Incommensurate Spin Ordering and Excitations in Underdoped La$_{2-x}$Ba$_x$CuO$_4$

The diverse magnetic properties of the La$_{2-x}$(Sr,Ba)$_x$CuO$_4$ transition metal oxides may be tuned in a controllable way by doping with mobile holes. In one interpretation, the holes are believed to organise into correlated static or dynamic stripes. We report the first observation of static {\it incommensurate} spin ordering in underdoped La$_{2-x}$Ba$_x$CuO$_4$ (x$\sim $0.05, x=0.08) using neutron diffraction. Elastic collinear incommensurate peaks are observed below the superconducting transition (T$_C \sim $27 K) in La$_{2-x}$(Sr,Ba)$_x$CuO$_4$ (x=0.08). In marked contrast, diagonal satellite peaks have been observed at low temperature in positions rotated by 45$^\circ $ within the (HK0) plane for La$_{2-x}$(Sr,Ba)$_x$CuO$_4$ (x$\sim $0.05). Our neutron scattering results are compared with analogous studies on La$_{2-x}$Sr$_x$CuO$_4$ which indicate that such a rotation of the spin structure may be a generic feature of the underdoped La-214 cuprates.