Intergranular exchange coupling field under multidomain state for stacked perpendicular recording media evaluated by ferromagnetic resonance

The intergranular exchange (coupling) field H _ex of single/stacked granular perpendicular magnetic recording media with a switching field (SF) distribution was measured by ferromagnetic resonance (FMR) by sweeping the external field H _DC through the internal effective field H _eff. H _eff consists of components corresponding to the anisotropy field H _k , the dipole interaction field, the self-demagnetization field, and the H _ex for up and down magnetic moments of magnetic grains classified by their SF. FMR measurements were performed for one kind of weak intergranular exchange coupled medium (medium I) and two kinds of strong intergranular exchange coupled media (medium II (single magnetic layer structure) and III (stacked structure)). These measurements revealed that the increase in H _ex with decreasing 4\pi M for media II and III was larger than that for medium I. In addition, H-ex/H _k was determined to be 0.25 (for grains with low SF) and 0.37 (for grains with high SF) for medium II and 0.34 (for both low and high SF) for medium III under M=0.3). The variation in H _eff for the multidomain state was smaller for the strong intergranular exchange coupled media than for the weak intergranular exchange coupled medium. The values normalized by H _k were approximately \pm 0.038-0.060, \pm 0.008-0.015, \pm 0.012-0.030 for media I, II, and III, respectively.