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Intergranular exchange in perpendicular recording media

by Martin P. Stehno


Transition shapes

Intergranular exchange plays a crucial role for medium performance in perpendicular recording. The shapes of the written transitions (transition widths, and side-writing) are the important parameters determining noise in the read-back signal. High intergranular exchange causes the written transitions to 'fray out' and have irregular shapes.

intergrain
Bit shapes after t = 2.2 ns simulation time. The reduced applied field is denoted by H0/Hk and sigma specifies the opening angle of the easy-axis distribution.


Switching times

Small switching times in the ns regime are required for high data rates. Two reversal characteristics have been found. The difference in switching time can be up to 0.5 ns. In weakly exchange-coupled grains the field rise profile (substituting for the field of a moving head, which is writing the transition) defines which switching characteristics applies.

switching time

left: Switching time tauS as a function of the reduced applied field H0/Hk for irregularly-shaped grains at different intergranular exchange. right: Switching time tauS as a function of the reduced applied field H0/Hk for regularly-shaped grains for easy-axis distributions with different opening angles.

switching time

Switching time tauS for different field rise profiles.


Coherent and incoherent switching

It depends on the material texture whether it is favorable for a grain to switch 'in phase' with its neighboring grains or incoherently. Coherent switching is faster than incoherent switching.

movie1 movie2

Coherent (left) and incoherent switching (right) in the center region of the bit. The blue region is magnetized in the direction of the applied field, the red one is magnetized in the opposite direction.

References:
M. Stehno, "Intergranular exchange in perpendicular recording media", Diploma thesis, Vienna University of Technology, 2002

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June 18, 2003