects of phase transitions driven by nucleation and growth as well. We discuss the experimental challenges and advantages of the investigation method using pulsed magnetic fields. We summarize examples for some of the most important material classes including Gd, Laves phases, La–Fe–Si, Mn–Fe–P–Si, Heusler alloys and Fe–Rh. Further, we present the recent developments in simultaneous measurements of temperature change, strain, and magnetization, and introduce a technique to characterize multicaloric materials under applied magnetic field and uniaxial load. We conclude by demonstrating how the use of pulsed fields opens the door to new magnetic-refrigeration principles based on multicalorics and the ‘exploiting-hysteresis’ approach.

The quest for better performance from magnetocaloric devices has led to the development of thermal control devices, such as thermal switches, thermal diodes, and thermal capacitors. These devices are capable of controlling the intensity and direction of the heat flowing between the magnetocaloric material and the heat source or heat sink, and therefore have the potential to simultaneously improve the power density and energy efficiency of magnetocaloric systems. We have developed a new type of thermal control device, i.e., a silicon mechanical thermal switch capacitor ( TSC). In this paper we first review recently developed thermal switches based on micro-electromechanical systems and present the operation and structure of our new TSC. Then, the results of the parametric experimental study on the thermal contact resistance, as one of the most important parameters affecting the thermal performance of the device, are presented. These experimental data were later used in a numerical model for a magnetocaloric device with a thermal switch-capacitor. The results of the study show that for a single embodiment, a maximum cooling power density of 970 W m ^−2 (510 W kg _mcm ^−1 ) could be achieved for a zero-temperature span and an operating frequency of 5 Hz. However, a larger temperature span could be achieved by cascading multiple magnetocaloric elements with TSCs. We have shown that the compact TSC can be used in caloric devices, even with small temperature variations, and can be used in a variety of practical applications requiring thermal regulation.

[Display omitted] • XRD profiles revealed the Ruddlesden-popper phase in La 1.4-x Sm x Ca 1.6 Mn 2 O 7 (0.0 ≤ x ≤ 0.4) compounds. • With increasing the Sm-concentration the unit cell volume drastically reduced from 290.51 Å3 for x = 0.0 to 290.19 Å3, 289.76 Å3 and 289.22 Å3 for x = 0.1, 0.2 and 0.3 compounds respectively. • The planer (O(3)-Mn-O(3)) and out of plane (O(3)-Mn-O(1)) bond angles have consistently decreased from 87.87°, 78.88° for parent compound up to 85.41°, 73.58° for x = 0.3 composition respectively. • Maximum entropy change obtained for x = 0.0, 0.1, 0.2, 0.3 and 0.4 samples are 2.52 J/kgK, 4.61 J/kgK, 4.2 J/kgK, 2.48 J/kgK, 1.2 J/kgK respectively upon applying 2.5 T magnetic field. • The highest values of TEC enhanced from 4.04 J/kgK (x = 0.0) up to x = 8.5 J/kgK (x = 0.1) and 7.77 J/kgK (x = 0.2) with a Δ T H - C = 5 K. Evolution in the structural, magnetic and magnetocaloric (MCE) properties for a series of samarium (Sm) doped La 1.4-x Sm x Ca 1.6 Mn 2 O 7 (0.0 ≤ x ≤ 0.4) Ruddlesden-Popper manganites prepared by using the solid-state reaction method have been investigated. Structural refinement of XRD profiles established that the MnO 6 octahedral structural units were distorted upon Sm-substitution. The effect of Sm-concentration on the microstructure was also very well observed in terms of increased average grain size and absolute density. Discrepancy in MCE parameters by implementing the conventional isothermal curve method was appeared which was resolved with a separate calorimetric method. The isothermal entropy change was enhanced from 2.52 J/kgK for x = 0.0 sample up to 4.61 J/kgK and 4.2 J/kgK for x = 0.1 and x = 0.2 samples respectively. The temperature averaged entropy change (TEC) was also remarkably improved from 4.04 J/kgK up to 8.5 J/kgK and 7.77 J/kgK for the same compounds respectively. [ABSTRACT FROM AUTHOR]

Magnetocaloric alloys are an important class of materials that enable non-vapor compression cycles. One promising candidate for magnetocaloric systems is LaFeMnSi, thanks to a combination of factors including low-cost constituents and a useful curie temperature, although control of the constituents' phase distribution can be challenging. In this paper, the effects of composition and high energy ball milling on the particle morphology and phase stability of LaFe11.71-xMnxSi1.29H1.6 magnetocaloric powders were investigated. The powders were characterized with optical microscopy, dynamic light scattering, X-ray diffraction (XRD), and differential scanning calorimetry (DSC). It was found that the powders retained most of their original magnetocaloric phase during milling, although milling reduced the degree of crystallinity in the powder. Furthermore, some oxide phases (<1 weight percent) were present in the as-received and milled powders, which indicates that no significant contamination of the powders occurred during milling. Finally, the results indicated that the Curie temperature drops as Fe content decreases (Mn content increases). In all of the powders, milling led to an increase in the Curie temperature of ~3-6 °C. [ABSTRACT FROM AUTHOR]

In this work, we present the first theoretical analysis of the elastic, electronic, magnetic and magnetocaloric properties of 1T-phase Janus MnXO (X = S, Se and Te) monolayers using first-principle calculations based on density functional theory combined with Monte Carlo simulation. The stability of the studied monolayers has been analyzed by using the elastic constants C ij and formation energy, which indicate that all monolayers are stable. Moreover, the electronic properties reveal the metal characteristics of all monolayers, with a total magnetic moment of 3.25 μ B , 3.81 μ B and 4.09 μ B for MnSO, MnSeO, and MnTeO, respectively. Further, the extracted critical temperature values (T c) from Monte Carlo simulation are T c = 115 K, T c = 130 K and T c = 82 K, for MnSO, MnSeO and MnTeO, respectively. Interestingly, the predicted maximum magnetic entropy change −ΔS max and maximum relative cooling power RCP (and adiabatic temperature change) are 10,38 J/kg.K and 763,43 J/kg (7,11 K) for MnSO, 6,79 J/kg.K and 515,57 J/kg (3,25K) for MnSeO, and 6,39 J/kg.K and 407,16 J/kg (3,46 K) for MnTeO at a magnetic field of 5T. These results suggest that the Janus MnXO (X = S, Se and Te) monolayers are a potential materials for use in magnetic nanodevices and low-dimensional magnetic refrigeration. [ABSTRACT FROM AUTHOR]

Magnetic refrigeration is a promising alternative to traditional gas compression refrigeration technologies, as it offers significantly higher energy efficiency. One factor hindering the widespread implementation of magnetic refrigeration is the dependence on expensive and critical rare earth elements needed to produce a strong magnetocaloric effect. Inside this work we explore substituting a non-critical rare earth element, Ce, into an established magnetocaloric material of choice, GdNi. Structural characteristics are investigated via x-ray powder diffraction, and the magnetocaloric effect is evaluated indirectly from heat capacity measurements as well as calculated using the mean field method. Our results show that small substitutions of Ce into GdNi results in the dilution of the magnetocaloric effect while maintaining relative cost effectiveness for small substitutions of Ce. As Ce content is increased, the cost effectiveness of the adiabatic temperature change (ΔT ad) increases as high as 20% for (Gd0.4Ce0.6)Ni while the magnetic entropy change (-ΔS m) shows minimal improvement in cost effectiveness, falling from 3.93 J/kg*K in GdNi to 1.75 J/kg*K in (Gd 0.4 Ce 0.6)Ni at 1 T applied field. This demonstrates that using Ce substitutions can be a powerful tool in both tuning Curie temperature and mitigating the reliance on critical elements as we look to develop towards a more sustainable future. • Substituting Ce for Gd in GdNi gradually lowers MCE and T c. • Cost effectiveness is increased with increasing Ce. • Weaker temperature dependent lattice parameter changes near T c compared to GdNi. • Heat capacity behavior is well-reproduced by MFT. [ABSTRACT FROM AUTHOR]

The study reports the synthesis and characterization of the magnetic and magnetocaloric effects of metal-oxide (MO) modified La0.7Ca0.3MnO3 perovskites manganite. The powder composite samples, with a nominal composition of (1 − x)La0.7Ca0.3MnO3-xMO (Wt.% x = 0.0, 2.5, 5.0), were prepared using the facile autocombustion method, followed by an annealing process. The phase purity and structure were confirmed by X-ray diffraction. Temperature and field-dependent magnetization measurements and Arrott analysis revealed mixed first- and second-order phase transition (ferromagnetic to paramagnetic) in composite samples. The phase transition temperature shifted to lower temperatures with the addition of MO in the composite. A large magnetic entropy change (4.75 JKg−1K−1 at 1T and 8.77 JKg−1K−1 at 5T) was observed in the La0.7Ca0.3MnO3 (LCMO) sample and was suppressed, due to the presence of the MO phase in the composite samples. On the other hand, the addition of MO as a secondary phase in the LCMO samples enhanced their relative cooling power (RCP). The RCP of all composite samples increased with respect to the pristine LCMO, except for LCMO–5%NiO. The highest RCP value of 267 JKg−1 was observed in LCMO–5%CuO samples, which was 23.4% higher than the 213 JKg−1 observed for the pure LCMO at a magnetic field of 5T. The enhanced RCP of these composites makes them attractive for potential refrigeration applications.

The transition toward a carbon-neutral society based on renewable energies goes hand in hand with the availability of energy-efficient technologies. Magnetocaloric cooling is a very promising refrigeration technology to fulfill this role regarding cryogenic gas liquefaction. However, the current reliance on highly resource critical, heavy rare-earth-based compounds as magnetocaloric material makes global usage unsustainable. Here, we aim to mitigate this limitation through the utilization of a multicaloric cooling concept, which uses the external stimuli of isotropic pressure and magnetic field to tailor and induce magnetostructural phase transitions associated with large caloric effects. In this study, La 0.7 Ce 0.3 Fe 11.6 Si 1.4 is used as a nontoxic, low-cost, low-criticality multiferroic material to explore the potential, challenges, and peculiarities of multicaloric cryocooling, achieving maximum isothermal entropy changes up to -28 J (kg K) -1 in the temperature range from 190 K down to 30 K. Thus, the multicaloric cooling approach offers an additional degree of freedom to tailor the phase transition properties and may lead to energy-efficient and environmentally friendly gas liquefaction based on designed-for-purpose, noncritical multiferroic materials.

This study explores the magnetic and magnetotransport behavior of polycrystalline ErAl 3 compound. The polycrystalline compound adopts HoAl 3 -type structures with the R-3m space group, No. 166-2 and hR60 configurations. Multiple magnetic orderings and two field-induced metamagnetic transitions are observed. ErAl 3 exhibits a significant magnetocaloric effect (MCE),-ΔSM= 15.25 J kg -1 K -1 and high relative cooling power of 383 J kg -1 with applied magnetic field change (Δ H ) of 70 kOe near the paramagnetic to ferromagnetic transition, showcasing its potential for magnetic refrigeration technology. The compound also demonstrates metallic behavior, with a notable magnetoresistance of 48.5%at 2 K due to the suppression of antiferromagnetism. The magnetic phase diagram reveals four distinct phases influenced by temperature and magnetic field, identified through the study of the MCE., (© 2024 IOP Publishing Ltd.)

Prototyping innovative energy devices is a complex multivariable dimensioning problem. For the case of magnetocaloric systems, one aims to obtain an optimized balance between energy conversion performance, useful power generated, and power consumed. In these devices, modeling is entering a mature phase, but dimensioning is still time consuming. We have developed a technique that dimensions any type of magnetocaloric system by training statistical learning classifiers that are used to simulate the computation of a very large number of systems with different combinations of parameters to be dimensioned. We used this method in the dimensioning of a magnetocaloric heat pump aiming at optimizing the temperature span, heating power, and coefficient of performance, obtaining an f-score of 95%. The respective classifier was used to mimic over 940 thousand computed systems. The gain in computation time was 300 times that of computing numerically the system for each combination of parameters.

A single-step, reactive hot-pressing technique was used to synthesize dense, polycrystalline samples of the nanolaminated MAB phase, Fe 2 AlB 2. The process yielded nearly single-phase stoichiometric Fe 2 AlB 2 materials containing only Al 2 O 3 as the impurity phase in a fraction of the synthesis time typically required for the more commonly used arc-melting methods. To determine crystalline, magnetic, and stoichiometric properties, samples were analyzed by X-ray diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and temperature-dependent vibrating sample magnetometry. The results of both magnetic techniques show magnetic and magnetocaloric properties similar to those obtained in the literature by other methods such as multi-day arc melting and annealing. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Modeled hydrogen liquefication from 80 to 20 K in a multi-stage AMR system using permanent magnets with 1-Tesla field. • Achieved over 60% of Carnot COP with the multistage design with optimized operation conditions. • Compared two types of 1D AMR models: steady state and time-dependent models. Hydrogen (H 2) is promising alternative to replace fossil fuels, but its transport and storage has been challenging. As H 2 fuel cell vehicles are gaining traction, the infrastructure for storing large amounts of liquid H 2 is needed. However, liquid H 2 would suffer from boil-off loss, and traditional vapor compression refrigeration systems would not be able to economically recover the lost H 2 due to the low efficiencies at cryogenic temperature. Magnetocaloric (MC) refrigeration systems could possess much higher coefficient of performance (COP) at cryogenic temperature compared to the vapor compression ones. Previous work on cryogenic MC systems, however, have only focused on large scale applications which use superconducting magnets to provide a large magnetic field but are prohibitively expensive to operate for small scale applications, such as that of a H 2 refilling station. In this work, we model the performance of a MC refrigeration cycle using 1-Tesla permanent magnets for H 2 liquefaction, with the objective of cooling H 2 from 80 K (using liquid nitrogen as the heat sink) to 20 K (boiling point of hydrogen). We evaluate main performance metrics including the total work input to the refrigeration system, COP, total MCM mass in the system, and total volume of the permanent magnets, etc. Our modeling results indicate that such a permanent magnet-based MC cooling system is feasible for small-scale H 2 liquefaction, with projected COP values significantly higher than those of vapor compression systems. This work provides design guidelines for future experimental efforts on permanent magnet MC cooling systems for cryogenic cooling. [ABSTRACT FROM AUTHOR]

Ferromagnetic-structural transformation has been studied widely in MnCoGe-based materials. However, the magnetostructural transition (MST) from antiferromagnetic (AFM) orthorhombic phase to ferromagnetic (FM) hexagonal phase, which may lead to a large inverse magnetocaloric effect (MCE), has rarely been reported. Here, the introduction of Mn vacancy lowers the structural transition temperature while retains the AFM state in the orthorhombic phase, thus successfully realizing the AFM-FM MST in Mn0.95Co0.75Cu0.25Ge. Moreover, successive inverse and normal MCEs are observed around the first-order AFM-FM MST and the second-order FM-paramagnetic (PM) transition, respectively. A thermostat is proposed based on this special feature, which could release heat above the critical temperature while absorb heat below the critical temperature by simply applying the same magnetization/demagnetization cycles. This thermostat can be very useful in many applications where a constant temperature is required, such as cryostats and incubators. [ABSTRACT FROM AUTHOR]

In this paper, we present studies of the influence of B-site substitution of Mn (5% and 10%) on magnetic and magnetocaloric properties of La 0.5 Sr 0.5 CoO 3. Polycrystalline samples of La 0.5 Sr 0.5 Co 1-x Mn x O 3 (x = 0, 0.05, 0.1) are synthesized via solid state reaction route. Ferromagnetism is observed in all the compounds with enhancement in magnetisation and simultaneous effect of lowering of T C from 243 K to 235 K through increase in Mn content from 0 to 10%. Ferromagnetic hysteresis is noticed in all cases with significant rise in the value of remanent magnetisation (M r) from 4180 emu/mol for x = 0–5750 emu/mol for x = 0.1. Magnetocaloric effect (MCE) is observed with peaks in temperature dependent magnetic entropy change curves (-ΔS M ~ T) appearing at T C. However, the peaks undergo shift to higher temperatures for higher field changes due to possible augmented short range interactions among ferromagnetic clusters taking place above T C that effectively modifies T C to a larger value. Both − Δ S M max and RCP (Relative Cooling Power) record increase with values lying in the ranges 2.64–2.74 J/kg/K and 159.48–177.66 J/kg respectively for ΔH = 6 T corresponding to Mn content x = 0–0.1. Such remarkable features predict their potential application in magnetic refrigeration technology. Moreover, these materials are quite exciting from basic physics point of view. [ABSTRACT FROM AUTHOR]

We present the processing of functionally graded multi-material freeze-cast structures, i.e. structures with varying material properties but continuous and homogeneous microchannels. We demonstrate this for stepwise, continuous, highly viscous and gelation freezing to achieve freeze-cast structures of graded La 0.66 Ca 0.24 Sr 0.09 Mn 1.05 O 3 (LCSM9) and La 0.67 Ca 0.27 Sr 0.06 Mn 1.05 O 3 (LCSM6) ceramics. The two phases are successfully distinguished from one another by the addition of 10 wt% Ce 0.9 Gd 0.1 O 2 (CGO) to the LCSM6 phase. The relative Ce-content facilitates tracking of the LCSM6 phase using energy-dispersive X-ray (EDS) elemental analysis. Coupled with scanning electron microscopy, we show well-defined interfaces with continuous microchannels between the two LCSM phases in both green and sintered samples. By implementing constant freezing rates of −1.5 K/min a consistent freezing front velocity of ∼13 μm/s is maintained, and we found that a structural continuity can be maintained across the LCSM9–LCSM6/CGO interface, however, with varying pore morphology depending on the various freezing procedures. [ABSTRACT FROM AUTHOR]

A series of vacant doping samples of La0.65−xCa0.35MnO3 (x = 0, 0.05, 0.10, 0.15) were prepared by sol-gel method. XRD analysis results show that all samples are a single-phase perovskite structure; XRD refinement demonstrates that La0.50Ca0.35MnO3 is a single phase with the space group Pbnm. When x = 0.15, the variation of magnetization versus temperature in a magnetic applied field of the sample indicates that La0.5Ca0.35MnO3 is converted from a ferromagnetic phase to a paramagnetic phase. The Curie temperature (Tc) of the sample increases with the concentration of vacancy doping. The value of Tc increases from 209.5 to 242.1 K when x increases from 0 to 0.15. The Arrott plot indicates that the phase transition occurring in the sample is a secondary phase transition. When x = 0.15, the Curie temperature of the sample reaches 242.1 K; when the magnetic field variation is equal to 5 T, the maximum magnetic entropy changes (− ΔSM) of the sample become 1.77 J kg−1 K−1 and the relative cooling capacity (RCP) is 95.58 J•kg−1. [ABSTRACT FROM AUTHOR]

The sintering behaviour of LaFe11.0Si1.2Co0.8 powder produced by gas atomisation was examined to provide a basis for the application of powder metallurgical shaping technologies to magnetocaloric La(Fe,Si)13-alloys. The aim was to establish sintering parameters for attaining both high densification and good magnetocaloric properties for the investigated particle sizes <10 µm and <25 µm. Dilatometry measurements and sintering trials were carried out and density, microstructure and entropy change ΔS of the sintered samples were analysed. For the fine particles <10 µm, the lowest investigated sintering temperature 1150°C results in a relative density of 97%, a low α-Fe content and a high ΔS = −5 J kg−1 K−1 (ΔH=2 T). For powder <25 µm, a two-stage process is required to achieve similar properties. [ABSTRACT FROM AUTHOR]

The sintering behaviour of LaFe11.0Si1.2Co0.8 powder produced by gas atomisation was examined to provide a basis for the application of powder metallurgical shaping technologies to magnetocaloric La(Fe,Si)13-alloys. The aim was to establish sintering parameters for attaining both high densification and good magnetocaloric properties for the investigated particle sizes <10 µm and <25 µm. Dilatometry measurements and sintering trials were carried out and density, microstructure and entropy change ΔS of the sintered samples were analysed. For the fine particles <10 µm, the lowest investigated sintering temperature 1150°C results in a relative density of 97%, a low α-Fe content and a high ΔS = −5 J kg−1 K−1 (ΔH=2 T). For powder <25 µm, a two-stage process is required to achieve similar properties. [ABSTRACT FROM AUTHOR]

We investigate the magnetocaloric effect obtained by the rotation of a magnetic field applied to an exchange-coupled multilayer system composed of two different ferromagnetic (FM) materials. We specifically consider a system in which the two FMs have perpendicular uniaxial anisotropy axes and utilize conditions which yield a reorientation of the total magnetization when compensation between the anisotropies of the two layers occurs. We calculate the consequent entropy change associated with the "artificial" reorientation. By using known parameters from MnBi and Co we predict an entropy change of Δ s = 0. 34 J kg−1 K−1 for perfect coupling. Lastly, we study the behavior of the multilayer under a rotating magnetic field via a micromagnetic model. When the layer thicknesses are of the order of the local domain wall width, the magnetic field-induced entropy change can be obtained with magnetic fields one order of magnitude lower than in the uncoupled case. • We investigate the rotational magnetocaloric effect (MCE) obtained in an exchange-coupled multilayer system. • The system has two ferromagnetic materials with mutually perpendicular uniaxial anisotropy axes. • The available MCE is highly dependent on the layer thickness and can reach Δ s/ μ 0 Δ H ∼ 10 J kg−1 K−1 T−1. • Layered materials with optimized anisotropy constants should yield yet higher MCEs. [ABSTRACT FROM AUTHOR]

La(Fe,Si) 13 alloys are prone to display burst-like phenomena at their first-order ferromagnetic transition, such as a latent heat decomposed into a succession of heat flux avalanches. For some extreme cases, these phenomena are so pronounced that they can yield recalescence/antirecalescence events. It turns out that these features are also highly sensitive to the technical specificities of the employed measurement systems. Here, we compare the thermal events at the first-order magnetic transition of LaFe 11.4 Si 1.31 Co 0.29 using magnetization, two differential scanning calorimeters and a thermometry probe specially designed for the present study. These techniques are complementary to each other, and allow to make a distinction between the properties influenced by thermal coupling and those more intrinsic to the compound or to the sample. A simple thermal model is built to account for the thermal lags in the different devices, and to provide insights into the dependence of the duration of the transition on the thermal sweep rate. Looking in closer detail at the development of the transition, we found evidence of a rate independent initial regime, followed by a second one along which the same shape of temporal profile is spread over a more or less long duration depending on the sweep rate. Possible underlying mechanisms corresponding to these regimes are discussed. [Display omitted] • Comparative study including rarely considered Tzero DSC and thermometry probes. • A thermal model describes the complex dependency of transition duration on sweeping rate. • Detailed thermometry measurements confirm the reproducibility of the transition path. • A rate independent initial regime is observed from DSCs and thermometry measurements. [ABSTRACT FROM AUTHOR]

To fully assess and improve the performance of near room-temperature magnetic refrigerants, it is crucial to measure and compare their three most relevant thermodynamic properties, and their dependence on temperature (T) and applied magnetic field (H) : the isothermal entropy change, ∆ S iso (T , H) ; the adiabatic temperature change, ∆ T ad (T , H) ; and the heat capacity, C p (T , H). Typically, each thermodynamic property requires its own specialized measurements, which are very time-demanding and can be challenging to setup. In this work, we report a complete thermomagnetic characterization of a benchmark magnetocaloric material, gadolinium, using a single and commercially available SQUID magnetometer. By improving a recently reported method with incremental field ramping steps for measuring temperature through magnetization, we obtained a ∆ T ad (T) curve under a 1 T field change with its peak amplitude and maximizing temperature respectively within 2 % and 0.8 % of previously reported values for gadolinium. We were also able to estimate the temperature dependent heat capacity, C p (T , μ 0 H = 0.85 T) , using the ∆ T ad (T) measurements from magnetometry combined with magnetization versus temperature curves at different field values. This estimate of C p around the transition temperature of gadolinium is within a relative error of 11 % of the experimental and reported values. The reported methodology allows the complete characterization of a second-order magnetocaloric material (∆ S iso (T , H) , ∆ T ad (T , H) , C p (T , H)) around its Curie temperature using a single and widely available device, which can accelerate studies of different magnetocaloric materials' performance, and approximate their implementation in magnetic refrigeration and/or waste heat energy harvesting industries. • Enhanced near-adiabatic conditions in a time-dependent magnetometry protocol. • Incremental field protocol improves adiabatic temperature change estimative. • Novel approach: heat capacity from magnetization and adiabatic temperature change. • Complete magnetocaloric materials characterization using a single magnetometer. [ABSTRACT FROM AUTHOR]

Recent studies have pointed out that first-order magnetic transitions can be affected by defects. It however remains unclear which precise microstructural features are involved and how to pragmatically control their formation. Here, we show that a genuine technical parameter such as the compaction pressure employed prior to the sintering can induce subtle differences on magnetization data and yet correspond to a large evolution of the magnetocaloric performances in MnFe 0.95 P 0.575 Si 0.36 B 0.065 compounds prepared by powder metallurgy, resulting in an enhancement of the isothermal entropy change (Δ S) in 1 T up to ∼40 %. This outcome is unexpected as Δ S normalized per mass should not be affected by what is at first glance a matter of compaction or density. Our detailed structural and microstructural investigations indicate that the sharpening of the transition for the highest compaction pressures originate from different distributions of the secondary phase at the grain boundaries. This work not only bring into light an interesting fundamental question how microstructural details can affect the development of a magnetic transition, it has also important practical consequences to ensure a reproducibility of large magnetocaloric or thermomagnetic effects in MnFe(P,Si,B) materials for possible applications. [Display omitted] • Magnetocaloric performances of MnFe(P,Si,B) increase with higher green compaction. • Final density not affected by green pressure with common 24 h high temperature sintering. • Increase in ΔS is not related to secondary phase content but rather to its distribution. [ABSTRACT FROM AUTHOR]

Magnetocaloric alloys are an important class of materials that enable non-vapor compression cycles. One promising candidate for magnetocaloric systems is LaFeMnSi, thanks to a combination of factors including low-cost constituents and a useful curie temperature, although control of the constituents’ phase distribution can be challenging. In this paper, the effects of composition and high energy ball milling on the particle morphology and phase stability of LaFe11.71-xMnxSi1.29H1.6 magnetocaloric powders were investigated. The powders were characterized with optical microscopy, dynamic light scattering, X-ray diffraction (XRD), and differential scanning calorimetry (DSC). It was found that the powders retained most of their original magnetocaloric phase during milling, although milling reduced the degree of crystallinity in the powder. Furthermore, some oxide phases (

Spherical powder is key for achieving good heat transfer and maintaining low pressure drop for heat transfer fluid in magnetocaloric regenerator. Obtaining relatively large spherical powder of rare earth metals that are extremely sensitive to oxygen is challenging. Comparing to gas atomization and plasma rotating electrode methods, rotating disk atomization (RDA) method is more robust for obtaining a small amount of powder with a short turn-around time, thereby, more suitable for research and development of new magnetocaloric materials. In this work, commercially bulk gadolinium was successfully atomized to produce spherical powder in the targeted 150–250 μm range. The obtained powder exhibited the expected magnetocaloric performance with a maximum change of magnetic entropy of 12.1 J/kg K and a maximum adiabatic temperature change of 12.8 K with an applied magnetic field of 7 T. Unlabelled Image • Spherical Gd powder produced for use in magnetocaloric refrigeration. • Rotating disk atomized powder showed good yield in targeted size range and morphology. • Magnetocaloric performance of powder showed little change compared to stock material. [ABSTRACT FROM AUTHOR]

Magnetic and magnetocaloric properties of LaCr 1-x Mn x O 3 (x = 0, 0.05, 0.1) synthesized by solid state reaction have been investigated. They exhibit PM to CAFM transition with Néel temperature (T N) and paramagnetic Curie-Weiss temperature (θ) undergoing shift towards lower temperature upon increasing concentration of Mn. Significant enhancement in value of magnetisation has also been noticed due to development of ferromagnetic nature which is corroborated in hysteresis measurement where loops are observed upon substitution of Mn. Arrott plots confirm the magnetic transition as second order. MCE is observed for all the compounds with maximum value of change in entropy, − Δ S M max , falling in the range of near-room-temperature region. − Δ S M ∼ T curves were seen to peak at T N which register rise as a function of applied fields. Relative cooling power (RCP) were calculated for all the compositions. It was found to increase with fields applied with value of RCP recorded, for example, for x = 0.1 equal to 1.81 J/Kg at 9 T. [ABSTRACT FROM AUTHOR]

Designing multicaloric single-phase materials with combined electro- and magnetocaloric effects is still at its initial stage and presents a number of challenges. One of the main challenges encountered so far is to reduce the excessive electrical conductivity, which leads to the appearance of Joule heating that might completely degrade the electrocaloric response. In this work, multicaloric Pb(Fe 0.5 Nb 0.5)O 3 material was successfully prepared exhibiting pronounced electrocaloric effect above room temperature and maximum magnetocaloric effect at cryogenic temperature. The conductivity was suppressed by controlling the sintering temperature. The ceramic sintered at 1000 °C exhibits maximum electrocaloric effective cooling of 0.88 °C at 28 °C and maximum magnetocaloric effect of 0.14 °C at −271 °C. The caloric properties can be further improved by doping Pb(Fe 0.5 Nb 0.5)O 3 with manganese. In comparison to the undoped sample, Pb(Fe 0.5 Nb 0.5)O 3 doped with 0.5 mol% of manganese exhibits three times higher maxima of electrocaloric effective cooling (2.47 °C at 80 °C) and magnetocaloric temperature change (0.44 °C at −271 °C). [ABSTRACT FROM AUTHOR]

We present a comprehensive study of the three-metal boride alloys Mn 0.95 − δ Fe 1.05 + δ − x Co x B (δ = 0 − 0.15 and x = 0 − 1.05) synthesized via arc melting and annealing. The alloys crystallize in a body-centered tetragonal unit cell in the same space group (I 4 / m c m) as the parent compound Mn 2 B. A small amount of MB (M = Mn, Fe, Co) impurity forms in the as-cast samples which is removed by acid treatment. Rietveld refinement on the powder X-ray data reveals an anisotropic variation of lattice parameters in the series. The c -axis increases with increasing Co content but the a -axis decreases. Magnetic measurements show a non-monotonic variation of both T c and the saturated magnetization M s as a function of x. These findings highlight the tunable nature of Mn 0.95 − δ Fe 1.05 + δ − x Co x B and motivate further studies of metal-rich boride alloys. [ABSTRACT FROM AUTHOR]

Dielectric and magnetocaloric measurements are carried out for the chromite TmCrO 3. This oxide was reported to be multiferroic below the Néel temperature (T N) of ∼125 K, likely due to a structural transformation. The dielectric response shows large dielectric constants below 300 K. However, from the analyses of loss tangent, AC conductivity and dielectric modulus, this behavior is rooted in hopping of charge carriers rather than electric dipoles, as proposed for some other chromites. No dielectric anomaly is found at T N. The magnetocaloric effect shows that the magnetic transitions at T N as well as the spin reorientation temperature are of a second order. This result strongly suggests the absence of magnetostructural transition at T N in accord with no observation of ferroelectric transition at this temperature. • Dielectric and magnetocaloric properties of TmCrO 3. • Dielectric relaxation due to charge hopping rather than electric dipole. • Second order magnetic transitions suggesting weak magnetoelectric coupling. [ABSTRACT FROM AUTHOR]

• A state of the art of all the developed solid-state caloric prototypes was presented. • The devices were classified as magnetocaloric, electrocaloric, elastocaloric or barocaloric. • The main design features and the energy performances shown were reported for each device. • A point of view of the future perspectives is proposed in the conclusions. The solid-state processes for cooling and heat pump technologies are based on the well-known caloric effects (magneto-, electro-, elasto- and baro-caloric). With an interest arisen in 1976, year of the development of the first room-temperature magnetic refrigerator, up to a few years ago, magnetocaloric was among the most investigated caloric cooling techniques and it was considered among the main solid-state alternatives to vapor-compression cooling and heat-pumping. During such period, a remarkable number of prototypes of magnetic refrigerators or heat pumps was built. Recently, the attention toward all the four caloric effects has recently grown; therefore, an increase in projects on solid-state prototype developing (not only on magnetocaloric) was observed. The intention of this paper is to offer a state-of-the-art of all the solid-state prototypes for cooling and heat pumps processes, devoted to room temperature operations, developed before the year 2019. [ABSTRACT FROM AUTHOR]

Freeze‐casting is a technique used to produce structures with anisotropic porosity in the form of well‐defined microchannels throughout a sample. Here, this technique is used on the magnetocaloric ceramic La0.66Ca0.26Sr0.07 Mn1.05O3. We show that a dynamic freezing profile, where the temperature is decreased continuously at −10 K/min, results in homogeneous, lamellar channels with widths of ∼15 µm, while static freezing, where the temperature is kept constant at 177 K, results in channels of increasing size away from the initial ice crystal nucleation site. The effect of gelation before freeze‐casting is also investigated. Gelation inhibits ice crystal growth, which significantly changes the morphology by making channel cross sections less elongated, while additionally introducing more dendrites and ceramic bridges in the structure. The latter significantly dominates the flow path through the gelated structures, affecting the calculated tortuosity, which increases to τ ≈ 4 when compared to non‐gelated samples where calculated tortuosities are in the range of ∼1.3 to ∼3. Finally, we present a systematic and automatic approach for evaluating channel and wall sizes and calculating tortuosities. This is based on analysis of images obtained by scanning electron microscopy using a continuous particle size distribution method and the TauFactor application in MATLAB®. [ABSTRACT FROM AUTHOR]

Approaching the border of the first order transition and second order transition is significant to optimize the giant magnetocaloric materials performance. The influence of vanadium substitution in the Mn 1.2- x V x Fe 0.75 P 0.5 Si 0.5 alloys is investigated for annealing temperatures of 1323, 1373 and 1423 K. By tuning both the annealing temperature and the V substitution simultaneously, the magnetocaloric effect can be enhanced without enlarging the thermal hysteresis near the border of the first to second order transition. Neutron diffraction measurements reveal the changes of site occupation and interatomic distances caused by varying the annealing temperature and V substitution. The properties of the alloy with x = 0.02 annealed at 1323 K is comparable to those found for the MnFe 0.95 P 0.595 Si 0.33 B 0.075 alloy, illustrating that Mn 1.2- x V x Fe 0.75 P 0.5 Si 0.5 alloys are excellent materials for magnetic heat-pumping near room temperature. • Low field performance can be enhanced without enlarging the thermal hysteresis. • V prefers to occupy 3 f site and shortens the intra-atomic distances. • Decreasing effect of V on T C is stronger at lower annealing temperature. • An increased Δ T ad of 2.7 K and a decreased hysteresis of 1.8 K at 1 T are achieved. [ABSTRACT FROM AUTHOR]

In this work, we highlight the occurrence of different physical mechanisms that independently control the saturation magnetization and the ferro-paramagnetic transition temperature of Ni-Mn-based Heusler compounds, opening new possibilities in mastering the functional properties of this wide class of magnetic materials. We present the magnetic, structural and magnetocaloric features of a complete Ni 48 Mn 36 In 16−x Sn x (x = 0–16) series. The observed different trends of the critical temperature and of the saturation magnetization on varying the Sn to In ratio are discussed with the help of first-principles calculations of the electronic structure and magnetic interactions of the compound. Unlabelled Image [ABSTRACT FROM AUTHOR]

The use of magnetic shape memory alloys (MSMAs) in manufacturing industry has increased significantly in recent years. This is mainly due to their great interest in their potential applications in smart devices, because of the reversible distortions suffered. The well-known example of these combinations is the Heusler type. A review is given of experimental works concerning the examination of magnetic field, structural phase transitions, and the magnetocaloric impact in Heusler Ni–Mn–X (X = In, Sn, Sb) and Ni–Co–Mn–Y (Y = In, Sn, Sb) alloys. This type of compounds has excellent properties, for example, the presence of coupled magnetostructural (coïncident magnetic and martensitic transitions) and metamagnetostructural phase transitions (coïncident metamagnetic (ferromagnetic-antiferromagnetic) and martensitic transitions), the magnetocaloric impact (MC), and the large magnetoresistance change (MR). The conceivable difficulties and remaining problems are briefly discussed. [ABSTRACT FROM AUTHOR]

Antiperovskite materials based on M n 3 C u N with Cu partially replaced by Ag or Ni and simultaneous substitution of N by C were synthesized and their magneto-structural transition was studied. By replacing Cu partially with Ag or Ni, Curie temperatures increase, the magnetostructural transition becomes isosymmetric and presents a more second order character. Simultaneous replacement of N by C reverses this trend as the Curie temperature decreases with increasing Ag/Ni content while the structural transition remains non-isosymmetric. Additionally, moderate magnetocaloric effects with magnetic entropy changes around 6 J/Kkg in the range of 130–150 K at a 2 T field change are observed. Finally, the influence of the change in number of valence electrons and cell size by substitution on the transition is discussed. • 8 previously unknown antiperovskite materials based on M n 3 C u N have been studied. • Low T XRD reveals that symmetry below T T depends on degree of N substitution by C. • Also, C content reverses trends seen in T C when Cu is substituted by Ag/Ni. • A Δ S M around 6 J/Kkg in the range of 130–150 K at a 2 T field changes is observed. • The influence of N(VE) and cell size on the FOMST is comprehensively discussed. [ABSTRACT FROM AUTHOR]

Magnetic materials with strong local interactions but lacking long-range order have long been a curiosity of physicists. Probing their magnetic interactions is crucial for understanding the unique properties they can exhibit. Metal-organic frameworks have recently gathered more attention as they can produce more exotic structures, allowing for controlled design of magnetic properties not found in conventional metal-oxide materials. Historically, magnetic diffuse scattering in such materials has been overlooked but has attracted greater attention recently, with advances in techniques. In this study, we investigate the magnetic structure of metalorganic formate frameworks, using heat capacity, magnetic susceptibility and neutron diffraction. In Tb(DCO2)3, we observe emergent magnetic order at temperatures below 1.2 K, consisting of two k-vectors. Ho(DCO2)3 shows diffuse scattering above 1.6 K, consistent with ferromagnetic chains packed in a frustrated antiferromagnetic triangular lattice, also observed in Tb(DCO2)3 above 1.2 K. The other lanthanides show no short- or long-range order down to 1.6 K. The results suggest an Isinglike one-dimensional magnetic order associated with frustration is responsible for the magnetocaloric properties, of somemembers in this family, improving at higher temperatures. [ABSTRACT FROM AUTHOR]

Highlights • Magnetic transition temperature of AlT 2 X 2 materials largely dictated by the c -axis. • The a -parameter has little effect on the magnetic transition temperature of AlT 2 X 2. • (T-T) bonds responsible for magnetostructural phase transition response. • Describes lattice-magnetic transition temperature interactions in AlFe 2 B 2. Abstract Systematic trends connect detailed composition, lattice parameters and magnetic transition temperatures in the ferromagnetic intermetallic compound AlT 2 X 2 with the Mn 2 AlB 2 -type crystal structure, where T = Mn, Fe, Ni, Co and X = B, C. Data were derived from both literature reports and from experiments performed on synthesized samples (T = (Fe 1−x Ni x) 2 , x = 0, 0.05, 0.1; X = (B 0.9 C 0.1) 2). It is observed that compositional variation alters specific bonds responsible for the magnetic phase transition response, which ranges from 200 K ≤ T t ≤ 310 K. Elemental composition that provides changes in the c -axis length and the associated (T-T) c -axis interatomic distance contribute the largest bonding effects to magnetic phase transition temperature T t , alterations. Overall, these results are attributed to the dependence of T t on the specifics of the Fe sublattice occupancy, electronic state and T-T bonding. In contrast, T t is found to be largely independent of the (b/a) axial ratios and the associated (T-X) b -axis /(T-X) (ac)-plane interatomic distance ratios, indicating that interatomic interactions along the a -axis have little effect on the T t. [ABSTRACT FROM AUTHOR]

Highlights • La 0.65 Nd 0.05 Ba 0.3 Mn 1−x Cr x O 3 (0 ≤ x ≤ 0.15) crystallize in rhombohedral crystal structure with R -3 c as a space group. • Our samples undergo a second order paramagnetic (PM) – ferromagnetic (FM) transition. • Our samples can be used as a potential magnetic refrigeration material near room temperature. • The TEC (10 K, 5 T) shows a decreased magnetocaloric performance of the Cr doped manganites. Abstract In this paper, we report the structural, magnetic and magnetocaloric properties of La 0.65 Nd 0.05 Ba 0.3 Mn 1−x Cr x O 3 (0 ≤ x ≤ 0.15) system synthesized by conventional solid-state reaction method. Phase identification is performed by X-ray diffraction analysis, which shows the rhombohedra crystal structure with R -3 c as a space group. Magnetic measurements show that all our samples undergo a second order paramagnetic (PM) – ferromagnetic (FM) transition when the temperature decreases. However, The M(T) shows a decrease in a Curie temperature (T C) from 330 K for x = 0 to 275 K for x = 0.15 as a Cr ion increases. This decrease can be explained by the decreasing number of hopping electrons, which reduces the double exchange (DE) interaction of Mn3+-O-Mn4+. The role of the trivalent ions Cr3+ is merely to dilute the DE interactions between Mn sites. Moreover, the maximum magnetic entropy change −ΔS M calculated from the M(μ 0 H) measurements are found to be 4.26, 4.03, 3.97 and 3.55 Jkg−1K−1 under an external field change of 5 T for x = 0, 0.05, 0.1 and 0.15, respectively. However, the temperature-averaged entropy change (TEC) over 10 K temperature span under same field, TEC (10 K, 5 T), are 4.19, 3.91, 3.86 and 3.47 Jkg−1K−1. This finding suggests that our samples can be used as a magnetic refrigeration material around room temperature. Furthermore, the ΔS M –T curves collapse onto a single master curve confirming the second order phase transition. Additionally, a phenomenological universal model is used to predict the magnetocaloric properties for Cr substituted samples. [ABSTRACT FROM AUTHOR]