  Micromagnetic simulation results on Co-based recording media are presented
which examine the impact of a modified near-neighbor exchange interaction
between grains of the form Jz(MizMjz), reflecting the hexagonal crystal
symmetry. Both out-of-plane and in-plane M-H loops are calculated, with an
emphasis on a model fit to data reported by Wang et al. [IEEE Trans. Magn. vol.
43, 682 (2007)] on exchange coupled composite perpendicular media. The
principle effect of Jz is to increase the coercivity and slope of both hard and
soft layers. Improved agreement with experimental data for the in-plane loops
is achieved by assuming a substantial value for Jz. Possibilities for
measurement of Jz through spin-wave excitations are discussed. Thermal
fluctuation effects are also examined through simulations of the magnetization
vs temperature.
