We observe a thermally induced spontaneous magnetization reversal ofepitaxial ferromagnet/antiferromagnet heterostructures under a constant appliedmagnetic field. Unlike any other magnetic system, the magnetizationspontaneously reverses, aligning anti-parallel to an applied field withdecreasing temperature. We show that this unusual phenomenon is caused by theinterfacial antiferromagnetic coupling overcoming the Zeeman energy of theferromagnet. A significant temperature hysteresis exists, whose height andwidth can be tuned by the field applied during thermal cycling. The hysteresisoriginates from the intrinsic magnetic anisotropy in the system. Theobservation of this phenomenon leads to open questions in the generalunderstanding of magnetic heterostructures. Moreover, this shows that ingeneral heterogeneous nanostructured materials may exhibit unexpected phenomenaabsent in the bulk.
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