Competition of magnetism and superconductivity in underdoped (Ba1-xKx) Fe2As2

Polycrystalline samples of underdoped (Ba1-xKx)Fe2As2 (x <= 0.4) were synthesized and studied by x-ray powder diffraction, magnetic susceptibility, specific heat and Fe-57-Mobbauer spectroscopy. The structural phase transition from tetragonal to orthorhombic lattice symmetry shifts towards lower temperatures, becomes less pronounced at x = 0.1-0.2 and is no longer present at x = 0.3. Bulk superconductivity is observed in all samples except (Ba0.9K0.1)Fe2As2 by resistivity and magnetic susceptibility measurements. Specific heat data show a broad spin-density-wave (SDW) phase transition in (Ba0.9K0.1) Fe2As2, which is hardly discernible in (Ba0.8K0.2) Fe2As2. No SDW anomaly is found in the specific heat of optimally doped (Ba0.6K0.4) Fe2As2, where C changes by 0.1 JK(-1) at T-c = 37.3 K. Fe-57-Mobbauer spectra show full magnetic hyperfine field splitting, indicative of antiferromagnetic (AF) ordering at 4.2K in samples with x = 0-0.2, but zero magnetic hyperfine field in samples with x = 0.3. The spectra of (Ba0.9K0.1)Fe2As2 and (Ba0.8K0.2) Fe2As2 in the phase transition regions are temperature-dependent superpositions of magnetic and non-magnetic components, caused by inhomogeneous potassium distribution. Our results suggest the co-existence of AF ordering and superconductivity without mesoscopic phase separation in the underdoped region and show unambiguously homogeneous superconducting phases close to optimal doping. This is in contrast to recently reported results about singlecrystal (Ba1-xKx)Fe2As2.