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

A complete experimental study of the physical properties playing a relevant role in the 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 observed for these materials: a paramagnetic to ferromagnetic transition in the range of 182-282 K and a spin reorientation transition in the range of 26-76 K. As a consequence, two peaks related to a direct magnetocaloric effect (DMCE) appear with 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^-1 K^-1 around 182 K. For the same applied field the other compounds show moderate values around room temperature (2.88-4.53 J kg^-1 K^-1). However, the superposition of the two peaks results in huge refrigerant capacity values, up to RC_FWHM(5 T) = 1103.04 J kg^-1 in the case of Tb₂Dy₄FeSb₂. The thermal diffusivity, thermal effusivity, thermal conductivity and specific heat capacity 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 of 1.3-2.3 mm² s^-1, which are good for magnetic refrigerators at high working frequencies. The study is completed with a rigorous critical behavior analysis 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 of 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 exhibiting 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 in the properties of the compounds is discussed.