Summary

A Mn-based compound with composition MnFeP1-xAsx has shown a magnetocaloric effect (MCE) and promising properties for magnetic refrigeration applications at room-temperature. However, due to arsenic (As) toxicity its substitution for other elements such as silicon (Si), germanium (Ge) and aluminum (Al) was proposed to be strategic. The samples were synthesized by a solid state reaction by mechanical alloying processing using a high energy ball mill and further heat treatment at high temperatures. The compounds MnFeP0.6Si0.4-xGex were synthesized and they crystallized on the hexagonal Fe2P-struture, which shows the MCE, but secondary phases were also formed. However, these compounds show a magnetic phase transition around room-temperature. Besides, it was researched the ball milling time during the mechanical alloying for the compound MnFeP0.6Si0.3Ge0.1, in which an occurrence of a reaction was detected after 20 hours milling due to the phase transformation of the raw materials into the Fe2P-structure. After heat treatment, all the samples with more than one hour ball milling crystallized on the desired structure. Magnetic characterization of these samples showed an improvement of the properties when the ball milling time is increased. The magnetic entropy change was calculated on a 2 T apply magnetic field revealed a value of 14 J/kg K. It was also studied the variation of the transition-metals ratio on the compounds Mn1-xFe1+xP0,6Si0,3Ge0,1, where it was found that after 20 hours ball milling and heat treatment they crystallized on the hexagonal structure with a small proportion of secondary phase. Thermo-magnetic results confirmed that the addition of Fe leads to an increase of the Curie temperature, as well as a decrease of thermal hysteresis. Although, when modifying the stoichiometric alloy by the addition of Fe and Mn, the absolute value of the magnetic entropy change is lowered. On the last part of this work, it was carried out a substitution of Si and Ge by atoms of Al on alloys with known MCE. However, addition of this element promotes the formation of secondary phases and the drop of the magnetocaloric properties of the alloy.

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