Multiple magnetic transitions and complex magnetic structures in ?Fe₂SiSe_4?with the sawtooth lattice

The sawtooth lattice shares some structural similarities with the kagome lattice and may attract renewed research interest. Here we report a comprehensive study on the physical properties of Fe2SiSe4, an unexplored member in the olivine chalcogenides with the sawtooth lattice of Fe. Our results show that Fe2SiSe4 is a magnetic semiconductor with band gap of 0.66 eV. It first undergoes an antiferromagnetic transition at T-m1 = 110 K, then an ferrimagneticlike one at T-m2 = 50 K, and finally a magnetic transition at T-m3 = 25 K, which is likely driven by the thermal populations of spin-orbit manifold on the Fe sites. Neutron diffraction analysis reveals a noncollinear antiferromagnetic structure with propagation vector q1 = ( 0, 0, 0) at T-m2 < T < Tm1. Interestingly, below T-m2, an additional antiferromagnetic structure with q2 = (0, 0.5, 0) appears, and Fe2SiSe4 exhibits a complex double-q magnetic structure which has never been observed in sawtooth olivines. Density functional theory calculations suggest this complex noncollinear magnetic structure may originate from the competing antiferromagnetic interactions for both intra- and interchain in the sawtooth lattice. Furthermore, band-structural calculations show that Fe2SiSe4 has quasi-flat-band features near the valence and conduction band structure. Our results have shown that Fe2SiSe4 could serve as a new material playground for further research on magnetic devices and the flat-band effect through chemical doping.