The transition-metal based alloy system YNi _4-xCo _xSi shows a second-order ferromagnetic-to-paramagnetic transition near room temperature. Here, the magnetic structure, the magnetocaloric properties and the magnetic anisotropy of YNi _4-xCo _xSi (x = 0–4) are investigated. For x = 3.5, 3.75 and 4.0 a Curie temperature near room temperature is observed with T _C = 250, 283 and 310 K, respectively. In orientated YNi _4-xCo _xSi powder samples the c axis of the hexagonal crystal structure is found to be the easy magnetic axis, with a large dominant K ₂ anisotropy constant (K ₂ > K ₁ > 0). The magnetic structure and the preferred atomic position for Ni are demonstrated by neutron diffraction measurements. We have found a dramatic decrease in the magnetic moment at the 3 g site in the CaCu ₅-type structure (space group P6/mmm), the saturation magnetization and the Curie temperature with increasing Ni concentration.

The Fe₂P type Mn–Fe–P–Si alloys exhibit a giant magneto-elastic first-order transition, but the large hysteresis limits their performance. Crystal structure evolution and magnetocaloric performance were investigated by varying the Mn and Fe contents at a constant V substitution of 0.02 in Fe₂P-type (Mn_1.17-xFe_0.73-yV_0.02) (P_0.5Si_0.5) (where x + y = 0.02). The V substitution of Fe content shows a larger reduction of hysteresis compared with the same substitution amount of Mn content. During magnetoelastic phase transition, V-substitution reduces the volume change and the volumetric stresses, providing a superior mechanical stability. Compound with the V substitution of Fe (y = 0.02) shows the best magnetocaloric effect with a low thermal hysteresis of 0.6 K. Our developed Mn_1.17-xFe_0.73-yV_0.02P_0.5Si_0.5 alloys are excellent materials for room-temperature magnetic heat-pumping applications by using a permanent magnet.

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.

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.

Zero thermal expansion (ZTE) materials with the advantage of an invariable length with varying temperatures are in high demand for modern industry but are relatively rare for metals. Fe-based Laves phases attract significant attention due to the rich and intriguing physical properties resulting from the coupling between crystal, electric, and magnetic structures. In this work, the structural, magnetic transition, thermal expansion, and magnetocaloric effect of single-phase Fe_2-xHf_0.80Nb_0.20 Laves phase alloys were investigated by means of macroscopic magnetic measurements, Mössbauer spectroscopy, and X-ray diffraction at the temperature range of 4.2-400 K. With the introduction of Fe vacancies, the ZTE coefficient of −1.2 ppm/K is smaller than that (1.7 ppm/K) of stoichiometric Fe₂Hf_0.80Nb_0.20 alloy. Meanwhile, the magnetic entropy change experiences an enhancement from 0.39 to 0.50 J/kg K at a magnetic field change of 2 T. These improved properties are attributed to the vacancy-induced coexistence of ferromagnetic and antiferromagnetic phases, as evidenced by variable-temperature X-ray diffraction and Mössbauer spectroscopy. This work unveils a promising avenue for new zero thermal expansion materials by controlling the vacancies at magnetic atom positions in Fe-based Laves phase alloys.

The magnetocaloric properties of Mn₅Si_1-xP_xB₂ (0 ≤ x ≤ 1) compounds were studied for energy harvesting applications. The crystal structure and the magnetic structure were characterized by powder X-Ray Diffraction and powder Neutron Diffraction. The results indicate that these magnetocaloric materials crystallize in the tetragonal Cr₅B₃-type crystal structure. The introduction of P causes a stretching of the c axis and compression of the a-b plane, leading to a decrease in the unit-cell volume V. In the ferromagnetic state the magnetic moments align within the a-b plane, and the magnetic moment of the Mn1 atom on the 16 l site is larger than that of the Mn2 atom on the 4c site. The Curie temperature T_C can be adjusted continuously from 305 K (x = 1) to 406 K (x = 0) by replacing Si with P. The corresponding magnetic entropy change varies from 1.90 Jkg⁻¹K⁻¹ (x = 0) to 1.35 Jkg⁻¹K⁻¹ (x = 1) for a magnetic field change of 1 T. The PM-FM transition in these compounds corresponds to a second-order phase transition. Mn₅Si_1-xP_xB₂ compounds exhibit a magnetization difference of 28.1 - 31.3 Am²kg⁻¹ for a temperature span of 30 K around T_C in an applied magnetic field of 1 T. The considerable change in magnetization, the tunable T_C near and above room temperature and the absence of thermal hysteresis make these compounds promising candidates for magnetocaloric energy harvesting materials.

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.

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.

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.

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.

The effect of the heat treatment on the magnetism, magnetocaloric effect and microstructure formation has been systematically studied in Fe-rich (Mn,Fe)_y(P,Si) melt-spun ribbons (1.80 ≤ y ≤ 2.00). XRD, SEM and EDS measurements demonstrate that a metal deficiency prompts the stable (Mn,Fe)Si phase, whereas in the metal-rich region the (Mn,Fe)₃Si phase is formed. It is found that the annealing temperature influences the composition and lattice parameters of the (Mn,Fe)_y(P,Si) alloys, which greatly affects the Curie temperature (T_C). For the optimal metal/non-metal ratio y the magnetic entropy change (|ΔS_m|) is found to increase from 5.5 to 15.0 Jkg⁻¹K⁻¹ in a magnetic field change of 2 T by varying the annealing temperature from 1313 to 1433 K, indicating an enhancement of the first-order magnetic transition (FOMT). The presented results reveal that the secondary phase and magnetic properties in the (Mn,Fe)_y(P,Si) system can be tuned by varying the annealing temperature and by adjusting the metal/non-metal ratio y.

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.