We analyzed the effect of a Ti underlayer on the magnetic properties and read-write characteristics of perpendicular Co–Cr media, using a merged magnetoresistive head to investigate the latter. The output at 100 kfci for Co78Cr22 was decreased by using a Ti underlayer, whereas that for Co72Cr28 was increased. Such phenomena cannot be clearly explained only by the improvement in the crystalline orientation, the increase in the anisotropy field Hk⊥, and the reduction in the thickness of the initially grown layer. The maximum gradient in the curve showing the dependence of ΔM⊥ on the external field for the Co72Cr28 was about four times larger than that for the Co78Cr22. This result indicates that Co72Cr28 has strong interparticle interaction that aligns the magnetization direction of the particles coherently, and that the magnetization in the initial layer has a strong effect on the magnetization near the top surface through the exchange force. Consequently, the use of a Ti underlayer increases the reproduced output of perpendicular media by 60% at 100 kfci. Co72Cr28 media with a Ti underlayer have a D50 of 150 kfci at a magnetic spacing of 62 nm and a medium thickness of 200 nm. © 1997 American Institute of Physics.