Off-axis electron holography of patterned magnetic nanostructures.

Magnetization reversal processes in lithographically patterned magnetic elements that have lateral dimensions of 70-500 nm, thicknesses of 3-30 nm and a wide range of shapes and layer sequences have been followed in situ using off-axis electron holography in the transmission electron microscope. This technique allows domain structures within individual elements and the magnetic interactions between them to be quantified at close to the nanometre scale. The behaviour of 30 nm-thick Co elements was compared with that of 10 nm-thick Ni and Co elements, as well as with Co/Au/Ni trilayers. The hysteresis loops of individual elements were determined directly from the measured holographic phase images. The reproducibility of an element's domain structure in successive cycles was found to be affected by the out-of-plane component of the applied magnetic field and by the exact details of its initial magnetic state. Close proximity to adjacent elements led to strong intercell coupling, and remanent states with the in-plane magnetic field removed included domain structures such as solenoidal (vortex) states that were never observed during hysteresis cycling. Narrow rectangular bars reversed without the formation of end domains, whereas closely separated magnetic layers within individual elements were observed to couple to each other during field reversal.