Comparison of the ferromagnetic phase transitions in La_0.7Ca_0.3MnO₃and single crystal nickel by micromagnetic imaging

There is considerable controversy surrounding the nature of the paramagnetic to ferromagnetic phase transition in La0.7Ca0.3MnO3. We have used transmission electron microscopy to perform micromagnetic imaging in order to determine whether the phase change is first or second order. On warming through the transition point, the ferromagnetic phase retreats from the sample surface as it is replaced by the paramagnetic phase. This coexistence of ferromagnetic and paramagnetic phases indicates a primarily first-order transition. However, there is also a continuous loss of magnetization which precedes the phase transition. We compare this with the ferromagnetic transition in nickel which displays a purely continuous phase change. We discuss the accuracy and range of applicability of the micromagnetic imaging techniques of electron holography and Fresnel imaging which were used in this investigation.