|The origin of exchange bias in our model is the antiferromagnetic
exchange energy stored in AF domain walls when the external field reverses
the ferromagnet. Therefore, weak exchange interactions between AF grains
are essential in order to find a shift of the hysteresis loop.
The small simulation below illustrates the processes leading to exchange
bias. The green spins in the front represent the ferromagnetic layer. For
the antiferromagnet only the spins of one sublattice are shown. The easy
axes of the antiferromagnetic grains are more or less perpendicular to
the film plane except that of the middle grain with the red spin. When
the external field reverses the ferromagnet, only AF spins with easy directions
almost parallel to the interface switch irreversibly. In this simulation
the red spin undergoes an irreversible transition during cycle 1. The AF
exchange energy rises considerably and leads to asymmetric remanent states
and thus to exchange bias. When the ferromagnet rotates back, the
middle spin maintains its new direction due to intergranular exchange coupling.
So, exchange bias results from the combination of switching and non-switching
AF grains or the difference of the AF domain configuration between the
positive and the negative remanent state.
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Mar. 05, 2003