FeMn3Ge2Sn7O16 : a spin-liquid candidate with a perfectly isotropic 2-D kagomé lattice

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Australian Institute of Physics
The compound Fe4Si2Sn7O16 has a hitherto unique crystal structure, consisting of ionic oxide layers based on edge-sharing FeO6 and Sn4+O6 octahedra alternating with layers of intermetallic character based on FeSn2+6 octahedra, separated by covalent SiO4 tetrahedra. [1,2] The ionic layers contain kagomé lattices of magnetic Fe2+ cations (octahedral crystal field, high-spin [HS] d6, S = 2) with perfect trigonal symmetry; while the intermetallic layers are non-magnetic because the Fe2+ is in the low-spin (S = 0) state. The formula is more correctly written as Fe4Si2Sn7O16 to differentiate the one LS-Fe2+ per formula unit in the intermetallic layer from the three HS-Fe2+ per formula unit in the kagomé oxide layer. Fe4Si2Sn7O16 also has a unique magnetic ground state below a Néel ordering temperature TN = 3.5 K, in which the spins on 2/3 of the Fe2+ sites in the kagomé oxide layers order antiferromagnetically, while 1/3 remain disordered and fluctuating down to at least 0.1 K. [3] 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-Fe2+ (t2g4eg2) plays a role. To test this possibility, we have now synthesised an isotropic analogue with a kagomé lattice of HS Mn2+ (t2g3eg2), by co-substituting Ge4+ for Si4+ in the bridging/stannite layers to match the lattice dimensions between layers. We found that FeMn3Ge2Sn7O16 has the same “striped” magnetic ground state as Fe4Si2Sn7O16, in the same temperature range, ruling out this explanation. However, the zero-field striped structure is collinear for FeMn3Ge2Sn7O16 vs. non-collinear for Fe4Si2Sn7O16, 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 Fe4Si2Sn7O16. 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.
Angular momentum, Chalcogenides, Configuration, Crystal lattices, Crystal structure, Energy levels, Oxygen compounds, Particle properties, Transition element compounds
Allison, M. C., Wurmehl, S., Büchner, B., Valla, J., Söhnel, T., Avdeev, M., Schmid, S., & Ling, C. D. (2020). FeMn3Ge2Sn7O16 : a spin-liquid candidate with a perfectly isotropic 2-D kagomé lattice. Paper presented to the 44th Condensed Matter and Materials Meeting, Holiday Inn, Rotorua, New Zealand, 4-7 February 2020, (p.35). Retrieved from: https://physics.org.au/wp-content/uploads/cmm/2020/CMM20_ConferenceHandbook(04Feb2020).pdf