We report herein the synthesis and characterization of exchange-biased Co_xFe_3−xO₄@CoO@PMMA nanohybrids in which the inorganic core comprised a highly crystallized polyol-made ferrimagnetic cobalt iron spinel oxide nanoparticles perfectly epitaxied to an very thin antiferromagnetic polycrystalline cobalt monoxide CoO shell, whereas the organic coating comprised a layer of dense poly(methyl methacrylate) (PMMA) chains. PMMA brushes were grown by atom transfer radical polymerization (ATRP). The polymerization was monitored by combining infrared absorption (FTIR) and X-ray photoelectron (XPS) spectroscopy. The resulting nanohybrids are considered as potential building blocks for flexible magnetic recording devices. Their magnetic properties were then investigated by low temperature field cooling (FC) and zero field cooling (ZFC) magnetometry experiments. The obtained results agree fairly with a direct dependency of the coercivity and the remanence with the polymer chain length and then with the interparticle distance, emphasizing the role of material processing in the design of tailored flexible polymer based hybrid materials.