Speaker
Description
The compound Fe$_4$Si$_2$Sn$_7$O$_{16}$ has a hitherto unique crystal structure, consisting of ionic oxide layers based on edge-sharing FeO$_6$ and Sn$^{4+}$O$_6$ octahedra alternating with layers of intermetallic character based on FeSn$^{2+}_6$ octahedra, separated by covalent SiO$_4$ tetrahedra. The ionic layers contain kagomé lattices of magnetic Fe$^{2+}$ cations (octahedral crystal field, high-spin [HS] d$^6$, $S = 2$) with perfect trigonal symmetry; while the intermetallic layers are non-magnetic because the Fe$^{2+}$ is in the low-spin ($S = 0$) state. The formula is more correctly written as Fe$_4$Si$_2$Sn$_7$O$_{16}$ to differentiate the one LS-Fe$^{2+}$ per formula unit in the intermetallic layer from the three HS-Fe$^{2+}$ per formula unit in the kagomé oxide layer.
Fe$_4$Si$_2$Sn$_7$O$_{16}$ also has a unique magnetic ground state below a Néel ordering temperature T$_N$ = 3.5 K, in which the spins on 2/3 of the Fe$^{2+}$ sites in the kagomé oxide layers order antiferromagnetically, while 1/3 remain disordered and fluctuating down to at least 0.1 K. The nature and origin of this unique “striped” partial spin-liquid state is unclear. The fact that it breaks trigonal symmetry, which the more conventional q = 0 or √3×√3 kagomé states would not, raises the possibility that the anisotropic distribution of the 6 unpaired spins on HS-Fe$^{2+}$ (t$_{2g}^4$e$_g^2$) plays a role. To test this possibility, we have now synthesised an isotropic analogue with a kagomé lattice of HS Mn$^{2+}$ (t$_{2g}^3$e$_g^2$), by co-substituting Ge$^{4+}$ for Si$^{4+}$ in the bridging/stannite layers to match the lattice dimensions between layers.
We found that FeMn$_3$Ge$_2$Sn$_7$O$_{16}$ has the same "striped" magnetic ground state as Fe$_4$Si$_2$Sn$_7$O$_{16}$, in the same temperature range, ruling out this explanation. However, the zero-field striped structure is collinear for FeMn$_3$Ge$_2$Sn$_7$O$_{16}$ vs. non-collinear for Fe$_4$Si$_2$Sn$_7$O$_{16}$, which may indeed be a consequence of the change in anisotropy on the magnetic kagomé site, and suggests that FeMn3Ge2Sn7O16 is an even more ideal spin-liquid candidate than Fe$_4$Si$_2$Sn$_7$O$_{16}$. We also found that an external applied magnetic field lifts the degeneracy on the disordered site, giving rise to another ordered magnetic structure never before observed nor predicted on a kagomé lattice.
| Level of Expertise | Experienced Researcher |
|---|---|
| Travel Funding | No |
| Do yo wish to take part in the poster slam | No |
| Speakers Gender | Male |
