First-order magnetoelastic transitions usually involve mechanisms unique to each family of materials. For (Mn,Fe)₂(P,Si) compounds, it is generally predicted that the unit cell distortion occurring at the ferromagnetic transition leads to a strong electronic reconstruction of the Fe d states accompanied by a notable change in magnetic moment. However, there is no experimental consensus on this mechanism. Here, we use x-ray emission spectroscopy (XES) complemented by first-principles calculations, high-energy resolution fluorescence detected x-ray absorption (HERFD-XAS), and resonant inelastic x-ray scattering (RIXS) experiments, to clarify the nature of the first-order transition in a Mn_0.74Fe_1.23P0.71Si_0.32 crystal. HERFD-XAS and RIXS data show a minor evolution of the spectral features in the upper part of the K-edge for Mn and Fe, consistent with the calculated 4p density of states and fingerprinting the transition. In contrast, no significant evolution of the XES spectra is observed when the transition is crossed. In Fe-rich compositions, the calculations indicate that Fe at the 3g site develops a magnetic moment (2.43 μ_B) that is smaller than that of Mn at the 3g site (2.93 μ_B), but larger than that of Fe at the 3f site (1.48 μB). Quantitative XES analysis using the IAD method gives a reasonable agreement with the magnetic moments for different (Mn,Fe)₂(P,Si) compositions. However, the reduction of the Fe moment predicted by theory (approx. −0.6μB) is not observed around the transition. This study indicates that the Fe moment collapse at the transition may be weaker than the theoretically predicted value or more gradual in temperature, suggesting a secondary role for this moment instability in the giant magnetocaloric effect of (Mn,Fe)₂(P,Si) compounds.

We herein provide a combined experimental investigation and theoretical calculations on the impact of Mn doping and Fe off-stoichiometry on the magnetoelastic transition and the magnetocaloric properties of Laves phase Hf_0.82Ta_0.18Fe₂ alloys. Mn substitution led to an increase in unit-cell volume while Fe vacancies induced lattice contraction. By adjusting the Mn and Fe content, we achieved a table-like magnetocaloric response with a magnetic entropy change of 1.7–2.2 J/(kg K) at a magnetic field change of 2 T over a wide temperature range from 190 to 260 K. Mössbauer spectroscopy, neutron powder diffraction and density functional theory calculations all reveal that both Mn atoms and Fe vacancies preferentially occupy the 6h crystallographic site of the lattice structure with space group P6₃/mmc, and that the shortest intralayer Fe-6h interatomic distance governs the magnetoelastic transition in (Hf, Ta)Fe₂ Laves phases. The tunable magnetic transition is ascribed to the slight change of the electronic state of the Fe-6h site and limited hybridization between Mn and Fe atoms. These findings offer new insight into the site-specific control for optimizing the magnetocaloric properties of Fe-based Laves phase alloys and inspire the design of other promising magnetocaloric materials with magnetoelastic transitions.

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.

Materials with zero thermal expansion (ZTE) or negative thermal expansion (NTE) are critical for precision applications. Magnetocaloric materials exhibiting a strong spin-lattice coupling often undergo lattice changes near a magnetic transition, offering a route to ZTE behavior via magnetoelastic effects. This study examines the effect of Boron doping on the magnetoelastic transition, thermal expansion and magnetocaloric properties in Fe1.98Hf0.85Ta0.15B ₓ (x = 0.00, 0.01, 0.02, 0.03, 0.04) Laves phase alloys. Boron doping enhances hardness and increases the field sensitivity of the transition temperature. The second-order transition in the undoped alloy evolves into a first-order ferromagnetic-antiferromagnetic transition upon doping. First-principles calculations show that B occupies the 2 a sites, modifying the Fe-Hf 3 d -5 d hybridization and strengthening the spin-lattice coupling. In the Fe1.98 Hf0.85Ta0.15B0.01 alloy a near-zero thermal expansion with a coefficient of −0.17 ppm/K is observed in a temperature range of 133–213 K below the magnetoelastic transition at T t = 266 K, which is ascribed to the enhanced magnetoelastic transition by light-element doping with B. Our findings highlight a promising strategy to optimize the ZTE behavior through targeted light-element doping in magnetocaloric Laves phase systems.

Magnetostrictive materials are widely used in actuators, sensors, and energy-harvesting systems, but many high-performance compounds rely on heavy rare-earth elements or require high magnetic fields to develop giant magnetostrains. Here, we present Fe₂P/epoxy composites, exploiting an anisotropic first-order ferromagnetic transition (FOMT) to generate giant magnetostrains. A parametric model based on structural discontinuities and thermodynamic considerations is proposed to guide composition selection. Textured MnFe_0.95P_0.55Si_0.40B_0.05/epoxy composites were prepared by magnetic field alignment and characterized by strain-gauge dilatometry measurements as a function of temperature and magnetic field. Near the FOMT, despite matrix dilution effects, linear magnetostrains up to 0.22% at 2 T (0.37% at 7 T) are achieved. In particular, at intermediate fields, the magnetostrain shows a nearly linear increase with the field of about 0.1%/T (1000 ppm/T) with limited hysteresis. These results demonstrate that Fe₂P-type compounds, previously developed for magnetocaloric applications, can be adapted into scalable, low-cost magnetostrictive composites with tunable transition temperatures that rely only on abundant elements.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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 influence of chromium and aluminium doping on the over-reduction during activation of iron-oxide-based water-gas shift catalysts was investigated using Mössbauer spectroscopy for the first time. In situ Mössbauer spectra of catalysts exposed to industrially relevant gas compositions were recorded with increasingly reducing R factors R = [CO]*[H₂]/[CO₂]*[H₂O]. Whereas α-Fe and cementite formed during exposure of a non-doped iron-oxide catalyst to process conditions with an R factor of 2.09, such phases were only observed at R = 4.60 for a chromium-doped catalyst, showing that chromium stabilizes the catalyst. Over-reduction was enhanced to R = 2.88 in a chromium-copper co-doped catalyst. α-Fe was already observed at R = 1.64 in an aluminium-doped catalyst, while cementite formation occurred at R = 2.09, showing that over-reduction was enhanced, the presence of aluminium delaying carburization. Co-doping copper in the aluminium-doped catalyst showed cementite formation at R = 2.09, the same as a non-doped catalyst.

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.

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.

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.