  Here we present a combined study of the slightly underdoped novel pnictide
superconductor Ba(1-x)K(x)Fe(2)As(2) by means of X-ray powder diffraction,
neutron scattering, muon spin rotation (muSR), and magnetic force microscopy
(MFM). Commensurate static magnetic order sets in below Tm ~ 70 K as inferred
from the emergence of the magnetic (1 0 -3) reflection in the neutron
scattering data and from the observation of damped oscillations in the
zero-field-muSR asymmetry. Transverse-field muSR below Tc shows a coexistence
of magnetically ordered and non-magnetic states, which is also confirmed by MFM
imaging. We explain such coexistence by electronic phase separation into
antiferromagnetic and superconducting/normal state regions on a lateral scale
of several tens of nanometers. Our findings indicate that such mesoscopic phase
separation can be considered an intrinsic property of some iron pnictide
superconductors.
