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A New Paradigm for Exchange Bias in Polycrystalline Films

Kevin, O’Grady, The university of York, IEEE Distinguished Lecture


The phenomenon of exchange bias has remained something of a mystery since it was first discovered in core shell particles in 1956 [1]. Over the subsequent years many different models have been attempted to try and explain this effect most of which agree with some experimental data that can be found in the literature. However no single theory has ever been able to put a theoretical line consistently through data for different systems.

 

In this lecture the reason for our inability to explain exchange bias will be reviewed. Subsequently a new paradigm to explain exchange bias in sputtered polycrystalline films will be presented. This new paradigm is based on an original granular model due to Falcomer and Charap [2]. The basis of the new paradigm is that very careful thermal and magnetic cycling is required to ensure that the order in the antiferromagnetic grains is controlled carefully. Without such careful control reproducible data cannot be obtained.

 

These measurement procedures which are time consuming and complex, we refer to as the York Protocol and have been developed over the last 9 years. It will be shown that using the York Protocol and an extension of the former granular model, effects such as the film thickness dependence and grain size dependence of exchange bias can be fully explained with an excellent fit between theory and experiment [3]. The York Protocol also allows for the measurement of the anisotropy constant of antiferromagnetic grains [4].

 

The above model allows for an understanding of the behaviour of the individual AF grains in detail. Since the behaviour of the "bulk" of the antiferromagnetic grains is now known preliminary data describing the behaviour of the interface spins can now be distinguished from the behaviour of the bulk. Possible mechanisms for the behaviour of the interfaces themselves will also be presented.

 

[1] W. H. Meiklejohn, and C. P. Bean, Phys. Rev. 102 (1956) 1413

[2] E. Fulcomer, and S. H. Charap, J. Appl. Phys. 43 (1972) 4190.

[3] G. Vallejo-Fernandez, L. E. Fernandez-Outon, and K. O’Grady, J. Phys. D : Appl. Phys.41 (2008) 112001.

[4] G. Vallejo-Fernandez, L. E. Fernandez-Outon, and K. O’Grady, Appl. Phys. Lett. 91 (2007) 212503.

 

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