Itinerant electron metamagnetism and magnetocaloric effect in RCo2-based Laves phase compounds

By virtue of the itinerant electron metamagnetism (IEM), the RCo2 compounds with R=Er, Ho and Dy are found to show first-order magnetic transition at their ordering temperatures. The inherent instability of Co sublattice magnetism is responsible for the occurrence of IEM, which leads to interesting magnetic and related physical properties. The systematic studies of the variations in the magnetic and magnetocaloric properties of the RCo2-based compounds show that the magnetovolume effect plays a decisive role in determining the nature of magnetic transitions and hence the magnetocaloric effect (MCE) in these compounds. It is found that the spin fluctuations arising from the magnetovolume effect reduce the strength of IEM, which subsequently lead to a reduction in the MCE. Most of the substitutions at the Co site are found to result in a positive magnetovolume effect, leading to an initial increase in the ordering temperature. Application of pressure, on the other hand, causes a reduction in the ordering temperature due to the negative magnetovolume effect. A comparative study of the magnetic and magnetocaloric properties of RCo2 compounds under various substitutions and applied pressure is presented. Analysis of the magnetization data using the Landau theory of magnetic phase transitions has shown that there is a strong correlation between the Landau coefficients and the MCE. The variations seen in the order of magnetic transition and the MCE values seem to support the recent model proposed by Khmelevskyi and Mohn for the occurrence of IEM in RCo2 compounds. Metastable nature of the transition metal sublattice in RCo2-based compounds and its role in determining the magnetic and magnetocaloric properties is explained.