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Lorentz electron microscopy study of magnetisation reversal mechanism in exchange-coupled bilayers

A series of images observed during the magnetisation reversal of S10 with a sweep rate of 0.5 Oe/s are shown in Fig. 1. The direction of H, and of the UEA are indicated. Along the forward branch of the magnetisation reversal loop (Figures 1(a)- 1(e)), the coupling between the two layers leads to the CoFe layer remaining saturated in the positive direction until a field of -91 Oe. Reversal of the CoFe layer proceeds slowly via coherent moment rotation (seen as an anticlockwise rotation of the magnetisation ripple) followed by the nucleation of domains with near 90° domain walls. As the magnitude of the field increases the domains grow and the domain wall angle increases to near 180° before the walls disappear. Negative saturation of the CoFe layer, antiparallel to the UEA, is achieved at -158 Oe via a further coherent moment rotation of about 20°. The magnitude of H, was increased rapidly to -400 Oe and then decreased in order to follow the recoil reversal (Figs. 1(f)- 1 (/)). The CoFe layer remained at negative saturation until -138 Oe. The recoil process is achieved by coherent rotation (clockwise) of the magnetisation, followed by incoherent rotation and the formation of low angle walls (seen as splitting of the magnetisation ripple) as the CoFe moments relax back towards the unidirectional easy axis. The two split magnetisation directions are not symmetric with respect to H, - one rapidly rotates towards Ha resulting in the formation of 90° walls at about -113 Oe at an angle to Ha, which then move relatively slowly across the film with respect to the forward reversal. The walls are finally pinned at some microstructure features or form 360° walls. As -Ha decreases slightly the walls left at microstructural features disappear. Some of the 360° walls only disappear at positive Ha.