Robust ferromagnetism in the compressed permanent magnet ${\mathrm{Sm}}_2{\mathrm{Co}}_{17}$

The compound ${\mathrm{Sm}}_2{\mathrm{Co}}_{17}$ displays magnetic properties amenable to permanent magnet applications owing to both the $3d$ electrons of Co and the $4f$ electrons of Sm. The long-standing description of the magnetic interactions between the Sm and Co ions implies a truly ferromagnetic configuration, but some recent calculations challenge this axiom, suggesting at least a propensity for ferrimagnetic behavior. We have used high-pressure synchrotron x-ray techniques to characterize the magnetic and structural properties of ${\mathrm{Sm}}_2{\mathrm{Co}}_{17}$ to reveal a robust ferromagnetic state. The local Sm moment is at most weakly affected by compression, and the ordered moments show a surprising resilience to volumetric compressions of nearly 20%. Density functional theory calculations echo the magnetic robustness of ${\mathrm{Sm}}_2{\mathrm{Co}}_{17}$.

Figure 1

The experimental (closed symbols) and calculated (open symbols) crystal structure parameters of ${\mathrm{Sm}}_2{\mathrm{Co}}_{17}$ under pressure: (a) basal plane $a$-axis lattice parameter, (b) $c$-axis lattice parameter, (c) $c/a$ ratio, and (d) the unit-cell volume. Lines in (a)–(c) are guides to the eye, while those in (d) are fits to the third-order Birch-Murnaghan equation of state. Error bars returned from structural refinements are smaller than the experimental points.

Figure 2

The ambient-pressure, spin-resolved, partial densities of states (DOS) from both Sm and Co states plotted versus binding energy $E-E_F$. The spin-up states are plotted on the positive, vertical axis, while the spin-down states are plotted along the negative, vertical axis. Inset: the spin-summed contribution from the Co $3d$ and Sm $5d$ states, suggesting Sm-Co hybridization.

Figure 3

The computed, total density of states (DOS) decomposed into the spin-up (red line, positive axis) and spin-down (blue line, negative axis) spins for different calculated pressures as functions of energy $E$. The Fermi level ($E_F$) is marked with a vertical, black dashed line; shaded regions denote the occupied DOS. The sharp edge in the spin-up DOS, predominantly composed of Co $3d$ states, is demarcated by a red, dashed line.

Figure 4

(a)–(c) Representative Sm $L{\gamma }_1$ x-ray emission spectra under pressure. The low-pressure (2.4 GPa) spectrum is plotted as a solid, black line in each panel, while the open symbols represent the spectra at 16.4, 36.1, and 60.3 GPa. The differences between the high- and low-pressure spectra are plotted as solid, colored lines. The dotted, horizontal black line is zero. (d) A schematic representation of the atomic XES process for the Sm $L{\gamma }_1$ line. A photon with an energy above the Sm $L$ edge excites a $2p$ electron into the continuum, and that $2p$ core hole is filled by a decay from the $4d$ manifold. The spin of the final-state $4d$ core hole interacts with the magnetic configuration of the $4f$ shell, lifting the degeneracy of the $4f$ state and manifesting two transitions separated by $\Delta \mathrm{E}$. (e) The IAD (see text) as a function of pressure implies a decreasing Sm $f$ moment under pressure; the solid line is a guide to the eye. Error bars are estimated from the noise in the XES signals.

Figure 5

The Sm $L_3$-edge XANES (a) and XMCD (b) as well as the Co $K$-edge XANES (c) and XMCD (d) spectra as a function of incident photon energy for the four different pressures investigated.

Figure 6

The theoretical and measured (normalized to theory at $P=0$) Sm (a) and Co (b) moments as functions of elemental volume (see text). The evolution of the Co moment under pressure is included for comparison in (b). Error bars are estimated from the noise in the XMCD spectra far from the edge. The lowest compression (highest pressure) data point in each pane corresponds to 54 GPa.

Figure 7

Normalized magnetization determined from XMCD measurements for the Sm $L{}_3$ (left panel) and Co $K$ edges (middle panel) plotted as functions of applied magnetic field. The hysteresis loops are offset vertically for clarity. Ambient-pressure magnetization of an aligned powder embedded in epoxy determined at 100 K using a SQUID magnetometer (right panel) and displaying a coercivity near 2 kOe.