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The Origin of Exchange Bias

by Markus Kirschner

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.
Cycle 2 leaves even the blue spin unaffected, reflecting the training effect. The remanent states are symmetric, and no bias occurs. 

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.

Cycle 1
Cycle 2

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Mar. 05, 2003