Advances in thin-film synthesis and characterization have enabled the development of electrically switched magnetic materials for novel memory applications. These devices are typically switched by substrate-strain transfer or electrostatic field effects, but such mechanisms are slow and of limited utility. Writing in the June issue of Nature Nanotechnology (DOI: 10.1038/NNANO.2013.96; p. 411), researchers Uwe Bauer, Satoru Emori, and Geoffrey Beach at the Massachusetts Institute of Technology (MIT) now report an electric and magnetic field coupling mediated by magnetic domain wall pinning. The researchers describe new insight into fundamental defect-domain wall interactions and believe that such a mechanism may form the basis for faster, more efficient memories.
A series of polar magneto-optical Kerr effect microscopy maps showing expansion of a magnetic domain (blue) under an applied magnetic field of 170 Oe with increasing time (from left to right). Row (a) shows the material in its virgin state, without electric-field poling. In this case the domain propagates unperturbed. Row (b) shows the material after poling the dashed region, which then acts to pin the propagating domain. Reproduced with permission from Nature Nanotech. (2013), DOI: 10.1038/NNANO.2013.96. © 2013 Macmillan Publishers Ltd.
The researchers first deposited thin-film heterostructures of Ta(4 nm)/ Pt(3 nm)/Co(0.9 nm)/GdOx (3 nm) using dc magnetron sputtering and capped them with an array of Ta/Au gate electrodes. They chose this particular compound because the perpendicular magnetic anisotropy (PMA) of Co is sensitive to interfacial O2– ions; by applying an electric field it is possible to displace the ions at the interface and reversibly switch the PMA of Co. To visualize domains, the researchers used a technique called magneto-optical Kerr effect (MOKE) microscopy, which relies on the change in polarization and phase of a laser reflected from the film. The researchers first nucleated a magnetic domain on the surface of the film using an external tungsten microprobe and then mapped the expansion of the domain. Their results directly reveal magnetic domain wall growth under the application of an external magnetic field.
The researchers find that by poling the Ta/Au contact, it is possible to locally change the interface anisotropy and pin magnetic domain walls around the contact. In a series of elegant measurements, the group finds that it is possible to create a nonvolatile memory cell based on a series of domain wall traps with increasing pinning strength. These results may lead to magnetic memories based on electric-field pinning of magnetic domain walls, while the techniques developed for this study may be extended to other studies of domain wall kinetics.
