Effect of Si doping and applied pressure upon magnetostructural properties of Tb${}_5$(Si${}_x$Ge${}_{1-x}$)${}_4$ magnetocaloric compounds

The composition- and pressure-dependent magnetostructural properties of Tb${}_5$(Si${}_x$Ge${}_{1-x}$)${}_4$ ($x$ $=$ 0.4, 0.485, 0.625, and 0.7) were investigated using x-ray powder diffraction and x-ray magnetic circular dichroism in a diamond anvil cell, respectively. Substituting the smaller-size Si for Ge stabilizes a single-phase, ferromagnetic (FM) orthorhombic O(I) structure for $x$ ⩾ 0.7. Similarly, application of external pressure causes a canted antiferromagnetic orthorhombic O(II) sample ($x$ $=$ 0.4) to transform into an FM O(I) phase at 4 GPa. The element- and orbital-specific x-ray absorption data indicate that the Tb 4$f$ orbital occupation changes with external pressure, likely through 4$f$-5$d$ electronic mixing, yet no changes in Tb 4$f$ electronic structure are observed with Si doping. The results point to different mechanisms behind the enhancement of FM exchange interactions in Tb${}_5$(Si${}_x$Ge${}_{1\hspace{0.5em}x}$)${}_4$ with chemical and applied pressure, respectively.

Figure 1

X-ray diffraction patterns of (a) Tb${}_5$(Si${}_{0.4}$Ge${}_{0.6}$)${}_4$, (b) Tb${}_5$(Si${}_{0.485}$Ge${}_{0.515}$)${}_4$, (c) Tb${}_5$(Si${}_{0.625}$Ge${}_{0.375}$)${}_4$, and (d) Tb${}_5$(Si${}_{0.7}$Ge${}_{0.3}$)${}_4$ at $T$ $=$ 10 K, together with results of Rietveld refinements using models of single-phase O(II) (a), mixed-phase M$+$O(I) (b) and (c), and single-phase O(I) (d) crystal structures. Gray lines at the bottom of the plot represent the difference between the experimental and theoretical intensities.

Figure 2

Isothermal $M$-$H$ data for Tb${}_5$(Si${}_{0.4}$Ge${}_{0.6}$)${}_4$, Tb${}_5$(Si${}_{0.485}$Ge${}_{0.515}$)${}_4$, Tb${}_5$(Si${}_{0.625}$Ge${}_{0.375}$)${}_4$, and Tb${}_5$(Si${}_{0.7}$Ge${}_{0.3}$)${}_4$, collected at $T$ $=$ 10 K. Arrows indicate the directions of the change of magnetic field in Tb${}_5$(Si${}_{0.4}$Ge${}_{0.6}$)${}_4$. The inset shows $M_s$ (filled squares, left-hand axis) and $M_r$ (open squares, right-hand axis) as a function of Si content. The O(I) concentrations for the four samples are marked on top of the open squares.

Figure 3

Temperature-dependent magnetization data of (a) Tb${}_5$(Si${}_{0.4}$Ge${}_{0.6}$)${}_4$, (b) Tb${}_5$(Si${}_{0.485}$Ge${}_{0.515}$)${}_4$, (c) Tb${}_5$(Si${}_{0.625}$Ge${}_{0.375}$)${}_4$, and (d) Tb${}_5$(Si${}_{0.7}$Ge${}_{0.3}$)${}_4$ measured on warming at $H$ $=$ 0.45 T, after field cooling (open circles) and zero-field cooling (filled circles).

Figure 4

Temperature-dependent integrated XMCD data of (a) Tb${}_5$(Si${}_{0.4}$Ge${}_{0.6}$)${}_4$ taken at various pressures and (b) Tb${}_5$(Si${}_{0.4}$Ge${}_{0.6}$)${}_4$, Tb${}_5$(Si${}_{0.485}$Ge${}_{0.515}$)${}_4$, Tb${}_5$(Si${}_{0.625}$Ge${}_{0.375}$)${}_4$, and Tb${}_5$(Si${}_{0.7}$Ge${}_{0.3}$)${}_4$ taken at ambient pressure. Both data sets were measured on warming. The lines are guides to the eye.

Figure 5

(a) Doping- and (b) pressure-dependent ($x$ $=$ 0.4) Tb $L_3$-edge XANES (upper curves) and XMCD (lower curves) data. Arrows indicate the numeral scales for XANES (right axis) and XMCD (left axis). The XMCD data are normalized to the absorption jump. Data were collected at $H$ $=$ 0.45 T and $T$ $=$ 10(2) K. The insets show the reversal of XMCD signal upon reversal of the applied field. For clarity, XANES data in (b) are only shown at ambient pressure and 1.5 and 7.0 GPa.

Figure 6

Pressure dependencies of $T_t$ for Gd${}_5$(Si${}_{0.375}$Ge${}_{0.625}$)${}_4$ (open circles, taken from Ref. 23) and Tb${}_5$(Si${}_{0.4}$Ge${}_{0.6}$)${}_4$ (filled circles). Dashed line shown for Tb${}_5$(Si${}_{0.4}$Ge${}_{0.6}$)${}_4$ corresponds to a ${\mathit{dT}}_t/\mathit{dP}$ of ∼1.35 K kbar${}^{\hspace{0.5em}1}$ for $P$ > 4 GPa, which is close to 1.5 K kbar${}^{-1}$ observed in Gd${}_5$(Si${}_{0.375}$Ge${}_{0.625}$)${}_4$. Solid line for Tb${}_5$(Si${}_{0.4}$Ge${}_{0.6}$)${}_4$ is the extrapolation of the low-pressure linear behavior.