Study of colossal magnetocaloric effect in MnAs1-xAx compounds and derived compounds

Abstract

The MnAs compound is very interesting because it presents a first order magnetic transition coupled to a crystallographic one from a hexagonal (ordered, low T) to orthorhombic (disordered, high t), and this induces the giant magnetocaloric effect. Under pressure, the compound shows the colossal magnetocaloric effect. Compounds derived from MnAs, as MnAsSb, estabilize the hexagonal structure and present a second order magnetic transition, but with a giant magnetocaloric effect. Another advantage of the Sb containing compounds is that the Curie temperature can be tuned by the Sb content. Preliminary substitutions of As by S, Se and Te indicate the occurrence of a giant effect larger than the one for MnAs. Another important feature of the MnAs and MnAsSb compounds is that both show the colossal effect when submitted to hydrostatic pressure, and this can be particularly important to aplications in active magnetic regenerators. Other compounds derived from MnAs, as the ones coming from doping in the Mn site with the Fe, Cu and Ag atoms, show the colossal magnetocaloric effect at ambient pressure, and this makes concrete the possibility of practical application of the colossal effect. The reason these dopings originate the colossal effect at ambient pressure is that the Fe and Cu atoms are smaller than the Mn atom, and when replacing it, play the role of external pressure (what we call chemical pressure). However, the roles of Fe and Cu are not exactly the same. Fe, when entering the hexagonal lattice, cause the decrease of both lattice parameters and an overal decrease of the unit cell volume, effectively acting as a pressure. Cu, when entering the lattice, cause an increase of the a parameter and a bigger decrease of the c parameter, with an overal decrease of the unit cell volume, and this again is equivalent the effect of an external pressure. The Fe doping cause a decrease of the Curie temperature and an increase of the thermal hysteresis, while the Cu substitution does not alter either the Curie temperature neither the thermal hysteresis. Silver has similar effects as Cu, but we observe the presence of a Ag-rich phase in the microstructure of the samples.Given the importance of the colossal magnetocaloric effect for practical applications, and aiming to obtain other materials with these effects and with low or null thermal hysteresis, we propose to do substitutions at the doping level in the As site of the MnAs compound with several atoms, as: 1)P and Bi, elements of the same colum as As. P can lead to the same equivalent pressure effect as observed in the case of Fe doping, once it is an atom smaller than Mn. Bismuth can have the effect of Sb when substituting in greater amounts, or the Ag effect when in doping levels. 2)S, Se and Te: when in great amounts, this substitutions let to the giant effect. We propose study the effect of the doping with these elements in the magnetic and magnetocaloric properties of the compound. 3)Mixed substitutions: as one of the aims of the research is to obtain the colossal effect without thermal hysteresis, there is interest in the mixed substitutions, as Fe in the Mn site and Sb in the As site, or other elements in this site, as S, Se and Te. 4)Other substitutions/dopings: a survey of the As binary phase diagrams reveals that there is at least 2 elements of the IVA colunm of the periodic table that present significative solubility in As: Ge and Sn. We propose to do substitutions/dopings with these elements and study the magnetic and magnetocaloric properties of the resulting compounds.