Effect of Hydrogen on the Structure, Magnetic and Magnetocaloric Properties of Laves Phase Type-Compounds RE(Fe0.25Co0.75)2Hy with RE = Ho and Er, y = 0.0, 3.0, 3.5

The crystal structure and overall magnetic properties of RE(Fe0.25Co0.75)2Hy with RE = Ho and Er, were analyzed versus y, the hydrogen content. The single phase C15 type compounds, synthesized using cold crucible HF melting, were hydrogenated and controlled using a PCI apparatus. The impact of hydrogen insertion on the cell parameter, the Curie temperature TC and the magnetization saturation were determined. All compounds and hydrides were found to be ferrimagnets, the magnetic moment of RE and 3d elements being opposite since Ho and Er belong to the 2nd row of rare earth elements. Depending on the formula, a typical compensation point was in evidence. The 2nd order character of the ferri ↔ paramagnetic transition was established using the Arrott plot method. Magnetization isotherms vs. magnetic field gives quantified results for the magnetic entropy variation by application of the Maxwell relation. Control of y, the hydrogen content, shows it is interesting for improvement of the MCE of the starting compounds. However increasing y leads to decreased TC at the lowest temperatures, suggesting potential cryogenic uses.