Hydrogenation effects on the magnetic and crystal-field interactions in theR2Fe14BHx(R=Gd,Pr,Dy)compounds

Abstract

The magnetic properties of the ${R}_{2}{\mathrm{Fe}}_{14}{\mathrm{BH}}_{x}$ $(R=\mathrm{Gd},\mathrm{Pr},$ and Dy) compounds have been studied by measuring low-field ac magnetic susceptibility vs temperature and magnetization vs temperature and applied magnetic field on magnetically aligned samples. A strong reduction in the magnetocrystalline anisotropy of the Gd-based compounds is observed, and the compounds remain axial in the measured temperature range. A hydrogen induced spin reorientation transition (HISRT) takes place in the ${\mathrm{Dy}}_{2}{\mathrm{Fe}}_{14}{\mathrm{BH}}_{x}$ series for $x>~1,$ with the transition temperature ${T}_{s}$ increasing with increasing hydrogen content. In the ${\mathrm{Pr}}_{2}{\mathrm{Fe}}_{14}{\mathrm{BH}}_{x}$ compounds no HISRT is found, however for $0<x<~3$ we observe first-order magnetization processes (FOMP), with the critical field ${H}_{c}$ decreasing with increasing hydrogen content. For the ${\mathrm{Pr}}_{2}{\mathrm{Fe}}_{14}{\mathrm{BH}}_{5.5}$ compound the reduction is so strong that ${H}_{c}\ensuremath{\approx}0,$ and the compound remains in a conical phase in the studied temperature range (5 -- 470 K). We have combined analytical methods and a crystalline electric field-mean field model to obtain a quantitative evaluation of the effect of hydrogenation on the magnetic and crystal-field interactions. We have found that the observed behavior in the Gd, Dy, and Pr series can be explained by a decrease of the Fe sublattice anisotropy and by a decrease (in the Dy and Pr series) of the crystal-field parameters ${B}_{n0}$ $(n=2,$ 4, and 6) under hydrogenation. The hydrogen induced variations of the R-Fe exchange interaction seem to have a minor influence on the observed magnetic behavior.