To continue the rapid increase of recording density, most observers believe the next generation of hard drives will use perpendicular media, in which the magnetization is perpendicular to the plane of the disk, rather than parallel as in existing "longitudinal" media.
Here is a link to a 18 MB quicktime movie showing the switching of a grain by the passage of a monopole head.
At the lower left is a grain of a magnetic-recording medium, divided into 4x4x4 cells. On the upper right is a model recording head, whose magnetic field is modeled as that of a linear pole density (the red cylinder labeled N, for North). The pole field felt by the cubical grain is shown as a blue vector labeled H_ext, and the anisotropy field (black vector, H_anis) due to the crystalline anisotropy of the material. As the head approaches the grain, its magnetic field becomes stronger and more vertical, canceling the anisotropy field and allowing the magnetization to switch from up to down. This actually takes place through a spin wave instability mechanism, which leaves the magnetizations rotating rapidly as the poles recede; they eventually settle down as a result of magnetic damping and point downward, but damping is not included in this simulation.
This work was presented at the May 2002 Intermag Europe conference in Amsterdam, as GV03: "Switching Simulations in Perpendicular Media: Spin Wave Instabilities" (pdf file) and has been accepted for publication in IEEE Transactions on Magnetism.
Additional movie: