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Reactive single-step hot-pressing at 1473 K and 35 MPa for 4 h produces dense, bulk, near single-phase, low-cost and low-criticality Fe$_2$Al$_{1.15}$B$_2$ and Fe$_2$Al$_{1.1}$B$_2$Ge$_{0.05}$Ga$_{0.05}$ MAB samples, showing a second-order magnetic phase transition with favorable magnetocaloric properties around room temperature. The magnetic as well as magnetocaloric properties can be tailored upon Ge and Ga doping, leading to an increase of Curie temperature $T_C$ and spontaneous magnetization $m_S$. The maximum isothermal entropy change $\Delta s_{T,max}$ of hot-pressed Fe$_2$Al$_{1.15}$B$_2$ in magnetic field changes of 2 and 5 T amounts to 2.5 and 5 J(kgK)$^{-1}$ at 287.5 K and increases by Ge and Ga addition to 3.1 and 6.2 J(kgK)$^{-1}$ at 306.5 K, respectively. The directly measured maximum adiabatic temperature change $\Delta T_{ad,max}$ is improved by the composition modification from 0.9 to 1.1 K in magnetic field changes of 1.93 T. Overall, we demonstrate that hot-pressing provides a much faster, more scalable and processing cost reducing alternative compared to conventional synthesis routes to produce heat exchangers for magnetic cooling devices. Therefore, our criticality assessment shows that hot-pressed Fe-based MAB phases provide a promising compromise of material and processing cost, criticality and magnetocaloric performance, demonstrating the potential for low-cost and low-criticality magnetocaloric applications around room temperature.

A multi-stimuli cooling cycle can be used to increase the cyclic caloric performance of multicaloric materials like Ni-Mn-In Heusler alloys. However, the use of a uniaxial compressive stress as an additional external stimulus to a magnetic field requires good mechanical stability. Improvement of mechanical stability and strength by doping has been shown in several studies. However, doping is always accompanied by grain refinement and a change in transition temperature. This raises the question of the extent to which mechanical strength is related to grain refinement, transition temperature, or precipitates. This study shows a direct comparison between a single-phase Ni-Mn-Sn and a two-phase Gd-doped Ni-Mn-In alloy with the same transition temperature and grain size. It is shown that the excellent magnetocaloric properties of the Ni-Mn-In matrix are maintained with doping. The isothermal entropy change and adiabatic temperature change are reduced by only 15% in the two-phase Ni-Mn-In-Heusler alloy compared to the single-phase alloy, which is resulting from a slight increase in thermal hysteresis and the width of the transition. Due to the same grain size and transition temperature, this effect can be directly related to the precipitates. The introduction of Gd precipitates leads to a 100% improvement in mechanical strength, which is significantly lower than the improvement observed for Ni-Mn-In alloys with grain refinement and Gd precipitates. This reveals that a significant contribution to the improved mechanical stability in Gd-doped Heusler alloys is related to grain refinement., Comment: Keywords: metal matrix composites, magnetocaloric, magneto-structural phase transitions, microstructure, mechanical properties, Heusler alloy

Utilizing the molecular beam epitaxy technique, a nanoscale thin-film magnet of c-axis-oriented Sm2Co17 and SmCo5 phases is stabilized. While typically in the prototype Sm(Co, Fe, Cu, Zr)7.5-8 pinning-type magnets, an ordered nanocomposite is formed by complex thermal treatments, here, a one-step approach to induce controlled phase separation in a binary Sm-Co system is shown. A detailed analysis of the extended X-ray absorption fine structure confirmed the coexistence of Sm2Co17 and SmCo5 phases with 65% Sm2Co17 and 35% SmCo5. The SmCo5 phase is stabilized directly on an Al2O3 substrate up to a thickness of 4 nm followed by a matrix of Sm2Co17 intermixed with SmCo5. This structural transition takes place through coherent atomic layers, as revealed by scanning transmission electron microscopy. Highly crystalline growth of well-aligned Sm2Co17 and SmCo5 phases with coherent interfaces result in strong exchange interaction, leading to enhanced magnetization and magnetic coupling. The arrangement of Sm2Co17 and SmCo5 phases at the nanoscale is reflected in the observed magnetocrystalline anisotropy and coercivity. As next-generation permanent magnets require designing of materials at an atomic level, this work enhances our understanding of self-assembling and functioning of nanophased magnets and contributes to establishing new concepts to engineer the microstructure for beyond state-of-the-art magnets.

Recently 1:12 magnets of Sm-(Fe,V) have shown promising coercivities and the potential to be alternative rare-earth-lean permanent magnets. In this work, we investigated the effects of partial substitution of Cu, Mo and Ti for V in the magnets prepared by hot compaction and hot deformation of mechanically milled powders. The microstructure of the Sm-Fe-(V,Cu) and Sm-Fe-(V,Ti) hot-deformed magnets consisted in fine grains with sizes between 50 and 150 nm. The Sm-Fe-(V,Cu) magnet showed the best performance with μ0H c = 0.96 T, μ0M r = 0.49 T, (BH) max = 42 kJ m − 3 and T C = 362 ∘ C. Atom probe tomography of this magnet revealed the presence of a thin Sm17.5Fe71.5V8Cu3intergranular phase of 3-6 nm surrounding the 1:12 nanograins. The addition of a small amount of Cu, not only improved the magnetic properties but also hindered the grain growth during hot deformation. Micromagnetic simulations of the magnetization reversal agreed with the experimental values of coercivity. The presence of the intergranular phase reduces the number of grains that switch simultaneously.

Ni-Mn-based Heusler alloys, in particular all-d-metal Ni(-Co)-Mn-Ti, are highly promising materials for energy-efficient solid-state refrigeration as large multicaloric effects can be achieved across their magnetostructural martensitic transformation. However, no comprehensive study on the crucially important transition entropy change Δs exists so far for Ni(-Co)-Mn-Ti. Here, we present a systematic study analyzing the composition and temperature dependence of Δst. Our results reveal a substantial structural entropy change contribution of approximately 65 J(kgK)-1, which is compensated at lower temperatures by an increasingly negative entropy change associated with the magnetic subsystem. This leads to compensation temperatures Tcomp of 75 K and 300 K in Ni35Co15Mn50-yTiy and Ni33Co17Mn50-yTiy, respectively, below which the martensitic transformations are arrested. In addition, we simultaneously measured the responses of the magnetic, structural and electronic subsystems to the temperature- and field-induced martensitic transformation near Tcomp, showing an abnormal increase of hysteresis and consequently dissipation energy at cryogenic temperatures. Simultaneous measurements of magnetization and adiabatic temperature change ΔTad in pulsed magnetic fields reveal a change in sign of ΔTad and a substantial positive and irreversible ΔTad up to 15 K at 15 K as a consequence of increased dissipation losses and decreased heat capacity. Most importantly, this phenomenon is universal, it applies to any first-order material with non-negligible hysteresis and any stimulus, effectively limiting the utilization of their caloric effects for gas liquefaction at cryogenic temperatures.

Ferromagnetic Fe3Sn, Fe5Sn3 and Fe3Sn2 single crystals were synthesized using the reactive flux technique. Derived from single crystal x-ray diffraction and Transmission Electron Microscopy (TEM), a new structural model is proposed for the Fe5Sn3 crystals - the threefold twinning of an orthorhombic unit cell with (3 + 1) dimensional space group Pbcm(α00)0s0. The spontaneous magnetization (Ms) and the anisotropy constants K1 and K2 of Fe3Sn, Fe5Sn3 and Fe3Sn2 single crystals were determined in a wide temperature range using M(H) dependencies and a modified Sucksmith-Thompson technique. Ms and K1 were also evaluated in the framework of Density Functional Theory (DFT) and an overall good agreement was observed between the calculated and experimental results. Furthermore, a critical evaluation of different analytical models for the assessment of magnetocrystalline anisotropy was performed, which are restricted to the analysis of uniaxial magnetic domain patterns, and it is shown that such high-throughput techniques can lead to unrealistic results. Finally, a DFT high-throughput screening of the Fe-Sn phase diagram was used to identify Fe-Sn based phases with potential to be stabilized upon alloying, and their magnetization and magnetocrystalline anisotropy were evaluated. The results show that a similar strong anisotropy as observed in Fe3Sn may also be found in other Fe-Sn based phases, having higher potential to be used as hard magnetic material.

Materials with a large magnetocaloric response require a large magnetic moment. However, we show in this paper that it is possible to retain both the magnetic entropy change and the adiabatic temperature change even using dopants that reduce the magnetic moment of the parent alloy, provided that the first order character of the transition is enhanced. In this work, a combination of first-principles calculations, experimental determination of the magnetocaloric response (direct and indirect) as well as a new criterion to determine the order of the phase transition are applied to Cr-doped La(Fe,Si) 13 compounds. Despite a reduction in magnetic moment, the magnetocaloric response is retained up to x≈0.3 in LaFe 11.6-x Cr x Si 1.4 . Unlike other transition metal dopants, Cr occupy 8b sites and couple antiferromagnetically to Fe atoms. The cross-over of first to second order transition is achieved for a Cr content of x=0.53, larger in comparison to other dopants (e.g. Ni or Mn). A direct relation between the first order character and the hysteresis is observed.

In order to find a promising trade-off permanent magnet material regarding a performance/cost-ratio, the Ce 1 − x Sm x Fe 11 − y Ti 1 V y -phase (x = 0–1; y = 0, 1) is analyzed in detail. In the first part, its existence range is studied (1000 °C) and the intrinsic magnetic properties are comprehensively determined. Diffraction experiments localize both structure-stabilizing transition metals on 8i-sites, explaining the measured reduction in saturation polarization as V is added. Curie temperatures increase upon Sm-substitution with a negligible dependence on V. Annealings of nanocrystalline material produced via intensive milling and melt-spinning show that V especially raises the obtainable maximum coercivities for Sm-rich phases (924 kA/m). In the second part, the promising magnetic properties of the nanocrystalline material are successfully transferred to the bulk state via hot-pressing. The isotropic Ce 0 . 5 Sm 0 . 5 Fe 10 Ti 1 V 1 -magnet (coercivity = 425 kA/m) is characterized by various means. Magnetic measurements, structural investigations and calculations of the elastic constants consider necessary factors for a successful texturing by die-upsetting (as accepted for Nd - Fe -B). The results are fundamental for further considerations in this active field of research.

We report on the magnetic properties and magnetocaloric effect of dysprosium ribbons obtained from a bulk sample by means of cold rolling. A magnetic entropy change ΔS ≈ 5.5 J/(kg·K) and an adiabatic temperature change ΔT ≈ 1.65 K for a bulk sample of dysprosium were observed for in external magnetic field changes of 3 T and 1.9 T, respectively. As compared to the bulk sample, ΔS and ΔT in the cold rolled ribbons is smaller by ∼30% and ∼12%, respectively. These changes in magnetocaloric effect of the cold rolled ribbons are fully reversible and can be restored by additional annealing.

The adiabatic temperature change ΔTad of a Mn1:3Fe0:7P0:5Si0:55 Fe2P-type alloy was measured under different magnetic field-sweep rates from 0:93 Ts-1 to 2870 Ts-1. We find a field-sweep-rate independent magnetocaloric effect due a partial alignment of magnetic moments in the paramagnetic region overlapping with the magnetocaloric effect of the first-order phase transition. Additionally, the first-order phase transition is not completed even in fields up to 20T leading to a non-saturating behavior of ΔTad. Measurements in different pulsed fields reveal that the first-order phase transition cannot follow the fast field changes as previously assumed, resulting in a distinct field-dependent hysteresis in ΔTad.

Hydrogen has the largest gravimetric energy density among all chemical fuels. At the same time, the density of gaseous ${\mathrm{H}}_{2}$ is extremely low, which makes its compression to high pressures, liquefaction, or solid-state storage necessary for transport purposes. Liquid hydrogen ($\mathrm{L}{\mathrm{H}}_{2}$) can be transported in a dewar under atmospheric pressure, but this requires energy-intensive cooling down to $20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Magnetocaloric materials have great potential to revolutionize gas liquefaction to make $\mathrm{L}{\mathrm{H}}_{2}$ more competitive as fuel. In this paper, we investigate a series of Laves-phase materials regarding their structural, magnetic, and magnetocaloric properties in high magnetic fields. The three compounds $\mathrm{Ho}{\mathrm{Al}}_{2}$, ${\mathrm{Ho}}_{0.5}{\mathrm{Dy}}_{0.5}{\mathrm{Al}}_{2}$, and $\mathrm{Dy}{\mathrm{Al}}_{2}$ are suited for building a stack for cooling from liquid-nitrogen temperature ($77\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) down to the boiling point of hydrogen at $20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This is evident from our direct measurements of the adiabatic temperature change in pulsed magnetic fields, which we compare with calorimetric data measured in a static field. With this methodology, we are now able to study the suitability of magnetocaloric materials down to low temperatures up to the highest magnetic fields of 50 T.

Measurements of the magnetization in quasistatic and pulsed magnetic fields with different sweep rates, measurements of the specific heat in various magnetic fields, and direct measurements of the adiabatic temperature change have been employed to study the metamagnetic phase transition from an antiferromagnetic (AF) to the ferromagnetic (FM) state in an ${({\mathrm{Fe}}_{0.98}{\mathrm{Ni}}_{0.02})}_{49}{\mathrm{Rh}}_{51}$ alloy with a critical AF-FM transition temperature, ${T}_{\mathrm{tr}}$, reduced to 266 K. Based on the obtained results, a magnetic phase diagram for this alloy has been constructed. The AF-FM transition induced by the magnetic field below 10 K is found to occur in a steplike fashion in contrast to smooth behavior at $10\phantom{\rule{0.16em}{0ex}}\mathrm{K}lTl{T}_{\mathrm{tr}}$. The adiabatic temperature change $\mathrm{\ensuremath{\Delta}}{T}_{ad}$ in the magnetic field of 2 T exceeds 6.5 K in pulsed fields (\ensuremath{\sim}100 T/s) and in the Halbach setup (\ensuremath{\sim}0.5 T/s), which is in agreement with the estimation from the S-T diagram constructed based on the specific heat measurements. The reversible $\mathrm{\ensuremath{\Delta}}{T}_{ad}$ reaches \ensuremath{-}4.6 K under cyclic conditions in the Halbach setup (2 T). A complete transformation to the FM state in the whole temperature range requires a magnetic field of 14 T. Direct measurements of $\mathrm{\ensuremath{\Delta}}{T}_{ad}$ in pulsed fields of 14 T revealed an irreversible part of the magnetocaloric effect associated with the presence of magnetic hysteresis and respective losses during the magnetization process.

International audience; Anisotropy constants are obtained from an analysis of single crystal magnetization curves measured up to high fields. The anisotropy of the 3d transition metal (M) sublattice is considered, as well as molecular exchange field coupling between the rare-earth (R) and transition metal sublattices (M). This procedure allows for non colinear R and M magnetic moments, meaning that their angles with respect to the easy axis are independent variables. With this approach we obtain anisotropy constants that are larger than those reported in the literature, which reflects the anisotropy of the isolated R sublattice. Results for Co and/or Ce doped Nd2Fe14B single crystals are presented, showing the influence of such substitutions on the magnetocrystalline anisotropy. These results indicate that the enhanced performance of NdFeB-based magnets co-doped with Ce and Co is due to an improvement in intrinsic properties.

Novel multicaloric cooling utilizing the giant caloric response of Ni-Mn-based metamagnetic shape-memory alloys to different external stimuli such as magnetic field, uniaxial load and hydrostatic pressure is a promising candidate for energy-efficient and environmentally-friendly refrigeration. However, the role of microstructure when several external fields are applied simultaneously or sequentially has been scarcely discussed in literature. Here, we synthesized ternary Ni-Mn-In alloys by suction casting and analyzed the microstructural influence on the response to magnetic fields and uniaxial load. SEM-EBSD reveals a distinct core-shell microstructure with a radially symmetric solidification texture resulting in a two-step martensitic transformation. In correlation with temperature-dependent XRD a significant effect of grain orientation on the stress-induced martensitic transformation is demonstrated. The influence of microstructure on the magnetic-field-induced transformation dynamics is studied by strain measurements in static and pulsed fields. Temperature-stress and temperature-magnetic field phase diagrams are established and single caloric performances are characterized in terms of ${\Delta}s_{T}$ and ${\Delta}T_{ad}$. The cyclic ${\Delta}T_{ad}$ values are compared to the ones achieved in the multicaloric exploiting-hysteresis cycle. It turns out that a tailored microstructure and the combination of both stimuli enable outstanding caloric effects in moderate external fields which can significantly exceed the single caloric performances. In particular for Ni-Mn-In, the maximum cyclic effect in magnetic fields of 1.9 T is increased by more than 200 % to -4.1 K when a moderate sequential stress of 59 MPa is applied. Our results illustrate the crucial role of microstructure for multicaloric cooling using Ni-Mn-based metamagnetic shape-memory alloys.

Pinning-type magnets with high coercivity at high temperatures are at the core of thriving clean-energy technologies. Among these, Sm2 Co17 -based magnets are excellent candidates owing to their high-temperature stability. However, despite intensive efforts to optimize the intragranular microstructure, the coercivity currently only reaches 20-30% of the theoretical limits. Here, the roles of the grain-interior nanostructure and the grain boundaries in controlling coercivity are disentangled by an emerging magnetoelectric approach. Through hydrogen charging/discharging by applying voltages of only ≈1 V, the coercivity is reversibly tuned by an unprecedented value of ≈1.3 T. In situ magneto-structural characterization and atomic-scale tracking of hydrogen atoms reveal that the segregation of hydrogen atoms at the grain boundaries, rather than the change of the crystal structure, dominates the reversible and substantial change of coercivity. Hydrogen reduces the local magnetocrystalline anisotropy and facilitates the magnetization reversal starting from the grain boundaries. This study opens a way to achieve the giant magnetoelectric effect in permanent magnets by engineering grain boundaries with hydrogen atoms. Furthermore, it reveals the so far neglected critical role of grain boundaries in the conventional magnetization-switching paradigm of pinning-type magnets, suggesting a critical reconsideration of engineering strategies to overcome the coercivity limits.

Ni-Mn-based metamagnetic shape-memory alloys exhibit a giant thermal response to magnetic fields and uniaxial stress which can be utilized in single caloric or multicaloric cooling concepts for energy-efficient and sustainable refrigeration. However, during cyclic operation these alloys suffer from structural and functional fatigue as a result of their high intrinsic brittleness. Here, we present based on Fe-doping of Ni-Mn-In a microstructure design strategy which simultaneously improves cyclic stability and maintains the excellent magnetocaloric and elastocaloric properties. Our results reveal that precipitation of a strongly Fe-enriched and In-depleted coherent secondary gamma-phase at grain boundaries can ensure excellent mechanical stability by hindering intergranular fracture during cyclic loading. In this way, a large elastocaloric effect of -4.5 K was achieved for more than 16000 cycles without structural or functional degradation, which corresponds to an increase of the cyclic stability by more than three orders of magnitude as compared to single-phase Ni-Mn-In-(Fe). In addition, we demonstrate that the large magnetocaloric effect of single-phase Ni-Mn-In-(Fe) can be preserved in the dual-phase material when the secondary gamma-phase is exclusively formed at grain boundaries as the martensitic transformation within the Heusler matrix is barely affected. This way, an adiabatic temperature change of -3 K and an isothermal entropy change of 15 $Jkg^{-1}K^{-1}$ was obtained in 2 T for dual-phase Ni-Mn-In-Fe. We expect that this concept can be applied to other single caloric and mutlicaloric materials, therewith paving the way for solid-state caloric cooling applications.

International audience; Nd2Fe14B-type materials exhibit the highest energy product around room temperature and hence dominate the high-performance permanent magnet market. Intensive research efforts aim at alternative material systems containing less critical elements with similar or better magnetic properties. Nd-and Sm-based compounds with a ThMn12-type structure exhibit intrinsic properties comparable or even superior to Nd2Fe14B. However, it has not been possible to achieve technically relevant coercivity and remanent magnetization in ThMn12-based bulk sintered magnets. Using SmFe11Ti as a prototypical representative, we demonstrate that one important reason for the poor performance is the formation of twins inside micro-crystalline grains. The nature of the twins in SmFe11Ti was investigated in twinned "single crystals" and both bulk and thin film poly-crystalline samples, using advanced electron microscopy and atom probe tomography as well as simulations and compared with benchmark Nd2Fe14B. Both micro-twins and nano-twins show a twin orientation of 57±2 • and an enrichment in Sm, which could affect domain wall motion in this 2 material. Micromagnetic simulations indicate that twins act as nucleation centers, representing the magnetically weakest link in the microstructure. The relation between twin formation energies and geometrical features are briefly discussed using molecular dynamic simulations.

We have investigated synthesis, magnetic properties and microstructure of Sm5Fe17-based hard magnetic phase with a Sm20Fe70Ti10 composition. Ultrahigh coercivities, μ0Hc=7.18 T at room temperature and μ0Hc=8.86 T at 10 K, have been achieved. The room-temperature coercivity, determined from high-field pulse measurements, reaches 35% of the anisotropy field μ0Ha=20.7±0.8 T. Further, it is demonstrated that a coercivity of 2.18 T is maintained even at 500 K. The Curie temperature TC of Sm20Fe70Ti10 is 577 K and the calculated exchange stiffness parameter A is 7.72 pJ/m. Detailed transmission electron microscopy investigations show a two-phase microstructure consisting of the Sm5Fe17-based hard magnetic matrix phase with grain size below 200 nm and finer, below 100 nm, Fe2Ti grains. Majority of the Fe2Ti phase is located at the grain boundaries with some finer inclusions found also inside the 5:17 grains. Despite the high fraction of the Fe2Ti grains, nearly single-phase demagnetization loops are observed. In order to enhance Ms, the effect of Ti content on phase constitution, magnetic properties and microstructure was studied in detail. Ms increases and Hc decreases for the Ti-lean compositions.

The inverse magnetocaloric effect in Ni-Mn based Heusler compounds occurs during the magnetostructural transition between low-temperature, low-magnetization martensite and high-temperature, high-magnetization austenite. In this study, we analyze the metamagnetic transformation of a Ni49.8Mn35In15.2 compound by simultaneous adiabatic temperature change ΔTad and strain Δl/l0 measurements in pulsed magnetic fields up to 10 T. We observe a ΔTad of −10 K and a Δl/l0 of −0.22% when the reverse martensitic transition is fully induced at a starting temperature of 285 K. By a variation of the magnetic field-sweep rates between 316, 865, and 1850 T s−1, the transitional dynamics of the reverse martensitic transformation have been investigated. Our experiments reveal an apparent delay upon the end of the reverse martensitic transformation at field rates exceeding 865 T s−1 which is related to the annihilation of retained martensite. As a consequence, the field hysteresis increases and higher fields are required to saturate the transition. In contrast, no time-dependent effects on the onset of the reverse martensitic transformation were observed in the studied field-sweep range. Our results demonstrate that kinetic effects in Heusler compounds strongly affect the magnetic cooling cycle, especially when utilizing a multicaloric “exploiting-hysteresis cycle” where high magnetic field-sweep rates are employed.

Defects of various kinds play a crucial role on coercivity in rare-earth-free permanent magnetic alloys with L10 structure. In this work, the negative effect of twin structure and the positive effect of dislocations on the coercivity are clarified in a systematic experimental study of L10-MnAl alloys, because the former defect can induce the nucleation of reversal domain and the latter can act as a pinning center. Thus, the combination of eliminating twin structure and introducing high-density dislocations could overcome the present bottleneck in magnetic performance. This work may inspire avenues for the development of L10 rare-earth-free permanent magnetic alloys.

In this study, molecular beam epitaxy is utilized to stabilize a nanostructured thin-film magnet consisting of a soft magnetic Y2Co17 exchange coupled to hard magnetic YCo5. While, typically, a phase decomposition can be obtained in rare-earth cobalt systems only by the addition of further elements like Cu, Fe, and Zr and complex heat treatments, here we directly induce phase separation by growth kinetics. The resulting nanoscale architecture, as revealed by cross-sectional transmission electron microscopy, is composed of a network of coherently interlinked and aligned Y2Co17 and YCo5 building blocks. The formation of coherent precipitations is facilitated by the perfectly matching lattice constants, atomic species, and crystal symmetry of the two phases with vastly different magnetocrystalline anisotropies. The hard magnetic phase induces an aligned uniaxial anisotropy in Y2Co17, resulting in substantial coercivity associated with enhanced energy products. This work highlights the importance of thin-film epitaxy in understanding magnetic hardening mechanisms and suggests strategies for a rational design of future sustainable magnetic systems.

Cerium-based intermetallics are currently attracting much interest as a possible alternative to existing high-performance magnets containing scarce heavy rare-earth elements. However, the intrinsic magnetic properties of Ce in these systems are poorly understood due to the difficulty of a quantitative description of the Kondo effect, a many-body phenomenon where conduction electrons screen out the Ce-4f moment. Here, we show that the Ce-4f shell in Ce–Fe intermetallics is partially Kondo screened. The Kondo scale is dramatically enhanced by nitrogen interstitials suppressing the Ce-4f contribution to the magnetic anisotropy, in striking contrast to the effect of nitrogenation in isostructural intermetallics containing other rare-earth elements. We determine the full temperature dependence of the Ce-4f single-ion anisotropy and show that even unscreened Ce-4f moments contribute little to the room-temperature intrinsic magnetic hardness. Our study thus establishes fundamental constraints on the potential of cerium-based permanent magnet intermetallics.

$\mathrm{SmF}{\mathrm{e}}_{11}\mathrm{Ti}\text{\ensuremath{-}}\mathrm{based}$ alloys have potential as permanent magnet materials; however, until now, crystallographically textured bulk permanent magnets have not yet been produced from this alloy system. This is partly due to the lack of information on the morphology and composition of grain boundary phases present in the Fe-rich Sm-Fe-Ti alloys. Here we investigated the microstructure of a $\mathrm{S}{\mathrm{m}}_{1.25}\mathrm{F}{\mathrm{e}}_{11}\mathrm{Ti}$ alloy by using correlative transmission electron microscopy and atom-probe tomography, combined with magneto-optical Kerr effect (MOKE) probing to relate the material's micro- and nanostructure to its properties. The grains of the $\mathrm{Sm}{(\mathrm{Fe},\mathrm{Ti})}_{12}$ matrix phase are separated by grain boundaries exhibiting a different composition over 3--4 nm width. They contain $g75\phantom{\rule{0.16em}{0ex}}\mathrm{at}%$ of the ferromagnetic element Fe, with an enrichment of Sm of up to 16.6 at% and a depletion in Ti, down to approx. 3.4 at%. We believe that the grain boundary is ferromagnetic at room temperature, which makes the magnetic decoupling of the grains practically impossible, which, in turn, leads to a low coercivity of $\mathrm{SmF}{\mathrm{e}}_{11}\mathrm{Ti}$-based alloys. MOKE measurements reveal the strong ferromagnetic coupling across the grain boundary, causing the nucleation of reversal magnetic domains when exposed to low magnetic fields. In a triple-junction area we identified three other ferromagnetic phases: $\mathrm{S}{\mathrm{m}}_{3}{(\mathrm{Fe},\mathrm{Ti})}_{29},\mathrm{SmF}{\mathrm{e}}_{2}$, and $\mathrm{F}{\mathrm{e}}_{2}\mathrm{Ti}$. These details bring out the scope of further adjustment of the coercivity in the Sm-Fe-Ti alloy system by grain boundary segregation engineering through the reduction of the presence of ferromagnetic phases to ensure a magnetic decoupling of the micrometer-sized $\mathrm{Sm}{(\mathrm{Fe},\mathrm{Ti})}_{12}$ grains.

The possibilities of application of known models of domain structure to multiphase alloys are analyzed with the aim to assess the anisotropy constants and to search for magnetically uniaxial highly anisotropic phases. It is shown in what cases and how the standard models of domain structure may be applied to such alloys. Recommendations on the necessary amendments of the model to be applied to multiphase materials are given.

Despite significant progress in the study of materials undergoing first-order magnetic phase transitions accompanied by the so-called giant magnetocaloric effect (MCE), Gd metal still remains the most widely used material in prototypes of magnetic refrigerators due to its significant MCE, good machinability and reasonable mechanical and chemical stabilities. Alloying of Gd enables fine-tuning the Curie temperature of Gd-based solid solutions (all show second-order phase transitions), for graded magnetocaloric materials. Commonly, Gd packed spheres are used as a magnetocaloric working substance in the active magnetic regenerator (AMR) cycle. In this work, we show that the optimized stacking parallel-plate geometry of AMR bed made of Gd is more effective for application at frequencies 1–10 Hz then the packed spheres. We also give a short review on magnetocaloric properties of cold-rolled Gd-X (X = Y, In, Zr, Ga, B) solid solutions. These materials can be produced in the form of thin (∼100 μm) foils/plates to ensure rapid heat exchange between to the heat transfer fluid. Although the magnetocaloric effect decreases in the as-rolled foils, it can be recovered by thermal treatment of the final stacked-plates regenerators. Gd-Y, Gd-In and Gd-Zr solid solutions have magnetocaloric properties, comparable to the MCE of pure Gd in a wide temperature working span up to 37 K, 36 K, and 16 K respectively, which makes them suitable magnetocaloric material systems for testing the fundamental heat exchangers geometries at ambient temperature and in frequencies of 1–10 Hz.

We report on the spontaneous magnetization Ms, the exchange stiffness constant A and the magnetocrystalline anisotropy constants K1, K2, K3 and K4 of Nd5Fe17 and Nd5Fe17H16 single crystals. Field dependencies of magnetization M(H) were measured along a, b' and c principal crystallographic directions within the temperature range of 10–600 K and magnetic fields up to 40 T. Large anisotropies of spontaneous magnetization and high-field susceptibility were revealed for both compounds. The exchange stiffness parameter A was determined using Bloch's T3/2 law. In order to provide high accuracy detection of K1(T), K2(T), K3(T) and K4(T), we used two different approaches: the modified Sucksmith- Thompson technique and the Neel's phase method.

Microstructures of magnetocaloric Ni-Mn-In-based Heusler alloys, Ni50.2Mn35.0In14.8 and Ni46.1Mn37.9Fe3.0In13.0, were studied to understand the origin of a large difference in thermal hysteresis in these two alloys. In-situ transmission electron microscopy (TEM) observation showed that the Fe containing sample with a large hysteresis shows a discontinuous phase transition due to the existence of nano-scale Fe-rich bcc phase, along with Fe-lean B2 and L21 phases in the austenite state. The Fe-free sample with a low hysteresis shows a uniform phase transition from martensite to austenite initiated by the nucleation of austenite at the twin boundaries. Ni segregation was found at the twin boundaries of the low hysteresis sample that is considered to facilitate the nucleation of the austenite. The phase transition progresses by the growth of the nucleated austenite to the neighboring twins. 5M and 7M modulated martensites in the low hysteresis sample give rise to a slight difference in the phase transition temperatures in the twin bands contributing to the small hysteresis of 4.4 K in the Fe-free sample. Based on these results, we conclude that to minimize the thermal hysteresis of the Ni-Mn-In based magnetocaloric compounds, one of the key factors is to achieve a uniform composition and crystal structure in the alloy.

International audience; Rare-earth free permanent magnets were produced by consolidation of cobalt nanorods synthesized by the polyol process exhibiting a mean diameter in the range 10e30 nm. Compactions of magnetically prealigned rod assemblies at various pressures and temperatures were carried out to make dense materials. Bulk magnets exhibiting a very good mechanical strength and an energy product as high as 65 kJ m À3 were obtained. The best results were obtained when the compaction conditions were soft enough to preserve the morphology and alignment of the rods in the final material, as revealed by X-ray diffraction and neutron scattering. For the first time the bottom-up approach is convincingly reported to produce bulk magnets without the addition of any matrix, the obtained nanostructured materials exhibit coercivity much higher than the AlNiCo magnets and can fill the performance " gap " between hexaferrites and rare-earth based magnets.

Ferromagnetic-shape-memory (FSM) Heusler compounds are an important class of multifunctional materials having promising applications in a vast variety of areas such as actuating, sensing, energy harvesting, spintronics and multicaloric cooling. Their multifunctionality stems from a reversible martensitic phase transition. However, their full exploitation is prevented by some undesirable characteristics of the martensitic transition: thermal hysteresis and broad transition. We studied here the role of specific martensitic configurations on the transition characteristics. By advanced magnetic force microscopy imaging in a wide temperature (260–350 K) and magnetic field range (up to 14 T) we directly observed the nucleation and the self-accommodation of the martensitic twinning configurations under zero-field, isofield and isothermal conditions. The experiments were performed on Ni-Mn-Ga epitaxial thin films with martensitic twinning configurations made of both X- and Y-type, which are characterized by different orientations of the twinning planes (i.e. at 45° and 90° degrees to the (001) MgO substrate, respectively). We have found that between the two possible twinning configurations, the Y-type, which nucleates first, shows a significantly smaller thermal hysteresis as well as a sharper phase transition with respect to X-type twinning configuration, for all the three investigated conditions.

We report on the magnetocaloric effect in Gd 100- x B x ( x = 0, 10, 15) cold rolled ribbons. A moderate entropy change Δ S = 5 J/kg·K and adiabatic change of Δ T = 4.8 K were observed for the as-cast materials in an external magnetic field of 3 T which is less by 20% in comparison with the pure gadolinium metal. It was found that a significant (up to 70%) depression of magnetization and magnetocaloric properties developed in the course of plastic deformation can completely be restored by means of a high temperature heat treatment. It is concluded that cold rolling is one promising technique for producing foil shaped magnetocaloric materials suitable for designing heat exchangers of magnetic cooling devices.

Hydrogenated La(Fe,Mn,Si)13-based alloys have an excellent magnetocaloric properties, but poor mechanical and chemical stability. This hinders their direct machining and implementation in an active magnetic regenerator (AMR). In this paper, we show how machinability and corrosion protection of the particles can be improved by a hot-dip coating. To avoid the loss of hydrogen during the coating process, a low melting temperature eutectic Bi–Sn–In alloy was selected as a metal binder. The coated particles were used to build a packed bed regenerator as an array of fixed particles, avoiding such negative effects as sedimentation, segregation, and channel deformation. Similarity theory, combined with unsteady heat transfer approach was applied in order to calculate the optimal operation frequency and to estimate the maximal cooling power of the magnetocaloric regenerators. Two different geometries of heat exchangers were theoretically compared: stacked flat plate/channel structure and packed bed of equidimensional spherical particles. It is shown that, operating at low frequency, the same amount of magnetocaloric material can expel bigger amount of heat, when formed as packed bed heat exchangers. The metal-bonded packed bed regenerator made from La(Fe,Mn,Si)13H x powder was tested in a home-build versatile testing device in a magnetic field change of 10 kOe. The maximal achievable temperature span as a function of both parameters—hot end temperature and length of regenerator—was explored. The largest thermal span of 8 K was produced by the regenerator with 40 mm length.

Metal-filled carbon nanocomposites containing 20 wt% of metallic FeCo nanoparticles were synthesized by means of infrared heating of precursors (polyacrylonitrile – iron acetylacetonate - cobalt acetate). This fabrication approach shows promise for making radiation-absorbent materials in short one-step process with ability to control the size of nanoparticles and attune the composition of the metallic components. In this work the magnetic behavior of reaction products obtained at different stages of the synthesis have been investigated in detail. We report on the influence of the annealing temperature on evolution of the structure, chemical composition, size, surface morphology, spontaneous magnetization and coercivity of the FeCo nanoparticles.

Due to their excellent magnetocaloric properties hydrogenated La(Fe,Mn,Si)13 are considered as promising and cost efficient materials for active magnetic regenerators operating near room temperature. However, due to their poor mechanical and chemical stability this alloys can not be directly implemented in a cooling machine. A solution of the problem is the production of a composite La(Fe,Mn,Si)13Hx magnetocaloric materials by using adhesive-bonding techniques similar to those used for production of polymer-bonded permanent magnets. Upon bonding one has to consider that the thermal stability of the polymer binder is rather low. Main disadvantage of a polymer-bonded composite is the fatigue due to the mechanical stress caused by the large magnetovolume effect in La(Fe,Mn,Si)13Hx. Our article reports on a new method and equipment to produce metal-bonded magnetocaloric material using the low melting eutectic Field's alloy as a binder. A comprehensive investigation of the magnetocaloric, mechanical, chemical and thermal transport properties of polymer-bonded and metal-bonded magnetocaloric material is presented.

Spin-wave and exchange stiffness constants of 22 ferromagnetic compounds have been deduced from their spontaneous magnetization, $$M_{\mathrm{s}}$$ , by using an improved technique. The improvement consists in utilizing the entire $$M_{\mathrm{s}}(T)$$ curve, up to the Curie point, rather than just its low-temperature part, with $$T\ll T_{\mathrm{C}}$$ . For 17 of the 22 ferromagnets, literature data have been used, while 5 compounds have been studied anew, on single crystals grown for the purpose.

We report on the impact of magnetoelastic coupling on the magnetocaloric properties of ${\mathrm{LaFe}}_{11.4}{\mathrm{Si}}_{1.6}{\mathrm{H}}_{1.6}$ in terms of the vibrational (phonon) density of states (VDOS), which we determined with $^{57}\mathrm{Fe}$ nuclear resonant inelastic x-ray scattering (NRIXS) measurements and with density functional theory (DFT) based first-principles calculations in the ferromagnetic (FM) low-temperature and paramagnetic (PM) high-temperature phase. In experiments and calculations, we observe pronounced differences in the shape of the Fe-partial VDOS between nonhydrogenated and hydrogenated samples. This shows that hydrogen not only shifts the temperature of the first-order phase transition, but also affects the elastic response of the Fe subsystem significantly. In turn, the anomalous redshift of the Fe VDOS, observed by going to the low-volume PM phase, survives hydrogenation. As a consequence, the change in the Fe-specific vibrational entropy $\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{lat}}$ across the phase transition has the same sign as the magnetic and electronic contribution. DFT calculations show that the same mechanism, which is a consequence of the itinerant electron metamagnetism associated with the Fe subsystem, is effective in both the hydrogenated and the hydrogen-free compounds. Although reduced by 50% as compared to the hydrogen-free system, the measured change $\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{lat}}$ of $(3.2\ifmmode\pm\else\textpm\fi{}1.9)\phantom{\rule{0.16em}{0ex}}\frac{\mathrm{J}}{\mathrm{kg}\phantom{\rule{0.16em}{0ex}}\mathrm{K}}$ across the FM-to-PM transition contributes with $\ensuremath{\sim}35$% significantly and cooperatively to the total isothermal entropy change $\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{iso}}$. Hydrogenation is observed to induce an overall blueshift of the Fe VDOS with respect to the H-free compound; this effect, together with the enhanced Debye temperature observed, is a fingerprint of the hardening of the Fe sublattice by hydrogen incorporation. In addition, the mean Debye velocity of sound of ${\mathrm{LaFe}}_{11.4}{\mathrm{Si}}_{1.6}{\mathrm{H}}_{1.6}$ was determined from the NRIXS and the DFT data.

We combined experimental investigations and theoretical calculations to unveil an abnormal magnetic behavior caused by addition of the nonmagnetic element Cu in face-centered-cubic FeNiCoMn-based high-entropy alloys (HEAs). Upon Cu addition, the probed HEAs show an increase of both Curie temperature and saturation magnetization in as-cast and homogenized states. Specifically, the saturation magnetization of the as-cast HEAs at room temperature increases by 77% and 177% at a Cu content of 11 and 20 at. %, respectively, compared to the as-cast equiatomic FeNiCoMn HEA without Cu. The increase in saturation magnetization of the as-cast HEAs is associated with the formation of an Fe-Co rich phase in the dendritic regions. For the homogenized HEAs, the magnetic state at room temperature transforms from paramagnetism to ferromagnetism after 20 at. % Cu addition. The increase of the saturation magnetization and Curie temperature cannot be adequately explained by the formation of Cu enriched zones according to atom probe tomography analysis. Ab initio calculations suggest Cu plays a pivotal role in the stabilization of a ferromagnetic ordering of Fe, and reveal an increase of the Curie temperature caused by Cu addition which agrees well with the experimental results. The underlying mechanism behind this phenomenon lies in a combined change in unit-cell volume and chemical composition and the related energetic stabilization of the magnetic ordering upon Cu alloying as revealed by theoretical calculations. Thus, the work unveils the mechanisms responsible for the Cu effect on the magnetic properties of FeNiCoMn HEAs, and suggests that nonmagnetic elements are also crucial to tune and improve magnetic properties of HEAs.