We report direct domain observations of Barkhausen avalanches at criticality in Co and MnAs thin films investigated by means of a magneto-optical microscope magnetometer, capable of time-resolved domain observation in real time. Through a statistical analysis of the fluctuating size of Barkhausen jump from numerous repetitive experiments for each sample, the distribution of Barkhausen jump size is found to exhibit a power-law scaling behavior with the critical exponent of similar to 1.33 in both systems. This value is consistent with the two-dimensional prediction of a theoretical model proposed by Cizeau et al. [Phys. Rev. Lett. 79 (1997) 4669]. (c) 2006 Elsevier B.V. All rights reserved.