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This work presents ab-initio self-consistent calculations performed with the TB-LMTO code to study the different phases of the Fe-Al phase diagram, corresponding to the ordered structures B2, DO3 and B32 and for Fe50Al50 and Fe3Al compositions. Both, unpolarized and spin-polarized calculations have been performed to deduce the energetic difference between the paramagnetic and ferromagnetic state of the corresponding structure. Calculations for the disordered structures have also been performed for the previously mentioned compositions. These results show that by disordering the alloy magnetism is enhanced and that the equilibrium lattice parameter increases.En este trabajo se presentan cálculos autoconsistentes ab-initio realizados con el método TB-LMTO (Tight Binding Linear Muffin Tin Orbital) con el fin de estudiar las diferentes estructuras que se presentan en el diagrama de fases de las aleaciones Fe-Al. Se han estudiado las estructuras ordenadas B2, DO3 y B32 para las siguientes concentraciones: Fe50Al50 y Fe3Al. Asimismo, se han realizado cálculos teniendo y sin tener en cuenta la polarización de spin con el fin de poder deducir la diferencia energética entre los estados ferromágneticos y paramágneticos de la misma estructura. Por otra parte se han realizado estos mismos cálculos para estructuras desordenadas y las mismas concentraciones. Los resultados muestran que mediante el desorden aumenta el magnetismo de estas aleaciones y crece el parámetro de red.

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 RCFWHM(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. [ABSTRACT FROM AUTHOR]

The influence of the different structures on the evolution of the magnetism in the order-disorder transition produced by mechanical milling in the intermetallic Fe70Al30-xSix alloys has been systematically studied by means of XRD measurements, Mössbauer spectroscopy and magnetic measurements. These techniques show that there are two different stages in the evolution of the magnetism during the order-disorder transition and both stages are related to the presence of ordered B2 in annealed alloys. In a first stage the magnetization of the alloys decreases and this decrease corresponds to the presence of B2 structure and the disordering of the D03 structure. In the second stage the magnetization increases corresponding to the disordering of the B2 structure. Therefore, the increase of the magnetization is related to the disordering of B2 structure, but not to the disordering of D03 structure. Besides, when plotting the normalized variation of magnetization produced by the disorder versus the normalized variation of lattice parameter we observe a linear relationship, showing that lattice parameter and magnetic properties are also related.

The order-disorder transition produced by ball milling in the intermetallic Fe[SUB70]Al[SUB30] has been systematically studied by x-ray diffraction, calorimetry, and Mössbauer experiments. These techniques show a monotonous transformation that ends after 6 h of milling. In the transition, the lattice parameter increase amounts to 0.7% and there is a large enhancement of the alloy's magnetism. [ABSTRACT FROM AUTHOR]