Magnetic order in ${\text{TbCo}}_2{\text{Zn}}_{20}$ and ${\text{TbFe}}_2{\text{Zn}}_{20}$

We report neutron diffraction studies of ${\text{TbCo}}_2{\text{Zn}}_{20}$ and ${\text{TbFe}}_2{\text{Zn}}_{20}$, two isostructural compounds which exhibit dramatically different magnetic behavior. In the case of ${\text{TbCo}}_2{\text{Zn}}_{20}$, magnetic Bragg peaks corresponding to antiferromagnetic order are observed below $T_N\approx 2.5\text{K}$ with a propagation vector of (0.5 0.5 0.5). On the other hand, ${\text{TbFe}}_2{\text{Zn}}_{20}$ undergoes a ferromagnetic transition at temperatures as high as 66 K which shows a high sensitivity to sample-to-sample variations. Two samples of ${\text{TbFe}}_2{\text{Zn}}_{20}$ with the same nominal compositions but with substantially different magnetic ordering temperatures ($T_c\approx 51$ and 66 K) were measured by single-crystal neutron diffraction. Structural refinements of the neutron diffraction data find no direct signature of atomic site disorder between the two ${\text{TbFe}}_2{\text{Zn}}_{20}$ samples except for subtle differences in the anisotropic thermal parameters. The differences in the anisotropic thermal parameters between the two samples are likely due to very small amounts of disorder. This provides further evidence for the extreme sensitivity of the magnetic properties of ${\text{TbFe}}_2{\text{Zn}}_{20}$ to small sample variations, even small amounts of disorder.

Figure 1

(Color online) The schematic crystal structure of ${\text{TbCo}}_2{\text{Zn}}_{20}$ shows that both Tb and Co ions are surrounded exclusively by Zn atoms as illustrated by the coordination polyhedron of Tb and Co.

Figure 2

(Color online) ${\text{TbCo}}_2{\text{Zn}}_{20}$ single-crystal measurements with the crystal oriented in the $\left(hhl\right)$ scattering plane. (a) Temperature dependence of the integrated scattering intensity of (0.5 0.5 0.5) magnetic reflection (left) and the magnetic susceptibility (right) measured with $H\parallel \left[111\right]$ and $H=0.1\text{T}$. Note the temperature of onset magnetic scattering is close to the temperature (2.5 K) of maximal $d\left(\chi T\right)/dT$ value in Ref. 5. (b) Typical elastic long scans along the $\left(hhl\right)$ direction measured at $T=1.4\text{K}$ and $T=10\text{K}$, respectively. Aluminum reflection contributions ($2\theta =60.79°$, $71.5°$, and $111.44°$) from the aluminum sample holder are marked.

Figure 3

(Color online) (a) Neutron powder diffraction patterns in log scale measured at $T=1.4\text{K}$ (solid circles) and $T=10\text{K}$ (open circles). Aluminum reflections $\left(2\theta =60.79°,71.5°\right)$ from aluminum sample container are marked. Observed neutron powder diffraction pattern (circles), calculated profile (solid line) and their difference (bottom line) between $5<2\theta <105$ measured at (b) $T=10\text{K}$, and (c) $T=1.4\text{K}$.

Figure 4

(Color online) Physical properties of ${\text{TbFe}}_2{\text{Zn}}_{20}$ crystals, s1 and s2. (a) M/H as a function of temperature. Inset: magnetization versus applied magnetic field H at $T=2\text{K}$ with $\text{H}\parallel \left[111\right]$ (b) Electrical resistivity $\rho$ as a function of temperature. Inset: $d\rho /dt$ between 1.5 and 165 K.

Figure 5

(Color online) Diffraction data for ${\text{TbFe}}_2{\text{Zn}}_{20}$ (s2). Representative data collected at 100 K with the SCD instrument.