In situ Lorentz TEM magnetization studies on a Fe–Pd–Co martensitic alloy

Understanding domain wall pinning centers and the resultant mobility of ferroic (ferromagnetic and ferroelastic) walls under an applied magnetic field is of central importance to actuator applications of magnetic shape memory alloys. The movement of ferroic boundaries in a twinned Fe–Pd–Co martensite was analyzed by means of Lorentz mode transmission electron microscopy. An in situ magnetizing sample-holder was used to record the evolution of the magnetic domain structure as a function of the applied field. Fresnel images were recorded at different field values and a phase reconstruction algorithm was used to map the magnetization configuration inside the foil. The motion of magnetic domain walls was found to be strongly influenced by the presence of twin boundaries. Free-standing domain walls became mobile at fields as low as 15 Oe, whereas an order of magnitude higher field was required to depin domain walls that coincided with twin boundaries. The domain wall motion was completely reversible with a notable hysteresis.