The use of Lorentz microscopy for the determination of magnetic reversal mechanism of exchange-biased Co30Fe70/NiMn bilayer

Lorentz Transmission Electron Microscopy (LTEM) combined with in situ magnetizing experiments is a powerful tool for the investigation of the magnetization reversal process at the micron scale. We have implemented this tool on a conventional Transmission Electron Microscope (TEM) to study the exchange anisotropy of a polycrystalline Co35Fe65/NiMn bilayer. Semi-quantitative maps of the magnetic induction were obtained at different field values by the Differential Phase Contrast (DPC) technique adapted for a TEM (SIDPC). The hysteresis loop of the bilayer has been calculated from the relative intensity of magnetic maps. The curve shows the appearance of an exchange bias field reveals with two distinct reversal modes of the magnetization: the first path corresponds to a reversal by wall propagation when the applied field is parallel to the anisotropy direction whereas the second is a reversal by coherent rotation of magnetic moments when the field is applied antiparallel to unidirectional anisotropy direction.