Ultra‐Clean Hinge States in Three‐Dimensional CuF 6 Sr 2 : An Ideal Magnetic Second‐Order Topological Insulator

Despite the intense interest in 2D magnetic second‐order topological insulators (SOTIs) over the past 5 years, realistic 3D magnetic SOTI materials remain extremely limited. Based on first‐principles calculations, we demonstrate that bulk ferromagnetic CuF 6 Sr 2 is an intrinsic three‐dimensional magnetic second‐order topological insulator. Notably, only the spin‐up channel hosts a nontrivial bulk band gap, whereas the spin‐down channel remains topologically trivial. Specifically, only the spin‐up channel in CuF 6 Sr 2 carries a quantized fractional topological charge ( Q (2) = e /4), whereas the spin‐down channel is topologically trivial ( Q (2) = 0), highlighting a spin‐selective fractional topological response. According to the higher‐order bulk–boundary correspondence, we further confirm that, in the absence of SOC, CuF 6 Sr 2 hosts topologically protected, ultra‐clean hinge states in the spin‐up channel, which are clearly distinguishable from the bulk bands. Remarkably, unlike previously reported nonmagnetic SOTI materials, the hinge states in CuF 6 Sr 2 are spin‐polarized, providing a magnetic platform for exploring spin‐dependent higher‐order topological phenomena and their potential relevance to topological spintronics.