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An Intuitive Picture of Exchange Bias

by Markus Kirschner


A unidirectional anisotropy in the ferromagnet requires the so-called field cooling process: The bilayer system is cooled below the Néel temperature (TN) of the antiferromagnet, starting at a temperature T (TN < T < TC, where T is the Curie temperature of the ferromagnet). Usually an external field is applied to fix the direction of the ferromagnet during the cooling process (1). 

Below the Néel temperature the antiferromagnet chooses the state that minimizes the energy due to the coupling to the ferromagnet (2). When the external field reverses the ferromagnet, the antiferromagnetic spins do not switch if the assumption of sufficiently high anisotropy is fulfilled (3-4). The AF spins at the interface try to keep the ferromagnetic spins in their original direction. Consequently the external field needed to reverse an exchange biased ferromagnet is larger than for single ferromagnetic layers.

For increasing H the ferromagnet switches back at lower external fields due to the codirectional AF torque (5-2). Altogether the magnetization curve exhibits a shift along the field axis,  i.e. exchange bias. 

Although this model of Meiklejohn and Bean gives an intuitive insight into the phenomena of exchange bias, it predicts bias fields several orders of magnitude larger than all experimentally measured values. Moreover, exchange bias was found experimentally for uncompensated as well as for fully compensated interfaces.

intuitive

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Mar. 05, 2003