Microwave absorption study of polycrystalline SmO_1-xF_xFeAs

Measurements of nonresonance microwave absorption have been used to determine the position of the irreversibility line and to estimate the critical current density for the three SmO_1−xF_xFeAs samples with fluorine concentrations x = 0.06, 0.08, and 0.1. The irreversibility lines of all samples are characterized with a sharp slope, indicating a strong pinning up to T_c. A weak field dependence of the critical current density and an estimate of parameters characterizing the vortex matter allow us to suggest the presence of additional pinning centers in underdoped samples. It is likely that these pinning centers are nanoscale inclusions of a possibly magnetic phase occurring in the fluorine concentration range of 0.06÷0.08.