Measurement of the remanent magnetization of single Co/Cu and Ni nanowires by off-axis TEM electron holography

The off-axis TEM electron holography technique allows to recover the amplitude and phase of an electron wave at the exit plane of a region filled by magnetic and/or electric fields. The phase image shows lines of equal phase called equiphase lines that are closely related to the field lines of the magnetic (electric) field. This technique has been applied to the study of the remanent magnetization of ferromagnetic nanowires of radius ranging from 20 to 150 nm and a few microns in length. In order to observe the remanent magnetization of the nanowires the objective lens of the electron microscope is switched off and the spatial resolution of the reconstructed phase images is presently limited to 70 nm. The measurement of the phase shift induced by Co and Ni nanowires has been performed with a precision better than 3%. As a result, for the first time by electron microscopy, values of the remanent magnetization of single nanowires have been obtained with an accuracy better than 15%. The measurement precision of the nanowire cross-section was mainly responsible for the larger uncertainty. For the smallest radii, the remanent magnetization state is uniform along the wire axis and its value corresponds to complete saturation magnetization. For larger radii, non-uniform remanent magnetization states have been observed as indicated by a reduced magnetization compared to the saturation value. Several nanowires with domains of opposite magnetization were also observed. Multilayered nanowires formed by a periodic arrangement of Co segments separated by Cu segments showed several magnetic configurations, from the parallel to antiparallel alignment of the magnetic segments. The interpretation of these experimental results is confirmed by the direct calculation of equiphase lines images.