Multiple magnetic transitions and complex magnetic structures in Fe2SiSe4 with the sawtooth lattice

Abstract

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 ${\mathrm{Fe}}_{2}{\mathrm{SiSe}}_{4}$, an unexplored member in the olivine chalcogenides with the sawtooth lattice of Fe. Our results show that ${\mathrm{Fe}}_{2}{\mathrm{SiSe}}_{4}$ is a magnetic semiconductor with band gap of 0.66 eV. It first undergoes an antiferromagnetic transition at ${\mathrm{T}}_{m1}=110$ K, then an ferrimagneticlike one at ${\mathrm{T}}_{m2}=50$ K, and finally a magnetic transition at ${\mathrm{T}}_{m3}=25$ 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 ${\mathbf{q}}_{\mathbf{1}}=(0,0,0)$ at ${\mathrm{T}}_{m2}<\mathrm{T}<{\mathrm{T}}_{m1}$. Interestingly, below ${\mathrm{T}}_{m2}$, an additional antiferromagnetic structure with ${\mathbf{q}}_{\mathbf{2}}=(0,0.5,0)$ appears, and ${\mathrm{Fe}}_{2}{\mathrm{SiSe}}_{4}$ exhibits a complex double-$\mathbf{q}$ 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 ${\mathrm{Fe}}_{2}{\mathrm{SiSe}}_{4}$ has quasi-flat-band features near the valence and conduction band structure. Our results have shown that ${\mathrm{Fe}}_{2}{\mathrm{SiSe}}_{4}$ could serve as a new material playground for further research on magnetic devices and the flat-band effect through chemical doping.