Tailoring the magnetocaloric, magnetic and thermal properties of Dy6(Fe,Mn)X2 intermetallics (X Sb, Te, Bi)

ABSTRACT The structural, magnetic, magnetocaloric (MCE) and thermal properties of seven Fe2P-type Dy6(Fe,Mn)X2 (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 μ0ΔH = 5 T, with the tableau in-between being around 4 J/kgK, for Dy6FeSb2 and Dy6FeSbTe). The calculation of the Thermal Average Entropy Change allows to place the properties of two compounds (Dy6FeSb2 and Dy6FeSbTe) 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 (μ0ΔH = 5 T) for Dy6FeSbTe. 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.