Selecting optimal R6TX2 intermetallics (R = Gd, Tb, Dy; T = Mn, Fe, Co, Ni; X = Sb, Te) for magnetic refrigeration

Abstract

A complete experimental study of the physical properties playing a relevant role on magnetic refrigeration application (structural, magnetic, magnetocaloric and thermal) has been performed over nine selected Fe2P-type R6TX2 (R= Gd, Tb, Dy; T= Mn, Fe, Co, Ni; X=Sb, Te) intermetallic compounds, to work close to room temperature. Two magnetic phase transitions are present on these materials: a paramagnetic to ferromagnetic transition in the range 182-282 K and a spin reorientation transition in the range 26-76 K. As a consequence, two peaks related to a direct magnetocaloric effect (DMCE) appear in the magnetic entropy change, generating a wide table-like plateau region in between both peaks, which is required to improve the efficiency of refrigerators following an Ericsson cycle. The highest magnetic entropy peak value for μ0ΔH = 5 T is found for Tb2Dy4FeSb2, with 7.72 J/kg K around 182 K. For the same applied field the other compounds show moderate values around room temperature (2.88-4.53 J/Kg K). However, the superposition of the two peaks results in huge refrigerant capacity values, up to RCFWHM(5 T)=1103.04 J/kg in the case of Tb2Dy4FeSb2. The thermal diffusivity, effusivity conductivity and specific heat have been measured at room temperature, and the temperature dependence of the former has been obtained around the relevant magnetic phase transition region, with values in the range 1.3-2.3 mm2/s, which are good for magnetic refrigerators under high working frequencies. The study is completed with a rigorous critical behavior analisis of the second order PM-FM transition. The critical exponent γ points to long range order interactions, in general, while β values are in the range (0.59-0.90), indicating a deviation from theoretical models as a reflection of the magnetic complexity in these compounds. The critical exponents have been used to confirm the scaling relations of magnetocaloric properties, and the scaling of refrigerant capacity (RC) values in materials presenting two magnetic phase transitions is addressed, concluding that for a correct scaling of RC the magnetic entropy change peak must be considered symmetric. The role of each atom on the properties of the compounds is discussed.