Highlights

Imaging Magnetization Reversal in Magnetic Nanostructures

Scientific Achievement

Magnetic nanoscale heterostructures are of active scientific interest because of size confinement effects which produce very different behavior than in the bulk.  The focus of this project is to  correlate the magnetization reversal behavior of heterostructures consisting of layered ferromagnet/antiferromagnet (FM/AF) patterned elements with their microstructure and chemistry.  In addition, we are pursuing novel phase retrieval techniques that are applied to magnetization mapping in the TEM, with the aim of developing a quantitative picture of the microstructure-property relationship in these structures.

Current magnetic data storage technology is based on devices with single-domain magnetization and exchange bias set through a Field-Cooling (FC) thermomagnetic treatment.  We have used in situ TEM magnetizing experiments to study magnetization reversal in vortex-supporting patterned bilayer ferromagnet/antiferromagnet disks in which exchange bias has been set through a Zero Field-Cooling (ZFC) thermomagnetic treatment.  This imprints a circular  exchange bias with a preferred chirality, which profoundly influences the magnetic behavior of the FM layer.  The circular exchange bias stabilizes the vortex state against externally-applied fields, and leads to reproducible magnetization reversal behavior for both Py/IrMn and CoFe/IrMn disks.  For Py/IrMn heterostructures reversal always occurs with a single chirality, whereas for CoFe/IrMn structures, reversal takes place through a combined mechanism of vortex+domain walls, which allows both chiralities of the vortex to be accessed reliably during one field cycle.  Collaboration with Prof M De Graef led to an understanding of how both the chirality and polarity of a magnetic vortex could be determined from a single Lorentz TEM images, which cannot be achieved using any other technique.

Significance

Our research offers the first insight into the magnetization reversal mechanisms associated with circular exchange bias, a configuration first analyzed in its ground state by our collaborators from Univ. Autonoma de Barcelona.  We have revealed two different mechanisms of magnetization reversal, which occur as a result of the competing effects of exchange bias and microstructure.  Our research offers insights into the possibility of controlling the chirality of vortices through materials and device design, significant for memory and data storage applications.  Future work on this project includes the influence of FIB-induced defects, interactions between patterned elements and the study of full magnetic tunnel junctions.  A further extension of this work may be investigation of multiferroic systems using vortices as nanoscale magnetic probes.  This work has been published in Phys. Rev. B, 79(1), 014436 (2009).

Performers

M. Tanase, A. K. Petford-Long (Argonne-MSD); K. Buchanan (Argonne-CNM); O. Heinonen (Seagate Technology); J. Sort, J. Nogues (Univ. Autonoma de Barcelona); M. De Graef, C. D. Phatak (Carnegie Mellon Univ)