Diagonal and collinear incommensurate spin structures in underdoped La2-xBaxCuO4

We have studied incommensurate spin ordering in single-crystal underdoped La2-xBaxCuO4 with x similar to 0.08, 0.05, and 0.025 using neutron-scattering techniques. Static incommensurate magnetic order is observed in the La2-xBaxCuO4 (x = 0.05 and 0.025) compounds with ordering wave vectors which are rotated by 45 about the commensurate (0.5,0.5,0) position, with respect to that in the superconducting x = 0.08 material. These spin modulations are one dimensional in the x = 0.05 and 0.025 samples, with ordering wave vectors lying along the orthorhombic b* direction. Such a rotation in the orientation of the static spin ordering as a function of increasing Ba doping, from diagonal to collinear, is roughly coincident with the transition from an insulating to a superconducting ground state and is similar to that observed in the related La2-xSrxCuO4 system. This phenomenon is therefore a more generic property of underdoped La-214 cuprates.