to be 30 or more greater than that of a conventional medium having about the same effective thickness for a medium for which the reversal of the magnetization of the magnetic underlayer takes place at a negative magnetic field strength when the magnetization curve is measured at 135k the resolution became less than 60 disks on which recording had been carried out were removed from the magnetic disk drive and the magnetic transition width of an isolated transition was observed using a magnetic force microscope mfm whereupon it was found that the magnetic transition width changes in accordance with the ratio hkb hka of the anisotropic magnetic field strength hkb of the magnetic underlayer of the afc medium to the anisotropic magnetic field strength hka of the recording magnetic layer as shown in fig1 comparing with a mono layer longitudinal medium it is undesirable for the ratio hkb hka to exceed 0 7 since then the magnetic transition width becomes wider than that of a mono layer longitudinal medium of thickness the same as that of the recording magnetic layer moreover if the ratio hkb hka is between 0 4 and 0 65 then a magnetic transition width about as steep as that of a mono layer longitudinal medium having the same effective thickness is obtained when the magnetization curve was evaluated at 30k for a case in which rearrangement.