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• Change of ions tunes the magnetocaloric effects in the range 2–400 K. • 2 Direct MCE with a table-like effect in-between, interesting for Ericsson cycle. • Inverse MCE at low temperatures. • PM-FM transition ruled by long-range magnetic interactions. The structural, magnetic, magnetocaloric (MCE) and thermal properties of seven Fe 2 P-type Dy 6 (Fe,Mn)X 2 (X Sb, Bi, Te) intermetallics (space group P 6 ¯ 2 m , N 189, hP9) have been experimentally studied. They present a paramagnetic to ferromagnetic transition (in the range 129–370 K), followed, as temperature decreases, by a spin-reorientation one (from 52 to 170 K) and a ground magnetic state at 2 K with antiferromagnetic components. This state turns into a ferromagnetic state when a magnetic field is applied. The critical exponents β,γ,δ related to the PM-FM transition point to long range order interactions but in most compounds their values severely deviate from the Mean Field class, presenting an unconventional critical behavior, probably due to magnetocrystalline anisotropies. This magnetic complexity has the consequence that in every intermetallic three MCE effects arise: Two direct magnetocaloric effects (DMCE) with a table-like effect in between (from 40 K to more than 400 K), with moderate values of the magnetic entropy maxima (up to 6.9 J/kgK for μ 0 Δ H = 5 T, with the tableau in-between being around 4 J/kgK, for Dy 6 FeSb 2 and Dy 6 FeSbTe). The calculation of the Thermal Average Entropy Change allows to place the properties of two compounds (Dy 6 FeSb 2 and Dy 6 FeSbTe) close to other rare earth based high entropy alloys described in literature. The seven compounds present a relevant third MCE, inverse, below 25 K, with a value as high as 17.8 J/kgK (μ 0 Δ H = 5 T) for Dy 6 FeSbTe. The maximum of the magnetic entropy change at the Curie temperature has been shown to scale with the critical exponents found and universal curves have been built. Finally, the thermal diffusivities in the range of the DMCE have been measured, with the result that they present good values (between 1 and 3 mm2/s) to be used in real magnetocaloric refrigeration systems. [ABSTRACT FROM AUTHOR]

The magnetic and magnetocaloric properties of the intermetallic family (Gd,Tb) 6 (Fe,Mn)Bi 2 have been studied from 2 K to temperatures above the respective Curie temperatures T C. The substitution of Gd by Tb (Gd 6 FeBi 2 , Gd 3 Tb 3 FeBi 2 , Tb 6 FeBi 2) tunes T C in the range 350-250 K and favors the apparition of a metamagnetic transition at very low temperature (below 10 K) from a complex magnetic state to a ferromagnetic one, as well as a spin reorientation transition below T m = 72 K. As a consequence, an important inverse magnetocaloric effect (IMCE) appears below 20 K and an interesting direct magnetocaloric effect (DMCE) appears over a wide temperature span between T C and T m with maxima at those temperatures. The partial substitution of Fe by Mn in Tb 6 Fe 0.5 Mn 0.5 Bi 2 shifts these effects upwards in temperature while expanding the region of the direct magnetocaloric effect between 70 and 400 K. The combination of adjoint IMCE and DMCE as well as the wide span of the latter shows that tuning this family allows to locate the magnetocaloric effect in different regions of interest. The critical behavior of the PM-FM transitions has been studied obtaining the critical exponents α, β, γ, δ and checking that the respective magnetocaloric effects also scale with the critical parameters n and δ. The transition in Gd 6 FeBi 2 belongs to the Heisenberg universality class with deviations due to magnetocrystalline anisotropies; the critical exponents for Gd 3 Tb 3 FeBi 2 (in agreement with the Mean Field model) suggest the presence of long range order magnetic interactions, while Tb 6 FeBi 2 and Tb 6 Fe 0.5 Mn 0.5 Bi 2 present an unconventional critical behavior aligned with long range order interactions. • Gd by Tb substitution tunes T C and improves the magnetocaloric properties. • Tb samples present two main transitions and extended temperature working range. • Adjoint inverse and direct magnetocaloric effects in Td-doped compounds. • Critical exponents α, β, γ, δ found for the PM-FM transition. [ABSTRACT FROM AUTHOR]

Four polycrystaline Fe 2 P-type Ho 6 (Fe,Mn)Bi 2 intermetallic compounds (space group P 6 ‾ 2 m , No. 189, hP 9) have been studied using magnetic techniques in order to explore their ability as magnetocaloric materials, and study the critical behavior of the paramagnetic (PM) to ferromagnetic (FM) transitions to obtain a deeper understanding of the range of the magnetic interactions. The obtained critical exponents β , γ and δ for the four compounds studied (Ho 6 MnBi 2 , Ho 6 FeBi 2 , Ho 6 (Mn 0.5 Fe 0.5)Bi 2 , Ho 6 (Mn 0.75 Fe 0.25)Bi 2) point to long-range order interactions, as they are close to those of the Mean Field Universality class. All of the compounds show relevant magnetocaloric properties over a very broad temperature range, limited by the PM-FM transition and a spin-reorientation one, well separated in all cases. They present very high values of the refrigerant capacities (from 520 J/kg to 709 J/kg at 5 T), good magnetic entropy changes (from 3.4 to 5.7 J/(kgK) at 5 T), and a flat and wide temperature span for the working temperature range (nearly 200 K for Ho 6 MnBi 2 , 80 K for Ho 6 FeBi 2 at 5T). The change in properties with composition proves that the magnetocaloric properties can be tuned in Fe 2 P-type compounds to accommodate different refrigeration applications. Finally, the magnetocaloric scaling laws have been successfully tested and universal curves for the magnetic entropy change have also been obtained in the PM-FM transition region. • Second order FM to PM transition, long-range order interactions involved. • Very high values of the refrigerant capacities from 520 J/kg to 709 J/kg at 5T • Good magnetic entropy changes from 3.4 to 5.7 J/(kgK) at 5T • Flat and wide temperature span for the working temperature range (up to 200 K). • Magnetocaloric scaling laws tested and universal curves obtained. [ABSTRACT FROM AUTHOR]