Tetragonal magnetic phase in Ba1-xKxFe2As2 from x-ray and neutron diffraction

Combined neutron and x-ray diffraction experiments demonstrate the formation of a low-temperature minority magnetic tetragonal phase in Ba0.76K0.24Fe2As2 in addition to the majority magnetic, orthorhombic phase. The coincident enhancement in the magnetic (1/2 1/2 1) peaks shows that this minority phase is of the same type that was observed in Ba1-xNaxFe2As2 (0.24 <= x <= 0.28), in which the magnetic moments reorient along the c axis. This is evidence that the tetragonalmagnetic phase is a universal feature of the hole-doped iron-based superconductors. The observations suggest that in this regime the energy levels of the C-2 and C-4 symmetric magnetic phases are very close.