Hydrogenation and the effect of H absorption on structural and magnetic properties of nanocrystalline Pr2Co7 alloys

Abstract We have studied the hydrogenation, structural and magnetic properties of nanocrystalline Pr 2 Co 7 H x ( 0 ⩽ x ⩽ 10.8 ) hydrides.We attempted to observe hydrogen absorption - desorption phenomena and understand the evolution of microstructure from the nanocrystalline Pr 2 Co 7 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 Pr 2 Co 7 compound transforms from hexagonal ( P 6 3 / mmc ) to orthorhombic ( Pbcn ) and monoclinic ( C 2/ c ) symmetry in hydrogenation.The absorption of hydrogen by Pr 2 Co 7 compound leads to the enhancement of the Curie temperature T C 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 Pr 2 Co 7 H x and deduce the exchange interactions J ij 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 μ s increases from 8.32 to 14 μ B /f.u with increasing H content from x = 0 to 2.5, then decreases.Similarly, the calculation of the anisotropy constant K 1 reveals an increase from 5.24 × 10 7 to a maximum of 5.69 × 10 7 erg/cm 3 .The decrease of coercivity H c and the maximum energy product ( BH ) max as a function H content could be mainly due to the decrease of magnetocrystalline anisotropy H r resulting from the absorption of H. Therefore, it appears that the Pr 2 Co 7 H 0.25 hydride has optimal magnetic properties: H c  = 0.61 T, ( BH ) max  = 5.8 MGOe, M r  = 40 emu/g, remanence ratio M r / M max  = 0.62 and T C  = 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.