Ultra-Clean Hinge States in Three-Dimensional CuF₆Sr₂: 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 CuF₆Sr₂ 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 CuF₆Sr₂ 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, CuF₆Sr₂ 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 CuF₆Sr₂ are spin-polarized, providing a magnetic platform for exploring spin-dependent higher-order topological phenomena and their potential relevance to topological spintronics.