We report simulations of the writing process of isolated transitions with a magnetometer for perpendicular recording. The fields of three different head types, a pole head, a shielded pole head and a ring head were used to study the writing of transitions on two perpendicular media, one with a very square loop and the other one with a shallow loop. It is found that the shielded head records the narrowest transitions, followed by the ring and then by the pole head.