Ferrimagnetic coupling between cobalt and light rare-earth samarium induced by dense hydrogenation of SmCo5 permanent magnet under high pressures

X-ray magnetic circular dichroism, x-ray powder diffraction and first-principles calculations revealed that dense hydrogenation of a Gd-doped ${\mathrm{SmCo}}_{5}{\mathrm{H}}_{x}$ compound $({\mathrm{Sm}}_{0.92}{\mathrm{Gd}}_{0.08}{\mathrm{Co}}_{5}{\mathrm{H}}_{x})$, ranging up to $x\ensuremath{\approx}13.5$, flips the direction of Sm $4f$ magnetic moments. Consequently, the magnetic coupling of Sm $4f$ with Co $3d$ moments changes to ferrimagnetic from the conventional ferromagnetic coupling. The ferrimagnetic structure possessed significantly lower Curie temperatures of ${T}_{\text{C}}\ensuremath{\le}131$ K, which is in contrast to the extremely high ${T}_{\text{C}}\ensuremath{\sim}1300$ K of a ${\mathrm{SmCo}}_{5}$ permanent magnet. Structural parameters of two hydrogenation-induced crystal structures were determined carefully by x-ray powder diffraction. Large volume expansion exceeding $V/{V}_{0}=1.3$ indicates that hydrogen occupation in the Sm-3Co tetrahedron occurred above 1 GPa. The hydrogen in the tetrahedron dominates the ferromagnetic coupling between the interatomic Co 3$d$--Sm 5$d$ electron orbitals, which plays an important role in the induction of the ferrimagnetic structure of ${\mathrm{SmCo}}_{5}{\mathrm{H}}_{x}$.