Using the magnetocaloric effect in nanoparticles holds great potential for efficient refrigeration and energy conversion. The most promising candidate materials for tailoring the Curie temperature to room temperature are rare-earth-based magnetic nanoalloys. However, only few high-nuclearity lanthanide/transition-metal nanoalloys have been produced so far. Here we report, for the first time, the observation of magnetic response in spark-produced LaFeSi nanoalloys. The results suggest that these nanoalloys can be used to exploit the magnetocaloric effect near room temperature; such a finding can lead to the creation of unique multicomponent materials for energy conversion, thus helping toward the realization of a sustainable energy economy.

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The influence of Co (Ni) and B co-doping on the structural, magnetic and magnetocaloric properties of (Mn,Fe)2(P,Si) compounds is investigated by X-ray diffraction (XRD), differential scanning calorimetry, magnetic and direct temperature change measurements. It is found that Co (Ni) and B co-doping is an effective approach to tune both the Curie temperature and the thermal hysteresis of (Mn,Fe)2(P,Si) materials without losing either the giant magnetocaloric effect or the positive effect of the B substitution on the mechanical stability. An increase in B concentration leads to a rapid decrease in thermal hysteresis, while an increase in the Co or Ni concentration hardly changes the thermal hysteresis of the (Mn,Fe)2(P,Si) compounds. However, the Curie temperature decreases slowly as a function of the Co or Ni content, while it increases dramatically for increasing B concentration. Hence, the co-substitution of Fe and P by Co (Ni) and B, respectively, offers a new control parameter to adjust the Curie temperature and reduce the thermal hysteresis of the (Mn,Fe)2(P,Si) materials.@en

Mn_1.000Fe_0.950P_0.595Si_0.330B_0.075compounds have been synthesized by high-energy ball milling and a subsequent solid-state reaction. The influence of the sintering conditions on the magnetic phase transition of Mn_1.00Fe_0.95P_0.595Si_0.33B_0.075samples has been systematically investigated using X-ray diffraction and magnetic measurements. The experimental results show that all the samples obtained after different heat treatment conditions crystallize in the Fe₂P-type hexagonal structure. The annealing temperature has a strong influence on the Curie temperature, which can be tuned between 265 and 298 K by sintering the samples at different temperatures between 1273 and 1373 K. The annealing time, however, does not significantly affect the Curie temperature. Both the annealing temperature and the annealing time have a significant effect on the magnetic entropy change. The magnetic entropy change under a magnetic field change of 1 T increases from 2.7 to 6.5 J kg⁻¹K⁻¹by increasing the annealing temperature from 1273 to 1373 K. By increasing the annealing time, the magnetic entropy change at first increases and then saturates after 20 h of heat treatment at 1373 K.

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Temperature dependent high-resolution x-ray diffraction measurements were used to characterize the magneto-elastic ferromagnetic transition of (Fe,Mn)2(P,Si,B) compounds. Across the transition, apart from a change in lattice parameters across the transition, the internal coordinates of Mn and Fe also change. This intrinsic degree of freedom allows Fe in the tetrahedral coordination to decrease the two interatomic distances with the 2c position and increase the two distances with the two 1b position, while the Fe–Mn distance remains constant. For Mn in the square based pyramidal coordination, all interatomic distances effectively remain constant. Electron density plots show that for second-order transitions, the observed changes are smaller and continuously extending over a wide temperature range in the ferromagnetic and paramagnetic states, due to short-range order. This study shows that the mechanism behind the phase transition in Fe2P-based materials is an isostructural transition that is equal for both first- and second-order transitions.@en

Given the potential applications of (Mn,Fe₂(P,Si))-based materials for room-temperature magnetic refrigeration, several research groups have carried out fundamental studies aimed at understanding the role of the magneto-elastic coupling in the first-order magnetic transition and further optimizing this system. Inspired by the beneficial effect of the addition of boron on the magnetocaloric effect of (Mn,Fe₂(P,Si))-based materials, we have investigated the effect of carbon (C) addition on the structural properties and the magnetic phase transition of Mn _1.25Fe _0.70P _0.50Si _0.50C _z and Mn _1.25Fe _0.70P _0.55Si _0.45C _z compounds by x-ray diffraction, neutron diffraction and magnetic measurements in order to find an additional control parameter to further optimize the performance of these materials. All samples crystallize in the hexagonal Fe ₂P -type structure (space group P-62m), suggesting that C doping does not affect the phase formation. It is found that the Curie temperature increases, while the thermal hysteresis and the isothermal magnetic entropy change decrease by adding carbon. Room-temperature neutron diffraction experiments on Mn _1.25Fe _0.70P _0.55Si _0.45C _z compounds reveal that the added C substitutes P/Si on the 2c site and/or occupies the 6k interstitial site of the hexagonal Fe ₂P -type structure.

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We tailored the size distribution of Pt nanoparticles (NPs) on graphene nanoplatelets at a given metal loading by using low-temperature atomic layer deposition carried out in a fluidized bed reactor operated at atmospheric pressure. The Pt NPs deposited at low temperature (100 °C) after 10 cycles were more active and stable towards the propene oxidation reaction than their high-temperature counterparts. Crucially, the gap in the catalytic performance was retained even after prolonged periods of time (>24 hours) at reaction temperatures as high as 450 °C. After exposure to such harsh conditions the Pt NPs deposited at 100 °C still retained a size distribution that is narrower than the one of the as-synthesized NPs obtained at 250 °C. The difference in performance correlated with the difference in the number of facet sites as estimated after the catalytic test. Our approach provides not only a viable route for the scalable synthesis of stable supported Pt NPs with tailored size distributions but also a tool for studying the structure-function relationship.

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Amongst magnetic materials that exhibit a giant magnetocaloric effect near room temperature, the(Mn,Fe)2(P,Si) system is one of the most promising candidates for magnetic refrigeration. Although the(Mn,Fe)2(P,Si) materials hold many advantages, controlling the magnetic entropy changeDSm, theadiabatic temperature changeDTad, the thermal hysteresis and the mechanical stability across theferromagnetic transition requires a delicate tuning of the composition. This work investigates theaddition of nitrogen, as an interstitial or substitutional element, as a new parameter to tune the prop-erties of (Mn,Fe)2(P,Si) materials. We found that the nitrogen addition results in a decrease of the Curietemperature, consistent with the observed increase in thec/aratio. The introduction of nitrogen in(Mn,Fe)2(P,Si) materials also results in a strong enhancement of the mechanical stability.@en

Magnetic refrigeration near room temperature is considered as environmentally benign alternative for the current compressor based cooling technology. Two materials that are based on ferromagnetic transition metal compounds are considered as the most promising candidates to be used in real world applications. Here we discuss the main features of these materials.@en

Magnetocaloric nanoparticles based on the Fe2P system have been prepared by ball milling in the presence of the organic solvent heptane and the surfactant oleic acid. The surfactant is used to prevent the re-welding of crushed particles during the ball milling and to enhance the dispersion of nanoparticles in the solvent [1]. A size selection process using centrifugation and ultrasonic vibration was employed after ball milling to obtain nanoparticles with a narrow size distribution. The XRD peak intensity decreases while the peaks broaden in the presence of surfactant and solvent, indicating a decrease in particle size and an increase in the degree of amorphization. The magnetization of the nanoparticles obtained by surfactant-assisted high-energy ball milling decreased compared to the bulk. This can be attributed to a larger contribution of the surface states due to the size reduction, amorphization and contamination such as residual surfactant. The effect of milling parameters on Fe2P-based compounds as nano-scale particles as well as their dynamic response by nanosizing will be discussed in detail.@en