OBSERVATION OF MODULATED MAGNETIC LONG-RANGE ORDER IN LA1.88SR0.12CUO4

Magnetic superlattice peaks are observed in single-crystal neutron-dif fraction measurements on orthorhombic La1.88Sr0.12CuO4 at reciprocal p oints of (1/2+/-epsilon,1/2,0) and (1/2,1/2+/-epsilon,0) in the tetrag onal notation where epsilon = 0.126 +/- 0.003. The La NMR measurement reveals a broadening of the field-swept spectrum below similar to 45 K corresponding to the existence of magnetic order. The remarkable soft ening of longitudinal sound waves along [110] is observed in the same crystal. The features observed in the neutron diffraction, NMR, ana ul trasonic measurements suggest that the dynamical incommensurate spin c orrelation is pinned by a lattice instability toward the low-temperatu re tetragonal phase.