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.

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.

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.

Functional multicaloric materials exploit the strong coupling between multiple thermodynamic fields and offer a promising route toward compact and environmentally friendly solid-state refrigeration. Here, composition engineering is employed to develop a series of hexagonal MM′X-type MnFeNiCoGeSi alloys exhibiting giant magnetocaloric and barocaloric effects over a wide temperature range. A first-order magnetostructural transition produces concurrent discontinuities in magnetization and lattice volume, yielding both notable magnetocaloric and barocaloric response across 260–340 K with entropy changes above 47.3 J/kgK (magnetic field variation of 5 T) and 36.3 J/kgK (low pressure of 60 MPa), respectively, both surpassing most representative multicaloric materials. The large disparity between the sensitivity of the transition temperatures to hydrostatic pressure (T_C/dp = –49.4 K/GPa) and magnetic field (dT_C/dμ₀H = 1.2 K/T) provides exceptional tunability of the magnetostructural transition under coupled external stimuli. Consequently, under 1.16 GPa and a magnetic field change of 2 T, the refrigerant capacity increases from 65.14 to 73.53 J/kg relative to ambient pressure, accompanied by a remarkable transition-temperature shift of approximately 60 K. These findings establish multicomponent MnFeNiCoGeSi alloys as a promising platform for designing multicaloric refrigeration technologies.

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.

Microalloyed low-carbon steels strengthened by vanadium carbide (VC) nanoprecipitates are receiving increasing attention, particularly in the automotive industry. A clear understanding of the nanoprecipitate chemistry is essential for optimizing the alloy composition and processing routes, thereby enhancing the mechanical properties of such advanced steels. The chemical evolution of VC precipitates, especially regarding the incorporation of iron into the nanoprecipitates, remains uncertain. Here, a model vanadium-microalloyed low-carbon steel is studied by atomic-resolution scanning transmission electron microscopy (STEM) techniques. The steel contains nanoscale VC precipitates formed either as interphase precipitates (IP) at the austenite/ferrite interface during the austenite-to-ferrite phase transformation, or as randomly distributed precipitates (RP) in the ferrite matrix during bainite tempering. The first-time observation of carbon sublattice atoms in VC is achieved using integrated differential phase-contrast STEM (iDPC-STEM). Non-equilibrium compositions are identified under both precipitation mechanisms, with no correlation between precipitate size and associated elemental contents. Most interphase VC nanoprecipitates contain higher amounts of not only iron but also manganese compared to random VC nanoprecipitates. Complementary ex-situ small-angle neutron scattering (SANS) analysis and solute-drag effect (SDE) modeling support the co-segregation of iron and manganese into the precipitates. Manganese typically appears to form a core–shell-like structure within VC. Experimental evidence is presented for the SDE-assisted formation of manganese-rich–core (fibrous) interphase VC precipitates, and a mechanism is proposed for iron–manganese co-enrichment in random VC precipitates. This study offers new insights into future strategies to tune nanoprecipitate chemistry in microalloyed steels.

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.

The formation of nanoscale vanadium carbide (VC) precipitates is reported in steels subjected to two different thermal treatments. The thermal treatments lead to either interphase precipitation (IP) or random precipitation (RP). Small-angle neutron scattering measurements coupled with transmission electron microscopy analysis are performed to determine the VC precipitate volume fraction and size distribution. It is seen that the samples exhibiting IP show a higher number density of VC precipitates compared to those undergoing RP. Moreover, a broader size distribution of the precipitate radii is observed in the samples with RP, where lens-shaped nanoscale VC precipitates are found predominantly at grain boundaries (GBs) and sub-grain boundaries (SGBs), with smaller precipitates dispersed within the matrix. It is seen that the addition of carbon and vanadium does not increase the VC precipitate number density when the mechanism of precipitation is IP, whereas an increase in the VC precipitate number density with carbon and vanadium addition is seen in case of RP.

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.

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.

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.

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.

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.

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 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.