Hydrogenation and the effect of H absorption on structural and magnetic properties of nanocrystalline [formula omitted] alloys

Abstract

We have studied the hydrogenation, structural and magnetic properties of nanocrystalline Pr2Co7Hx(0⩽x⩽10.8) hydrides.We attempted to observe hydrogen absorption - desorption phenomena and understand the evolution of microstructure from the nanocrystalline Pr2Co7 alloy to the full-hydride phase using X-ray diffraction along the P-C isotherm.Two plateaus were clearly observed during the absorption-desorption process in the P-C isotherm.The crystal structure of Pr2Co7 compound transforms from hexagonal (P63/mmc) to orthorhombic (Pbcn) and monoclinic (C2/c) symmetry in hydrogenation.The absorption of hydrogen by Pr2Co7 compound leads to the enhancement of the Curie temperature TC from 600 K at x = 0 to 691 K at x = 3.75.Mean field theory was used to describe the magnetization curves M(T) of Pr2Co7Hx and deduce the exchange interactions Jij and Curie temperature.We have also applied the approach to saturation magnetization to our compounds.The results are discussed on the basis of the Random magnetic anisotropy modeling.The saturation magnetic moment μs increases from 8.32 to 14 μB/f.u with increasing H content from x = 0 to 2.5, then decreases.Similarly, the calculation of the anisotropy constant K1 reveals an increase from 5.24×107 to a maximum of 5.69×107 erg/cm3.The decrease of coercivity Hc and the maximum energy product(BH)max as a function H content could be mainly due to the decrease of magnetocrystalline anisotropy Hr resulting from the absorption of H. Therefore, it appears that the Pr2Co7H0.25 hydride has optimal magnetic properties: Hc = 0.61 T, (BH)max = 5.8 MGOe, Mr = 40 emu/g, remanence ratio Mr/Mmax = 0.62 and TC = 607 K. It follows from this that hydrogenation can be one of the effective means of raising the magnetic stiffness of materials for permanent magnets and magnetic recording.