Universality of magnetic susceptibility in the conical state of kagome ferromagnet Fe$_3$Sn$_2$
We report universal behavior of the differential magnetic susceptibility (DMS) in the conical phase that mediates the spin-reorientation (SR) transition of the kagome ferromagnet Fe$_3$Sn$_2$. Within the SR temperature range, the DMS isotherms exhibit extremely narrow crossing regions, forming isosbestic points. Using an isosbestic-invariance analysis, we show that the isotherms collapse onto a single temperature-independent curve, revealing quadratic-in-temperature corrections to the susceptibility. Complementary field-dependent magnetic-force-microscopy measurements uncover evolution of spin textures from stripe-like domains at low fields to isolated bubble-like domains near the isosbestic field ($\sim 0.6$~T), a behavior not previously reported in bulk Fe$_3$Sn$_2$ within the conical state. These findings point to a universal mechanism for the emergence of complex magnetic textures near isosbestic points, driven by the competition between magnetocrystalline anisotropy, dipolar interactions, and external magnetic field.