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Phys Rev Lett ; 96(13): 137201, 2006 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-16712025


We observe a thermally induced spontaneous magnetization reversal of epitaxial ferromagnet/antiferromagnet heterostructures under a constant applied magnetic field. Unlike any other magnetic system, the magnetization spontaneously reverses, aligning antiparallel to an applied field with decreasing temperature. We show that this unusual phenomenon is caused by the interfacial antiferromagnetic coupling overcoming the Zeeman energy of the ferromagnet. A significant temperature hysteresis exists, whose height and width can be tuned by the field applied during thermal cycling. The hysteresis originates from the intrinsic magnetic anisotropy in the system. The observation of this phenomenon leads to open questions in the general understanding of magnetic heterostructures. Moreover, this shows that in general heterogeneous nanostructured materials may exhibit unexpected phenomena absent in the bulk.

Phys Rev Lett ; 94(5): 057203, 2005 Feb 11.
Artigo em Inglês | MEDLINE | ID: mdl-15783688


The size dependence of exchange bias field HE and coercivity Hc was studied by measuring exchange biased Fe-FeF2 dot arrays in comparison with an unstructured exchange biased Fe-FeF2 bilayer. The domain sizes in the ferromagnet (FM) and the antiferromagnet (AFM) play an important role for exchange bias (EB), and thus interesting phenomena may be expected when the size of an EB system becomes comparable to these sizes. We observe drastic changes of HE and Hc in nanostructured Fe-FeF2, which are unexpected because they appear even at a structure size which is too large for matching with AFM or FM domain size to play a role. We propose that under certain conditions the hysteresis loop is affected differently in the two branches of the reversal by shape anisotropy due to patterning. This is possible because the EB induces a reversal asymmetry already in the unpatterned bilayer system.