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Pulse Width (10-10s)

increasing intergranular exchange coupling will initially slow down the reversal process because of larger correlated magnetization fluctuations. The switching time normally is increased in the presence of exchange and this effect is increased with increasing field rise time [161, 162]. However, similar to static properties, the applied field required to switch the highly exchanged coupled medium is lower than that required to switch the zero exchange coupled system [122].

The dynamic coercivity parameters have been investigated as a function of the characteristics of the applied field. Different types of the field-rise pulse with different durations were supposed. For example, uniform exponential field for switching was investigated in Ref. [161]. Although experimentally, the switching time normally is considered as inversely proportional to the switching field, numerical simulations showed that the field rise time has a major effect on the switching time of the medium and the above relation is true for the head field rise time larger than 0.3 ns or in the range Hmax/HK 0.5-0.6.

For magnetic recording media switching, it has been shown that, in the case of a squared field pulse [162] and for small damping a < 0.4, the perpendicular media switches slower than the longitudinal. The fundamental reason why the longitudinal media switches faster than the perpendicular one is the demagnetization field of the media, which produce an additional torque. In longitudinal media, this field is perpendicular to the reversal direction and provides an additional reversal torque. For perpendicular media, the demagnetizing field is parallel to the reversal direction and does not give an additional reversal torque.

Surface irregularities and grain structure drastically change the reversal mechanisms [112]. For perfect magnetic elements, the high stray field at the particle ends causes the nucleation and determines the switching process. For granular structured material, the nucleation inside a particle occurs, which determines a faster switching process [120]. The surface roughness further decreases the switching time [120, 163, 164], since, at rough surfaces, vortices could be nucleated at particle ends.

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