Recently, the promising multi-component magnetocaloric materials (Mc-MCMs) are found to have a tunable giant magnetocaloric effect (GMCE) near room-temperature and manifest fruitful functionalities like multi-caloric effects, which are candidates for solid-state caloric applications. Introducing vacancy defects is found to be an efficient method to optimize its GMCE property. However, the responsible mechanism and especially the characteristics of the atomic vacancies are far from being elucidated. Here, we produce direct-solidified MnCoNiGeSi-based Mc-MCMs which exhibit the distinct shift in transition temperature (T_t) upon introducing Mn/Ni vacancies. It is found that T_t decreased significantly in the Mn vacancy materials and increased in the Ni vacancy materials. The first-order transition is maintained and the strength of the magnetic entropy change (Δs_m) was unchanged without degradation. For the Mn vacancy sample the decreased Mn-Mn atomic distance and strengthened covalent bonding can stabilize the high-temperature hexagonal phase, while for the Ni vacancy sample the decreased interatomic distances among different pairs (Mn-Ge, Mn-Mn and Mn-Ni) promote the stabilization of the low-temperature orthorhombic phase. Additionally, the introduced vacancy defects have directly been observed through HAADF-STEM. Positron annihilation results clarified the mono-vacancy nature for these vacancies, and indicate that the Ni positions around the Ni vacancies could partially be occupied by Mn atoms. Our study reveals that introducing atomic vacancy defects can effectively regulate the magnetocaloric properties and provide important fundamental insights into defect engineering of Mc-MCMs.

Compared with traditional techniques, solid-state magnetocaloric phase transition materials (MPTMs), based on the giant magnetocaloric effect (GMCE), can achieve a higher energy conversion efficiency for caloric applications. As one of the most promising MPTMs, the hexagonal (Mn,Fe)₂(P,Si)-based compounds host some advantages, but the existing hysteresis and relatively unstable GMCE properties need to be properly tackled. In this study, it is found that substitutions with Ni, Pd, and Pt can maintain and even enhance the GMCE (≈8.7% maximum improvement of |Δs_m|). For a magnetic field change of Δμ₀H = 2 T, all samples obtain a |Δs_m| in the range of 20–25 J kg⁻¹ K⁻¹ with a low thermal hysteresis ΔT_hys (≤5.6 K). The performance surpasses almost all other (Mn,Fe)₂(P,Si)-based materials with ΔT_hys (<10 K) reported until now. The occupancy of substitutional Ni/Pd/Pt atoms is determined by X-ray diffraction, neutron diffraction, and density functional theory calculations. The difference in GMCE properties upon doping is understood from the competition between a weakening of the magnetic exchange interactions and the different degrees of orbital hybridization among 3d-4d-5d elements. The studies elaborate on the responsible mechanism and provide a general strategy through d-block doping to further optimize the GMCE of this materials family.

Solid-state caloric effects as intrinsic thermal responses to different physical external stimuli (magnetic-, uniaxial stress-, pressure-, and electric-fields) can achieve a higher energy efficiency compared with traditional gas compression techniques. Among these effects, magnetocaloric energy conversion is regarded as the best available alternative and has been exploited extensively for promising application scenarios in the last decades. This review systematically introduces the magnetocaloric effect and its applications, and summarizes the corresponding representative magnetocaloric materials, as well as important progress in recent years. Specifically, the review focuses on some key understandings of the magnetocaloric effect by utilizing state-of-the-art technical tools such as synchrotron X-ray, neutron scattering, muon spin spectroscopy, positron annihilation spectroscopy, high magnetic fields, etc., and highlights their importance toward advanced materials design and development. An overview of the basic principles and applications of these advanced techniques on magnetocaloric materials is provided. Finally, the challenges and perspectives on further developments in this field are discussed. Further in-depth understanding and manufacturing technology advancement combined with fast-developed artificial intelligence and machine learning are expected to advance the magnetocaloric energy conversion technology closer to real applications.

The emerging all-d-metal Ni(Co)MnTi-based Heusler compounds attract extensive attention because it can potentially be employed for solid-state refrigeration. However, in comparison to the abundant physical functionalities in bulk conditions, the hidden properties related to the NiCoMnTi-based Heusler nanoparticles (NPs) have not yet been investigated experimentally. Here, we present NiCoMnTi Heusler NPs that have been manufactured by spark ablation under Ar gas flow, and the related magnetic and microstructural properties have been studied. Compared with the bulk sample, it is found that the magneto-structurally coupled transition in the bulk sample has collapsed into a magnetic transition for the NPs sample. Superparamagnetic NPs with widely distributed dislocations have directly been observed by high-resolution transmission electron microscopy. For the NPs, the magnetocrystalline anisotropy constant is 3.54 × 10⁴ J/m³, while the saturation magnetization after post-treatment has been estimated to be around 26 Am² kg⁻¹. Our current research reveals that Ni-Co-Mn-Ti-based quaternary NPs could show interesting properties for future nano-application, and the produced NPs will further expand the functionalities of this material family.

Recently, the all-d-metal Ni(Co)MnTi based Heusler compounds are found to have a giant magnetocaloric effect (GMCE) near room temperature and manifest different functionalities like multicaloric effects, which can be employed for solid-state refrigeration. However, in comparison to other traditional Heusler compounds, the relatively large thermal hysteresis (ΔT_hys) and moderately steep ferromagnetic phase transition provides limitations for real applications. Here, we present that fast solidification (suction casting) can sufficiently tailor the GMCE performance by modifying the microstructure. Compared with the arc-melted sample, the magnetic entropy change of the suction-casted sample shows a 67% improvement from 18.4 to 29.4 Jkg⁻¹K⁻¹ for a field change (∆μ₀H) of 5 T. As the thermal hysteresis has maintained a low ΔT_hys value (5.5 K) for the enhanced first-order phase transition, a very competitive reversible magnetic entropy change of 21.8 Jkg⁻¹K⁻¹ for ∆μ₀H = 5 T is obtained. Combining high-resolution transmission electron microscopy (HRTEM) and positron annihilation spectroscopy (PAS) results, the difference in lattice defect concentration is found to be responsible for the significant improvement in GMCE for the suction-cast sample, which suggests that defect engineering can be applied to control the GMCE. Our study reveals that fast solidification can effectively regulate the magnetocaloric properties of all-d-metal NiCoMnTi Heusler compounds without sacrificing ΔT_hys.

In this work, the magnetocaloric effect and negative thermal expansion in melt-spun Fe₂Hf_0.83Ta_0.17 Laves phase alloys were studied. Compared to arc-melted alloys, which undergo a first-order magnetoelastic transition from the ferromagnetic to the antiferromagnetic phase, melt-spun alloys exhibit a second-order transition. For Fe₂Hf_0.83Ta_0.17 ribbons, we observed a large volumetric coefficient of negative thermal expansion of −19 × 10⁻⁶ K⁻¹ over a wide temperature range of 197 – 297 K and a moderate adiabatic temperature change of 0.7 K at 290 K for a magnetic field change of 1.5 T. The magnetic field dependence of the transition temperature (dT_t/dµ₀H = 4.4 K/T) for the melt-spun alloy is about half that of the arc-melted alloy (8.6 K/T). The origin of second-order phase transition of the melt-spun alloy is attributed to the partially suppressed frustration effect, which is due to the atomic disorder introduced by the rapid solidification.

Magnetic refrigeration (MR) is a cutting-edge technology that promises high energy efficiency and eco-friendliness, making it an exciting alternative to traditional refrigeration systems. However, the main challenge to its widespread adoption is cost competitiveness. In this context, the use of liquid metals as heat transfer liquids in the MR has been proposed as a game-changing solution. Unfortunately, the toxicity and flammability of these liquid metals have raised serious concerns, limiting their practical use. In this study, we investigate the compatibility of a nontoxic and nonflammable GaInSn-based liquid metal with a magnetocaloric material, La(Fe,Mn,Si)₁₃H_z, over a 1.5 year period. Our findings reveal nearly a 14% reduction in specific cooling energy and peak-specific isothermal magnetic entropy change for the considered magnetocaloric material. Our study provides valuable insights into the long-term stability of magnetocaloric materials and their compatibility with liquid metals, facilitating the development of more cost-effective and sustainable MR systems.

The transition-metal based Laves phase materials represent an extended family of alloys with rich and fascinating physical properties. In this work, we have investigated the negative thermal expansion and magnetocaloric effect in arc-melted and melt-spun Fe₂Hf_1-xTi_x (x = 0.15, 0.27, 0.30, 0.33, 0.36, 0.40) alloys. For x = 0.30–0.40, two hexagonal phases with different compositions share the same P6₃/mmc lattice symmetry, but have slightly different lattice parameters. The saturation magnetization and Curie temperature both follow a decreasing trend with the average unit-cell volume. For Fe₂Hf_0.6Ti_0.4 melt spinning improves the saturation magnetization from 48.7 to 59.6 Am²/kg and the magnetic entropy change from 0.46 to 0.54 J/kgK at a magnetic field change of 2 T. These enhanced values are attributed to an improved homogeneity caused by a suppression of phase segregation during rapid solidification. We have utilized neutron powder diffraction and Mössbauer spectroscopy to illustrate the correlation between the magnetic order and the negative thermal expansion in single-phase Fe₂Hf_0.85Ti_0.15. The magnetic moments of Fe align below 400 K in the a-b plane and a moment change for the Fe atoms is responsible for the large volumetric coefficient of thermal expansion of −25 × 10⁻⁶ K⁻¹ over a wide temperature range of 300–400 K.

The influence of off-stoichiometry and of doping with the 5d transition metal Ta has been studied in the quaternary (Mn,Fe)₂(P,Si)-based compound, which is one of the most promising materials systems for magnetic refrigeration. It is found that Ta substitution can decrease the transition temperature T_tr, while the thermal hysteresis ∆T_hys remains about constant. A low Ta doping enhances the magnetocaloric effect (MCE). For Mn_0.6Fe_1.27-yTa_yP_0.64Si_0.36 with y = 0.01 the magnetic entropy change ∆S_m shows and enhancement of 30.7% compared to the undoped material for a low magnetic field change of 1 T. The occupancy of substitutional Ta atoms is determined by XRD and DFT calculations. The T_tr shift and enhanced MCE upon Ta doping are ascribed to the competition between a weakening of the magnetic exchange interactions and a strengthening of the hybridization. Our studies provide a good strategy to further optimize the MCE of this material family.

The orthorhombic MnCoSi compounds have been found to present a large magnetoelastic coupling, which is regarded as the source for the magnetocaloric effect (MCE) and the magnetostrictive effect. As a result, these compounds are potential materials for caloric applications such as solid-state refrigeration. In the present study, we offer fundamental insights in the magnetoelastic coupling in these compounds based on their structural, metamagnetic, and MCE behavior. The directly measured adiabatic temperature change (ΔTad) in different initial temperatures (down to 18 K) and pulsed magnetic fields (up to 40 T) presents a moderate MCE performance (the maximum ΔTad=-3.1K for a field change of 13 T), which results from the metamagnetic behavior of these compounds. Furthermore, the magnetization measurements in pulsed (and static) magnetic fields indicate that the magnetoelastic coupling is significantly enhanced for increasing fields resulting in an improved saturation magnetization. The metamagnetic transition is continuously pushed to lower temperatures in higher fields. The phase diagram constructed from the experimental transition temperatures Tt and the critical magnetic fields μ0Hcr indicate that the transition is terminated below 18 K and that ferromagnetism is stabilized for fields above 22.3 T. Our results provide unique insights into the strong magnetoelastic coupling under high pulsed magnetic fields, providing guidelines for the design of giant magnetocaloric materials for future caloric applications.

Structural, magnetic and magnetocaloric properties of Mn₃Sn_1-xZn_xC antiperovskite carbides have been studied. With increasing Zn content the first-order magnetic transition (FOMT) is weakened. The Curie temperature (T_C) reduces first from 273 to 197 K and when x > 0.3, T_C increases, reaching its maximum of 430 K for x = 1.0. An increase in T_C is accompanied by pronounced changes in magnetic behaviour and a significant rise in magnetization from 21.82(4) to 76.2(2) Am²kg⁻¹ for x = 0.8 in the maximum applied magnetic field of 5 T. Neutron powder diffraction (NPD) was employed to study the magnetic structure of Mn₃Sn_1-xZn_xC compounds. The refinement of the NPD data for x = 0.3 revealed a magnetic structure with propagation vector k = (½,½,0) with a decrease in the canted antiferromagnetic (AFM) moment, which results in a reduction of the negative volume change at the magnetic transition and a decrease in the magnetocaloric effect (MCE). For x = 0.4, the magnetic structure is described by a propagation vector k = (½,½,½) for the AFM moment which dominates at low temperature, with the presence of a minor ferromagnetic (FM) component with a k = (0, 0, 0) propagation vector, which confirms the presence of the ferrimagnetic (FiM) state. For a higher Zn content (x = 0.6), the magnetic moment originates mainly from the FM component found on three independent Mn positions and an additional AFM moment oriented in the a-b plane. The results presented confirm the presence of competing AFM-FM interactions in Mn₃Sn_1-xZn_xC antiperovskite carbides.

The influence of doping with the 5d transition metal W has been studied in the quaternary (Mn,Fe)₂(P,Si) based giant magnetocaloric compounds, which is one of the most promising systems for magnetic refrigeration. It is found that W substitution can separately decrease the Curie temperature T_C and retain the thermal hysteresis ∆T_hys at an almost constant level (∼5 K) for Mn_0.6Fe_1.27-xW_xP_0.64Si_0.36 (x ≤ 0.02). Low-content W doping conserves the good magnetocaloric effect (MCE) without an obvious degradation. For x ≤ 0.02 the average magnetic entropy change |∆S_m| amounts to 11.4 Jkg⁻¹K⁻¹ for an applied magnetic field change of 2 T and the adiabatic temperature change ∆T_ad amounts to 3.9 K for an applied magnetic field change of 1.5 T. The occupancy of substitutional W atoms is determined by XRD experiments and DFT calculations. Our studies provide a good strategy to further optimize the MCE of this material family.

The quarternary (Mn,Fe)₂(P,Si)-based materials with a giant magnetocaloric effect (GMCE) at the ferromagnetic transition T_C are promising bulk materials for solid-state magnetic refrigeration. In the present study we demonstrate that doping with the light elements fluorine and sulfur can be used to adjust T_C near room temperature and tune the magnetocaloric properties. For F doping the first-order magnetic transition (FOMT) of Mn_0.60Fe_1.30P_0.64Si_0.36F_x (x = 0.00, 0.01, 0.02, 0.03) is enhanced, which is explained by an enhanced magnetoelastic coupling. The magnetic entropy change |ΔS_m| at a field change (Δμ₀H) of 2 T markedly improved by 30% from 14.2 Jkg⁻¹K⁻¹ (x = 0.00) at 335 K to 20.2 Jkg⁻¹K⁻¹ (x = 0.03) at 297 K. For the F doped material the value of |ΔS_m| for Δμ₀H = 1 T reaches 11.6 Jkg⁻¹K⁻¹ at 294 K, which is consistent with the calorimetric data (12.4 Jkg⁻¹K⁻¹). Neutron diffraction experiments reveal enhanced magnetic moments by F doping in agreement with the prediction of DFT calculation. For S doping in Mn_0.60Fe_1.25P_0.66-ySi_0.34S_y (y = 0.00, 0.01, 0.02, 0.03, 0.04) three impurity phases have been found from microstructural analysis, which reduce the stability of the FOMT in the main phase and decrease T_C, e.g. the |ΔS_m| reduces from 7.9(12.6) Jkg^-1K^-1 (332 K) for the undoped sample to 3.4(6.2) Jkg^-1K^-1 (313 K) for the maximum doped sample for Δμ₀H = 1(2) T. Neutron diffraction experiments combined with first-principles theoretical calculation, distinguish the occupation of F/S dopants and the tuning mechanism for light element doping, corresponding to subtle structural changes and a strengthening of the covalent bonding between metal and metalloid atoms. It is found that the light elements F and S can effectively regulate the magnetocaloric properties and provide fundamental understanding of (Mn,Fe)₂(P,Si)-based intermetallic compounds.

In the field of nanoscale magnetocaloric materials, novel concepts like micro-refrigerators, thermal switches, microfluidic pumps, energy harvesting devices and biomedical applications have been proposed. However, reports on nanoscale (Mn,Fe)₂(P,Si)-based materials, which are one of the most promising bulk materials for solid-state magnetic refrigeration, are rare. In this study we have synthesized (Mn,Fe)₂(P,Si)-based nanoparticles, and systematically investigated the influence of crystallite size and microstructure on the giant magnetocaloric effect. The results show that the decreased saturation magnetization (M_s) is mainly attributed to the increased concentration of an atomically disordered shell, and with a decreased particle size, both the thermal hysteresis and T_c are reduced. In addition, we determined an optimal temperature window for annealing after synthesis of 300–600 °C and found that gaseous nitriding can enhance M_s from 120 to 148 Am²kg⁻¹ and the magnetic entropy change (ΔS_m) from 0.8 to 1.2 Jkg⁻¹K⁻¹ in a field change of Δμ₀H = 1 T. This improvement can be attributed to the synergetic effect of annealing and nitration, which effectively removes part of the defects inside the particles. The produced superparamagnetic particles have been probed by high-resolution transmission electron microscopy, Mössbauer spectra and magnetic measurements. Our results provide important insight into the performance of giant magnetocaloric materials at the nanoscale.

The all-d-metal Ni-(Co)-Mn-Ti-based Heusler alloys are found to show a giant magnetocaloric effect near room temperature and are thereby potential materials for solid-state refrigeration. However, the relative large thermal hysteresis and the moderate ferromagnetic magnetization provides limitations for real applications. In the present study, we demonstrate that introducing interstitial B atoms within Ni36.5Co13.5Mn35Ti15 alloys can effectively decrease the thermal hysteresis ΔThys (down to 4.4 K), and simultaneously improve the saturation magnetization (maximum 40% enhancement) for low concentrations of B doping (up to 0.4 at. %). In comparison to the undoped reference material, the maximum magnetic entropy change (ΔSm) for the Ni36.5Co13.5Mn35Ti15B0.4 alloy shows a remarkable improvement from 9.7 to 24.3 J kg-1K-1 for an applied magnetic field change (Δμ0H) of 5 T (30.2 J kg-1K-1 for Δμ0H = 7 T). Additionally, due to the obtained low thermal hysteresis ΔThys, the maximum reversible ΔSmrev amounts to 18.9 J kg-1K-1 at 283 K for Δμ0H = 5 T (22.0 J kg-1K-1 at 281 K for Δμ0H = 7 T), which is competitive to the traditional Ni-Mn-X-based Heusler alloys (X = Ga, In, Sn, Sb). The enhancement of the magnetic moments by B doping is also observed in first-principles calculations. These calculations clarify the atomic occupancy of B and the changes in the electronic configuration. Our current study indicates that interstitial doping with a light element (boron) is an effective method to improve the magnetocaloric effect in these all-d-metal Ni-Co-Mn-Ti-based magnetic Heusler compounds.

The effect of Co and Ni doping on the structure, magnetic and magnetocaloric properties of Fe-rich (Mn,Fe)₂(P,Si) compounds was studied. With increasing Co and Ni content, both the Curie temperature (T_c) and the thermal hysteresis (ΔT_hys) decreased, whereas the hexagonal P-62 m crystal structure was maintained. A pronounced reduction in hysteresis was observed upon Co doping, while a significant reduction in Curie temperature was found upon Ni doping. Mössbauer spectroscopy measurements and DFT calculations indicated the substitution of Fe at the 3f site for both Co and Ni doping. Rietveld refinement of the X-ray diffraction data showed that Co substitute atoms in the main phase and the impurity phase, while Ni exhibits an affinity to the main phase. Magnetization measurements on the Co doped samples revealed an increase in magnetization for 2 at.% of Co, followed by a decrease for higher concentrations. DFT calculations showed that the magnetic moment on the 3f site is enhanced by Co substitution, whereas an opposite trend was observed for Ni substitution.

Magnetocaloric materials undergoing reversible phase transitions are highly desirable for magnetic refrigeration applications. (Mn,Fe)₂(P,Si) alloys exhibit a giant magnetocaloric effect accompanied by a magnetoelastic transition, while the noticeable irreversibility causes drastic degradation of the magnetocaloric properties during consecutive cooling cycles. In the present work, we performed a comprehensive study on the magnetoelastic transition of the (Mn,Fe)₂(P,Si) alloys by high-resolution transmission electron microscopy, in situ field- and temperature-dependent neutron powder diffraction as well as density functional theory calculations (DFT). We found a generalized relationship between the thermal hysteresis and the transition-induced elastic strain energy for the (Mn,Fe)₂(P,Si) family. The thermal hysteresis was greatly reduced from 11 to 1 K by a mere 4 at.% substitution of Fe by Mo in the Mn_1.15Fe_0.80P_0.45Si_0.55 alloy. This reduction is found to be due to a strong reduction in the transition-induced elastic strain energy. The significantly enhanced reversibility of the magnetoelastic transition leads to a remarkable improvement of the reversible magnetocaloric properties, compared to the parent alloy. Based on the DFT calculations and the neutron diffraction experiments, we also elucidated the underlying mechanism of the tunable transition temperature for the (Mn,Fe)₂(P,Si) family, which can essentially be attributed to the strong competition between the covalent bonding and the ferromagnetic exchange coupling. The present work provides not only a new strategy to improve the reversibility of a first-order magnetic transition but also essential insight into the electron-spin-lattice coupling in giant magnetocaloric materials.

The influence of excess Mn on the magnetoelastic ferromagnetic-to-antiferromagnetic transition T_t in the magnetocaloric compound (Mn,Cr)₂Sb has been studied. With increasing excess Mn the magnetoelastic transition temperature for (Mn,Cr)₂Sb initially increases and then decreases. This trend is accompanied by a strong reduction of the (Mn,Cr)Sb secondary phase. With increasing excess Mn a higher Cr content was found in the (Mn,Cr)Sb secondary phase in comparison to the matrix phase. This competition for Cr leads to a nonlinear dependence of T_t with increasing excess Mn at a fixed nominal Cr content. However, we observed that T_t depends linear on the c/a ratio for a wide range of temperatures from 170 to 350 K. A compositional diagram of the c/a ratio was constructed to assist the selection of (Mn,Cr)₂Sb alloys with a desired transition temperature.