In an attempt to control the defect morphology and density of the ZMR-SOI thin film, the temperature distribution across the melting zone was modified by a dual heat source consisting of the two tungsten-halogen lamps focused by a dual elliptical mirror. The intensity distribution of the focused beam was calculated to estimate the asymmetry of the temperature distribution. The intensity gradient at the solidifying interface became less steep when the right lamp power increased and left lamp power decreased keeping the sum of the each lamp power constant. On this condition, the defect spacing increased to the critical point and decreased dramatically, while the spacing decreased when the left lamp power exceeded the right. A model was proposed to explain the change of defect morphology and spacing.