  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 anti-parallel 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.
