Structural study of MnAs compounds and derivatives by X-ray diffraction with colossal magnetocaloric effect

Abstract

Recently (Gama et al., Phys. Rev. Lett. (2004) 93(23) 237202-1) it has been observed that the MnAs compound, when submitted to an hydrostatic pressure, presents the colossal magnetocaloric effect. The pressure decreases the magnetic transition temperature (TC) causing a remarkable thermal hysteresis and then, the entropic colossal effect which is considered when the maximum isothermic entropy change overcomes the value Rln(2J+1), where R is the gas constant and J is the total angular momentum of the magnetic ion. The aim of the present project is to use the X-ray diffraction techniques, mainly the Rietveld method for the crystalline structure refinement, in the study of structural properties of the MnAs based compounds such as Mn1-xTxAs and MnAs1-xAx and considering T = V, Cr, Ni, Zr, Nb, Mo among others and A = P, Bi, S, Te and Se as possible dopants. Through these chemical substitutions, one intends to produce the same external pressure effect, always aiming to optimize the conditions to obtain the materials which present the giant or colossal magnetocaloric effect. It is important to report that in some cases, the preliminary measurements of the replacement of Mn by Fe, Cu, Ag and Ti have already shown surprisingly results, in which the colossal effect was obtained for Fe, Cu and Ag whereas, the giant effect was obtained for Ti. The X-ray analyses will be performed in the two distinct crystalline phases: hexagonal and orthorhombic, where the transition occurs close to the room temperature (T= 40oC for MnAs compound). Furthermore, it is important to note this project is associated to our main original project (Fapesp 01/05883-0), and a essential part of its development, since it implies in the structural characterization of the magnetocaloric materials. It becomes very important the preparation of these compounds under optimized conditions since they involve low cost materials and processing and they will be certainly used in the magnetic regenerators as soon as the solution to the thermal hysteresis problem has been found.