Rwc

=0.0

Applied Field

Figure 16. Simulated hysteresis cycles in a Co-based granular material with different strength of the exchange interactions modeled by an effective parameter C*, introduced by the group of R. Chantrell (see text) (the effective exchange decreases from the top remanence to bottom). The decrease of remanence and coercivity is clearly observed. Reprinted with permission from J. D. Hannay et al., J. Magn. Magn. Mater. 193, 245 (1999). © 1999, Elsevier Science.

boundary [111]. More detailed micromagnetic calculations [111, 138, 144, 145] show that the decrease of coercivity occurs with increasing magnetostatic interactions. The effect is smaller for uniform grain structure [111]. However, if there is a distribution of grain sizes, the decrease of the coer-civity is more pronounced. It has been demonstrated that the magnetostatic interactions are responsible for a reduction in the low-temperature remanence [111, 138]. However, in a Co-based film used for longitudinal magnetic recording with random anisotropy distribution and uniform grain sizes, it has been found that the dipolar interactions tend to enhance the remanence [144, 145] that was attributed to in extra effective easy axis arising from dipolar interactions and lately was suggested to be the case of systems with regular lattice (no grain size distribution) [146].

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