The (Mn,Fe)₂(P,Si) compounds are one of the rare materials systems that exhibit an isostructural first-order ferromagnetic transition (FOMT) near ambient temperature. Since the discovery of its giant magnetocaloric effect (GMCE), this system is garnering ongoing interest, both for its promising performances for applications and for the scientific interest in uncovering the fundamental mechanisms driving the FOMT. This study examines the evolution of the structure, the microstructure, the thermal and magnetic properties in Mn_0.60+x Fe_1.3-x P_0.66-y Si_0.34+y (0 ≤ x ≤ 0.08, x = 2y ) compounds prepared by the melt-spun technique. The simultaneous increase in Mn and Si concentrations leads to a 40 % enhancement in the isothermal entropy change (|Δ S _max|) compared to parent compound. Furthermore, we propose a method to separate the latent heat ( L ) from the reversible specific heat. This allows us to establish a convincing correlation between two intrinsic quantities, the latent heat ( L ) and the elastic strain energy ( U _e). Our results demonstrate that both latent heat ( L ) and thermal hysteresis (Δ T _hys) are proportionally linked and vanish simultaneously at a critical end point.

In-situ time-resolved small-angle neutron scattering (SANS) experiments were conducted on homogenised cold-rolled ternary Fe-Au-W alloys during aging for 12 h at temperatures of 650 to 700 °C in order to study the kinetics of the nanoscale precipitation. For comparison the precipitation kinetics in the binary counterparts Fe-Au and Fe-W alloys were also studied. In the ternary Fe-Au-W alloy nanoscale Au-rich precipitates were observed by both transmission electron microscopy (TEM) and SANS, while no significant W-rich precipitation was observed. The SANS pattern of the cold-rolled Fe-Au-W alloy clearly reveals a preferred orientation for the plate-shaped nanoscale Au-rich precipitates. As these Au-rich precipitates have a fixed orientation relation with the matrix lattice this preferred orientation originates from the texture of the bcc matrix grains, as confirmed by X-ray diffraction (XRD) pole figure measurements. The effect of texture on the nuclear and the magnetic SANS signal during the precipitation kinetics was included in the data analysis. This enables us to monitor the temperature dependence of the precipitation kinetics for the Au-rich precipitates in the Fe-Au-W alloy during aging at temperatures of 650, 675 and 700 °C. It is found that an increase in aging temperature results in a faster kinetics and a lower final precipitate fraction.

Magnetocaloric refrigeration is one of the most promising next-generation solid-state caloric techniques to revolutionize the traditional air-compression technique. The La(Fe,Si)13-based materials are recognized as candidates with potential for practical applications. However, flexible strategies to improve the Curie temperature (TC) and further achieve the tunable giant magnetocaloric effect (GMCE) still need to be developed. Here, the systematic experimental investigation on a series of light elements (C, F, S) modified LaFe11.6Si1.4 compounds are presented. It is found that all modified samples exhibit a higher TC, with a negligible impact on the thermal hysteresis. The GMCE performance in C- and S-modified samples is significantly degraded, but the maximum magnetic entropy change |Δ sm| for the optimally doped F sample can be well maintained at 19.2 J kg−1 K−1 for a field change of 2 T. The preferential site occupancy of dopants is determined, and the microstructural observation and metastable atomic changes have also been analyzed. It is concluded that interstitial doping is more efficient to shift TC. The first-order transition can however not be maintained upon doping due to changes in the hybridization. These findings highlight the importance of the interplay between the lattice pressure effect and the covalent hybridization for this material family.

The hexagonal Mn3−xFexSn compounds possess several desirable properties that make them suitable magnetocaloric materials, including a ferromagnetic (FM)-to-paramagnetic (PM) transition near room temperature and soft magnetic behavior. In this study, we use themelt-spinning technique to explore the Mn-Fe-Sn ternary system. By combining magnetization measurements,Mössbauer spectroscopy, neutron diffraction (ND), oriented powder x-ray diffraction, and density functional theory (DFT) calculation, the magnetocaloric effect, spin structures, and the intrinsic magnetic properties of polycrystallineMn3−xFexSn (x = 0.8 − 1.4) compounds are determined. The FM-to-PM transition temperature TC ranges from 253 K (x = 0.8) to 394 K (x = 1.4). At low temperature, a spin reorientation at TS is observed, where below TS a coexistence of FM order with spins along the c axis and antiferromagnetic order with spins within the a − b plane occurs for x = 0.8 and 1.0. However, for compounds with x = 1.2 and 1.4, only FM order with spins along the c axis has been found below TS. Above TS, the spin structure corresponds to FM order with spins aligned within the a − b plane for all compositions. The magnetic moments of Mn and Fe were evaluated using DFT, demonstrating good agreement with the ND results.

Grasping (electronic) structure changes during photochromic processes is crucial for fully understanding the photochromic effect in rare-earth oxyhydride films. In this study, we employ in situ UV illumination positron annihilation lifetime spectroscopy (PALS) to investigate the time evolution of open-volume defects and metallic domains during photodarkening and bleaching in yttrium oxyhydride films. The PALS depth profiles before and after a photodarkening-bleaching cycle reveal a light-induced increase in open-volume defects, that occurs homogeneously throughout the oxyhydride layer. The time-dependent PALS measurements show that upon photodarkening, a fast initial formation of metallic domains occurs, as well as a fast release of loosely bounded hydrogen from vacancy clusters and nanopores. During further photodarkening, the concentration of divacancy-like defects gradually increases due to the aggregation of light-induced hydrogen vacancies with preexisting yttrium monovacancies. After the UV illumination is stopped, two subsequent bleaching phases are observed. During the first bleaching phase, a strong correlation between the shortest positron lifetime 𝜏1 and the photochromic contrast is seen in both samples, suggesting that metallic domains disappear and, correspondingly, positron trapping at yttrium monovacancies and divacancy-like defects increases. During the second bleaching phase, a subsequent correlation between 𝜏1 and the photochromic contrast is observed in the more H-rich sample, which is related to the disappearance of larger metallic domains. After bleaching, most of the metallic domains and the photoexcited electrons in the matrix have disappeared, while the formed small vacancy complexes and larger vacancies remain stable. Our PALS study suggests that the formation of metallic domains is the cause of photodarkening, and the formed vacancy defects are important for understanding the memory effect.

Zero thermal expansion (ZTE) materials, which maintain a constant length despite temperature variations, are highly desirable for advanced industrial applications. This review highlights recent progress in exploring ZTE behavior in Fe-based Laves phases, La–Fe–Si(Al)-based alloys, and rare-earth-based systems exhibiting the magnetocaloric effect (MCE). The abnormal lattice expansion observed in giant magnetocaloric materials, driven by magnetic interactions, provides a natural foundation for designing ZTE materials. This review offers new insights into the design and discovery of novel ZTE materials within MCE systems. Furthermore, key properties such as mechanical strength, thermal and electrical conductivity, and cycling stability are also discussed, paving the way for ZTE advancements in functional materials.

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.

Zero thermal expansion (ZTE) materials, which maintain a constant length despite temperature variations, are highly desirable for advanced industrial applications. This chapter highlights recent progress in exploring ZTE behaviour in Fe-based Laves phases, La-Fe-Si(Al)-based alloys, rare-earth-based alloys, hexagonal MM′X alloys and Mn-based antiperovskite with a giant magnetocaloric effect. The abnormal lattice expansion observed in giant magnetocaloric materials, driven by magnetic interactions, provides a natural foundation for the design of ZTE materials. Furthermore, key properties such as the mechanical strength, thermal and electrical conductivity, and plasticity are discussed. This chapter offers new insights into the design and discovery of novel ZTE magnetic materials, paving the way for advancements in functional materials.

Magnetothermal stimulation is key in biomedical applications like tumor ablation, drug delivery, and regenerative therapies. A common method involves injecting magnetic particles that heat under an alternating magnetic field (AMF). However, uncontrolled heating can damage healthy tissues. Maintaining temperatures below 45 °C is critical. Using materials with a Curie temperature (Tc) near this limit offers a self-regulating solution, as magnetization—and thus heating—drops sharply at Tc. This study explores Mn0.65Fe1.30P0.65Si0.37 (MCM), a magnetocaloric material composed of non-toxic elements and featuring a tunable Tc. It is engineered to exhibit a Tc of 43 °C, close to the safe physiological threshold. MCM particles are encapsulated in a wax matrix to form a composite that responds to AMF exposure. Heat generated by MCM particles triggers the wax phase transition, while the obtained Tc enables the composite to achieve self-limiting thermal regulation under magnetic field exposure. Biocompatibility tests using human umbilical vein endothelial cells (HUVECs) show over 90% cell viability in direct and indirect contact. Stability tests in phosphate buffers at 37 °C confirm controlled degradation over 28 days. These results demonstrate that MCM is a promising, burn-free magnetic material for safe, localized heating, supporting its use in self-regulating, temperature-responsive biomedical systems.

We present the design and comprehensive investigation of stable para-substituted triarylamine–2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) radical ion pairs (RIPs) generated via single-electron transfer (SET). We quantified the degree of SET in both solution and solid phases, utilising a suite of spectroscopic techniques including IR, EPR, NMR, and single-crystal X-ray diffraction (SC–XRD). Our findings reveal that the extent of SET is significantly influenced by the nature of the substituents (MeO > tBu > Br) and the polarity of the solvent (MeCN > DCM > toluene). The radical ion pair [(pMeOPh)3N]⋅+[DDQ]⋅− was unambiguously identified using EPR and UV–vis spectroscopy, and its structure was confirmed by SC–XRD. Detailed analysis indicates an open-shell singlet ground state with a thermally accessible triplet state, as corroborated by EPR, magnetic susceptibility measurements, and DFT calculations. This study offers crucial insights into the mechanistic pathways of RIP formation and tuning both in solution and solid states, laying the groundwork for future exploration of their reactivity and potential 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.

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.

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 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.

Cycling stability of the photochromic effect in rare-earth oxyhydride thin films is of great importance for long-term applications such as smart windows. However, an increasingly slower bleaching rate upon photochromic cycling was found in yttrium oxyhydride thin films; the origin of this memory effect is yet unclear. In this work, the microstructural changes under six photodarkening-bleaching cycles in YHxOy and GdHxOy thin films are investigated by in situ illumination Doppler broadening positron annihilation spectroscopy, complemented by positron annihilation lifetime spectroscopy (PALS) investigations on YHxOy films before and after one cycle. For the first three cycles, the Doppler broadening S parameter after bleaching increases systematically with photodarkening-bleaching cycle, and correlates with the bleaching time constant extracted from optical transmittance measurements. This suggests that the microstructural evolution that leads to progressively slower bleaching involves vacancy creation and agglomeration. PALS suggests that during a photodarkening-bleaching cycle, divacancies are formed that are possibly composed of illumination-induced hydrogen vacancies and preexisting yttrium monovacancies, and vacancy clusters grow, which might be due to local removal of hydrogen. If bleaching is a diffusion-related process, the formed vacancy defects induced by illumination might affect the diffusion time by reducing the diffusion coefficient. Hydrogen loss could also be a key factor in the reduced bleaching kinetics. Other microstructural origins including domain growth, or formation of OH- hydroxide groups, are also discussed with respect to the slower bleaching kinetics. During the fourth to sixth photodarkening-bleaching cycle, reversible shifts in the Doppler S and W parameters are seen that are consistent with the reversible formation of metallic-like domains, previously proposed as a key factor in the mechanism for the photochromic effect.

Phosphorus recovery via vivianite extraction from digested sludge has recently gained considerable interest. The separation of vivianite was demonstrated earlier at the pilot scale, and operational parameters were optimized. In this study, we tested the robustness of this technology by changing the sludge characteristics, such as dry matter, and via that, sludge viscosity, and vivianite particle size. It was proven that the main factor influencing recovery was the concentration of vivianite in the feed. The technology can extract vivianite even when the sludge has higher dry matter (1.8% - 3.3%) and, therefore, higher viscosity. Smaller vivianite sizes (< 10 µm) can still be recovered but at a lower rate. This made magnetic separation applicable to a wide range of wastewater treatment plants.

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.

The performance of a magnetocaloric heat pump (MCHP) consisting of active magnetocaloric regenerators (AMR) of 12 layers of MnFePSi magnetocaloric materials (MCM) with a linear distribution of Curie temperatures was investigated using a 1D numerical model. The model predicted the heating power and coefficient of performance (COP) of the AMR for a fixed temperature span of 27 K, between 281 K and 308 K, and variable flow rate and AMR cycle frequency. A maximum applied magnetic field strength of 1.4 T was used. A well-insulated house with a maximum heating power demand of 3 kW (under quasi steady state conditions) was considered. Ambient temperature in The Netherlands was taken as a reference for the estimation of the seasonal heating power demand. Without optimizing the design of the AMR, the model predicts a maximum single-AMR heating power equal to 43.5 W when the AMR operates at 3 Hz and 3 L min^-1, and a maximum COP equal to 5.8 when it operates at 1.5 Hz and 1 L min^-1 Considering the maximum heating power of a single AMR, approximately 69 AMRs are needed to provide the design heating power demand of the house. It was found that it is possible to achieve an AMR seasonal COP of 5.6 by continuously adjusting the flow rate and frequency of operation of the MCHP along with the ON/OFF switching of some groups of AMRs in order to adjust the heating power of the MCHP to the heating power demand of the house.

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.