This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.-- et al. ; Magnetic vortices have generated intense interest in recent years due to their unique reversal mechanisms, fascinating topological properties, and exciting potential applications. In addition, the exchange coupling of magnetic vortices to antiferromagnets has also been shown to lead to a range of novel phenomena and functionalities. Here we report a new magnetization reversal mode of magnetic vortices in exchange coupled Ir20Mn80/Fe20Ni80 microdots: distorted viscous vortex reversal. In contrast to the previously known or proposed reversal modes, the vortex is distorted close to the interface and viscously dragged due to the uncompensated spins of a thin antiferromagnet, which leads to unexpected asymmetries in the annihilation and nucleation fields. These results provide a deeper understanding of the physics of exchange coupled vortices and may also have important implications for applications involving exchange coupled nanostructures. ; This work was supported by the US NSF (DMR-1008791 and ECCS-1232275), the 2014-SGR-1015 project of the Generalitat de Catalunya, and MAT2010-20616-C02, CSD2007-00041 and MAT2012-35370 projects of the Spanish Ministerio de Economía y Competitividad (MinECO). Work at Argonne was supported by the U. S. Department of Energy, Office of Science, Materials Science and Engineering Division. Fabrication was performed at the Center for Nanoscale Materials, which is supported by DOE, Office of Science, Basic Energy Science under Contract No. DE-AC02-06CH11357. KL acknowledges support from the NSFC (11328402). AS acknowledges a grant from the ICREA Academia, funded by the Generalitat de Catalunya. ICN2 acknowledges support from the Severo Ochoa Program (MinECO, Grant SEV-2013-0295). ; Peer Reviewed