Donut- or ring-shaped Co nanomaterials were successfully synthesized using the polymer-modified method, by which PVP is used as a surface modifier reagent. The synthesized products were structurally and morphologically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), whereas the magnetic characterization was performed by temperature- and field-dependent magnetization measurements. The results show that the rings have a h-Co nanostructure. The outer diameter is about 400−500 nm, inner diameter 150−250 nm, and the ring plane thickness ∼120 nm. The magnetization measurements show a typical coercivity of about 112 Oe at 300 K and an almost vanishing coercivity, HC ≈ 0 at T = 5 K. Numerical calculations using the packeage of object oriented micromagnetic framework (OOMMF) reveal a magnetization ground-state of flux closure structure, as expected for the geometric structure of the submicrometer-sized Co rings. This explains the observed hy...