In this paper, ternary Co-Pt-P thin films were prepared by magnetron sputtering with platinum and home-made Co-P chips pasted on the cobalt target. The structure and magnetic properties were investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM), respectively. With increasing phosphor content, the coercivities of Co-12 at.% Pt-P films increase from 1034 to 1525 Oe owing to the exchange decoupling among magnetic grains. The decrease of inter-granular exchange coupling was confirmed by delta-M curve measurement and magnetic force microscopy (MFM). Transmission electronic microscopy (TEM) with nano- beam composition analysis shows that phosphor segregated at cobalt-based grain boundaries is responsible for the exchange decoupling. It is thus suggested that the phosphor addition is e?ective to tune the exchange coupling of magnetic grains, particularly for perpendicular recording media. In this paper, ternary Co-Pt-P thin films were prepared by magnetron sputtering with platinum and home-made Co-P chips pasted on the cobalt target. The structure and magnetic properties were investigated by X-ray diffraction (XRD) The increase of phosphor content, the coercivities of Co-12 at.% Pt-P films increase from 1034 to 1525 Oe owing to the exchange decoupling among magnetic grains. The decrease of inter-granular exchange coupling was confirmed by delta-M curve measurement and magnetic force microscopy (MFM). Transmission electronic microscopy (TEM) with nano-beam composition analysis shows that phosphor segregated at cobalt-based grain boundaries is responsible for the exchange decoupling. It is thus suggested that the phosphor addition is e? ective to tune the exchange coupling of magnetic grains, particularly for perpendicular recording media.