Ferrimagnetic coupling between cobalt and light rare-earth samarium induced by dense hydrogenation of ${\mathrm{SmCo}}_5$ 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 $x\approx 13.5$, 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 $T_{\text{C}}\le 131$ K, which is in contrast to the extremely high $T_{\text{C}}\sim 1300$ 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 ${\mathrm{SmCo}}_5{\mathrm{H}}_x$.

Figure 1

(a) Initial (bottom) and hydrogenated (top) magnetic configurations of ${\mathrm{SmCo}}_5$. (b) XANES and XMCD spectra of ${\mathrm{SmCo}}_5{\mathrm{H}}_x$ at $T=15$–16 K and selected pressures together with the reference spectra of ${\mathrm{SmAl}}_5$ and Co metal (dotted lines). The vertical lines indicate the absorption edge, $E_0$. The characteristic profiles are labeled by A–F with subscripts.

Figure 2

(a) Temperature evolution of XMCD spectra at the Sm $L_2$ edge XMCD under 3.8 and 15.0 GPa. (b) Temperature dependences of integrated XMCD intensity under selected pressures. Solid and dashed lines are guides and fitted lines to estimate $T_{\text{C}}$, respectively. (c) $P\text{−}T$ phase diagram of ${\mathrm{SmCo}}_5{\mathrm{H}}_x$.

Figure 3

(a) XRD patterns of ${\mathrm{SmCo}}_5{\mathrm{H}}_x$ at selected pressures. Diamonds and stars indicate newly observed Bragg reflections for phases II and III, respectively. (b)–(f) Structural parameters determined by Rietveld refinements. Note that each structural parameter of phases II and III is converted to the parameter of phase I to highlight the structural changes. The hatched area corresponds to the coexistence region of phase II and III. The parameters of nonhydrogenated ${\mathrm{SmCo}}_5$ are shown by cross marks for comparison. The solid line in panel (d) represents the fitted curves of the Birch-Murnaghan equations of state for the nonhydrogenated ${\mathrm{SmCo}}_5$. (g) Illustrations of the determined crystal structures of phases I, II, and III [30].

Figure 4

(a) Volume expansion (left axis) and the corresponding hydrogen content $x$ (right axis) of ${\mathrm{SmCo}}_5{\mathrm{H}}_x$ as a function of pressure. The relationship between volume expansion and hydrogen content was determined by extrapolation from the reported relationships [32]. (b) Four kinds of polyhedrons, O1, D1, T1, and T2, in the ${\mathrm{SmCo}}_5$ unit cell are shown as the hydrogen encapsulating structures. Their multiplicity in the cell is ignored, and the selected polyhedrons are shown.

Figure 5

(a) Schematic diagram of the magnetic interaction in ${\mathrm{SmCo}}_5$ leading to the ferrimagnetic coupling of the Sm $5d$ spin moment with Co $3d$ spin moments. States with the asterisks correspond to antibonding states. (b) The top and middle panels show calculated spin magnetic moments of Sm $5d$, Sm $4f$, and Co $3d$ states dependent on occupation of sites by H atoms in the case of the ferromagnetic coupling of Sm $4f$ and Co $3d$ spin moments. The bottom panel shows the total energy differences of the models between ferromagnetic and ferrimagnetic coupling of Sm $4f$ and Co $3d$ spin moments.

Figure 6

XAS and XMCD spectra at the Gd $L_{2,3}$ edges of doped Gd atoms in ${\mathrm{Sm}}_{0.92}{\mathrm{Gd}}_{0.08}{\mathrm{Co}}_5{\mathrm{H}}_x$.