Curie temperature study of Y(Fe1-xCox)2 and Zr(Fe1-xCox)2 systems using mean field theory and Monte Carlo method

Cubic Laves phases including YFe2, YCo2, ZrFe2, and ZrCo2 are considered as promising candidates for application in hydrogen storage and magnetic refrigeration. While YFe2 and ZrFe2 are ferromagnets, alloying with Co decreases magnetic moments and Curie temperatures (T-C) of pseudobinary Y(Fe1-xCox)(2) and Zr(Fe1-xCox)(2) systems, leading to the paramagnetic states of YCo2 and ZrCo2. The following study focuses on the investigation of Curie temperature of the Y(Fe1-xCox)(2) and Zr(Fe1-xCox)(2) system from first principles. To do it, Monte Carlo (MC) simulations and the mean field theory (MFT) based on the disordered local moments (DLM) calculations are used. The DLM-MFT results agree qualitatively with the experimental data from the literature and preserve the characteristic features of T-C(x) dependencies for both Y(Fe1-xCox)(2) and Zr(Fe1-xCox)(2). However, we have encountered complications in the Co-rich regions due to failure of the local density approximation (LDA) in describing the Co magnetic moment in the DLM state. The analysis of Fe-Fe exchange couplings for YFe2 and ZrFe2 phases indicates that the nearest-neighbor interactions play the main role in the formation of T-C.