  The potential of exchange spring bilayers and graded media is reviewed. An
analytical model for the optimization of graded media gives an optimal value of
the magnetic polarization of Js = 0.8 T. The optimum design allows for
thermally stable grains with grain diameters in the order of 3.3 nm, which
supports ultra high density up to 5 to 10 Tbit per inch2. The switching field
distribution is significantly reduced in bilayer media and graded media
compared to single phase media. For the graded media the switching field
distribution is reduced by about a factor of two. For bilayer media the minimum
switching field distribution is obtained for soft layer anisotropies about one
fifth of the hard layer anisotropy. The influence of precessional switching on
the reversal time and the reversal field is investigated in detail for magnetic
bilayers. Exchange spring bilayers can be reversed with field pulses of 20 ps.
